Planta (1989)177:178-184
Plal~_ta 9 Springer-Verlag 1989
Is a decreased water potential after withholding oxygen to roots the cause of the decline of leaf-elongation rates in Zea ma ys L. and Pbaseolus vulgaris L. ? Peter M. Schildwacht Department of Plant Ecology, University of Utrecht, Lange Nieuwstraat 106, NL-3512 PN Utrecht, The Netherlands
Abstract. Leaf-elongation rates of Zea mays L. and Phaseolus vulgaris L. were measured in plants grown for 4 d in nutrient solution bubbled with N2 and in soil-grown waterlogged Phaseolus plants. Leaf water potential in both species was lower 3-4 h after replacing aeration by N2-bubbling. In Zea, the water potential after 24 h or more was the same in control plants and plants with N2 treatment. In Phaseolus, the water potential of inundated plants and plants with N2 treatment was always lower than those of control plants. The leaf-elongation rate of both species was always lower in plants treated with N2, especially during light periods. In Zea, the elongation rate was lowest in the first 24 h, whilst in Phaseolus it was lowest on the last (fourth) day of treatment. There was no difference between N2 treatment and inundation experiments. It is concluded that during the first hours of treatment the leaf-elongation rate was reduced as a consequence of the lower water potential. Thereafter, however, elongation rates were lower than could be expected on the basis of the plant's water relations.
-
Key words: Leaf elongation - Oxygen with holding Phaseolus (water potential) - Root (oxygen deficiency) - Stomatal resistance - Water potential (leaf) - Waterlogging - Zea (water potential.
Introduction Withholding oxygen from roots gives rise to a variety of reactions in the shoot, e.g. a reduction of transpiration and leaf elongation (Kramer 1951; Abbreviations: LER=leaf elongation rate; PEG-200=polyethylene-glyeol 200; RWC = relative water content
Kramer and Jackson 1954; Brouwer 1977; Brouwer and Wiersum 1977; Crawford 1982). The reduction of leaf-elongation rate (LER) following interruption of the O2 supply to the roots was most pronounced in the light (Brouwer 1977), and was supposedly connected with a decreased water potential (Kramer 1951; Brouwer 1977). However, in waterlogged Lycopersicum esculentum plants (Jackson et al. 1978; Bradford and Hsiao 1982), in Triticum aestivum (Trought and Drew 1980) and in Zea mays (Wenkert et al. 1981) no such change in water potential was found. Similarly, Z. mays plants grown in nutrient solutions showed no change in leaf water potential in the first hours after oxygen depletion (Wenkert et al. 1981). Thus the growth reduction in plants with oxygen-deprived root systems does not seem to be connected with a change of water potential. However, a temporary decrease in water potential was found for Phaseolus vulgaris (Wadman-Van Schravendijk and Van Andel 1986). It is well known that the growth of P. vulgaris may be depressed by anaerobic soil (conditions in agricultural practices, depending on the amount of rainfall and soil density. However, the growth of the less sensitive species Zea mays can also be reduced on sandy soils after a period of heavy rain (Boone et al. 1987). Nearly all the previously reported experiments have involved plants growing in waterlogged soil, where the situation will be more complex than in a nutrient solution (Tanaka and Yoshida 1970; Rao and Mikkelsen 1977; Cannell and Lynch 1984). Therefore, the effects of an immediate oxygen depletion in nutrient-solution-grown Phaseolus plants were compared with those of inundation of soil-grown plants. The purpose of the present study was to deter-
P.M. Schildwacht: Water potential and elongation rate of roots m i n e w h e t h e r the r e d u c t i o n i n L E R s h o r t l y a f t e r the s t a r t o f the a n a e r o b i c c o n d i t i o n s c a n be a t t r i b u t e d to a d e c r e a s e i n w a t e r p o t e n t i a l o f the p l a n t i n b o t h species.
179 to flow out of the xylem onto the surface and bubbling continued was it certain that the correct pressure-chamber value had been reached. Otherwise the pressure was allowed to increase. Minus the pressure-chamber value is referred to as the xylem pressure potential (XPP; Richter 1976). Water potential was evaluated as XPP+the osmotic potential of the xylem fluid (Richter 1976).
Material and methods Plant material. Caryopses of Zea mays L. (cv. Caldeira 535) and seeds of Phaseolus vulgaris L. (cv. Hollandse stare) were germinated in moist sand in the greenhouse. After 9 d the plants were transferred to a growth room and placed in a nutrient solution (Steiner 1968) in glass tubes of 76 cm length and 4.5 cm width. The pH of the solution for Zea was between 4 and 5 and for Phaseolus between 6 and 7. Maize plants were grown until the seventh leaf was visible. Phaseolus plants were used when the third trifoliate leaf had been formed, after three weeks. The air and root temperatures were between 23 and 28~ C. The relative humidity (RH) was 40%_+10. Phaseolus plants were also germinated in square pots (9 cm diameter; 10 cm high; 1 seed per pot) containing in mixture of sand and leaf-mould (1:2, v/v). Plants were watered every day. Quantum flux density. Plants were grown at a quantum flux density of 230 gmol. m - 2. s- * (400-700 nm, fluorescent tubes, T133; Philips, Eindhoven, The Netherlands, for 16 h a day. Experimentalprocedure. Forty plants were used in each experiment. Experiments were conducted separately for Zea and Phaseolus plants grown in nutrient solution and Phaseolus plants grown in soil. Within each experiment shoots were cut in order to determine the exudation rate; thereafter, xylem pressure potential and osmotic potential of the xylem fluid of each plant were measured alternately while at the same time the diffusion resistance was measured on other plants. Leaf-elongation rate. The elongation of the youngest visible leaf of Zea was monitored continuously throughout the experiments using a rotary potentiometer (Kleinendorst and Brouwer 1970). The tip of the leaf was attached to a resistance wire (0.15 ram) which was led over a potentiometer spindle. For Phaseolus a horizontally stretched leaf was used to measure the LER in the direction of the leaf tip. The weight attached to the string was 15 g. Leaf growth was also determined by measuring the maximum length of the three leaflets of the trifoliate leaves. In a separate experiment, LER and xylem pressure potential of the xylem of Zea plants were decreased by lowering the osmotic potential of the nutrient solutions by adding different amounts of polyethylene-glycol 200 (PEG-200; BDH Chemicals, Poole, Dorset, UK). Also in this experiment, leaf elongation was measured continuously and the LER was evaluated before the addition of PEG and after 90 min. Bubbling of N 2 and inundation. The root system was deprived of oxygen by means of a stream of N2 gas. Waterlogging was carried out by placing around the pots buckets filled with tap water at the same temperature as the soil. Water potential. Pressure-chamber measurements were made with Zea on the seventh leaf. For Phaseolus the whole shoot was cut 5 cm below the cotyledonary node and placed in the pressure chamber. The rate of increase in pressure was 10 kPa. s -1. Measurements were sometimes disturbed by bubbles at values around 0.15 MPa, giving the impression that the pressure-chamber value had been reached. In that case the surface was wiped off twice. Thereafter, only when water continued
Osmotic potential of the xylem fluid. Xylem sap (10 gl) was collected by increasing the pressure on the leaf in the pressure chamber by 0.2 MPa. The osmotic potential of the xylem sap was measured with a dewpoint thermocouple psychrometer (Wescor, Logan, Utah, USA). Relative water content (RWC). The RWC of the elongation zone of the seventh leaf of Zea and the first trifoliate leaf of Phaseolus was measured. The fresh weight, dry weight and the weight after placing the tissue in water at 5~ for 24 h were obtained. The elongation zone of the seventh leaf is located 1-5 cm from the stem of the plant, and is enclosed by the leaf sheaths of the older leafs. Diffusion resistance. Diffusion resistance of the leaf was determined on the abaxial site of the two primary leaves of Phaseolus and of the seventh leaf of Zea, using a ventilated steady-state porometer (model Li-1600; Licor, Lincoln, Nebr.) USA. Exudation rate. Exudation rate was measured by placing a silicon tube over the remaining part of the stem after severing the above-ground parts at the time indicated. The exuded sap was collected with a syringe and weighed.
Results Z e a p l a n t s in n u t r i e n t solution. T h e L E R o f Z e a mays plants growing in n u t r i e n t solution decreased w i t h i n 20 m i n a f t e r f l u s h i n g the n u t r i e n t s o l u t i o n w i t h N2 gas, a n d d e c r e a s e d d u r i n g t h e n e x t 2.5 h (Fig. 1). I n a d d i t i o n , the L E R o f Z e a p l a n t s f l u s h e d w i t h N z gas w a s l o w e r e d d u r i n g the first d a r k p e r i o d o f t h e l i g h t p e r i o d s a f t e r 24 a n d 48 h (Fig. 2); h o w e v e r , t h e L E R w a s less r e d u c e d i n the s e c o n d d a r k p e r i o d a f t e r the s t a r t o f the N 2 t r e a t m e n t . T h e d i f f u s i o n r e s i s t a n c e to w a t e r v a p o u r o f N i t r e a t e d p l a n t s a f t e r 3.5 h was 13 k s . m - a , s i m i l a r to the v a l u e for c o n t r o l p l a n t s d u r i n g the d a r k p e r i o d w h e n s t o m a t a are s u p p o s e d l y c l o s e d (Fig. 3 A ) . T h e r e l a t i v e w a t e r c o n t e n t ( R W C ) o f the e l o n g a t i o n z o n e , m e a s u r e d 3.5 h a f t e r the s t a r t o f the t r e a t m e n t w i t h N z , h a d d e c r e a s e d f r o m 94 to 8 7 % (Fig. 3 B), b u t r e g a i n e d its o r i g i n a l v a l u e 20 h later. T h e x y l e m p r e s s u r e p o t e n t i a l w a s l o w e r after 3.5 h : -0.88 MPa compared with - 0 . 4 2 M P a i n c o n t r o l p l a n t s , w h e r e a s the o s m o t i c p o t e n t i a l o f the x y l e m sap w a s u n a f f e c t e d (Fig. 3 C). I n a s e p a r a t e e x p e r i m e n t L E R a n d x y l e m pressure p o t e n t i a l d e c r e a s e d after l o w e r i n g the o s m o t i c p o t e n t i a l o f the n u t r i e n t s o l u t i o n b y a d d i n g P E G 200 ( T a b l e 1). A f t e r 90 r a i n o f l o w e r e d o s m o t i c po-
t 80
P.M. Schildwacht: Water potential and elongation rate of roots
,n {ocotro, {: c0ntr0,N2,
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Fig. 1. A continuous registration of the elongation of the seventh leaves of four Zea plants. The arrow indicates the start of bubbling Nz through the nutrient solution
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Fig. 3A-C. Diffusion resistance (A), relative water content (B), xylem pressure potentials of the seventh leaf and osmotic potentials of the xylem fluid of the leaf (C) of Zea mays plants. Bubbling with N2 through the nutrient solution started immediately after the first measurement, as indicated by the arrow. Open symbols=controls; closed symbols= treated plants. At 33 and 57 h the diffusion resistance in the dark period is indicated in A. Bars indicate SE, n = 1 0 for A and n = 5 for B and C
3
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time h Fig. 2. Elongation rate of the seventh leaf of Zea mays. The start of the N2-bubbling through the nutrient solution is indicated. Points are means for the light or dark period of controls ( o - - 9 and treated plants ( e - - e ) . Dark periods are indicated by the bar on the x-axis. Vertical bars indicate SE, n = 15
tential of the solution the L E R had decreased to 40% of the value before the addition of P E G and the xylem pressure potential had decreased from 0.37 M P a to - 0 . 7 7 MPa. Water potential, being the sum of xylem pressure and osmotic potential, R W C and diffusion
resistance all had returned to control values after 1 d and no difference was observed in the following 2 d (Fig. 3).
Phaseolus plants. The L E R of Phaseolus decreased with the I h of N 2 treatment as in Zea (Fig. 4). Although Phaseolus showed some oscillation in growth, because of the circumnutation of the stem, it is obvious that the response to N2 was similar to that in Zea (compare Fig. 1). The L E R of N2treated plants was lower in the light period (Fig. 5) than in the dark period. This growth pattern was repeated for the next 3d (Table 2), The L E R of N2-treated plants decreased more in the light than in the dark periods and diminished in the following 2-3 d to very low values (Table 2). Only at the end
P.M. Schildwacht : Water potential and elongation rate of roots
18i
Table 1. Pressure potential of the xylem (XPP) and leaf-elongation rates (LER), expressed as % of the LER at the start of the treatment, of Zea mays plants grown in nutrient solutions. The XPP was measured 90 rnin after decreasing the osmotic potential of the rmtrient solution with different amounts of PEG-200. The LER were evaluated at the start and 90 nn~n decreasing the osmotic potential from continuous registrations of the LER. Mean_+SD, n = 4 - 9 . The values differing in the letter attached are significantly different, P < 0.05 by ' t ' test Nutrient solutions with PEG-200 LER (%) XPP (MPa)
0-10 -0.97_+0.17a
I
--
10 30 -0.83_+0.15a
PHASEOLUS
i
Controls 30-50 -0.77_+ 0.20a
50-70 -0.54
I
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70-90 -0.45_+0.08b
0.12b
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90-110 -0.37_+0.04c
PhASEdLUS
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20
94
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3
4
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Fig, 4. Continuous registration of the length of the center leaflet of the third trifoliate leaf of Phaseohts plants grown in nutrient solution. The lower two lines refer to the N2-treated plants
E r O.
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._
of the first dark period, the L E R of N2-treated plants was as high as that of control plants. The L E R of inundated plants showed the same trend (data not shown). The R W C of the trifoliate leaf of nutrient solution grown Phaseolus plants was lowered 3-4 h after the start of N2-bubbling (Fig. 6A), but reached the control level after 1 day. The same tendency was noticed for leaf R W C in inundated plants (data not shown), The xylem pressure potential was --0.4 M P a lower 3.5 h after the start of the N2 treatment. In contrast to Zea, the difference between treated and control plants remained during the following 3 d, the xylem pressure potential being invariably - 0 . 2 to - 0 . 3 M P a lower (Fig. 6B). No difference was found between the osmotic potentials of the xylem fluids 3.5 h after the start of N2 treatment (Fig. 6 B) or after the start of inundation (Table 3). In the plants grown in nutrient solution the osmotic potentials of the xylem decreased after 3 and 4 d (Fig. 6 B) and were somewhat lower than in soil-grown plants (Table 3).
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Fig. 5. The increase in length of the center leaflet of the third trifoliate leaf of Phaseolus vulgaris plants before and after the start of N2-bubbling through the root medium (indicated by the arrow) in light and dark. The dark period is indicated by the bar on the x-axis. Control (O) and treated (o, A) plants
Diffusion resistance was expected to be high 3.5 h after the start of the treatment because of the lowered water potential, but it remained unchanged during the first 36 h of treatment and then sharply increased (Fig. 6 C). Values were higher for inundated plants compared with N2-treated plants in nutrient solution (Table 3).
182
P.M. Schildwacht: Water potential and elongation rate of roots
Table 3. Xylem pressure potential (XPP), osmotic potential of the xylem, exudation rate and diffusion resistance after different periods of inundation of Phaseolus plants grown in soil. C = control, IN = inundated. Mean 4-SD, n = 10-15. The values differing in the letter attached are significantly different, P < 0.05, by ' t ' test; the comparisons are made vertically and horizontally within each parameter Period (h)
XPP (MPa)
3M 24 48 72
= EN
Osmotic potential (MPa)
C
IN
C
IN
Exudation rate (gg. s- a-(gDW shoot)- 1) C IN
-0.60• -0.58• -0.58• -0.75•
-0.82• -0.78• -0.91• -1.15•
-0.19• -0.08• -0.14• -0.19•
-0.14• -0.12• -0.18• -0.23•
7.2• 8.9• 12.3+6.2 6.5•
100
89
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Discussion
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,
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0
3
1
2
3
time d
Fig. 6A-C. Relative water content (A, xylem pressure potential of the shoot (o, e) and osmotic potential of the xylem sap (t,, 9 (B)), and diffusion resistance of the trifoliate leaf grown in nutrient solution (o, e) or grown in soil (zx, 9 (C) of Phaseolus plants. Arrows indicate start of bubbling with Nz or inundation. Open symbols = controls; closed symbols = treated plants. Measurements were made during the light period. Bars indicate SE; n = 5 (A), or 15 (B), or 10 (C)
Table 2. Mean leaf elongation rates (LER) of Phaseolus plants as measured with a ruler in the dark (D) and light (L) periods on three consecutive days of bubbling N2 through the nutrient solution. Bubbling of N2 started 4.5 h before the first dark period. M e a n • n=12. The values differing in the letter attached are significantly different, P < 0.05, by ' t ' test LER (p.m. s- 1) N2 treatment
Control D Dayl Day2 Day3
6.1• 3.9• 0.6+0.2 1.1• 3.0+1.2 170.0• 7.4• >180.0
i
g_
~8
(ks-m- 1) C IN
Xylem exudation rats, measured 3.5 h after the start of the N2 or inundation treatment (Table 3), were always zero.
.~00
.A 1
0.0 0.18• 0.0 0.0
Diffusion resistance
L
D
0.21_+0A2a 0.21-t-0.13a 0.19• 0.14• 0.15+0.09a 0.06• 0.12• 0.20+0.07a 0.08+0.04d
L 0.09 4- 0.06b --0.01 • -- 0.02 • 0.05e
Growth reduction. The reduction of the L E R during the first hours after N2-bubbling or inundation may be explained by the lowered water potential of the shoot. The L E R of Zea decreased significantly after 20-30 min of Nz-bubbling and by 50% within a few hours (Fig. 1). Leaf water potential was lowered by - 0 . 4 4 M P a and the plants started to wilt. Such a lowering of the water potential decreased the L E R by 62% 90 min after lowering the osmotic potential of the root bathing solution (Table 1). So, the reduction of L E R during the first hours after N2 treatment can be explained by the disturbance of the plant water balance. On the first day of treatment, the increase in stomatal resistance and the decrease in R W C are in accordance with this explanation. A decrease in flow rate across detopped maize roots after exposure to an anaerobic nutrient solution was measured by Everard and Drew (1987). Most of the decrease could be attributed to the reduced hydraulic conductivity. The measured time course for the decrease in hydraulic conductivity is in agreement with the decrease in L E R and water potential as reported here. The water potential of Phaseolus plants was also lowered 3.5 h after N2-bubbling, and by that time the plants were wilted. The R W C also decreased but, unlike in Zea, stomatal resistance was not affected. These results support the notion that, for Phaseotus also, L E R reduction can be explained by the change in the plant water balance, although the amount of reduction in L E R that can be expected for a lowering of plant water potential of - 0 . 2 to 0.4 M P a is not precisely known.
P.M. Schildwacht: Water potential and elongation rate of roots
183
Two phases for L E R reduction. After 24 h and more, a reduction of L E R in Zea was still noticeable, but leaf water potential, stomatal resistance and R W C all indicated that the plant water balance was restored. Therefore, I conclude that for Zea the reduction of L E R caused by an anaerobic root medium has two distinct phases: (i) a reduction that can be fully explained by the lowered leaf water potential, due to a reduced hydraulic conductivity of the roots, in the first hours after the start of the experiment, and (ii)a reduction that is independent of the water balance. In agreement with this second phase are data on Lycopersicum esculentum (Jackson et al. 1978) and other species, where no change in water potential was found. The discrepancy between these results and those of Drew and Sisworo (1977) and Trought and Drew (1980), who concluded that the water balance of the plant was not affected, may be explained by the greater time interval between their measurements. Only in short-term measurements, as done here, can the effect of inundation on the water potential of the plant be demonstrated. Only data on Zea mays (Wenkert et al. 1981) support the possibility of a reduction of L E R during the first hours without a decreased water potential.
10-4 M ABA to the elongation region of Zea mays plants did not have any effect on L E R (data not shown).
The water potential after one or more days. In N 2and inundated Phaseolus plants the water potential was still lower than that of the control plants in the 3 d that followed the start of the experiments. These results are at variance with those of Wadman-Van Schravendijk and Van Andel (1985) 1 and 2 d after flooding Phaseolus plants and those of Trought and Drew (1980) for Triticum aestivum. In their experiments, L E R continued to decrease during subsequent days whilst the leaf water potential was the same or higher than those of control plants after I or 2 d. The discrepancy may be explained by the higher air temperature and lower R H in the experiments reported here, giving a greater evaporational demand for the plants. treated
Abscisic acid as a possible factor in the second phase. Abscisic acid, which accumulates in leaves during and after a water stress, can reduce leaf growth of Triticum plants (Quarrie and Jones 1977). In flooded Pisum plants, the highest ABA concentrations were found in the light period and decreased in the dark periods (Zhang and Davies 1987). This difference in A B A concentrations in light and darkness was noticeable on the second day after the start of the flooding and was considerable from the third day on. However, application of 100 gl
Other explanations. Another possibility is that the L E R is reduced because some " r o o t factor", besides ABA, is transported in the transpiration stream; such a factor could be aminocyclopropane-carboxylic acid (ACC; Bradford and Hsiao 1982). The reduced transpiration during dark periods will bring less of this " r o o t factor" to the leaf-elongation zone if the concentration in the xylem has a maximal value. The enhanced L E R in the dark could be explained by reduced levels of a negative growth factor in the leaf zone as a consequence of lower transpiration rates in the dark. However, in an experiment with a split-root system, shoot-growth reduction of Triticum was alleviated when one root was maintained in an aerated nutrient solution. That experiment lasted 1 4 d (Trought and Drew 1981). This is strong evidence against the contribution of any root-produced toxin since the water uptake continued through both root parts. Unfortunately, the water uptake of each root part was not measured separately. Also the measured time course for ethylene levels in Phaseolus (Wadman-Van Schravendijk and van Andel 1986) seems to exclude ethylene and its precursor ACC as the cause for L E R reduction. Drew and Sisworo (1977) suggested that ion uptake may be impaired by N2 treatment or inundation. Exudation was indeed decreased in Zea and totally blocked in Phaseolus by these treatments, indicating a limited ion uptake. However, this hypothesis too fails to explain the difference between the light and dark period. Concluding remarks. The reduction of LER, leaf water potential and depression of exudation rates showed similar courses during N 2 treatment and inundation. Hence, factors which are supposed to play a role in the soil, but not in nutrient solution, e.g. short-chain carbonic acids and reduced, toxic ions are unlikely to have had an additional effect on L E R during the first days of inundation. In conclusion, N2 treatment and inundation lead to an L E R reduction that could be explained by the lowered water potential during the first hours. However, further testing of this hypotheses is needed before any conclusion can be drawn about the cause of the reduction in L E R after 24, 48 and 72 h. Recently, it was claimed that turgor did not limit L E R in Zea (Termaat et al. 1985). In their experiments water was pressed through the root
184
P.M. Schildwacht: Water potential and elongation rate of roots
by an excess pressure of 0.6 MPa. This technique infiltrates air spaces (B. Huisinga, this department, personal communication) and may lead to a very low oxygen level in the roots (De Boer and Prins 1985). The LER of plants in the experiments of Termaat et al. (1985) probably declined by the same unknown factor that was acting in phase two of the experiments reported in this paper.
Jackson, M.B., Gales, K., Campbell, D.J. (1978) Effect of waterlogged soil conditions on the production of ethylene and on water relationships in tomato plants. J. Exp. Bot. 29, 183-193 Kleinendorst, A., Brouwer, R. (1970) The effect of temperature of the root medium and of the growing point of the shoot on growth, water content and sugar content of maize leaves. Neth. J. Agric. Sci. 18, 140-148 Kramer, P.J. (1951) Causes of injury to plants resulting from flooding of the soil. Plant Physiol. 26, 72~736 Kramer, P.J., Jackson, W.T. (1954) Causes of injury to flooded tobacco plants. Plant Physiol. 29, 241-245 Quarrie, S.A., Jones, H.G. (1977) Effects of abscisic acid and water stress on development and morphology of wheat. J. Exp. Bot. 28, 192-203 Rao, D.N., Mikkelsen, D.S. (1977) Effects of acetic, propionic and butyric acids on rice seedling growth and nutrition. Plant Soil 47, 323-334 Richter, H. (1976) The water status in the plant. Experimental evidence. In: Water and plant life, pp. 42-58, Lange, O.L, Kappen, L., Schulze, E.-E. eds. Springer, Berlin Heidelberg New York Steiner, A.A. (1968) Soilless culture. Proc. 6th Coll. Int. Potash Inst. Florence, pp. 324-341 Tanaka, A., Yoshida, S. (1970) Nutritional disorders of the rice plant. Int. Rice Res. Inst. Tech. Bull. 10, 1-51 Termaat, A., Passioura, J.B., Munns, R. (1985) Shoot turgor does not limit shoot growth of NaCl-affected wheat and barley. Plant Physiol. 77, 869-872 Trought, M.C.T., Drew, M.C. (1980) The development of waterlogging damage in wheat seedlings (Triticum aestivum L). I. Shoot and root growth in relation to changes in the concentrations of dissolved gases and solutes in the soil solution. Plant Soil 54, 77-94 Trought, M.C.T., Drew, M.C. (1981) Alleviation of injury to young wheat plants in anaerobic solution culture in relation to the supply of nitrate and other inorganic nutrients. J. Exp. Bot. 32, 509-522 Wadman-Van Schravendyk, H., Van Andel, O.M. (1985) Interdependence of growth, water relations and abscisic acid level in Phaseolus vulgaris during waterlogging. Physiol. Plant. 63, 215-220 Wadman-Van Schravendyk, H., Van Andel, O.M. (1986) The role of ethylene during flooding of Phaseolus vulgaris. Physiol Plant. 66, 257-264 Wenkert, W., Fausey, N.R., Watters, H.D. (1981) Flooding responses in Zea mays L. Plant Soil 62, 351-366 Zhang, J., Davies, W.J. (1987) ABA in roots and leaves of flooded pea plants. J. Exp. Bot. 38, 649-659
The author thanks Professor O.M. van Andel, Dr. H. Konings and Professor H. Lambers for comments on the manuscript and helpful suggestions, and Rob Welschen for skillful technical assistance.
References Boone, F.R., Van der Werf, H.M.G., Kroesbergen, B., Ter Hag, B.A., Boers, A. (1987) The effect of compaction of the arable layer in sandy soils on the growth of maize for silage. 2. Soil conditions and plant growth. Neth. J. Agric. Sci. 35, 113 128 Bradford, K.J., Hsiao, T.C. (1982) Stomatal behaviour and water relations of waterlogged tomato plants. Plant Physiol. 70, 1508-1513 Brouwer, R. (1977) The effect of soil waterlogging on various physiological processes in maize. Phytotronic Newslett. 15, 75-80 Brouwer, R., Wiersum, L.K. (1977) Root aeration in relation to crop growth. In: Crop physiology, pp. 138-166, Gupta, U.S., ed. Oxford and TBH Publ, Oxford Cannell, R.Q., Lynch, J.M. (1984) Possible adverse effects of decomposing organic matter on plant growth. In: Organic matter and rice, pp. 455475. International Rice Research Institute, Los Banos, Philippines Crawford, R.M.M. (1982) Physiological responses to flooding. In: Encyclopedia of plant physiology, N.S., vol. 12B: Physiological plant ecology II. Water relations and carbon assimilation, pp. 453-479, Lange, O.L., Nobel, P.S., Osmond, C.B., Ziegler, H., eds. Springer, Berlin Heidelberg New York De Boer, A.H., Prins, H.B.A. (1985) Trans root-electrical potential in roots of Plantago media L. as affected by hydrostatic pressure; the induction of an Oz deficient root core. Plant Cell Physiol. 25, 643-655 Drew, M.V., Sisworo, E.J. (1977) Early effects of flooding on nitrogen deficiency and leaf chlorosis in barley. New Phytol. 79, 567 571 Everard, J.D., Drew, M.C. (1987) Mechanisms of inhibition of water movement in anaerobically treated roots of Zea mays L. J. Exp. Bot 38, 1154-1165
Received 3 October 1987; accepted 14 July 1988
Plal~_ta 9 Springer-Verlag 1989
Is a decreased water potential after withholding oxygen to roots the cause of the decline of leaf-elongation rates in Zea ma ys L. and Pbaseolus vulgaris L. ? Peter M. Schildwacht Department of Plant Ecology, University of Utrecht, Lange Nieuwstraat 106, NL-3512 PN Utrecht, The Netherlands
Abstract. Leaf-elongation rates of Zea mays L. and Phaseolus vulgaris L. were measured in plants grown for 4 d in nutrient solution bubbled with N2 and in soil-grown waterlogged Phaseolus plants. Leaf water potential in both species was lower 3-4 h after replacing aeration by N2-bubbling. In Zea, the water potential after 24 h or more was the same in control plants and plants with N2 treatment. In Phaseolus, the water potential of inundated plants and plants with N2 treatment was always lower than those of control plants. The leaf-elongation rate of both species was always lower in plants treated with N2, especially during light periods. In Zea, the elongation rate was lowest in the first 24 h, whilst in Phaseolus it was lowest on the last (fourth) day of treatment. There was no difference between N2 treatment and inundation experiments. It is concluded that during the first hours of treatment the leaf-elongation rate was reduced as a consequence of the lower water potential. Thereafter, however, elongation rates were lower than could be expected on the basis of the plant's water relations.
-
Key words: Leaf elongation - Oxygen with holding Phaseolus (water potential) - Root (oxygen deficiency) - Stomatal resistance - Water potential (leaf) - Waterlogging - Zea (water potential.
Introduction Withholding oxygen from roots gives rise to a variety of reactions in the shoot, e.g. a reduction of transpiration and leaf elongation (Kramer 1951; Abbreviations: LER=leaf elongation rate; PEG-200=polyethylene-glyeol 200; RWC = relative water content
Kramer and Jackson 1954; Brouwer 1977; Brouwer and Wiersum 1977; Crawford 1982). The reduction of leaf-elongation rate (LER) following interruption of the O2 supply to the roots was most pronounced in the light (Brouwer 1977), and was supposedly connected with a decreased water potential (Kramer 1951; Brouwer 1977). However, in waterlogged Lycopersicum esculentum plants (Jackson et al. 1978; Bradford and Hsiao 1982), in Triticum aestivum (Trought and Drew 1980) and in Zea mays (Wenkert et al. 1981) no such change in water potential was found. Similarly, Z. mays plants grown in nutrient solutions showed no change in leaf water potential in the first hours after oxygen depletion (Wenkert et al. 1981). Thus the growth reduction in plants with oxygen-deprived root systems does not seem to be connected with a change of water potential. However, a temporary decrease in water potential was found for Phaseolus vulgaris (Wadman-Van Schravendijk and Van Andel 1986). It is well known that the growth of P. vulgaris may be depressed by anaerobic soil (conditions in agricultural practices, depending on the amount of rainfall and soil density. However, the growth of the less sensitive species Zea mays can also be reduced on sandy soils after a period of heavy rain (Boone et al. 1987). Nearly all the previously reported experiments have involved plants growing in waterlogged soil, where the situation will be more complex than in a nutrient solution (Tanaka and Yoshida 1970; Rao and Mikkelsen 1977; Cannell and Lynch 1984). Therefore, the effects of an immediate oxygen depletion in nutrient-solution-grown Phaseolus plants were compared with those of inundation of soil-grown plants. The purpose of the present study was to deter-
P.M. Schildwacht: Water potential and elongation rate of roots m i n e w h e t h e r the r e d u c t i o n i n L E R s h o r t l y a f t e r the s t a r t o f the a n a e r o b i c c o n d i t i o n s c a n be a t t r i b u t e d to a d e c r e a s e i n w a t e r p o t e n t i a l o f the p l a n t i n b o t h species.
179 to flow out of the xylem onto the surface and bubbling continued was it certain that the correct pressure-chamber value had been reached. Otherwise the pressure was allowed to increase. Minus the pressure-chamber value is referred to as the xylem pressure potential (XPP; Richter 1976). Water potential was evaluated as XPP+the osmotic potential of the xylem fluid (Richter 1976).
Material and methods Plant material. Caryopses of Zea mays L. (cv. Caldeira 535) and seeds of Phaseolus vulgaris L. (cv. Hollandse stare) were germinated in moist sand in the greenhouse. After 9 d the plants were transferred to a growth room and placed in a nutrient solution (Steiner 1968) in glass tubes of 76 cm length and 4.5 cm width. The pH of the solution for Zea was between 4 and 5 and for Phaseolus between 6 and 7. Maize plants were grown until the seventh leaf was visible. Phaseolus plants were used when the third trifoliate leaf had been formed, after three weeks. The air and root temperatures were between 23 and 28~ C. The relative humidity (RH) was 40%_+10. Phaseolus plants were also germinated in square pots (9 cm diameter; 10 cm high; 1 seed per pot) containing in mixture of sand and leaf-mould (1:2, v/v). Plants were watered every day. Quantum flux density. Plants were grown at a quantum flux density of 230 gmol. m - 2. s- * (400-700 nm, fluorescent tubes, T133; Philips, Eindhoven, The Netherlands, for 16 h a day. Experimentalprocedure. Forty plants were used in each experiment. Experiments were conducted separately for Zea and Phaseolus plants grown in nutrient solution and Phaseolus plants grown in soil. Within each experiment shoots were cut in order to determine the exudation rate; thereafter, xylem pressure potential and osmotic potential of the xylem fluid of each plant were measured alternately while at the same time the diffusion resistance was measured on other plants. Leaf-elongation rate. The elongation of the youngest visible leaf of Zea was monitored continuously throughout the experiments using a rotary potentiometer (Kleinendorst and Brouwer 1970). The tip of the leaf was attached to a resistance wire (0.15 ram) which was led over a potentiometer spindle. For Phaseolus a horizontally stretched leaf was used to measure the LER in the direction of the leaf tip. The weight attached to the string was 15 g. Leaf growth was also determined by measuring the maximum length of the three leaflets of the trifoliate leaves. In a separate experiment, LER and xylem pressure potential of the xylem of Zea plants were decreased by lowering the osmotic potential of the nutrient solutions by adding different amounts of polyethylene-glycol 200 (PEG-200; BDH Chemicals, Poole, Dorset, UK). Also in this experiment, leaf elongation was measured continuously and the LER was evaluated before the addition of PEG and after 90 min. Bubbling of N 2 and inundation. The root system was deprived of oxygen by means of a stream of N2 gas. Waterlogging was carried out by placing around the pots buckets filled with tap water at the same temperature as the soil. Water potential. Pressure-chamber measurements were made with Zea on the seventh leaf. For Phaseolus the whole shoot was cut 5 cm below the cotyledonary node and placed in the pressure chamber. The rate of increase in pressure was 10 kPa. s -1. Measurements were sometimes disturbed by bubbles at values around 0.15 MPa, giving the impression that the pressure-chamber value had been reached. In that case the surface was wiped off twice. Thereafter, only when water continued
Osmotic potential of the xylem fluid. Xylem sap (10 gl) was collected by increasing the pressure on the leaf in the pressure chamber by 0.2 MPa. The osmotic potential of the xylem sap was measured with a dewpoint thermocouple psychrometer (Wescor, Logan, Utah, USA). Relative water content (RWC). The RWC of the elongation zone of the seventh leaf of Zea and the first trifoliate leaf of Phaseolus was measured. The fresh weight, dry weight and the weight after placing the tissue in water at 5~ for 24 h were obtained. The elongation zone of the seventh leaf is located 1-5 cm from the stem of the plant, and is enclosed by the leaf sheaths of the older leafs. Diffusion resistance. Diffusion resistance of the leaf was determined on the abaxial site of the two primary leaves of Phaseolus and of the seventh leaf of Zea, using a ventilated steady-state porometer (model Li-1600; Licor, Lincoln, Nebr.) USA. Exudation rate. Exudation rate was measured by placing a silicon tube over the remaining part of the stem after severing the above-ground parts at the time indicated. The exuded sap was collected with a syringe and weighed.
Results Z e a p l a n t s in n u t r i e n t solution. T h e L E R o f Z e a mays plants growing in n u t r i e n t solution decreased w i t h i n 20 m i n a f t e r f l u s h i n g the n u t r i e n t s o l u t i o n w i t h N2 gas, a n d d e c r e a s e d d u r i n g t h e n e x t 2.5 h (Fig. 1). I n a d d i t i o n , the L E R o f Z e a p l a n t s f l u s h e d w i t h N z gas w a s l o w e r e d d u r i n g the first d a r k p e r i o d o f t h e l i g h t p e r i o d s a f t e r 24 a n d 48 h (Fig. 2); h o w e v e r , t h e L E R w a s less r e d u c e d i n the s e c o n d d a r k p e r i o d a f t e r the s t a r t o f the N 2 t r e a t m e n t . T h e d i f f u s i o n r e s i s t a n c e to w a t e r v a p o u r o f N i t r e a t e d p l a n t s a f t e r 3.5 h was 13 k s . m - a , s i m i l a r to the v a l u e for c o n t r o l p l a n t s d u r i n g the d a r k p e r i o d w h e n s t o m a t a are s u p p o s e d l y c l o s e d (Fig. 3 A ) . T h e r e l a t i v e w a t e r c o n t e n t ( R W C ) o f the e l o n g a t i o n z o n e , m e a s u r e d 3.5 h a f t e r the s t a r t o f the t r e a t m e n t w i t h N z , h a d d e c r e a s e d f r o m 94 to 8 7 % (Fig. 3 B), b u t r e g a i n e d its o r i g i n a l v a l u e 20 h later. T h e x y l e m p r e s s u r e p o t e n t i a l w a s l o w e r after 3.5 h : -0.88 MPa compared with - 0 . 4 2 M P a i n c o n t r o l p l a n t s , w h e r e a s the o s m o t i c p o t e n t i a l o f the x y l e m sap w a s u n a f f e c t e d (Fig. 3 C). I n a s e p a r a t e e x p e r i m e n t L E R a n d x y l e m pressure p o t e n t i a l d e c r e a s e d after l o w e r i n g the o s m o t i c p o t e n t i a l o f the n u t r i e n t s o l u t i o n b y a d d i n g P E G 200 ( T a b l e 1). A f t e r 90 r a i n o f l o w e r e d o s m o t i c po-
t 80
P.M. Schildwacht: Water potential and elongation rate of roots
,n {ocotro, {: c0ntr0,N2,
i
30
"6~
g~
20
N2
d~
.-r
E E
* light
N2
O c
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l
10
-O
20
0 "5 105 ; ~ 100
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~8 ~g
95 go
10 I 0U
-10
I
t
I
C
"O
1
2
3 time h
4
5
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Fig. 1. A continuous registration of the elongation of the seventh leaves of four Zea plants. The arrow indicates the start of bubbling Nz through the nutrient solution
E -0.6 ~ -0,4
co
~ -0.2
t
0 4
E
I
"
I
48 time h
I
i
72
96
Fig. 3A-C. Diffusion resistance (A), relative water content (B), xylem pressure potentials of the seventh leaf and osmotic potentials of the xylem fluid of the leaf (C) of Zea mays plants. Bubbling with N2 through the nutrient solution started immediately after the first measurement, as indicated by the arrow. Open symbols=controls; closed symbols= treated plants. At 33 and 57 h the diffusion resistance in the dark period is indicated in A. Bars indicate SE, n = 1 0 for A and n = 5 for B and C
3
0
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-o 2 0
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.
.
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.
.
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time h Fig. 2. Elongation rate of the seventh leaf of Zea mays. The start of the N2-bubbling through the nutrient solution is indicated. Points are means for the light or dark period of controls ( o - - 9 and treated plants ( e - - e ) . Dark periods are indicated by the bar on the x-axis. Vertical bars indicate SE, n = 15
tential of the solution the L E R had decreased to 40% of the value before the addition of P E G and the xylem pressure potential had decreased from 0.37 M P a to - 0 . 7 7 MPa. Water potential, being the sum of xylem pressure and osmotic potential, R W C and diffusion
resistance all had returned to control values after 1 d and no difference was observed in the following 2 d (Fig. 3).
Phaseolus plants. The L E R of Phaseolus decreased with the I h of N 2 treatment as in Zea (Fig. 4). Although Phaseolus showed some oscillation in growth, because of the circumnutation of the stem, it is obvious that the response to N2 was similar to that in Zea (compare Fig. 1). The L E R of N2treated plants was lower in the light period (Fig. 5) than in the dark period. This growth pattern was repeated for the next 3d (Table 2), The L E R of N2-treated plants decreased more in the light than in the dark periods and diminished in the following 2-3 d to very low values (Table 2). Only at the end
P.M. Schildwacht : Water potential and elongation rate of roots
18i
Table 1. Pressure potential of the xylem (XPP) and leaf-elongation rates (LER), expressed as % of the LER at the start of the treatment, of Zea mays plants grown in nutrient solutions. The XPP was measured 90 rnin after decreasing the osmotic potential of the rmtrient solution with different amounts of PEG-200. The LER were evaluated at the start and 90 nn~n decreasing the osmotic potential from continuous registrations of the LER. Mean_+SD, n = 4 - 9 . The values differing in the letter attached are significantly different, P < 0.05 by ' t ' test Nutrient solutions with PEG-200 LER (%) XPP (MPa)
0-10 -0.97_+0.17a
I
--
10 30 -0.83_+0.15a
PHASEOLUS
i
Controls 30-50 -0.77_+ 0.20a
50-70 -0.54
I
'
70-90 -0.45_+0.08b
0.12b
I '
'1
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90-110 -0.37_+0.04c
PhASEdLUS
24 E 6 E o)
20
94
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._c
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2
I
I
3
4
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Fig, 4. Continuous registration of the length of the center leaflet of the third trifoliate leaf of Phaseohts plants grown in nutrient solution. The lower two lines refer to the N2-treated plants
E r O.
12
"6 AG 13) C
._
of the first dark period, the L E R of N2-treated plants was as high as that of control plants. The L E R of inundated plants showed the same trend (data not shown). The R W C of the trifoliate leaf of nutrient solution grown Phaseolus plants was lowered 3-4 h after the start of N2-bubbling (Fig. 6A), but reached the control level after 1 day. The same tendency was noticed for leaf R W C in inundated plants (data not shown), The xylem pressure potential was --0.4 M P a lower 3.5 h after the start of the N2 treatment. In contrast to Zea, the difference between treated and control plants remained during the following 3 d, the xylem pressure potential being invariably - 0 . 2 to - 0 . 3 M P a lower (Fig. 6B). No difference was found between the osmotic potentials of the xylem fluids 3.5 h after the start of N2 treatment (Fig. 6 B) or after the start of inundation (Table 3). In the plants grown in nutrient solution the osmotic potentials of the xylem decreased after 3 and 4 d (Fig. 6 B) and were somewhat lower than in soil-grown plants (Table 3).
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i
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8
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I
12 16 time h
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I
20
24
Fig. 5. The increase in length of the center leaflet of the third trifoliate leaf of Phaseolus vulgaris plants before and after the start of N2-bubbling through the root medium (indicated by the arrow) in light and dark. The dark period is indicated by the bar on the x-axis. Control (O) and treated (o, A) plants
Diffusion resistance was expected to be high 3.5 h after the start of the treatment because of the lowered water potential, but it remained unchanged during the first 36 h of treatment and then sharply increased (Fig. 6 C). Values were higher for inundated plants compared with N2-treated plants in nutrient solution (Table 3).
182
P.M. Schildwacht: Water potential and elongation rate of roots
Table 3. Xylem pressure potential (XPP), osmotic potential of the xylem, exudation rate and diffusion resistance after different periods of inundation of Phaseolus plants grown in soil. C = control, IN = inundated. Mean 4-SD, n = 10-15. The values differing in the letter attached are significantly different, P < 0.05, by ' t ' test; the comparisons are made vertically and horizontally within each parameter Period (h)
XPP (MPa)
3M 24 48 72
= EN
Osmotic potential (MPa)
C
IN
C
IN
Exudation rate (gg. s- a-(gDW shoot)- 1) C IN
-0.60• -0.58• -0.58• -0.75•
-0.82• -0.78• -0.91• -1.15•
-0.19• -0.08• -0.14• -0.19•
-0.14• -0.12• -0.18• -0.23•
7.2• 8.9• 12.3+6.2 6.5•
100
89
c
?
~ - m 80 >* 9.~_ 6C m -1.4
3
1
,
I r
r
160
Discussion
B
'E r2o
-1.2
i "e -1.6 5
inundation
& ~-0.8 a=
9~,
80
1
40 t +i 2
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,
1
2 time d
0
3
1
2
3
time d
Fig. 6A-C. Relative water content (A, xylem pressure potential of the shoot (o, e) and osmotic potential of the xylem sap (t,, 9 (B)), and diffusion resistance of the trifoliate leaf grown in nutrient solution (o, e) or grown in soil (zx, 9 (C) of Phaseolus plants. Arrows indicate start of bubbling with Nz or inundation. Open symbols = controls; closed symbols = treated plants. Measurements were made during the light period. Bars indicate SE; n = 5 (A), or 15 (B), or 10 (C)
Table 2. Mean leaf elongation rates (LER) of Phaseolus plants as measured with a ruler in the dark (D) and light (L) periods on three consecutive days of bubbling N2 through the nutrient solution. Bubbling of N2 started 4.5 h before the first dark period. M e a n • n=12. The values differing in the letter attached are significantly different, P < 0.05, by ' t ' test LER (p.m. s- 1) N2 treatment
Control D Dayl Day2 Day3
6.1• 3.9• 0.6+0.2 1.1• 3.0+1.2 170.0• 7.4• >180.0
i
g_
~8
(ks-m- 1) C IN
Xylem exudation rats, measured 3.5 h after the start of the N2 or inundation treatment (Table 3), were always zero.
.~00
.A 1
0.0 0.18• 0.0 0.0
Diffusion resistance
L
D
0.21_+0A2a 0.21-t-0.13a 0.19• 0.14• 0.15+0.09a 0.06• 0.12• 0.20+0.07a 0.08+0.04d
L 0.09 4- 0.06b --0.01 • -- 0.02 • 0.05e
Growth reduction. The reduction of the L E R during the first hours after N2-bubbling or inundation may be explained by the lowered water potential of the shoot. The L E R of Zea decreased significantly after 20-30 min of Nz-bubbling and by 50% within a few hours (Fig. 1). Leaf water potential was lowered by - 0 . 4 4 M P a and the plants started to wilt. Such a lowering of the water potential decreased the L E R by 62% 90 min after lowering the osmotic potential of the root bathing solution (Table 1). So, the reduction of L E R during the first hours after N2 treatment can be explained by the disturbance of the plant water balance. On the first day of treatment, the increase in stomatal resistance and the decrease in R W C are in accordance with this explanation. A decrease in flow rate across detopped maize roots after exposure to an anaerobic nutrient solution was measured by Everard and Drew (1987). Most of the decrease could be attributed to the reduced hydraulic conductivity. The measured time course for the decrease in hydraulic conductivity is in agreement with the decrease in L E R and water potential as reported here. The water potential of Phaseolus plants was also lowered 3.5 h after N2-bubbling, and by that time the plants were wilted. The R W C also decreased but, unlike in Zea, stomatal resistance was not affected. These results support the notion that, for Phaseotus also, L E R reduction can be explained by the change in the plant water balance, although the amount of reduction in L E R that can be expected for a lowering of plant water potential of - 0 . 2 to 0.4 M P a is not precisely known.
P.M. Schildwacht: Water potential and elongation rate of roots
183
Two phases for L E R reduction. After 24 h and more, a reduction of L E R in Zea was still noticeable, but leaf water potential, stomatal resistance and R W C all indicated that the plant water balance was restored. Therefore, I conclude that for Zea the reduction of L E R caused by an anaerobic root medium has two distinct phases: (i) a reduction that can be fully explained by the lowered leaf water potential, due to a reduced hydraulic conductivity of the roots, in the first hours after the start of the experiment, and (ii)a reduction that is independent of the water balance. In agreement with this second phase are data on Lycopersicum esculentum (Jackson et al. 1978) and other species, where no change in water potential was found. The discrepancy between these results and those of Drew and Sisworo (1977) and Trought and Drew (1980), who concluded that the water balance of the plant was not affected, may be explained by the greater time interval between their measurements. Only in short-term measurements, as done here, can the effect of inundation on the water potential of the plant be demonstrated. Only data on Zea mays (Wenkert et al. 1981) support the possibility of a reduction of L E R during the first hours without a decreased water potential.
10-4 M ABA to the elongation region of Zea mays plants did not have any effect on L E R (data not shown).
The water potential after one or more days. In N 2and inundated Phaseolus plants the water potential was still lower than that of the control plants in the 3 d that followed the start of the experiments. These results are at variance with those of Wadman-Van Schravendijk and Van Andel (1985) 1 and 2 d after flooding Phaseolus plants and those of Trought and Drew (1980) for Triticum aestivum. In their experiments, L E R continued to decrease during subsequent days whilst the leaf water potential was the same or higher than those of control plants after I or 2 d. The discrepancy may be explained by the higher air temperature and lower R H in the experiments reported here, giving a greater evaporational demand for the plants. treated
Abscisic acid as a possible factor in the second phase. Abscisic acid, which accumulates in leaves during and after a water stress, can reduce leaf growth of Triticum plants (Quarrie and Jones 1977). In flooded Pisum plants, the highest ABA concentrations were found in the light period and decreased in the dark periods (Zhang and Davies 1987). This difference in A B A concentrations in light and darkness was noticeable on the second day after the start of the flooding and was considerable from the third day on. However, application of 100 gl
Other explanations. Another possibility is that the L E R is reduced because some " r o o t factor", besides ABA, is transported in the transpiration stream; such a factor could be aminocyclopropane-carboxylic acid (ACC; Bradford and Hsiao 1982). The reduced transpiration during dark periods will bring less of this " r o o t factor" to the leaf-elongation zone if the concentration in the xylem has a maximal value. The enhanced L E R in the dark could be explained by reduced levels of a negative growth factor in the leaf zone as a consequence of lower transpiration rates in the dark. However, in an experiment with a split-root system, shoot-growth reduction of Triticum was alleviated when one root was maintained in an aerated nutrient solution. That experiment lasted 1 4 d (Trought and Drew 1981). This is strong evidence against the contribution of any root-produced toxin since the water uptake continued through both root parts. Unfortunately, the water uptake of each root part was not measured separately. Also the measured time course for ethylene levels in Phaseolus (Wadman-Van Schravendijk and van Andel 1986) seems to exclude ethylene and its precursor ACC as the cause for L E R reduction. Drew and Sisworo (1977) suggested that ion uptake may be impaired by N2 treatment or inundation. Exudation was indeed decreased in Zea and totally blocked in Phaseolus by these treatments, indicating a limited ion uptake. However, this hypothesis too fails to explain the difference between the light and dark period. Concluding remarks. The reduction of LER, leaf water potential and depression of exudation rates showed similar courses during N 2 treatment and inundation. Hence, factors which are supposed to play a role in the soil, but not in nutrient solution, e.g. short-chain carbonic acids and reduced, toxic ions are unlikely to have had an additional effect on L E R during the first days of inundation. In conclusion, N2 treatment and inundation lead to an L E R reduction that could be explained by the lowered water potential during the first hours. However, further testing of this hypotheses is needed before any conclusion can be drawn about the cause of the reduction in L E R after 24, 48 and 72 h. Recently, it was claimed that turgor did not limit L E R in Zea (Termaat et al. 1985). In their experiments water was pressed through the root
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P.M. Schildwacht: Water potential and elongation rate of roots
by an excess pressure of 0.6 MPa. This technique infiltrates air spaces (B. Huisinga, this department, personal communication) and may lead to a very low oxygen level in the roots (De Boer and Prins 1985). The LER of plants in the experiments of Termaat et al. (1985) probably declined by the same unknown factor that was acting in phase two of the experiments reported in this paper.
Jackson, M.B., Gales, K., Campbell, D.J. (1978) Effect of waterlogged soil conditions on the production of ethylene and on water relationships in tomato plants. J. Exp. Bot. 29, 183-193 Kleinendorst, A., Brouwer, R. (1970) The effect of temperature of the root medium and of the growing point of the shoot on growth, water content and sugar content of maize leaves. Neth. J. Agric. Sci. 18, 140-148 Kramer, P.J. (1951) Causes of injury to plants resulting from flooding of the soil. Plant Physiol. 26, 72~736 Kramer, P.J., Jackson, W.T. (1954) Causes of injury to flooded tobacco plants. Plant Physiol. 29, 241-245 Quarrie, S.A., Jones, H.G. (1977) Effects of abscisic acid and water stress on development and morphology of wheat. J. Exp. Bot. 28, 192-203 Rao, D.N., Mikkelsen, D.S. (1977) Effects of acetic, propionic and butyric acids on rice seedling growth and nutrition. Plant Soil 47, 323-334 Richter, H. (1976) The water status in the plant. Experimental evidence. In: Water and plant life, pp. 42-58, Lange, O.L, Kappen, L., Schulze, E.-E. eds. Springer, Berlin Heidelberg New York Steiner, A.A. (1968) Soilless culture. Proc. 6th Coll. Int. Potash Inst. Florence, pp. 324-341 Tanaka, A., Yoshida, S. (1970) Nutritional disorders of the rice plant. Int. Rice Res. Inst. Tech. Bull. 10, 1-51 Termaat, A., Passioura, J.B., Munns, R. (1985) Shoot turgor does not limit shoot growth of NaCl-affected wheat and barley. Plant Physiol. 77, 869-872 Trought, M.C.T., Drew, M.C. (1980) The development of waterlogging damage in wheat seedlings (Triticum aestivum L). I. Shoot and root growth in relation to changes in the concentrations of dissolved gases and solutes in the soil solution. Plant Soil 54, 77-94 Trought, M.C.T., Drew, M.C. (1981) Alleviation of injury to young wheat plants in anaerobic solution culture in relation to the supply of nitrate and other inorganic nutrients. J. Exp. Bot. 32, 509-522 Wadman-Van Schravendyk, H., Van Andel, O.M. (1985) Interdependence of growth, water relations and abscisic acid level in Phaseolus vulgaris during waterlogging. Physiol. Plant. 63, 215-220 Wadman-Van Schravendyk, H., Van Andel, O.M. (1986) The role of ethylene during flooding of Phaseolus vulgaris. Physiol Plant. 66, 257-264 Wenkert, W., Fausey, N.R., Watters, H.D. (1981) Flooding responses in Zea mays L. Plant Soil 62, 351-366 Zhang, J., Davies, W.J. (1987) ABA in roots and leaves of flooded pea plants. J. Exp. Bot. 38, 649-659
The author thanks Professor O.M. van Andel, Dr. H. Konings and Professor H. Lambers for comments on the manuscript and helpful suggestions, and Rob Welschen for skillful technical assistance.
References Boone, F.R., Van der Werf, H.M.G., Kroesbergen, B., Ter Hag, B.A., Boers, A. (1987) The effect of compaction of the arable layer in sandy soils on the growth of maize for silage. 2. Soil conditions and plant growth. Neth. J. Agric. Sci. 35, 113 128 Bradford, K.J., Hsiao, T.C. (1982) Stomatal behaviour and water relations of waterlogged tomato plants. Plant Physiol. 70, 1508-1513 Brouwer, R. (1977) The effect of soil waterlogging on various physiological processes in maize. Phytotronic Newslett. 15, 75-80 Brouwer, R., Wiersum, L.K. (1977) Root aeration in relation to crop growth. In: Crop physiology, pp. 138-166, Gupta, U.S., ed. Oxford and TBH Publ, Oxford Cannell, R.Q., Lynch, J.M. (1984) Possible adverse effects of decomposing organic matter on plant growth. In: Organic matter and rice, pp. 455475. International Rice Research Institute, Los Banos, Philippines Crawford, R.M.M. (1982) Physiological responses to flooding. In: Encyclopedia of plant physiology, N.S., vol. 12B: Physiological plant ecology II. Water relations and carbon assimilation, pp. 453-479, Lange, O.L., Nobel, P.S., Osmond, C.B., Ziegler, H., eds. Springer, Berlin Heidelberg New York De Boer, A.H., Prins, H.B.A. (1985) Trans root-electrical potential in roots of Plantago media L. as affected by hydrostatic pressure; the induction of an Oz deficient root core. Plant Cell Physiol. 25, 643-655 Drew, M.V., Sisworo, E.J. (1977) Early effects of flooding on nitrogen deficiency and leaf chlorosis in barley. New Phytol. 79, 567 571 Everard, J.D., Drew, M.C. (1987) Mechanisms of inhibition of water movement in anaerobically treated roots of Zea mays L. J. Exp. Bot 38, 1154-1165
Received 3 October 1987; accepted 14 July 1988
