Environmental Geochemistry and Health, 1991, 13(3) page 165

Accumulation of selenium in sugarcane

(Sachharum officinarumLinn.) in seleniferous areas of Punjab, India Karaj S. Dhillon and Surjit K. Dhillon Department of Soils, Punjab Agricultural University, Ludhiana- 141 004, India

Abstract

A survey was conducted during 1986-88 to assess the level and pattern of accumulation of selenium in sugarcane plants in seleniferous areas of Punjab (I.ndia). Total and water-extractable (available) selenium ranged from 0.55 to 2.58 (mean 1.43 _+ 0.67) mg kg-1 and from 0.02 to 0.05 (mean 0.033 _+0.007) mg kg-1, respectively, in seleniferous areas. Corresponding values from non-seleniferous areas were 0.23-0.55 (mean 0.36_+ 0.08) mg kg -1 and 0.015-0.025 (mean 0.020 _+ 0.003) mg kg -1, respectively. Sugarcane tops from seleniferous areas accumulated high levels of selenium ranging from 7.9 to 67.5 mg kg-'. These selenium levels were 6-14 times higher than those from non-seleniferous areas. During the early stages of growth (June), the selenium content was highest but decreased during the months of July and August and then did not change up to maturity. In the seleniferous areas sugarcane tops and canes at maturity contained 5.7-9.5 and 1.8-2.1 mg Se kg-', respectively. However, the tops and canes of plants growing near the permanent boundary (bundh) contained 9.5-18.8 and 2.1-2.4 mg Se kg -1, respectively. in a field experiment on sugarcane, application of gypsum up to 1 ton ha-1 resulted in a significant reduction of selenium content in sugarcane tops as well as in the cane. Selenium content in sugarcane tops at maturity was reduced from 15.16 to 5.08 mg kg -1 by applying gypsum of 1 ton ha-1.

Introduction

Materials and Methods

Amongst the nainera] elements, selenium is notable for its ability to accumulate in forage plants in quantities sufficient to make them lethal to grazing animals. The selenium content exceeding 4-5 nag kg -1 feed are considered toxic to livestock, whereas less than 0.1 nag kg-t has been associated with deficiency disorders like white muscle disease (Underwood, 1977). High Se concentrations in plants have recently been implicated in human (Yang et aL, 1983), wildlife (Mikkelsen el al., t986) and domestic animal (Dhillon and DhiUon, 1991) health problems.

Samples of sugarcane tops (fully mature green leaves from the top) at the harvesting stage along with surface soil (0-15 cm) were collected from the seleniferous and the adjoining non- seleniferous areas as identified by DhiUon and Dhillon (1991). To understand the pattern of Se accumulation, samples of sugarcane tops were also collected at monthly intervals up to maturity from four different locations earmarked in the study area. Three sites (one plant crop and two ratoon crops) were selected in the seleniferous area and one site (plant crop) in the non-seleniferous area. At each site, two samples were collected - one from near the permanent bundh (within 1 m) and the other 10 na away from the bundh (The bundh is a 15-20 cm wide ridge used as a permanent field boundary). This was done to study the accumulation of Se by sugarcane plants growing near the permanent bundhs and in the rest of the field. As in case of wheat, Se toxicity symptoms at first appeared in plants growing near the permanent bundhs and/or permanent water channels (Dhillon and Dhillon, 1991). In another experiment, different levels of gypsum (0, 0.5, 1, 2 and 4 ton ha-t) were applied to a sugarcane crop grown in a seleniferous soil. To monitor the effect of gypsum on Se absorption, samples of sugarcane tops were collected at monthly intervals and those of cane at maturity. From each sampling site, about 500 g of sugarcane

Although Se is not essential for growth of higher plants, the discovery of its essentiality for animals (Schwarz and Foltz, 1957) has stimulated surveys on the Se content of field crops in several countries. Recently Se toxic areas have been identified in some parts of Hoshiarpur and Jalandhar districts of Punjab, India (Dhillon and Dhillon, 1991). Sugarcane is one of the major crops in the region and farmers use the sugarcane tops as fodder for animals over a large part of the year. The present investigation was, therefore, undertaken to assess the level of Se and its pattern of accumulation in sugarcane plants. An attempt was also made to find ways and means to reduce the Se content in sugarcane tops to a level which is safe for animal consumption,

166

Accumulation of selenium in sugarcane Table 1 Some important characteristics of soils from the experimental sites.

Sampling site Non-seleniferous Plant crop

Seleniferous Plant crop

Ratoon crop I Ratoon crop II

Distance from bundh (m)

pH

Electrical Calcium conductivity carbonate (dS m-1) (%)

Organic carbon (%)

Selenium content (me kg-J )

Texture

1 10

8.50

0.21

0.80

8.30

0.25

0.55

0.66 0.67

0.64 0.48

Silty loam Silty loam

1 10 1 10 1 10

8.25 8.20 8.35 8.35 8.35 8.40

0.21 0.20 0.25 0.21 0.23 0.20

5.03 4.30 1.85 2.00 1.13 1.16

0.67 0.65 0.53 0.67 0.55 0.52

1.41 1.64 1.25 1o23 t.41 1.64

Silty loam Silty loam Silty loam Silty loam Silty loam Silty loam

tops and 1 kg of soil sample (in duplicate) were collected. The plant samples were washed with ordinary water followed by distilled and double distilled water. After draining off the excess water, the samples were dried to a constant weight in an oven at 55 -+ 5~ and ground to pass through a 40 mesh sieve in a Willey grinding mill. Soil samples were air dried and ground to pass through a 10 mesh sieve in a wood-pestle and mortar. For analysis of total Se, 1 g portion of plant samples and 2 g of soil samples were digested in a mixture of nitric and perchloric acids in conical flasks (250 mL) covered with water coolers (Sokkaro and Ohn, 1977). Water-extractable (available) Se was obtained by refluxing 50 g soil sample in 1:2 soil to water ratio on a water bath for half-an-hour and filtering the suspension through Whatman filter paper No.44. The acid digests and water extracts were analysed for Se (Cummins et ai., 1964). In general, a very good agreement was observed between duplicate analysis of Se in soil and plant samples. Results are, therefore, reported as average of the two replicates. Plant samples were also analysed for sulphur by turbidimetry (Chesnin and Yien, 1951), phosphorus by vanado-molybdate yellow colour method (Jackson, 1967) and micronutrients by atomic absorption spectrometry. Physical and chemical characteristics of soil samples from the experimental sites were determined using standard procedures outlined by Jackson (1967) and are reported in Table 1.

Results and Discussion

Large variations were observed in Se content of soil and sugarcane tops collected from different locations in the study area (Table 2). Total Se content of surface soil samples from seleniferous areas was 2.4-4.7 times higher than that of non- seleniferous areas. Only a very small fraction of total Se in seleniferous (2.31%) and non-seleniferous (5.55%) soils was present in water soluble form. Top leaves of sugarcane from seleniferous areas accumulated very high levels of Se (7.9-67.5 mg kg-1) which could be harmful to animals (Underwood, 1977). Farmers of the region have observed that their animals do not relish sugarcane tops from seleniferous areas. Moreover, other fodder crops from the region also accumulated Se in toxic levels and animals feeding on such fodder developed typical Se toxicity symptoms (Dhillon and Dhillon, 1991). A positive relationship of Se content of sugarcane leaves with the sulphur content of leaves (0.542*) and total (0.789**) and water soluble Se (0.770**) content of soils was also observed in the present investigation. The Se content of sugarcane leaves was higher at the early stages of growth (June), decreased considerably during the months of July and August and then did not change much up to maturity (Figure 1). The decrease in Se content during July and August is thought to be a dilution effect resulting from vigorous growth during this period. At various growth stages, the Se concentration in the leaves

Table 2 Selenium content of sugarcane tops and soit samples.

Sampling site

Number of samples

Non-seleniferous

11

Seleniferous

22

Figures in parentheses indicate (mean -+ SD).

Selenium content (me kg-i ) Sugarcane Soil tops Total Available 1.4--4.7 (2.5+_0.9) 7.9-67.5 (27.65:18.0)

0.23-0.55 (0.36+0.08) 0.55-2.58 (1.43_+0.67)

0,015-0.025 (0,020-&-0.003) 0.020--0,050 (0,033_+0,007)

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Figure 1 Pattern of selenium accumulation in sugarcane tops and cane. of plant crop (5.6-20.3 rag kg -]) in the seleniferous areas was 4 - 6 t i m e s h i g h e r than that o f l e a v e s from non-seleniferous areas (1.3-3.7 mg kg-1). The leaves of ratoon crop from seleniferous areas contained still higher amounts of Se (7.5-46.5 mg kg-1). This is also true for the Se content of cane at the harvesting stage (Figure 1). The concentration of other nutrients like P, S, Zn, Cu, Fe and Mn was also appreciably higher in tops of ratoon crop during the early stages of growth, but later on the differences narrowed down (Figure 2). The ratoon crop due to its extensive root system was probably able to exploit a larger volume of soil resulting in a higher accumulation of Se and other nutrients. Near the bundhs, the Se content of leaves was found to be 2.0-2.5 times higher in ratoon crop and 1.4-1.7 times higher in plant crop compared with that observed in leaf samples collected 10 m away from permanent bundhs in the seleniferous area. This difference was, however, not observed in leaves from the non- seleniferous area (Figure 1). The content of other nutrients like S, Zn, Mn and Fe in sugarcane leaves from the seleniferous area was also influenced by the distance from bundhs (Figure 2). This was more true for ratoon crop than plant crop. Higher contents of Se in sugarcane leaves from near the bundhs substantiate our previous observations with wheat where Se toxicity symptoms appeared at first in plants growing near the permanent bundhs and/or the water channels than the rest of the field (Dhillon and Dhillon, 1991). In cane, the portion of sugarcane generally consumed by human beings, Se content was much lower than the top leaves at the harvesting stage (Figure 1). At seleniferous sites, the Se concentration in cane was 3.9-8.7 and 3.2-4.2

times lower than that of leaves of ratoon and plant crops, respectively. Similar differences, though of lower magnitude (2.1-2.2 times), were also observed in Se content of cane and leaves from the non- seleniferous area. These results are in agreement with the findings reported by Hamilton and Beath (1963) with a variety of crops grown under different conditions.

Gypsum experiment Application of gypsum resulted in a significant reduction of Se content in sugarcane tops as well as in the cane with a corresponding increase in sulphur content (Figure 3). The percentage decrease in Se content varied from 52.0 to 67.8 at various stages of growth. A significant reduction in Se content was, however, observed only up to a level of I ton gypsum ha-1. Even with the application of the highest level of gypsum (4 ton ha-l), the Se content could not be reduced below approximately 1/4 that of the control. The sugarcane leaves still contained 4.92 mg Se kg-1 which is considered toxic for animals (Underwood, 1977). At any level of gypsum, the Se content of sugarcane tops was found to be 2-3 times higher than cane. Hurd-Karrer (1938) was the first to report that the addition of elemental S or gypsum could reduce the Se uptake by plants. Since then the antagonistic relationship between Se and S has been reported by several research workers (Gissel-Nielsen, 1973; Dhillon et al., 1977; Spencer, 1982) in various plants.

Conclusions The results of the study indicate that sugarcane leaves from seleniferous areas of Punjab accumulate Se to levels which

168

Accumulation of selenium in sugarcane

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