Biol Trace Elem Res (2013) 156:323–328 DOI 10.1007/s12011-013-9833-2
Nano-TiO2 Improve the Photosynthesis of Tomato Leaves under Mild Heat Stress Mingfang Qi & Yufeng Liu & Tianlai Li
Received: 21 June 2013 / Accepted: 30 July 2013 / Published online: 10 November 2013 # Springer Science+Business Media New York 2013
Abstract Nano-TiO2 has been reported to promote photosynthesis in some crops; however, the mechanism behind this action remains unknown. In this research, the effects of nanoTiO2 on leaf photosynthesis under mild heat stress were investigated. Results showed that the net photosynthetic rate, conductance to H2O, and transpiration rate of tomato leaves increased after application of an appropriate concentration of nano-TiO2. Nano-TiO2 also significantly decreased the minimum chlorophyll fluorescence and relative electron transport in leaves. Under mild heat stress, Nano-TiO2 increased regulated photosystem II (PS II) energy dissipation and decreased non-regulated PS II energy dissipation. These results indicate that nano-TiO2 plays a positive role in promoting photosynthesis in tomato leaves under mild heat stress. Keywords Nano- TiO2 . Tomato . Heat stress . Photosynthesis
Introduction Temperature is an important factor that influences photosynthesis in plants. Heat stress severely restricts plant growth and M. Qi : Y. Liu : T. Li Department of Horticulture, Shenyang Agricultural University, No.120, Dongling Road, Shenyang, Liaoning, China M. Qi e-mail: [email protected] Y. Liu e-mail: [email protected] M. Qi : Y. Liu : T. Li Key Laboratory of Protected Horticulture, Ministry of Education, No.120, Dongling Road, Shenyang, Liaoning, China M. Qi : Y. Liu : T. Li (*) Key Laboratory of Protected Horticulture of Liaoning Province, No.120, Dongling Road, Shenyang, Liaoning, China e-mail: [email protected]
productivity and is classified as one of the major abiotic adversities of many crops [1–4]. In China, the temperature in solar greenhouses is often beyond the appropriate range for horticultural crops, especially during clear weather in the summer and autumn. High temperatures in greenhouses occur mainly because of the lack of necessary equipment, such as environmental controllers; thus, most of the crops cultivated in solar greenhouses are subjected to varying degrees of heat stress. Tomato (Lycopersicon esculentum Mill.) is a widely cultivated vegetable in solar greenhouses in China. Suitable temperatures for plant growth and development range from 13 to 33 °C, and most cultivars are unable to grow in regions where the temperature during the growing season reaches 38 °C or higher, even when exposed for only short durations [5]. Our previous results show that tomato plants are damaged to varying degrees when the temperature reaches 35 °C or higher [6]. Effective prevention of heat stress is thus highly significant in improving the yield and quality of tomato in solar greenhouses. Photosynthesis is sensitive to heat stress. In general, photosystem II (PS II), carbon fixation by Rubisco, and ATPgenerating systems are considered as primary targets of thermal damage in plants [1, 3]. Comparing with PS II function, photosystem I (PS I) is more stable in heat stress [7]. High temperatures can produce reactive oxygen species (ROS), such as superoxide radicals, singlet oxygen, hydrogen peroxide, and hydroxyl radicals, all of which cause oxidative damage to the photosynthetic machinery. This characteristic is known to be an important effect of heat stress. Featuring excellent optical and biological properties, nanoTiO2 is resistant to reactive oxygen scavenging and has been reported to play an active role in photosynthesis. Some evidence indicate that nano-TiO2 can obviously increase photosynthetic effects in spinach, such as increased light absorbance, accelerated transport, and transformation of light energy, and activation of protective enzymes (such as superoxide dismutase (SOD), chloramphenicol acetyltransferase (CAT), ascorbate peroxidases (APX), etc.) [8–11].
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The role of nano-TiO2 in heat stress in crops has not been reported; however, we believe that nano-TiO2 has important functions in heat stress based on its optical and biological activities. Therefore, in the present study, to discuss the effects of nano-TiO2 in tomato photosynthesis under heat stress, we sprayed different concentrations of nano-TiO2 on tomato seedling leaves under 35 °C (mild heat stress) in a Chinese greenhouse. The methods and results are described below.
Qi et al.
WUE=Pn/Tr, and the stomatal limitation loss (Ls) was determined from the relation Ls=Ci/Ca [12]. Photochemical Efficiency Analysis of PS I and PS II
Nano-TiO2 solution was prepared according to the method of Gao et al. [12] via ultrasonic vibration for over 30 min until a milky nano-anatase TiO2 solution was obtained. The average grain size of nano-TiO2 was calculated using a transmission electron microscope (JEM100CX-II, Japan).
The chlorophyll fluorescence of PS II and absorbance changes in PS I were measured simultaneously using a saturation-pulse DUAL-PAM-100 fluorometer (Walz, Effeltrich, Germany). Prior to measurement, tomato leaves were dark-adapted for 30 min. Weakly modulated measuring light from a 620-nm light emitting diode (LED) and red actinic light of 600 μmol m−2 s−1 from LED arrays were delivered to upper leaf surfaces by a DUAL-DR measuring head. Saturating light pulses of 300-ms duration and 10,000 μmol m−2 s−1 intensity from 620-nm LED arrays were simultaneously applied to the upper and lower leaf sides by a DUAL-DR and a DUAL-E emitter unit, respectively. The oxidation/reduction status of PS I reaction centers was followed by changes in absorbance at 830 nm [14].
Plant Materials and Treatments
Data Processing
As described in [13], tomato seeds (L. esculentum Mill cv. Liaoyuanduoli) were germinated and grown in 12×12 cm nutrition pots inside heated solar greenhouses (25/15 °C, average day/night temperature) under natural light and with a relative humidity of 60 % in February 2012 at the Shenyang Agricultural University. When the seeds had grown to the six-leaf stage, three nano-TiO2 (supplied by a local company) treatments, i.e., T1 (0.05 g L−1), T2 (0.1 g L−1), and T3 (0.2 g L−1), were applied. Spring water was used as the control (CK). All treatments were done before exposure of the seedlings to moderately high temperature treatment of 35 °C in a phytotron. The environmental conditions were as follows: 12-h photoperiod with a photosynthetic photon flux density of 600 μmol m−2 s−1, temperature of 35/15 °C (day/night), and relative humidity of 60 %. Photosynthesis analyses were performed 7 days after the treatments.
Using Excel for data processing and mapping and SPSS19 software for statistical analysis, differences among treatments were statistically compared using the Duncan new multiple range method.
Materials and Methods Nano-TiO2 Solution
Results and Discussion Characteristics of Nano-TiO2 The function of nano-TiO2 is related to its size. Thus, we first determined the size of nano-TiO2. TEM micrographs of the nano-TiO2 samples are shown in Fig. 1. The average grain size was 16.04 nm, which is twice the size of materials used in Gao’s experiment [12].
Gas Exchange Parameter Analysis The photosynthetic gas exchange parameters of tomato leaves, including net photosynthetic rate (Pn), transpiration rate (Tr), intercellular CO2 content (Ci), and stomatal conductance (Gs), were measured using a portable photosynthesis system (Li-COR 6400; Li-COR Inc., Nebraska, USA) with constant irradiation (600 μmol photons m−2 s−1, photosynthetically active radiation (PAR)). The CO2 concentration within the chamber was maintained at 350 mmol mol−1; and the flow rate was kept at 400 mmol s−1. The instantaneous water use efficiency (WUE) was calculated using the formula
Fig. 1 Transmission electron microscopy (TEM) image of nano-TiO2
Nano-TiO2 Improve the Photosynthesis of Tomato Leaves
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Effects of Nano-TiO2 on the Photosynthetic Gas Exchange Parameters of Tomato Leaves under Mild Heat Stress
treatment, which means that nano-TiO2 protects the structure of the photosynthetic system under mild heat stress.
Different nano-TiO2 treatments showed varying functions on the photosynthetic gas exchange parameters of tomato leaves (Fig. 2). Pn increased by 9.56 and 18.21 % relative to CK after treatment with T1 and T2, respectively. By contrast, T3 decreased Pn by about 8.5 %. No significant difference between T2 and CK in terms of Ci was observed. However, Ci decreased markedly in T1 and T3. Maximum Gs and Tr were observed after T2 treatment. No significant difference in terms of Gs among T3, T1, and CK was observed. Tr values achieved by T3 where higher than those achieved by CK. Nano-TiO2 treatment generally yielded high Pn and Tr values as well as marked decreases in WUE. Ls levels showed changes following nano-TiO2 treatment. Ls increased in the T1 and T3 groups but decreased in the T2 group. All results showed that T2 can be used to improve tomato leaf photosynthesis under mild heat stress.
Effect of Nano-TiO2 on PS I and PS II Energy Regulation of Tomato Leaves under Mild Heat Stress
Nano-TiO2 treatment significantly decreased the relative electron transport of PS II [ETR (II)], which exhibited decreases of 17.96, 10.44, and 22.09 % following nano-TiO2 treatment
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Fig. 2 Effect of nano-TiO2 on the photosynthetic gas exchange parameters of tomato leaves under mild heat stress. Data are the means of three replicates with standard errors shown by vertical bars. * significant difference (P≤0.05), and ** highly significant difference (P≤0.01), and the same are as follows
Effects of Nano-TiO2 on the Relative Electron Transport of Tomato Leaves under Mild Heat Stress
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The maximum photochemical efficiency (Fv/Fm), an important parameter in stress physiology, ranges between 0.8 and 0.84 in normal plant leaves. We measured the Fv/Fm of tomato leaves after exposure to 7 days of mild heat stress and found that all values remained just above 8.0. No significant differences between the nano-TiO2 treatments and CK (Fig. 3) were observed. The initial minimum fluorescence value (Fo) decreased markedly following nano-TiO2
Intercellular CO2 concentration
Effects of Nano-TiO2 on the PS II Performance of Tomato Leaves under Mild Heat Stress
Under mild heat stress, the quantum yield of PS II [Y (II)] decreased slightly following nano-TiO2 treatment (Fig. 4); however, no obvious difference among T1, T2, and CK was observed. These results are primarily attributed to stronger regulated non-photochemical energy dissipation, as reflected by the higher Y (NPQ) values induced by nano-TiO2 treatment. Nano-TiO2 treatment decreased the quantum yield of non-regulated non-photochemical fluorescence quenching [as reflected by the higher Y (NO) values] obtained, which means that nano-TiO2 can afford protection against mild heat stress. In contrast to PS II, the photochemical yield of PS I [Y (I)] following T1 treatment was substantially lower than that obtained in the CK group; no significant differences among T2, T3, and CK were observed. The low Y (I) in T1 and T3 leaves is believed to arise from the higher donor side limitation of PS I [Y (ND)]. No difference in the quantum yield of non-photochemical energy dissipation was observed in PS I because of acceptor side limitations, Y (NA).
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Fig. 3 Effect of nano-TiO2 on the maximum photochemical efficiency (Fv/Fm) and the minimum chlorophyll fluorescence (Fo) of PS II of tomato leaves under mild heat stress
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compared with CK. No obvious difference in the relative electron transport of PS II [ETR (I)] was observed among T2, T3, and CK; however, T1 treatment showed much lower ETR (I) than CK treatment (Fig. 5). Differences in ETR (II) and ETR (I) changes show that nano-TiO2 may enhance the efficiency of electron transport from PS II to PS I. Effects of Nano-TiO2 on the Quenching of Variable Chlorophyll Fluorescence of Tomato Leaves under Mild Heat Stress We further investigated the photochemical (qP) and nonphotochemical (qN) quenching of variable chlorophyll fluorescence in tomato leaves (Fig. 6). No difference among T1, T2, and CK was found; however markedly decreased qP was observed after T3 treatment. Compared with CK, all nanoTiO2 treatments increased qN values by up to 37.05, 14, and 28.57 %, respectively. Thus, we can deduce that TiO2 induces non-photochemical processes to protect the photosystems of plants.
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Over the past 20 years, great advances in nanotechnology have been achieved. These advances have been accompanied by significant developments of biotechnology. The development of nanotechnology has significantly expanded the application domain of nanomaterials in various fields, such as in water treatment, environmental protection, food processing and packaging, industrial and household purposes, medicine, smart sensors, and agriculture, including plant germination and growth, plant protection and production, pathogen detection, and pesticide/herbicide residue detection [15]. Our data showed that proper concentrations of nano-TiO2 can increase the Pn of tomato leaves, consistent with some of the details described above. By contrast, Gao et al. [12] found that nanoTiO2 improves net photosynthesis of Ulmus elongata seedlings at low light intensity (PAR=800 μmol m−2 s−1), but it was versus at a higher light intensity (PAR=1,600 μmol m−2 s−1). Therefore, the impact of nano-TiO2 on plant photosynthesis is complex, and the applied concentration and other
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Fig. 4 Effect of nano-TiO2 on the quantum yield of PS (II) [Y (II)], regulated energy dissipation of PS II [Y (NPQ)], non-regulated energy dissipation of PS II [Y (NO)], quantum yield of PS (I) [Y (I)], quantum yield of nonphotochemical energy dissipation in PS I due to acceptor side limitation [Y(NA)], and quantum yield of non-photochemical energy dissipation in PS I due to donor side limitation[Y(ND)] of tomato leaves under mild heat stress
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Fig. 5 Effect of nano-TiO2 on the on the relative electron transport in PSII [ETR (II)] and PSI [ETR (I)] of tomato leaves under mild heat stress
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environmental conditions may affect nano-TiO2 activity to some degree. Considering the function of nano-TiO2 on photosynthesis, Hong et al. [8] proved that nano-TiO2 could reduce production rates of free radicals and MDA contents and enhance the SOD, CAT, and POD activities of chloroplasts in vivo to maintain membrane structure stability in spinach. Their study also provided evidence that the function of nano-TiO2 is related to the activation of the photochemical reaction of chloroplasts [9]. Under antioxidant stress, nano-TiO2 could clear large amounts of O2·- directly and remove ROS indirectly by activating SOD, CAT, GPX, and APX in spinach chloroplasts [11]. Several other studies have suggested that nanoTiO2 enhances Rubisco activity during the spinach growth stage [10, 16, 17]. Yang et al. [18] proved that improvements brought about by nano-TiO2 to the photosynthesis and growth of spinach is related to nitrogen metabolism because nanoTiO2 can increase nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminase activities. Gao’s [12] work proved that nano-TiO2 reduces the photosynthetic capacity of U. elongata seedlings; however, this result is believed to be caused by non-stomatal limitations. In the current study, we observed the effects of nano-TiO2 on the photosynthetic gas exchange parameters of tomato leaves. Nano-TiO2 significantly enhanced Gs and Tr under heat stress (Fig. 1), both of which are necessary for plants to dissipate extra energy. The findings indicate that nano-TiO2 induces plants to open their stomata and cool their leaves by transpiration during heat stress. However, the mechanism behind this action remains unclear. Measuring chlorophyll fluorescence has become a useful and non-invasive technique for obtaining rapid qualitative and quantitative information on photosynthesis [19]. In the current
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research, nano-TiO2 remarkably decreased Fo values, which indicates that PS II is protected from damaging forms of ROS. Increased values of Y (NPQ) and qN also show that PS II can develop more ways to dissipate excess energy under heat stress. The ability of plants to regulate energy distribution and prevent injury is based on the integrity of the cell structure. Thus, nano-TiO2 plays an important role in protecting plants from heat stress, which is related to its ability to generate superoxide ion radicals and hydroxide under light [20]. Plants must ameliorate heat stress because of the possible damage brought about by generation of ROS [21]. NanoTiO2 substantially reduced the ETR (II) in the current study, which indicates that photocatalyzed nano-TiO2 may have some function in electron production and transport. The advantages and disadvantages of the widespread use of manufactured nanomaterials have led to some controversy [22–28]. Kahru and Dubourguier recently summarized the toxicity of synthetic nanoparticles in environmentally relevant species (i.e., nano-TiO2, nano-ZnO, nano-CuO, nano-Ag, SWCNTs, MWCNs, and C60-fullerenes) and classified nano-TiO2 as harmful [L(E)C50 10–100 mg/L] [29]. However, the present work suggests that proper nano-TiO2 usage brings about more benefits than risks to plant cultivation.
Conclusion In summary, the data presented here suggests that appropriate concentrations of nano-TiO2 could efficiently improve photosynthesis in tomato leaves under mild heat stress. Nano-TiO2 enhanced leaf Tr, decreased Fo, and increased Y (NPQ) and qN values. Our results serve as a reference for predicting the effects of nano-TiO2 on plants exposed to heat stress.
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Fig. 6 Effect of nano-TiO2 on the photochemical quenching of variable chlorophyll fluorescence (qP) and non- photochemical quenching of variable chlorophyll fluorescence (qN) of tomato leaves under mild heat stress
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328 Acknowledgments This work was supported by the 12th Five-Year Support Project of China (Grant no.: 2011BAD12B03) the Natural Science Foundation of Liaoning Province of China (Grant no.: 201102192), the Program for Liaoning Excellent Talents in University (Grant no.: LJQ2011069), and the Foundation of Shenyang City of China (Grant no.: F12-277-1-25).
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Nano-TiO2 Improve the Photosynthesis of Tomato Leaves under Mild Heat Stress Mingfang Qi & Yufeng Liu & Tianlai Li
Received: 21 June 2013 / Accepted: 30 July 2013 / Published online: 10 November 2013 # Springer Science+Business Media New York 2013
Abstract Nano-TiO2 has been reported to promote photosynthesis in some crops; however, the mechanism behind this action remains unknown. In this research, the effects of nanoTiO2 on leaf photosynthesis under mild heat stress were investigated. Results showed that the net photosynthetic rate, conductance to H2O, and transpiration rate of tomato leaves increased after application of an appropriate concentration of nano-TiO2. Nano-TiO2 also significantly decreased the minimum chlorophyll fluorescence and relative electron transport in leaves. Under mild heat stress, Nano-TiO2 increased regulated photosystem II (PS II) energy dissipation and decreased non-regulated PS II energy dissipation. These results indicate that nano-TiO2 plays a positive role in promoting photosynthesis in tomato leaves under mild heat stress. Keywords Nano- TiO2 . Tomato . Heat stress . Photosynthesis
Introduction Temperature is an important factor that influences photosynthesis in plants. Heat stress severely restricts plant growth and M. Qi : Y. Liu : T. Li Department of Horticulture, Shenyang Agricultural University, No.120, Dongling Road, Shenyang, Liaoning, China M. Qi e-mail: [email protected] Y. Liu e-mail: [email protected] M. Qi : Y. Liu : T. Li Key Laboratory of Protected Horticulture, Ministry of Education, No.120, Dongling Road, Shenyang, Liaoning, China M. Qi : Y. Liu : T. Li (*) Key Laboratory of Protected Horticulture of Liaoning Province, No.120, Dongling Road, Shenyang, Liaoning, China e-mail: [email protected]
productivity and is classified as one of the major abiotic adversities of many crops [1–4]. In China, the temperature in solar greenhouses is often beyond the appropriate range for horticultural crops, especially during clear weather in the summer and autumn. High temperatures in greenhouses occur mainly because of the lack of necessary equipment, such as environmental controllers; thus, most of the crops cultivated in solar greenhouses are subjected to varying degrees of heat stress. Tomato (Lycopersicon esculentum Mill.) is a widely cultivated vegetable in solar greenhouses in China. Suitable temperatures for plant growth and development range from 13 to 33 °C, and most cultivars are unable to grow in regions where the temperature during the growing season reaches 38 °C or higher, even when exposed for only short durations [5]. Our previous results show that tomato plants are damaged to varying degrees when the temperature reaches 35 °C or higher [6]. Effective prevention of heat stress is thus highly significant in improving the yield and quality of tomato in solar greenhouses. Photosynthesis is sensitive to heat stress. In general, photosystem II (PS II), carbon fixation by Rubisco, and ATPgenerating systems are considered as primary targets of thermal damage in plants [1, 3]. Comparing with PS II function, photosystem I (PS I) is more stable in heat stress [7]. High temperatures can produce reactive oxygen species (ROS), such as superoxide radicals, singlet oxygen, hydrogen peroxide, and hydroxyl radicals, all of which cause oxidative damage to the photosynthetic machinery. This characteristic is known to be an important effect of heat stress. Featuring excellent optical and biological properties, nanoTiO2 is resistant to reactive oxygen scavenging and has been reported to play an active role in photosynthesis. Some evidence indicate that nano-TiO2 can obviously increase photosynthetic effects in spinach, such as increased light absorbance, accelerated transport, and transformation of light energy, and activation of protective enzymes (such as superoxide dismutase (SOD), chloramphenicol acetyltransferase (CAT), ascorbate peroxidases (APX), etc.) [8–11].
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The role of nano-TiO2 in heat stress in crops has not been reported; however, we believe that nano-TiO2 has important functions in heat stress based on its optical and biological activities. Therefore, in the present study, to discuss the effects of nano-TiO2 in tomato photosynthesis under heat stress, we sprayed different concentrations of nano-TiO2 on tomato seedling leaves under 35 °C (mild heat stress) in a Chinese greenhouse. The methods and results are described below.
Qi et al.
WUE=Pn/Tr, and the stomatal limitation loss (Ls) was determined from the relation Ls=Ci/Ca [12]. Photochemical Efficiency Analysis of PS I and PS II
Nano-TiO2 solution was prepared according to the method of Gao et al. [12] via ultrasonic vibration for over 30 min until a milky nano-anatase TiO2 solution was obtained. The average grain size of nano-TiO2 was calculated using a transmission electron microscope (JEM100CX-II, Japan).
The chlorophyll fluorescence of PS II and absorbance changes in PS I were measured simultaneously using a saturation-pulse DUAL-PAM-100 fluorometer (Walz, Effeltrich, Germany). Prior to measurement, tomato leaves were dark-adapted for 30 min. Weakly modulated measuring light from a 620-nm light emitting diode (LED) and red actinic light of 600 μmol m−2 s−1 from LED arrays were delivered to upper leaf surfaces by a DUAL-DR measuring head. Saturating light pulses of 300-ms duration and 10,000 μmol m−2 s−1 intensity from 620-nm LED arrays were simultaneously applied to the upper and lower leaf sides by a DUAL-DR and a DUAL-E emitter unit, respectively. The oxidation/reduction status of PS I reaction centers was followed by changes in absorbance at 830 nm [14].
Plant Materials and Treatments
Data Processing
As described in [13], tomato seeds (L. esculentum Mill cv. Liaoyuanduoli) were germinated and grown in 12×12 cm nutrition pots inside heated solar greenhouses (25/15 °C, average day/night temperature) under natural light and with a relative humidity of 60 % in February 2012 at the Shenyang Agricultural University. When the seeds had grown to the six-leaf stage, three nano-TiO2 (supplied by a local company) treatments, i.e., T1 (0.05 g L−1), T2 (0.1 g L−1), and T3 (0.2 g L−1), were applied. Spring water was used as the control (CK). All treatments were done before exposure of the seedlings to moderately high temperature treatment of 35 °C in a phytotron. The environmental conditions were as follows: 12-h photoperiod with a photosynthetic photon flux density of 600 μmol m−2 s−1, temperature of 35/15 °C (day/night), and relative humidity of 60 %. Photosynthesis analyses were performed 7 days after the treatments.
Using Excel for data processing and mapping and SPSS19 software for statistical analysis, differences among treatments were statistically compared using the Duncan new multiple range method.
Materials and Methods Nano-TiO2 Solution
Results and Discussion Characteristics of Nano-TiO2 The function of nano-TiO2 is related to its size. Thus, we first determined the size of nano-TiO2. TEM micrographs of the nano-TiO2 samples are shown in Fig. 1. The average grain size was 16.04 nm, which is twice the size of materials used in Gao’s experiment [12].
Gas Exchange Parameter Analysis The photosynthetic gas exchange parameters of tomato leaves, including net photosynthetic rate (Pn), transpiration rate (Tr), intercellular CO2 content (Ci), and stomatal conductance (Gs), were measured using a portable photosynthesis system (Li-COR 6400; Li-COR Inc., Nebraska, USA) with constant irradiation (600 μmol photons m−2 s−1, photosynthetically active radiation (PAR)). The CO2 concentration within the chamber was maintained at 350 mmol mol−1; and the flow rate was kept at 400 mmol s−1. The instantaneous water use efficiency (WUE) was calculated using the formula
Fig. 1 Transmission electron microscopy (TEM) image of nano-TiO2
Nano-TiO2 Improve the Photosynthesis of Tomato Leaves
325
Effects of Nano-TiO2 on the Photosynthetic Gas Exchange Parameters of Tomato Leaves under Mild Heat Stress
treatment, which means that nano-TiO2 protects the structure of the photosynthetic system under mild heat stress.
Different nano-TiO2 treatments showed varying functions on the photosynthetic gas exchange parameters of tomato leaves (Fig. 2). Pn increased by 9.56 and 18.21 % relative to CK after treatment with T1 and T2, respectively. By contrast, T3 decreased Pn by about 8.5 %. No significant difference between T2 and CK in terms of Ci was observed. However, Ci decreased markedly in T1 and T3. Maximum Gs and Tr were observed after T2 treatment. No significant difference in terms of Gs among T3, T1, and CK was observed. Tr values achieved by T3 where higher than those achieved by CK. Nano-TiO2 treatment generally yielded high Pn and Tr values as well as marked decreases in WUE. Ls levels showed changes following nano-TiO2 treatment. Ls increased in the T1 and T3 groups but decreased in the T2 group. All results showed that T2 can be used to improve tomato leaf photosynthesis under mild heat stress.
Effect of Nano-TiO2 on PS I and PS II Energy Regulation of Tomato Leaves under Mild Heat Stress
Nano-TiO2 treatment significantly decreased the relative electron transport of PS II [ETR (II)], which exhibited decreases of 17.96, 10.44, and 22.09 % following nano-TiO2 treatment
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Fig. 2 Effect of nano-TiO2 on the photosynthetic gas exchange parameters of tomato leaves under mild heat stress. Data are the means of three replicates with standard errors shown by vertical bars. * significant difference (P≤0.05), and ** highly significant difference (P≤0.01), and the same are as follows
Effects of Nano-TiO2 on the Relative Electron Transport of Tomato Leaves under Mild Heat Stress
(µmol CO2 mol )
The maximum photochemical efficiency (Fv/Fm), an important parameter in stress physiology, ranges between 0.8 and 0.84 in normal plant leaves. We measured the Fv/Fm of tomato leaves after exposure to 7 days of mild heat stress and found that all values remained just above 8.0. No significant differences between the nano-TiO2 treatments and CK (Fig. 3) were observed. The initial minimum fluorescence value (Fo) decreased markedly following nano-TiO2
Intercellular CO2 concentration
Effects of Nano-TiO2 on the PS II Performance of Tomato Leaves under Mild Heat Stress
Under mild heat stress, the quantum yield of PS II [Y (II)] decreased slightly following nano-TiO2 treatment (Fig. 4); however, no obvious difference among T1, T2, and CK was observed. These results are primarily attributed to stronger regulated non-photochemical energy dissipation, as reflected by the higher Y (NPQ) values induced by nano-TiO2 treatment. Nano-TiO2 treatment decreased the quantum yield of non-regulated non-photochemical fluorescence quenching [as reflected by the higher Y (NO) values] obtained, which means that nano-TiO2 can afford protection against mild heat stress. In contrast to PS II, the photochemical yield of PS I [Y (I)] following T1 treatment was substantially lower than that obtained in the CK group; no significant differences among T2, T3, and CK were observed. The low Y (I) in T1 and T3 leaves is believed to arise from the higher donor side limitation of PS I [Y (ND)]. No difference in the quantum yield of non-photochemical energy dissipation was observed in PS I because of acceptor side limitations, Y (NA).
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Fig. 3 Effect of nano-TiO2 on the maximum photochemical efficiency (Fv/Fm) and the minimum chlorophyll fluorescence (Fo) of PS II of tomato leaves under mild heat stress
Qi et al.
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compared with CK. No obvious difference in the relative electron transport of PS II [ETR (I)] was observed among T2, T3, and CK; however, T1 treatment showed much lower ETR (I) than CK treatment (Fig. 5). Differences in ETR (II) and ETR (I) changes show that nano-TiO2 may enhance the efficiency of electron transport from PS II to PS I. Effects of Nano-TiO2 on the Quenching of Variable Chlorophyll Fluorescence of Tomato Leaves under Mild Heat Stress We further investigated the photochemical (qP) and nonphotochemical (qN) quenching of variable chlorophyll fluorescence in tomato leaves (Fig. 6). No difference among T1, T2, and CK was found; however markedly decreased qP was observed after T3 treatment. Compared with CK, all nanoTiO2 treatments increased qN values by up to 37.05, 14, and 28.57 %, respectively. Thus, we can deduce that TiO2 induces non-photochemical processes to protect the photosystems of plants.
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Over the past 20 years, great advances in nanotechnology have been achieved. These advances have been accompanied by significant developments of biotechnology. The development of nanotechnology has significantly expanded the application domain of nanomaterials in various fields, such as in water treatment, environmental protection, food processing and packaging, industrial and household purposes, medicine, smart sensors, and agriculture, including plant germination and growth, plant protection and production, pathogen detection, and pesticide/herbicide residue detection [15]. Our data showed that proper concentrations of nano-TiO2 can increase the Pn of tomato leaves, consistent with some of the details described above. By contrast, Gao et al. [12] found that nanoTiO2 improves net photosynthesis of Ulmus elongata seedlings at low light intensity (PAR=800 μmol m−2 s−1), but it was versus at a higher light intensity (PAR=1,600 μmol m−2 s−1). Therefore, the impact of nano-TiO2 on plant photosynthesis is complex, and the applied concentration and other
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Fig. 4 Effect of nano-TiO2 on the quantum yield of PS (II) [Y (II)], regulated energy dissipation of PS II [Y (NPQ)], non-regulated energy dissipation of PS II [Y (NO)], quantum yield of PS (I) [Y (I)], quantum yield of nonphotochemical energy dissipation in PS I due to acceptor side limitation [Y(NA)], and quantum yield of non-photochemical energy dissipation in PS I due to donor side limitation[Y(ND)] of tomato leaves under mild heat stress
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Nano-TiO2 Improve the Photosynthesis of Tomato Leaves 25
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Fig. 5 Effect of nano-TiO2 on the on the relative electron transport in PSII [ETR (II)] and PSI [ETR (I)] of tomato leaves under mild heat stress
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environmental conditions may affect nano-TiO2 activity to some degree. Considering the function of nano-TiO2 on photosynthesis, Hong et al. [8] proved that nano-TiO2 could reduce production rates of free radicals and MDA contents and enhance the SOD, CAT, and POD activities of chloroplasts in vivo to maintain membrane structure stability in spinach. Their study also provided evidence that the function of nano-TiO2 is related to the activation of the photochemical reaction of chloroplasts [9]. Under antioxidant stress, nano-TiO2 could clear large amounts of O2·- directly and remove ROS indirectly by activating SOD, CAT, GPX, and APX in spinach chloroplasts [11]. Several other studies have suggested that nanoTiO2 enhances Rubisco activity during the spinach growth stage [10, 16, 17]. Yang et al. [18] proved that improvements brought about by nano-TiO2 to the photosynthesis and growth of spinach is related to nitrogen metabolism because nanoTiO2 can increase nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminase activities. Gao’s [12] work proved that nano-TiO2 reduces the photosynthetic capacity of U. elongata seedlings; however, this result is believed to be caused by non-stomatal limitations. In the current study, we observed the effects of nano-TiO2 on the photosynthetic gas exchange parameters of tomato leaves. Nano-TiO2 significantly enhanced Gs and Tr under heat stress (Fig. 1), both of which are necessary for plants to dissipate extra energy. The findings indicate that nano-TiO2 induces plants to open their stomata and cool their leaves by transpiration during heat stress. However, the mechanism behind this action remains unclear. Measuring chlorophyll fluorescence has become a useful and non-invasive technique for obtaining rapid qualitative and quantitative information on photosynthesis [19]. In the current
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research, nano-TiO2 remarkably decreased Fo values, which indicates that PS II is protected from damaging forms of ROS. Increased values of Y (NPQ) and qN also show that PS II can develop more ways to dissipate excess energy under heat stress. The ability of plants to regulate energy distribution and prevent injury is based on the integrity of the cell structure. Thus, nano-TiO2 plays an important role in protecting plants from heat stress, which is related to its ability to generate superoxide ion radicals and hydroxide under light [20]. Plants must ameliorate heat stress because of the possible damage brought about by generation of ROS [21]. NanoTiO2 substantially reduced the ETR (II) in the current study, which indicates that photocatalyzed nano-TiO2 may have some function in electron production and transport. The advantages and disadvantages of the widespread use of manufactured nanomaterials have led to some controversy [22–28]. Kahru and Dubourguier recently summarized the toxicity of synthetic nanoparticles in environmentally relevant species (i.e., nano-TiO2, nano-ZnO, nano-CuO, nano-Ag, SWCNTs, MWCNs, and C60-fullerenes) and classified nano-TiO2 as harmful [L(E)C50 10–100 mg/L] [29]. However, the present work suggests that proper nano-TiO2 usage brings about more benefits than risks to plant cultivation.
Conclusion In summary, the data presented here suggests that appropriate concentrations of nano-TiO2 could efficiently improve photosynthesis in tomato leaves under mild heat stress. Nano-TiO2 enhanced leaf Tr, decreased Fo, and increased Y (NPQ) and qN values. Our results serve as a reference for predicting the effects of nano-TiO2 on plants exposed to heat stress.
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Fig. 6 Effect of nano-TiO2 on the photochemical quenching of variable chlorophyll fluorescence (qP) and non- photochemical quenching of variable chlorophyll fluorescence (qN) of tomato leaves under mild heat stress
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328 Acknowledgments This work was supported by the 12th Five-Year Support Project of China (Grant no.: 2011BAD12B03) the Natural Science Foundation of Liaoning Province of China (Grant no.: 201102192), the Program for Liaoning Excellent Talents in University (Grant no.: LJQ2011069), and the Foundation of Shenyang City of China (Grant no.: F12-277-1-25).
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