Scholarly article on topic 'Effects of Preconditioning on Photosynthesis of Rice Seedlings under Water Stress'

Effects of Preconditioning on Photosynthesis of Rice Seedlings under Water Stress Academic research paper on "Biological sciences"

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{Photosynthesis / "drought stress" / re-watering / "oryza sativa"}

Abstract of research paper on Biological sciences, author of scientific article — Xuemei Li, Lihong Zhang, Lianju Ma

Abstract Rice seedlings (Oryza sativa L.) were grown in a controlled environment and divided into control seedlings (CK1: 80% field capacity was always held), preconditioned seedlings (PT, 6 days mild drought for preconditioning–3 days re-watering–intermediate drought stress) and non-preconditioned seedlings (CK2, 9 days 80% field capacity and immediately followed by intermediate drought). Photosynthetic CO2 exchange, photosynthetic pigment content were measured in CK1 and PT after preconditioning and re-watering as well as CK1, PT and CK2 after intermediate drought. After exposure to 6 days mild drought preconditioning, stomatal conductance (gs), transpiration rate (E) and chlorophyll content were lower, while Car content and Car/Chl ratio were higher in PT than those in CK1. PT had no significant differences in net photosynthetic rate (Pn) compared to CK1. After re-watering, Chla content was higher but Car/Chl ratio was lower in PT than those in CK1, and other parameters of PT were similar to CK1. After exposure to intermediate drought stress for 6 days, PT showed high water use efficiency (WUE) after intermediate drought stress. CK2 suffered more serious injuries than PT as indicated by lower Pn and pigment content, suggesting that the differences in response to intermediate drought stress in CK2 and PT seedlings are closely related to the effect of mild drought preconditioning. It may be concluded that preconditioning made rice seedlings modulate their metabolism such that they could acclimatize more successfully to the fluctuating water stress environment.

Academic research paper on topic "Effects of Preconditioning on Photosynthesis of Rice Seedlings under Water Stress"

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Environmental Sciences ELSEVIER Procedia Environmental Sciences 11 (2011) 1339 - 1345

Conference Title

Effects of Preconditioning on Photosynthesis of Rice Seedlings under Water Stress

Xuemei Li 1a Lihong Zhang2'b, Lianju Ma!'c

1College of Chemical and Life Science,, Shenyang Normal University, Shenyang, China 2Department of Environmental Science, , Liaoning University, Shenyang, China aLxmls132@163.com, bLihongzhang132@163.com, cmalianju@tom.com

Abstract

Rice seedlings (Oryza sativa L.) were grown in a controlled environment and divided into control seedlings (CK1: 80% field capacity was always held), preconditioned seedlings (PT, 6 days mild drought for preconditioning—3 days re-watering—intermediate drought stress) and non-preconditioned seedlings (CK2, 9 days 80% field capacity and immediately followed by intermediate drought). Photosynthetic CO2 exchange, photosynthetic pigment content were measured in CK1 and PT after preconditioning and re-watering as well as CK1, PT and CK2 after intermediate drought. After exposure to 6 days mild drought preconditioning, stomatal conductance (gs), transpiration rate (E) and chlorophyll content were lower, while Car content and Car/Chl ratio were higher in PT than those in CK1. PT had no significant differences in net photosynthetic rate (Pn) compared to CK1. After re-watering, Chla content was higher but Car/Chl ratio was lower in PT than those in CK1, and other parameters of PT were similar to CK1. After exposure to intermediate drought stress for 6 days, PT showed high water use efficiency (WUE) after intermediate drought stress. CK2 suffered more serious injuries than PT as indicated by lower Pn and pigment content, suggesting that the differences in response to intermediate drought stress in CK2 and PT seedlings are closely related to the effect of mild drought preconditioning. It may be concluded that preconditioning made rice seedlings modulate their metabolism such that they could acclimatize more successfully to the fluctuating water stress environment.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Intelligent Information Technology Application Research Association.

Keywords: Photosynthesis; drought stress; re-watering; oryza sativa

1. Introduction

Drought is one of the most severe constrains to crop production. Drought comes in many forms with respect to timing and severity, ranging from long drought seasons when the water supply by rain is lower than the demand, to short periods without rain when plants rely completely on the available water in the soil [1' 2]. As the most important world food crop, rice (Oryza sativa L.) demands tremendous amounts of water during growth, which results in a number of production challenges. In order to tackle such problems, researches are being made to seek effective method to enhance water stress tolerance of the plants. This is based on the fact that all plants apparently have the necessary genetic information to adapt to water stress to some extent.

1878-0296 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Intelligent Information Technology Application

Research Association.

doi:10.1016/j.proenv.2011.12.201

Drought stress has profound effects on plant physiology. Drought causes stomata closure with consecutive reduction in transpiration, decreases in chlorophyll content [3' 4]. Subsequently, photosynthesis is affected by internal water deficiency following stomatal closure. According to Price et al. [5], plant response to water deficit is the optimization of CO2 gain through stomatal aperture while minimizing water loss. Efficient regulation of transpiration can result in higher water use efficiency, thus resulting in a better post-drought recovery and performance of plants after early season drought [6' 7].

Upon appropriate stimulation (preconditioning), plants can increase their resistance against future stress exposure. This phenomenon is known as induced resistance. Since the discovery of priming in plant cell suspension cultures by Kauss et al. [8], priming has been demonstrated in different plant species against pathogens, insects, and abiotic stress [9]. Primed plants display either faster and, or stronger, activation of various defence responses that are induced following attack by either biotic or abiotic stress [10]. Priming can be elicited by exogenous chemicals as well as by exposure to the stress cues themselves [11' 12]. According to previous results [13], complete immersion of 5-day-old tomato seedlings for 12 h in low osmotic potential PEG solutions (-0.5, -0.75 and -1 MPa) induced greater vegetative growth of adult plants under 100 mM NaCl conditions, as well as adaptive physiological responses concerning ionic, nutritional and osmotic regulation. Such an effect might be attributed to the positive changes at the epigenetic level at an early stage of seedling treatment in the form of "stress imprint" that presumably played an important role in enabling the long-term modulation in the gene expression [14].

If rice has the capacity for some form of ''stress imprint'' from previous water stress, we can expect that rice would rapidly modulate their metabolism when water stress recurs. In this study, we have developed a new experimental approach: let rice exposed to mild drought stress (preconditioning) and re-watering, and then exposed to intermediate drought stress. The present study reveals that preconditioning treatment of mild drought stress modulates photosynthetic CO2 exchange and photosynthetic pigment content of rice seedlings, in order to enhance the tolerance in stress conditions.

2. Materials and methods

2. C. Plnmt dntaoinl nmd treatments

Rice seeds were surface sterilized in 2.65 % sodium hypochlorite for 10 min and then washed thrice thoroughly in distilled water. Seeds were then soaked in sterile deionized water at 28 °C for 6 h and then transferred to two sheets of sterile filter paper moistened with deionized water. The seeds were germinated at 28 C for 48 h in the dark. On the next day, the germinated seeds were grown in pots filled with vermiculite under well-watered conditions in a growth chamber (27 C day/20 C night, 16 h/8 h light/dark period, 800 ^mol m-2 s-1 PPFD and 80% relative air humidity). Seedlings were irrigated with nutrient solution.

Seedlings were grown for 8 days and then divided into three groups. (1) CK1: control seedlings, maintained at about 80% field capacity throughout the 15 days experiment by daily watering. (2) CK2: non-preconditioned seedlings, well watered 9 days and then stressed by 40% field capacity (denoted as Intermediate drought). (3) PT: preconditioned seedlings, preconditioned by exposure to mild drought (50% field capacity) for 6 days, re-watered to 80% field capacity for 3 days and then exposed to 40% field capacity for 6 days as for CK2.

2.2 Measurement cf ohlcocphyll ocmtemt

The second leaves (0.1g) were collected and soaked in 10 ml 95% (v/v) ethanol at 4°C in darkness until the tissues became white. Extracts was used to measure the absorbance at 663, 645, and 470 nm. Chlorophyll (Chl) and carotenoids (Car) content were calculated.

2.3 Mcosuacmrots of phEtEsyothctin gos rxnhoogc

Net photosynthesis rate (Pn) of second leaf were measured using a portable photosynthesis system (LI-6400, Li-Cor, Lincoln, NE, USA) with an open system and logged at CO2 concentration of about 370 ^mol mol-1 in the leaf chamber, and at a constant air flow rate of 500 ^mol s-1. Gas exchange was measured under saturated light between 10:00 and 12:00. Stomatal conductance (gs) and transpiration rate (E) were recorded simultaneously with Pn. Nine leaves were measured for each treatment.

2.4 Stotistinol ooolysis

The results were subjected to analysis of independent samples T Test between CK1 and CK2 or PT in each measuring date. The data analysis was carried out using statistical package SPSS 7.5. Comparisons with P < 0.05 were considered significantly different.

3. Results

3.1 Effrnts of pacsEoPitiEdiog oo phEtEsyothctin pigments

Chla and Chlb content declined significantly by the 6 days mild drought (Fig 1). After 3 days of re-watering, Chlb content reached to CK1 value and Chla content was higher than that of CK1. Following intermediate drought, Chla and Chlb content in CK2 showed a significant decrease in comparison with CK1 but Chla content in PT had no change.

Car content significantly increased in PT after mild drought and recovered to the value of CK1 after re-watering (Fig 2). After intermediate drought, Car content in PT was significant higher but was lower in CK2 than that of CK1.

Figure 1. Changes in chlorophyll a content (a), chlorophyll b content (b) and Car content (c) of rice leaves subjected to mild drought for 6 days, re-watering for 3 days and intermediate drought for 6 days. Data are expressed as the mean ± standard deviation (SD) of three replicates. An asterisk symbol denotes significance at p<0.05 between CK2 or PT vs CK1.

Preconditioning treatment of mild drought increased Car/Chl ratio, and after 3 d re-watering Car/Chl ratio significantly declined. Exposed to intermediate drought, non-preconditioned seedlings (CK2) had a significant high Car/Chl ratio, while preconditioned seedlings (PT) showed no significant change in Car/Chl ratio (Fig 2).

Figure 2. Changes in Car/Chl ratio of rice leaves subjected to mild drought for 6 days, re-watering for 3 days and intermediate drought for 6 days.

3.2 Effects of preconditioning on photosynthetic gas exchange

Net photosynthetic rate (Pn) was not significantly altered by the mild-drought preconditioning and subsequent re-watering (Fig 3). After intermediate drought, there was no significant difference between CK1 and PT, but was a significant reduction in CK2. This indicates that the mild-drought preconditioning

alleviated the adverse effect of recurred drought on net photosynthetic rate.

Mild drought Re-watering Intermediate drought

Figure 3. Changes in net photosynthetic rate (Pn) of rice leaves subjected to mild drought for 6 days, re-watering for 3 days and intermediate drought for 6 days.

Stomatal conductance (gs) and transpiration rate (E) decreased significantly by mild-drought but differences disappeared after re-watering (Fig 4). Mild-drought preconditioning did not alleviate the adverse effects of following intermediate drought on stomatal conductance and transpiration rate. PT and CK2 all showed significantly decreased in stomatal conductance and transpiration rate after the intermediate drought environment.

Figure 4. Changes in stomatal conductance, gs (a) and transpiration rate, E (b) of rice leaves subjected to mild drought for 6 days, re-watering for 3 days and intermediate drought for 6 days.

Mild drought and re-watering did not cause any change in water use efficiency (WUE, calculated as Pn/E) (Fig 5). After intermediate drought, preconditioned seedlings had a significant high WUE compared with CK1 and CK2. This indicates that preconditioning was found effective in improving WUE in the recurred drought.

Figure 5. Changes in water use efficiency (WUE) of rice leaves subjected to mild drought for 6 days, re-watering for 3 days and intermediate drought for 6 days.

4. Discussion

The present study was aimed to evaluate the stress alleviation potential of seedling preconditioning (mild drought) for increasing the stress tolerance of rice under intermediate drought stress.

Chl being highly sensitive to soil drought and drought-induced reductions in pigment contents were previously found in several crop species [15' 16]. However, Chla content did not change under intermediate drought stress in PT, which may be related to an acclimating effect of mild drought preconditioning. This is also supported by the fact that after intermediate drought, Chl contents in CK2 were significantly lower than those of CK1, which may be related to thylakoid membrane disintegration due to oxidative stress [17]. Chl loss has also been considered as an adaptive feature, which reduces the possibility of further damage to the photosynthetic machinery by the formation of ROS under an excess of excitation energy [18' 19].

Car, as a nonenzymatic protector dissipates excessive solar energy. In our study, Car/Chl ratio increased under drought stress, indicating that this energy dissipation mechanism helped to decrease the energetic overcharges in PS II and PS I[20], thus enhancing the photoprotection. Net photosynthesis rate indicated that in preconditioning treatment the effective stress is somehow diminish, and hence, the need for increased nonenzymatic protector accumulation is reduced.

The results reported here show that after 6 days exposure to intermediate drought, PT maintained net photosynthesis rate similar to the CK1, while CK2 significantly decreased their net photosynthesis rate (Fig 3). Contrary to previous studies that have related decrease in Pn with reduced Chl in different species [15' 21], we did not find reduction of Pn in PT under intermediate drought. We assume that the process of Pn reduction, possibly, has not already been developed at mild drought and our plants have developed various biochemical and physiological mechanisms to respond and adapt to the later intermediate drought stress. Partial stomatal closure was the dominant mechanism employed by plants to cope with drought [22]. Our findings agree with Lauriano et al. [3], who found that the gs and E values were markedly low in plants under drought stress as compared to CK1. Lang and Palva [23] and Knight et al. [24] suggested that if plants have previously undergone an acclimation process, their reaction to the following stress is more successful. In the present work, gs and E values were decreased in PT and so, WUE was significantly increased. Such an effect might be attributed to the positive changes at the epigenetic level at an early stage of seedling treatment in the form of "stress imprint" that presumably played an important role in adapt better to the intermediate drought stress.

In our experimental design, the only difference between CK2 and PT plants is that PT plants were pre-exposed to mild drought, while the CK2 were not. This experimental approach allowed us to evaluate the ability of rice to adapt to different soil conditions over a short, continuous period of time. The results

suggest that preconditioned plants display either faster and, or stronger, efficient metabolism that are induced by mild drought stress.

Acknowledgment

This study was supported by the National Natural Science Foundation of China (31070285, 30870205) and the Liaoning province Natural Science Foundation (20092070, 20102205). The authors wish to thank Prof. Tao for his help in revision of the manuscript.

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