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Validation and application of Salt Stress Unit in dealing with salt-affected growth and leaf proline accumulation of rice plants

Young-Sang Lee1, Soon-Ryang Park2, Seung-Hun Lee3 and Kyoung-Shim Cho4

1 Div. of Life Sciences, Soonchunhyang University, Asan, 336-745. South Korea, Email mariolee@sch.ac.kr
2 Div. of Life Sciences, Soonchunhyang University, Asan, 336-745. South Korea, Email qkrtnsfid@sch.ac.kr
3 Div. of Life Sciences, Soonchunhyang University, Asan, 336-745. South Korea, Email seung111@hotmail.com
4 Div. of Life Sciences, Soonchunhyang University, Asan, 336-745. South Korea, Email cbrs52@hotmail.com

Abstract

The validity of Salt Stress Unit (SSU) proposed as a quantitative parameter for the magnitude of salt stress was tested by exposing rice plants to non-steady salinity conditions mimicking the natural fluctuation of salinity. Changing levels of salinity (mM NaCl) were combined with different salt stress durations (days) and resultant magnitude of salt stresses were expressed in SSU (mM NaCl day). When salt-affected growth reduction relative to control was plotted as a function of SSU, linear growth reduction in proportion to SSU was observed in all measured growth parameters: shoot fresh and dry weight, plant height, leaf number, and tiller numbers. Application of SSU enabled quantitative comparisons of relative sensitivity to salt stress between varieties and growth stages of rice plants, as well as comparisons between rice and wheat seedlings. Application of SSU to relate salt-affected growth reduction and leaf proline content suggested that proline accumulation may not play an important role in preventing salt-induced growth reduction. All these results suggest that SSU calculated by the integration of salinity with respect to stress duration seems to be a practical parameter to represent the magnitude of salt stress in understanding the response of rice plants under salt stress conditions.

Media summary

Salt stress unit, calculated under fluctuating salinity condition can effectively represent the magnitude of salt stress in quantitatively interpreting salt-affected growth of rice plants.

Key Words

salt stress unit, stress magnitude, rice, salinity, stress duration

Introduction

Environmental stress is an important factor limiting crop growth and resultant yield loss. In dealing with environmental stress, the absence of proper parameter to quantify stress magnitude used to be a drawback in understanding the mechanism of plant response to stress. Based upon the assumption that environmental stress consists of two major factors: stress intensity and stress duration, the possibility of quantifying stress magnitude by integrating stress intensity with respect to stress duration was suggested, and a novel parameter ‘Salt Stress Unit (SSU)’ was proposed as a quantitative parameter indicating the magnitude of salt stress (Lee et al., 2004). However, such experimental limitation of previous report (Lee et al., 2004) that constant level of salinity was maintained during the induction of salt stress addressed further questions whether SSU theory could be also applied to field conditions in which soil salinity fluctuates dynamically according to weather and irrigation conditions. The objective of this research was to validate the practicality of SSU concept by applying SSU to the interpretation of rice responses under salt stress conditions of altering salinity, as well as by adopting SSU to quantitative comparisons in salt stress sensitivity between crops, varieties, and growth stages.

Methods

Results of 3 independent experiments with different objectives as followings are presented in this report; validity test for SSU under non-steady salinity conditions (Exp. 1), salt stress sensitivity comparison between rice cultivars and growth stages(Exp. 2), quantitative comparison between rice and wheat crops (Exp. 3).

Plant Materials

Rice (cv. Daean and cv. Chuchong) and wheat (cv. Eunpa) plants were hydroponically cultivated in a glass house by using IRRI nutrient solution (N: 40, P: 10, K: 40, Ca: 40, Mg: 40, Mn: 0.5, S: 40, Mo: 0.05, B: 0.2, Zn: 0.01, Cu: 0.01, Fe: 2.0, Si: 50 ppm). At young seedling or tillering stages, plants were transplanted to a styrofoam plate (25 x 25 cm; each plate contained 5 plants and was used for one treatment replication). After adaptation to transplanting, salt stress was initiated by adding NaCl into nutrient solutions.

Salt stress induction

Salt stresses of various magnitudes were induced by combining different levels of salinity and stress duration. As a salinity source NaCl was used and its concentrations were 0, 40, 60, 80, 100, and 120 mM for non-steady salinity experiment (Exp. 1), while consistent 0, 30, 60, 90, 120, 150, and 180 mM NaCl were used for experiments for cultivar and growth stage comparison (Exp. 2), and for comparing rice and wheat crops (Exp. 3). The stress durations ranged from 0 to 20 days according to experimental design.

Non-steady salinity fluctuation

To expose rice plants to changing level of salinities in Exp. 1, rice plants were transferred to different salinity levels everyday. For this purpose, one hydroponic bed (1,200 x 1,200 x 60 cm) was assigned for each salinity level, and on each transferring day the roots of rice plants to be transferred were washed with nutrient solution equivalent to control, and then moved into another hydroponic bed of different salinity. Sample collections and growth measurements were conducted on 4, 8, and 12th day after beginning of treatment. There were total 36 treatments and some examples of treatments were as followings: for treatment #11 the salinity of each day was 60-40-60-60-40-100-80-120-60-40-120-60 mM NaCl, resulting in SSU of 220, 560 and 840 mM NaCl day on 4, 8, and 12th day of treatment, respectively; and for treatment #35 the salinity of each day was 140-140-80-80- 100-10-40-40-40-60-120-120-100 mM NaCl, resulting in SSU of 440, 660, 1060 mM NaCl day on 4, 8, and 12th day of treatment, respectively.

Growth and leaf proline measurement and data analysis

Plants were harvested and growth parameters such shoot fresh and dry weight, plant height, leaf number, tiller number were measured. To compare the relative growth data collected at different days, measured data were converted into percentage values relative to control of the corresponding measurement day prior to data analysis. Linear regression analyses were adopted to clarify the relationship between relative growth and SSU. Leaf proline content was measured following the methods of Bates (1973).

Results

Validity of SSU under non-steady salinity conditions

Even though salinity was changed daily, relative growth of salt-affected rice plants showed statistically significant linear reduction in proportion to SSU in all measured growth parameters. These results suggested that rice plants exposed to salt stresses of the same SSU exhibited similar relative growth reduction regardless of stress duration or salinity history to which rice plants had been exposed. The sensitivity to salt stress of each growth parameters could be quantitatively compared as followings by using the slope of regression equations relating SSU and relative growth: fresh weight : dry weight : tiller number : leaf number : plant height = 496 : 481 : 453 : 253 : 173 in descending order.

Fig. 1. Relationship between salt stress unit and relative growth in shoot fresh weight (A), shoot dry weight (B), plant height (C), leaf number (D), tiller number (E), and water content (F) of rice plants as affected by salt stress. The salinity levels were changed everyday during stress induction to simulate the fluctuation of soil salinity under natural conditions.

Application of SSU in comparing salt stress sensitivity between varieties and growth stages

The feasibility of SSU was tested by applying SSU into quantitatively comparing salt stress sensitivities between rice varieties and growth stages. For this purpose some part of our previous report (Lee et al., 2004) were replotted (Fig. 2. A, B, C) after converting salinity unit from % NaCl into mM NaCl to make the unit of SSU (mM NaCl day) consistent with newly presented data (Fig. 2. D to I). All growth parameters showed linear reduction in proportion to SSU. Consequently, the slope of each linear regression equation could be interpreted as the degree of growth reduction per unit increment of SSU, i.e., the magnitude of salt stress and used for quantitative value representing the relative sensitivity to salt stress. The salt-affected growth reduction at tillering stage in shoot dry weight and shoot fresh weight were more rapid in cv. Daean compared with cv. Chuchong by 35% and 3%, respectively. The leaf number, however, exhibited rapid decrease in cv. Daean compared to cv. Chuchong by 8%. When different growth stages of cv. Chuchong were compared, all growth parameters such as shoot dry and fresh weight, and leaf number exhibited more rapid decrease at young seedling stage than at tillering stage by 17%, 23%, and 11%, respectively, indicating that rice plants at seedling stage is more sensitive to salt stress than at tillering stage.

Fig. 2. Relationship between salt-affected rice growth reduction and salt stress unit. Relative growth in shoot dry weight (A, D, G), shoot fresh weight (B, E, H), and leaf number (C, F, I) of two rice cultivars (cv. Daeanbyeo for A, B, C, and cv. Chuchongbyeo for D, E, F, G, H, I) at young seedling stage (D, E, F) and tillering stage (A, B, C, G, H, I) were plotted as a function of salt stress unit.

Application of SSU in elucidating relationship between leaf proline accumulation and growth reduction

Proline is an amino acid well known as an osmoticum that accumulates in plant tissues under water or salt stress conditions. The feasibility of SSU in understanding salt-affected physiological response was tested by measuring changes in leaf proline content and relating these data with growth reductions represented by SSU. As previously shown in Fig. 2, rice plants exhibited linear growth reduction in proportion to SSU at both young seedling and tillering stages. Leaf proline content, however, showed no linear relationship when plotted as a function of SSU (Fig. 3A, C); i.e., accumulation of proline could be observed only in rice plants exposed to high salinity over 120 mM NaCl even for a short stress duration, while relatively low salinity levels (≤90 mM NaCl) combined with long stress durations induced no distinctive proline accumulation. These results suggested that proline accumulation might be a salinity-dependent response rather than being proportional to the magnitude of salt stress. Besides, proline content plotted in relation to relative shoot dry weight (Fig. 3B, D) exhibited distinctive relationship at both young seedling and tillering stages. These results suggested that proline accumulation observed in our experiments could not effectively prevent salt-induced growth reduction.

Fig. 3. Leaf proline content of rice plants (cv. Chuchong) as affected by salt stress at young seedling (A, B) and tillering (C, D) stages. Proline content were plotted as a response to salt stress unit (A, C) or as a response of relative shoot dry weight status (B, D) affected by salt stress.

Application of SSU in comparing salt stress sensitivity between crop species

When SSU was applied in comparing salt stress sensitivities between rice and wheat plants, wheat seedlings exhibited higher growth reduction (Fig. 4); the quantitative sensitivity of rice plant relative to wheat was 1.20 and 1.04 in shoot fresh weight and shoot dry weight, respectively.

Fig. 4. Growth reduction in shoot fresh weight (A, C) and dry weight (B, D) of rice (A, B) and wheat (C, D) plants exposed to various magnitude of salt stress plotted as a response to salt stress unit.

Conclusion

Salt stress unit applied under both constant and fluctuating salinity conditions seems to be an effective parameter representing the magnitude of salt stress in quantitatively analysing physiological and growth responses of salt-affected plants as well as in quantitatively comparing relative sensitivity to salt stress between crops, varieties, and growth stages.

References

Lee YS, Park SR, Park HJ and Kwon YW (2004). Salt stress magnitude can be quantified by integration salinity with respect to stress duration. 4th International Crop Science Society.

Levitt J (1980). Responses of plants to environmental stress, Vo1.Ⅱ. Academic Press.

Francois LE, Grieve CM and Maas EV (1994). Time of salt stress affects growth and yield components of irrigated wheat. Agronomy J. 86, 100-107.

Munns R.and Termaat A (1986). Whole-plant responses to salinity. Aust. J. Plant Physiol. 13, 143-160.

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