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Salt stress magnitude can be quantified by integrating salinity with respect to stress duration

Young-Sang Lee1, Soon-Ryang Park2, Hyo-Jong Park3 and Yong-Woong Kwon4

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 gywhdqkr@hanmail.net
4
School of Plant Science, Seoul National University, Seoul, 151-742. South Korea, Email kwonyw@snu.ac.kr

Abstract

We hypothesized that salt stress consists of two major factors: stress intensity (salinity) and stress duration, and the magnitude of stress can be quantified by integrating salinity with respect to the duration of stress. The resultant value was termed as Salt Stress Unit (SSU), and it was proposed as a quantitative parameter to express the magnitude of stress. To verify our hypothesis, hydroponically cultured rice plants were exposed to salt stresses of various magnitudes which were induced by the combination of different levels of salinity and duration, and subsequent growth reduction was plotted and analysed as a function of SSU. Different parameters from salt-affected plants e.g., fresh and dry weight, plant height, number of leaves, and tiller numbers exhibited statistically significant linear reduction in proportion to SSU, indicating SSU is a practical quantitative parameter to interpret salt-affected growth of rice plants. By using SSU, various quantitative comparisons relating to salt stress were possible; e.g. NaCl exhibited 3.2 time higher growth reduction compared to artificial sea water; rice plants at seedling stage was 2.0 times more sensitive to salt stress than those at tillering stage; salt-affected dry weight reduction was 2.5 times faster than plant height reduction, etc. All these results suggested that growth reduction of rice plants under salt stress could be quantitatively handled by using salt stress unit, a novel quantitative parameter representing the magnitude of stress.

Media summary

Salt stress unit, calculated by integrating salt stress intensity with respect to stress duration, is a practical parameter to quantify the magnitude of salt stress.

Key Words

salt stress unit, quantification, stress magnitude, rice, growth,

Introduction

Under natural conditions, plants are exposed to various kinds of environmental stresses, which readily suppress the possible maximal growth of plants. Unlike the plant responses that can be quantitatively measured with various physiological or growth parameters, the stress itself could not be handled quantitatively due to the lack of a theory to measure the magnitude of environmental stress. Besides, the absence of a practical parameter to evaluate stress magnitude resulted in rare quantitative interpretation of plant responses in proportion to the amount of stress to which plants are exposed. Theoretically any environmental stresses consist of two major factors; one is the intensity and the other is duration of stress. However, most of the plant stress physiology experiments have been conducted by comparing the response of plants exposed to different level of stress intensities for the same stress duration, or by evaluating time-series plant responses at consistently maintained stress intensity levels. This is not the case of natural condition, in which the stress intensity is always in fluctuating condition. In analogy with ‘integrated temperature’ which is commonly used in understanding plant response to temperature, we hypothesized that the magnitude of stress could be quantified by integrating the intensity of stress during the period of stress. The objective of the research proposed here is to develop a theory of Salt Stress Unit (SSU) as a parameter to quantify the magnitude of salt stress, which can simultaneously consider both intensity and duration components of stress. As a model case, the rice plant and salt stress have been selected in this experiment.

Methods

Plant Materials

Rice plants (cv. Daeanbyeo, cv. Chuchongbyeo) 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). Prior to salt stress treatment, rice plants were transferred to a styrofoam plate (25 x 25 cm) and each plate containing 5 plants was used for one treatment.

Salt stress treatment

Salt stresses of various magnitudes were induced by combining salinity and stress duration. As a source of salt, NaCl (in Experiment I) or combination of salts corresponding to artificial sea water composition (in Experiments II and III; NaCl: 29.2, MgSO4: 6.6, MgCl2: 5.5., CaCl2: 1.47, KNO3: 1.0, KH2PO4: 0.058, NaHCO3: 0.042 g L-1,) were used. More details on salinity and stress duration are shown in Table 1.

Calculation of salt stress unit

Salt stress unit was defined as following:

where salinity indicates the intensity of salt stress and t1 and t2 represent initial and end time of stress, respectively. Based upon combination of salinity and stress duration salt stress treatments of the same SSU could be induced; e.g., salt stresses induced by the combination of 0.2 % NaCl for 12 days, 0.4 % NaCl for 6 days, and 0.6 % NaCl for 4 days, and 1.2 % NaCl for 2 days had the same (2.4 NaCl% day) SSU value. The unit of SSU were NaCl% day in Experiment I, and dS m-1 day in Experiments II and III.

Table 1. Summary of experimental conditions

 

Experiment I

Experiment II

Experiment III

Variety

Daean

Chuchong

Chuchong

Growth stage

Tillering

Seedling

Tillering

Salt stress inducer

NaCl

Artificial sea water

Artificial sea water

Stress intensity
(NaCl% or dS m-1)

0, 0.1, 0.2, 0.3, 0.4, 0.6, 1.2 (NaCl%)

0, 3.3, 5.0, 7.5, 10.0, 15.0, 20.0 (dS m-1)

0, 3.3, 5.0, 73.5, 10.0, 15, 20.0 (dS m-1)

Stress duration (days)

0, 1, 2, 3, 4, 6, 12, 20

0, 2, 4, 6, 8, 10

0, 3, 6, 9, 12, 15

Salt Stress Unit

NaCl% day

(dS m-1) day

(dS m-1) day

Range of SSU

0 – 24

0 - 200

0 - 300

Growth measurement and data analysis

On the last day of stress treatment, plants were harvested and shoot fresh and dry weight, root fresh and dry weight, plant height, leaf number, tiller number were measured. To compare relative growth reduction of treated plants measured at different days, measured data were converted into percentage value relative to control of measurement day, and were used for data analysis. Linear regression analyses were adopted to clarify the relationship between relative growth reduction and SSU.

Results

Rice plants treated with salt stress of the same salt stress unit exhibited similar growth reduction in all measured growth parameters such as plant height, leaf and tiller numbers, shoot and root fresh and dry weight. Salt-affected rice plants showed linear growth reduction in proportion to salt stress unit (Table 2). Based upon the slope of linear regression equations relating growth reduction and SSU, quantitative comparisons between growth stages, the origin of salts, growth parameters etc. were possible. Rice plants at seedling stage were 2.0 times more sensitive to salt stress than those at tillering stage. NaCl (%) induced 3.2 times higher growth reduction than artificial sea water (dS m-1). Estimated relative sensitivity to salt stress of plant height : leaf number : tiller number : shoot FW : root FW : total FW : shoot DW : root DW : total DW was 2.52 : 3.65 : 4.21 : 6.30 : 5.10 : 6.03 : 6.09 : 5.75 : 6.06.

Fig. 2. Relative growth in plant height (A), leaf number (B), tiller number (C), shoot fresh weight (D), root fresh weight (E), total fresh weight (F), shoot dry weight (G), and root dry weight (H) of rice plants exposed to salt stress of various magnitudes plotted as a function of salt stress unit.

Table 2. Results of linear regression analysis relating the relative growth of salt-affected rice plants (Y) and salt stress unit (X) at young and tillering stage.

Experiment

Unit of SSU (X)

Growth parameter (Y)

Regression equation

R2

Significance

I

NaCl% day

plant height

Y= 95.3 - 2.5X

0.5

<0.01

leaf number

Y= 97.2 - 3.7X

0.81

<0.01

tiller number

Y= 95.4 - 4.2X

0.66

<0.01

shoot fresh weight

Y= 89.0 - 6.3X

0.72

<0.01

root fresh weight

Y=194.6 - 5.1X

0.57

<0.01

total fresh weight

Y= 93.8 - 6.0X

0.73

<0.01

shoot dry weight

Y= 95.1 – 6.1X

0.7

<0.01

root dry weight

Y=104.0 - 5.8X

0.65

<0.01

total dry weight

Y= 97.0 - 6.1X

0.7

<0.01

II

(dS m-1) day

plant height

Y=101.6 - 0.2X

0.83

<0.01

shoot dry weight

Y=100.9 - 0.4X

0.85

<0.01

root dry weight

Y=103.6 - 0.3X

0.81

<0.01

III

(dS m-1) day

plant height

Y= 98.4 - 0.1X

0.95

<0.01

tiller numbers

Y=100.0 - 0.2X

0.83

<0.01

shoot dry weight

Y= 91.3 - 0.1X

0.86

<0.01

root dry weight

Y= 99.3 - 0.1X

0.79

<0.01

Conclusion

The proposed salt stress unit, defined as the integrated salinity with respect to stress duration is a practical parameter representing the quantitative magnitude of salt stress to understand the salt-affected growth reduction of rice plants.

References

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.

Soussi M, Ocana A and Lluch C (1998). Effects of salt stress on growth, photosynthesis and nitrogen fixation in chick-pea (Cicer arietinum L.). J. Exp. Bot. 49, 1329-1337.

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