Meta-analysis of the effect of salinity stress on grain yield, yield components and photosynthetic pigments in rice (Oryza sativa L.)

Introduction
Rice is often very sensitive to salinity in both of the seedling and reproductive stages, and excess salt/solutes in the soil or irrigation water can cause great damage to it. If a general conclusion has been obtained from the statistical analysis of the results, meta-analysis, we can look at the findings of these studies with more confidence. The purpose of this study is to conduct a meta-analysis to review the studies on the effects of salinity stress in rice.
Materials and methods
In order to obtain the necessary data for the present research, from the agricultural research information banks, it were searched the keywords "rice", "salinity stress", "yield", "yield components", "NaCl", "pigment", "content of elements", "sodium", "vegetative" and "reproductive" or the English synonym of each of these keywords. In this regard, out of 50 published studies, 38 articles were selected on the effect of salinity stress on growth, yield, yield components, content of elements and photosynthetic characteristics of rice and their data were analyzed after extraction. Among the articles, salinity was considered at two levels including mild (4 dS.m-1) and severe (8 dS.m-1). All calculations and drawing of plots were done with comprehensive meta-analysis (CMA) software.
Research findings
According to the statistics evaluating the homogeneity of studies (Q, I2 and Tau2), for grain yield and panicle number in plant under severe salinity stress and plant height under mild stress, the heterogeneity between studies was low, therefore the fixed model was used in the meta-analysis. Random model was used for other traits in both salinity stress conditions due to the heterogeneity between studies. Overall effect size for grain yield, plant height, panicles per plant, filled grains per panicle and potassium content were on the left side of the zero axis in both of severe and mild stress, which indicates a significant reduction of these traits under salinity stress. The overall effect size for severe and mild stress on sodium content were on the right side of the zero axis, which indicates the increasing effect of salinity stress on sodium content. The overall effect size showed a decrease in chlorophyll a and chlorophyll b in both salinity stress conditions and carotenoids in mild stress condition. The grain yield was the most sensitive and vulnerable trait to stress with a decrease of 72.8 and 118.9 percent in salinity of 4 and 8 dS.m-1, respectively. In salinity of 4 and 8 dS.m-1, the decrease in the number of panicles per plant was 30.42 and 35.29%, respectively, and the decrease in the number of filled grains per panicle was 25.16 and 57.84%, respectively. This finding showed that in mild salinity, the decrease in the number of panicles per plant was greater than the decrease in the number of filled grains per panicle; however, with increasing salinity level, the number of filled grains per panicle was more affected and this trait was more sensitive to high levels of salinity stress.
Conclusion
In general, improving number of fertile panicles, number of filled grains per panicle, higher level of potassium and lower level of sodium, reducing the ratio of Na+/K+, maintaining the amount of chlorophyll pigments at the optimal level and prevention of pigment destruction, increasing proline accumulation and activity of catalase and guaiacol peroxidase enzymes, prevention of cell membrane destruction and reduction of ion leakage were suitable criteria for rice salinity tolerance. These criteria can be used to identify and introduce of rice salinity tolerant genotypes. Cultivars incuding Gharib Siah Rihani, Xudao9, Nagdong, Pokkali, Zayandehroud, Seng Tarem and a mutant line of Nemat were genetic resources tolerant to salinity that can be used for breeding programs.