Modeling the impact of salinity stress on yield of hybrid and improved varieties of rice during different growth stages

Salinity of soil and water resources are the most important factors reducing yield in many crop plants, including rice. Considering the increase in salinity of soil and water resources in many paddy farms in Iran, and considering the different sensitivity of rice plants at different growth stages to water and soil salinity, it is necessary to manage saline water resources in these areas. In this experiment, the effects of different levels of irrigation water salinity on grain yield of two hybrid (Bahar) and improved (Dorfak) rice varieties at different growth stages were investigated. The objective of this experiment was to obtain the best application time of saline water with the minimum reduction of rice grain yield.

This study was conducted as a pot experiment in factorial based on completely randomized design with three factors and three replications in the Rice Research Institute of Iran (RRII), Rasht, Iran. The experimental factors included irrigation water salinity in four levels as the first factor, 2, 4, 6, and 8 dS/m, application time of salinity stress in four levels as the second factor, tillering, panicle formation, heading and maturity growth stages, and rice variety in two levels as the third factor, the hybrid variety of Bahar and the improved variety of Dorfak. Moreover, irrigation with normal water with a salinity of 0.4 dS/m was also considered as a control treatment for comparison with the other salinity treatments. A mix of NaCl+CaCl2 salts in a ratio of 2:1 were used to apply salinity treatments. After the end of each growth stage, leaching was carried out and irrigation with normal water was continued until the end of the growth stage. At the end of the growth and harvesting grains, grain yield based on 14% moisture, biological yield and harvest index were measured. SPSS software was used for statistical analysis and Tukey's test was used to comparison of means at 1% probability level.

The results of analysis of variance indicated that the effect of different salinity levels and its application stage on grain yield of both rice varieties was significant at 1% probability level. The results of comparison of means revealed that the improved variety Doefak showed more salinity tolerance and produced more grain yield at different salinity levels compared to the hybrid variety Bahar. The most sensitive and resistant growth stages to salinity stress were tillering (with an average grain yield of 6.82 g/pot for the hybrid variety Bahar and 8.07 g/pot for the improved variety Dorfak) and maturity (with an average grain yield of 14.71 g/pot for the hybrid variety Bahar and 15.90 g/pot for the improved variety Dorfak) stages, respectively. The lowest biological yield (14.37 g/pot) and harvest index (2.53%) in the improved variety Dorfak was obtained at the tillering stage and the salinity level of 8 dS/m. Also, the lowest biological yield (12.89 g/pot) and harvest index (4.61) in the hybrid variety Bahar was observed at the salinity level of 8 dS/m and at the heading and panicle formation stages, respectively. Modeling the effect of different salinity levels on grain yield indicated that yield variations in different salinity levels and at the different growth stages followed a quadratic equation. Coefficients of determination of the regression equations of grain yield in different salinity levels for the improved variety Dorfak at the tillering (R2=0.99), panicle formation (R2=0.99), heading (R2=0.95) and maturity (R2=0.98) stages, and also for the hybrid variety Bahar at the tillering (R2=0.99), panicle formation (R2=0.99), heading (R2=0.99) and maturity (R2=0.99) stages, revealed that the application time of salinity stress is more important than the salinity levels.

Application of different salinity levels caused a significant difference in grain yield of both improved and hybrid varieties, so that a significant decrease was observed in grain yield of both Dorfak and Bahar varieties with the increasing salinity levels, and the lowest grain yield was obtained at the salinity level of 8 dS/m. Application time of salinity stress was also very important and caused a significant difference in reduction of grain yield. The results of this experiment showed that under the water limitation conditions or in the absence of suitable water quality, it is possible to prevent the reduction of rice grain yield by managing irrigation water at different growth stages and using lower quality water in the final and maturity growth stages.