Morphological, physiological and biochemical bases of salinity tolerance at reproductive stage in rice (Oryza sativa L.)

The study was conducted to assess the genetic variability in tolerance of salt stress in rice and to elucidate the physiological bases of tolerance in a selected set of contrasting genotypes. Using a set of 67 genotypes selected for their variation in salt tolerance during seedling stage, it was shown that there is a wide variation in tolerance during reproductive stage was among genotypes. Salinity caused considerable reduction in several growth and yield parameters including plant height, flag leaf length, tillering ability, days to flowering, grain yield and yield components. Among the growth parameters, flag leaf length was the most reduced and for yield components, spikelet fertility and grain filling were the most affected (reduced by 32% and 29%, respectively). These same parameters were also similarly reduced by salt stress in Experiment 11 by about 26%, 48% and 48%, respectively. Salinity also reduced panicle exsertion in most genotype, and panicle exsertion correlated with peduncle length and spikelet fertility but not with grain yield. Salinity also delayed flowering by about 1 to 12 d in Experiment 1 and by 2 to 6 d in Experiment 2. Photosynthesis, stomatal conductance, internal CO2 concentration and transpiration were also reduced to different extents by salinity. Correlation coefficient analysis revealed significant relationship of net photosynthesis with stomatal conductance (r=0.82*) and with internal CO2 concentration (Ci, r=0.80*), but not with transpiration. The effects of salinity on stomatal conductance and transpiration were more pronounced than on photosynthesis, Ci was the least affected. Chlorophyll a and total chlorophyll concentration were also reduced by salinity, but no chlorophyll b, however, there were significant differences among rice genotypes for chlorophyll a, chlorophyll b and total chlorophyll concentration, with tolerant genotypes showing less reduction than the sensitive genotypes, suggesting the importance of chlorophyll retention during salt stress. Accumulation of soluble sugars in the flag leaf was higher than in other plant parts while accumulation of starch was greater in developing panicles. Sensitive genotypes tend to accumulate more soluble sugars and starch than tolerant genotypes which appears to reflect greater salt injury to the sink rather than the source. Excess uptake affected the accumulation of other essential elements, but with significant variation between genotypes. The uptake of K+, Mg2+, and Ca2+ was reduced in most genotypes which might have affected internal ion homeostasis and hence tolerance of salt stress. Growth and yield of the tolerant breeding line IR67075, was consistently less affected by salt stress, making it a target for further studies. The study confirmed the existence of genetic variation in tolerance to salinity at the reproductive stage in rice and few tolerant genotypes were identified that can be used as donors in breeding and for further studies. Evidently salinity tolerance at reproductive stage is controlled by several interrelated mechanisms that help reduce salt uptake and mitigate its internal adverse effects on growth and yield processes.