12. Physiological Adaptation of Alfalfa Genotypes to Salt Stress (One of Deleterious Impacts of Climate Change)

Plant physiological processes are invariably linked to the deleterious influences of climate change. This study seeks to identify the physiological responses of some salt tolerant ecotypes of alfalfa when exposed to salt stress during the mature phase of growth. Five Iranian alfalfa ecotypes representative of three climatic zones were evaluated for their response to salt stress in a split plot trial, with three replications, where the main plots were five ecotypes and subplots were the levels of salinity. The seeds were planted in plastic pots filled with sand and immersed in nutrient solution. After more than 2 months when the plant became mature the EC of nutrient solution was increased gradually by adding sodium chloride. At every 15 days the EC increased by 3 dS m⁻¹, and the final EC was 18 dS m⁻¹. The EC at each stage of salt addition was 6, 12, and 18 dS m⁻¹. At each stage of salinity the physiological characteristics of alfalfa ecotypes were measured including: crop growth rate, photosynthesis rate, chlorophyll content, stomatal conductance, and chlorophyll fluorescence. The results showed that the physiological parameters were affected by salt concentration and there were varying responses between genotypes. Salt stress resulted in a linear decline in the photosynthetic rate from 21 μmol CO₂ m⁻² s⁻¹ in control to 5 μmol CO₂ m⁻² s⁻¹ at 18 dS m⁻¹. Ecotypes showed similar responses in photosynthesis and stomatal conductance under salt stress. A positive correlation between CO₂ net uptake and stomatal conductance was observed. The chlorophyll content decreased with increasing salinity and there is a significant difference between ecotypes in terms of chlorophyll content. The results of Fv/Fm, ratio showed that up to 12 dS m⁻¹ there were not significant changes in Fv/Fm ratio but after 12 dS m⁻¹ the Fv/Fm ratio decreased with increasing of salt stress. There was a significant difference in Fv/Fm ratio among the ecotypes with increasing salinity. The study indicated that physiological responses to salinity varied among ecotypes, indicating genotypic differences in salt tolerance. In terms of physiological response the ecotype Ghargholough was identified as the most salt tolerant and should be chosen for areas that have become saline as a result of climate change.