Salinity is an impeding factor in crop production in arid and semi arid regions. It affects crop yield in several ways. The first effect of salinity is a reduction in leaf area, which in turn results in a reduction of photosynthesis potential in plant. Also chlorophyll degradation by toxic ions such as Na will lead to reduction in photosynthesis. Alfalfa which is known as a moderately tolerant crop to salinity is often studied in three growth stages, namely: germination, seedling emergence along with shoot growth, and mature plant growth. In this study photosynthetic response of four Iranian alfalfa cultivars to salinity as well as relationship with crop shoot dry weight, leaf area and leaf chlorophyll content were investigated. Shoot dry weight in all cultivars any of the three growth stages (seedling, second, and third cut) was reduced with increase in salinity. Although cultivar Shirazi yielded the highest shoot dry weight at seedling stage, and the cultivar Hamedani yielded a higher dry weight at the second and third harvests, yet the most tolerant cultivars to salinity stress in these three stages are Shirazi, Hamedani and Bami cultivars respectively in which minimum shoot dry weight reduction was detected. Increase in salinity level reduced the leaf area in all cultivars at any of the three growth stages. In seedling stage, the leaf area in Hamedani and in Shirazi cultivars was reduced to a larger extent than in the other two cultivars. Thus it is confirmed that there is a direct relationship between leaf area and crop yield. The yield in all cultivars had an increasing trend in the three stages and in salinity conditions, which indicates increase in tolerance to salinity with progress in plant growth. Low level of salinity (7 dS!m) decreased chlorophyll content in leaves, but chlorophyll content the higher salinity increased in level (12 dS!m). In all studied cultivars, salinity caused an increase in CER (CO2 Exchange Rate). A significant increase in chlorophyll content, at the second level of salinity (12 dS!m), was accompanied by a significant increase in photosynthesis. Considering the importance of physiological processes in providing the needed tolerance mechanisms against salinity stress, it is recommended that the other physiological aspects such as carbohydrate as well as nitrogen retranslocation from the root to the shoot be investigated as well.
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