Effect of drought stress on leaf area index, photosynthesis, stomatal conductance and proline content in two pinto bean cultivars (Phaseolus vulgaris L.)

In order to investigate the effect of drought stress on leaf area index, photosynthesis rate, chlorophyll, stomatal conductance and proline content in bean cultivars, a factorial experiment was conducted in a randomized complete block design with four replications. Treatments included drought stress in three levels, complete irrigation (control), irrigation cut at two weeks after emergence and irrigation cut at flowering stage and two bean cultivars were Talash and Khomain. Leaf area index, chlorophyll a and b, photosynthesis rate and stomatal conduction were decreased due to drought stress but amount of proline was increased. By applying drought stress at flowering time and two weeks after emergence compared to full irrigation (control), leaf area index was 30.57 and 55.96 percent, content of chlorophyll a was 23.1 and 56.05 percent and the content of chlorophyll b was 13.67 and 30.76 percent, photosynthesis rate was 13.22 and 26.73 percent, stomatal conductance was 31.65 and 50.25 percent decreased, respectively and amount of proline increased by 51.11 and 88.14 percent respectively.The highest and the lowest leaf area index were in complete irrigation (control) and Talash cultivar with 5.29 and in irrigation cuttings in two weeks after emergence and Khomain cultivar with 2.08, respectively.The highest amounts of chlorophyll a and b belonged to complete irrigation (control) and Talash cultivar with 0.213 and 0.118 mg/g fresh leaf weight and the lowest amount of chlorophyll a and b was related to irrigation cut at two weeks after emergence and Khomain cultivar with 0.132 and 0.76 mg/g leaf fresh weight, respectively. Generally, the negative effect of drought stress on chlorophyll a was more than chlorophyll b. Drought stress significantly reduced the rate of photosynthesis in bean cultivars.This decrease was observed in irrigation cut at flowering stage and irrigation cut at two weeks after emergence in comparison full irrigation (control) in the cultivar of the Talash was 12.99 and 24.50 percent respectively and in Khomain cultivar was 13.56 and 29.19 percent respectively.Stomatal conductance in complete irrigation (control), irrigation cut at flowering stage and irrigation cut at two weeks after emergence in the Talash cultivar was 0.432, 0.321 and 0.223 mol/m2.s and Khomain cultivar was 0.365, 0.233 and 0.154 mol/m2.s, respectively.The results of mean comparison showed that drought stress in each of the stages of growth reduced stomatal conductance in both cultivars. However, the percentage reduction in Khomain was more than the Talash.The content of proline in complete irrigation (control), irrigation cut at flowering stage and irrigation cut at two weeks after emergence in the Talash cultivar was 1.47, 2.30 and 2.83 mg/g fresh weight and in Khomein cultivar was 1.24, 1.78 and 2.25 mg/g fresh weight, respectively. drought stress in both cultivars increased significantly in proline content. According to the results, bean varieties had no significant difference in leaf area index and chlorophyll b content. In drought stress condition, Talash cultivar compared with Khomain cultivar showed less decrease in stomatal conductance and leaf area index and more had chlorophyll content and photosynthesis rate and it is recommended to cultivate in areas with drought stress.