Investigation on source size limitation and water stress effects on photosynthetic pigmentsو enzymatic activity yield of barley cultivars ( Hordeum vulgare L.)

Introduction Barley (Hordeum vulgare L.) is one of the most important crops in southern provinces of Iran such as Fars province, which is one of the largest barley producing areas in Iran. (Emam, 2011). In crops, the physiological basis of dry matter production depends on the source-sink relationship, where the source is the potential capacity for photosynthesis and the sink is the potential capacity to store or metabolize the photosynthetic products. When the sink is small, higher yield could not be achieved and even if the sink is large, the yield might not be high when the source capacity is limited (Alam et al., 2008; Emam and Seghatoleslami, 2005; Borras et al., 2004(. Drought stress is an important limiting factor which can cause major effect on crop productivity in arid and semi-arid regions of Iran (Ahmadi et al., 2009). Optimizing the source size after anthesis and its utilization by the sink is one of the major factors enhancing the yield potential in crops especially under water deficit conditions (Bijanzadeh and Emam, 2011). The purpose of this experiment was to investigate the effect of defoliation at milk development stage of barley on biochemical traits of barley cultivars under drought stress conditions. Material and methods In order to evaluation the effect of defoliation and drought stress on biochemical traits and grin yield of barley cultivars, a greenhouse experiment was conducted at College of Agriculture and Natural Resources of Darab, Shiraz University during 2017-2018. The treatments included of two levels of irrigation regimes (full irrigation and cutting off irrigation at milk development stage of barley), four barley cultivars (Zehak, Nimroz, Reyhan, Khatam) and three defoliation treatments included of non-defoliation, defoliation except the flag leaf and defoliation except the flag leaf and the penultimate leaf, which laid out as a factorial experiment based on a randomized complete block design with three replicates. At crop maturity, plants were harvested to measure spike length, 100- grain weight, biological yield, harvest index and grain yield and two weeks after applying drought stress treatment, chlorophyll a, b, carotenoid, catalase, and peroxidase contents were measured. Results and discussion The results showed that drought stress, cultivars and defoliation interaction had a significant effect (p≤0.01) on chlorophyll a, carotenoid, peroxidase, catalase, and grain yield but it was not significant for chlorophyll b. The results showed that drought stress caused 56.6 % and 24.2 % reduction in grain yield and chlorophyll b, respectively. In agreement to our results, Rahbarian et al. (2011) reported that variation in photosynthetic pigments and grain yield affected by drought stress level, genotype and growth stages of crop. Also, interaction effect of drought stress and defoliation showed 87.71% decrease in the amount of carotenoid. Amini and Hadad (2013) asserted that carotenoid content decrease sharply when plants exposed to drought stress. Defoliation except the flag leaf and penultimate leaf caused 98.76% increase in catalase in Reyhan cultivar, under drought stress. Likewise, defoliation except the flag leaf increase the peroxidase enzyme in all of the cultivars except the Nimroz. Similar to our results, Zhang and Kirkham (1995) and Kafi and Damghanii (2000) declared that catalase and peroxidase activity increased in some wheat cultivars when plants exposed to drought stress and this increasing depended on drought stress level and type of cultivar. Conclusion It was concluded that, under water stress, increasing the enzymatic activity by defoliation could play a vital role in barley grain yield especially in Reyhan barley cultivar.