Evaluation of grain filling rate and stem soluble carbohydrate remobilization in barley cultivars under terminal drought

Introduction In semiarid regions with a Mediterranean climate, grain-filling period of barley (Hordeum vulgare L.) faces different kinds of biotic and abiotic stresses including water deficit, heat stress and foliar diseases. Current assimilation as a carbon source for grain filling is dependent on light absorptive green surfaces after anthesis. This source decreases normally due to natural senescence and different stresses. Simultaneously, demand of growing grain increases in addition to the demand of maintenance and respiration of live plant biomass. So stem reserves are one of the important carbon reserves for grain filling. (Blum, 1998). Stem reserves are mainly in the form of WSC (Kuhbauch and Thome, 1989; Przulj and Momcivilic, 2001; Schnyder, 1993). The final grain weight is one of the main determinant components of yield, which is determined by two factors: grain filling rate and grain filling period (Blum, 1998, Yang and Zhang, 2005). These two components are influenced by environmental and genetic factors and are physiologically completely independent of each other. Ehdaie et al. (2008) studied grain growth in 11 wheat genotypes. Under drought stress, 26% grain weight reduction was due to 29 percent reduction in grain filling rate and 50 percent grain filling period. The aim of this experiment was to investigate the changes in grain weight and carbohydrate concentration of four barley cultivars under end of season drought stress, and to investigate the role of stem reserves and current photosynthesis in grain weight stability under drought stress. Materials and methods A pot experiment was carried out in a factorial design with three replicates at the faculty of Agronomy, University of Shahid Chamran in the duration of 2010-2011 growing seasons. Four spring barley cultivars (Hordeum vulgare L.) selected from preliminary experiment including Nimruz, Jonub, Nosrat and Torkaman were grown under two water treatments including water stress vs. fully irrigation. The chlorophyll Content of flag leaf was assessed from anthesis to maturity at 5-day intervals from the same-age plants. Sampling was done from anthesis to maturity at 5-day intervals from the same-age plants. The spikes were removed and the weight of nine middle grain was measured. The linear rate of grain growth was estimated by The linear regression analysis for each cultivar in well-watered and droughted treatment. The penultimate internodes were separated from stems and then frozen in liquid nitrogen after removing leaf sheet. Five main spikes at maturity were harvested for assessment of main spike grain yield, the number of grains and grain weight. WSC were extracted based on Sonnewald et al. (1992) and measured according to Dubois et al. (1956). Results are expressed as milligrams of WSC per gram of fresh weight for WSC concentration. The mobilized WSC (MWSC) in penultimate internode was estimated as the difference between postanthesis maximum and minimum WSC concentration. Remobilization efficiency (Re) of WSC was estimated by the proportion (%) of postanthesis maximum WSC concentration of penultimate internode that mobilized. Result and discussion Significant difference was observed among cultivars in WSCc at anthesis. Jonub showed the highest and Nosrat showed the lowest concentrations. Inspite of lower WSC concentration (WSCc) at anthesis, Torkaman and Nimruz could be set in the same group with Jonub, based on maximum WSCc (Fig. 1). The ratio of preanthesis WSC to total reserved WSC was different among cultivars. Jonub had the highest ratio, while Torkaman had the lowest ratio. Water withholding elevated remobilisation efficiency by 10%. Nosrat was affected the most by water-stressed conditions by increase of 47% in mobilised carbohydrates and increase of 44% in remobilisation efficiency. On the other hand, Nimruz and Jonub had high level of WSC remobilisation in both conditions and these cultivars weren’t highly affected by drought. Water withholding increased the rate of chlorophyll loss by 66%. Nosrat and Nimruz showed the highest and the lowest rate of chlorophyll loss in both circumstances respectively. On the other hand, Jonub and Nosrat showed the highest and the lowest increase in this feature by entering drought. Drought decreased grain filling rate by 35 that resulted in decrease of 15% in grain weight. Reduction by 17% was observed for yield of main spike by entering drought that was mainly due to the decrease of grain weight , Because grains per spike didn’t change. Conclusion Cultivars were different in WSC-related traits such as WSC concentration at anthesis, maximum WSC concentration and rate of WSC accumulation in ten-day period after anthesis. Water withholding elevated WSC remobilisation efficiency. Water withholding increased the rate of chlorophyll loss by 66%. Drought decrease the the rate of grain filling by 35%. The decrease of 15% in grain weight led to decrease of 17% in grain yield of main stem. Cultivars showed different manner about use of current photoassimilates and reserved WSC in the second phase of grain filling that led to different changes in grain weight by entering drought conditions.