Effects of photosynthetic source limitation and post-anthesis water deficiency on grain filling rate, photosynthesis and gas exchange in bread wheat cultivars

Occurrence of drought stress at grain filling period is the prominent and inevitable characteristic of arid and semi-arid regions. The objectives of this research were to determine the effect of water deficiency and the role of current photosynthesis on grain filling, photosynthesis and gas exchange. Therefore, an experiment was carried out in a split-plot design based on a randomized complete block design with three replications at the Research Farm of Razi University in Iran during 2010-2011 cropping season. The irrigation treatments including non water stress (full irrigation over the whole season) and water deficiency stress (irrigation cessation after anthesis stage) were considered as main plots, while the factorial combination of eight wheat cultivars including Bahar, Parsi, Pishtaz, Pishgam, Chamran, Zarin, Sivand, Marvdasht and DN-11 line and the source manipulation treatments including control (Non source limitation), defoliation of flag leaf, defoliation of all leaves except the flag leaf, removing awns and shading over ears, were considered as sub-plots. The results showed that, post-anthesis water deficiency significantly decreased grain yield (22.2%) and 1000-seed weight (18.6%) compared to the control treatment and Removal of photosynthetic sources of assimilates reduced grain yield owing to grain weight reduction. The contribution of spike photosynthesis to grain filling (35.4%) was greater than the flag leaf (18.6%) and lower leaves (18.5%) and also, awns had the lowest contribution to grain filling (3.3%). Grain filling period was shortened during water deficiency stress, so that it reduced from 35 days under control conditions to 28 days under stress conditions, whereas it caused an increase in seed filling rate. Moreover, results indicated that photosynthetic source limitations caused an increase in photosynthetic rate and gas exchange. This increase reflects the compensatory role of photosynthetic rate of remained leaves in preventing yield loss.