Response of yield and source activity of wheat (Triticum aestivum L.) cultivars to post anthesis water deficit

Wheat, as the most important crop in the Southwest of Iran, encounters low precipitation and high temperatures during its grain filling period. Drought stress may occurs during whole crop growth period. However, the negative effect of drought stress on grain yield is so high when it occurs after anthesis. Therefore, selecting cultivars with high resistance to late season drought stress id in high importance for wheat production. In tropical and subtropical regions, which has heat stress in late season, wheat genotypes usually face with source restriction. On the other hand, in different irrigation conditions, it has suggested that grain per spike and spike number per unit area are the most sensitive grain components to drought stress. In Khuzestan province, water deficit is usually occurred during late season after wheat anthesis. Thus, for sustainable crop production, selecting cultivars with high resistance to late season drought stress is crucial. Therefore, the current research was aimed to evaluate the effect of late season drought stress on source activity, grain yield and yield components of wheat genotypes and, introducing the most tolerant genotype to after anthesis drought stress.
The experiment was carried out during 2005-2006 growing season in Ahwaz research center of agriculture and natural resource as a split plot based on RCBD with three replications where irrigation regimes (I1: control or fully irrigation and I2: no irrigation after anthesis) were assigned as main plot and wheat genotypes (Chamran, S-78-11, S-80-18 and S-82-10) were allocated to sub plots. In control treatment, plants were irrigated with a 10-days intervals. In order to evaluate the resource limitation, spikelet removal (removing 50% of total spikelet from one side of a spike) was used. After that, resource limitation was measured by dividing grain weight of removed spikelet by grain weight of control, then, obtained value was subtracted by one and multiplied by 100. At maturity, grain yield and yield component were measured.
Results of the experiment revealed that the highest grain yield was achieved by S-80-18 under fully irrigation regime. However, this genotype produced the lowest grain yield under drought stress conditions, where its grain yield under drought stress was 31 percent lower than its grain yield under fully irrigation. The highest spikelet per spike and grain number per spike were belong to S-78-11 genotype under fully irrigation regime. However, this genotype had the lowest grain weight under two irrigation regimes compared with other genotypes. Grain per plant and grain weight were two traits which were significantly reduced by drought stress. In this case, decreasing in photosynthetic materials which were partitioned to grain was effective. On the other hand, late season drought stress reduces grain filling period and, therefore, grain weight.
The grain yield of S-78-11 genotype was reduced four percent when faced with late season drought stress, suggesting that this genotype had the highest resistant to late season drought stress. The highest and the lowest source limitation were observed in S-78-11 (4.19 percent) and S-80-18 (5.10 percent) genotypes, respectively. However, except S-78-11 genotype, other genotypes were not significantly different. Source limitation was increased 50 percent in drought stress conditions compared with fully irrigation. In this case, all genotypes, except S-78-11 genotype, were not significantly difference. However, S-78-11 genotype (with 26.4 percent) and S-80-18 genotype (with 9.3 percent) had the highest and lowest source limitation, respectively.
The highest yield reduction under drought stress conditions was observed in S-80-18, where S-78-11 genotype showed the highest resistance to late season drought stress. In all genotypes, except S-80-18, resource limitation was increased by increasing in water limitation.