Field evaluation of short term heat stress pre and post flowering on the water soluble carbohydrates in spring bread wheat (Triticum aestivum L. ) under warm climate of Ahvaz

Short periods of heat stress in spring under Mediterranean climate (e.g. southern vast areas of Iran) are common and cereal crops often exposed to short periods of elevated temperatures which can have large effects on crop yields. Stem water-soluble carbohydrate (WSC) could be an important carbon bank for supporting grain filling in wheat especially when carbon assimilation is hampered by heat stress. In Iran, a large number of studies have been done on the effects of heat stress on grain yield of wheat and its components, based mainly on the change in planting date, and data from the effect of short-term heat wave on yield and grain growth and the role of WSC in developing grains do not exist under field conditions. The aim of this study was to determine the effects of short-term heat stress before flowering and the beginning of grain filling on peduncle WSC and grain filling pattern as well as determination of the contribution of peduncle WSC in genotypes with different amounts of carbohydrates in grain growth under short-term stress conditions.

This research was carried out at Research Farm of Khuzestan Agricultural Sciences and Natural Resources University, located 35 km northeast of Ahwaz, in 2014. Four wheat genotypes (Chamran. Maroon, Arvand and Atrak) were exposed to heat stress (maximum 35 °C) for a three-day in the field with a portable heat chamber at two different stages, near flowering (H1) and early grain set (H2). to investigate the changes in individual grain weight (IGW) during grain development, the total WSC changes in the peduncle of the main stem after exposure to short-term heat stress during the development of grain, three plants marked from day zero after flowering. The grains were harvested at intervals of seven days until the time of maturity. The total WSC were measured by anthrone method. When the dry weight of peduncles was maximized in each genotype, the concentration of peduncle carbohydrates (WSC-C) was considered as the maximum WSC-C. To calculate the maximum peduncle WSC content, the maximum WSC-C was multiplied by the maximum peduncle weight. The difference between peduncle WSC-C at maximum time and at maturity time was considered as the mobilized WSC from peduncle to grain. The ratio (%) of peduncle mobilized WSC to maximum peduncle WSC-C was calculated as the efficiency of peduncle in mobilized WSC. The analysis of variance of the measured traits in this study was performed using SAS 9.4 software. Duncan's test was done for means comparison.

The results of analysis of variance showed that there were significant differences between heat stress levels and the traits of genotypes. Short term heat stress (H1 or H2) caused a significant reduction in individual grain weight (IGW) and grain number of main spike. There was no detectable change in IGW until 14 days post-anthesis, after which time grain growth in the heat-stressed plants was reduced from 4.5% to 17%. Based on number of grain per spike and IGW, Atrak, Chamran and Arvand genotypes were recognized as tolerant gynotypes. Heat stress (average of H1 and H2) reduced maximum peduncle water soluble carbohydrate (WSC) content by 26% and mobilized WSC by 15%. The reduction of peduncle WSC content by the heat stress treatments appeared to start at 21 days post-anthesis Reductions in IGW and among genotypes were negatively correlated to the maximum peduncle WSC content.