Effects of Post Anthesis Water Deficit on Physiological Characteristics of Current and Storage Sources and Production Capacity of Temperate Wheat Cultivars

Wheat (Triticum aestivum L.) is one of the most important crops in the world, which plays the greatest role in feeding the world's population. The crop yield (such as wheat) is determined by the interaction between genetic and environmental factors. Among environmental factors, water deficit in many agricultural areas is the major obstacles for achieving optimal crop yield including wheat. The carbohydrates that are needed for grain growth are provided from two sources (1) during grain filling via leaves photosynthesis (current source) and (2) excess carbohydrates that are produced after and before anthesis and stored in the stem and then remobilized to the growing grains during grain filling stage (storage source). The decline of photosynthetic activity is the main sign of leaf senescence. It is generally accepted that cultivars that are able to preserve photosynthesis rate of leaves for a longer time, also, have more grain yield. Drought stress that occurs after anthesis can significantly affect on the power of both current and storage sources of wheat. These effect via reduction of grain filling rate and grain filling duration can lead to the reduction of grain size and then grain yield.

This research was conducted to evaluate the effect of post-anthesis water deficit on physiological characteristics of current and storage sources and also to identify strategies for increasing grain yield of two wheat growth types include 1) spring growth type (Sivand and Sirvan) and facultative growth type (Pishgam and Orum). This research was applied to the research farm of Razi University. The experiment was laid out as split plot based on randomized complete block design (RCBD) in three replications. Water deficit was conduct in two levels (control and water deficit from anthesis till maturity) in main plot and wheat cultivars in subplot. Density was considered 400 plants per square meter. Sowing date was at 21 October 2016. To evaluate the power of current sources, gas exchange of leaves (photosynthesis rate, stomatal conductance, transpiration rate, temperature, CO2 concentration in sub stomatal cavity, fluorescence chlorophyll and stomatal conductance) and also for evaluating the power of storage sources, remobilization of carbohydrates from stem to the growing grains was measured.

With respect to the results, in control and water deficit conditions, Sivand and Orum cultivars with 8435 and 5255 kg.h-1 had the highest and the lowest grain yield respectively. Application of post-anthesis water deficit caused 36, 28, 24 and 26 percent reduction in grain yield of Sivand, Sirvan, Pishgam and Orum cultivars respectively. In control treatment, 7 and 21 days after anthesis, Sivand and Sirvan had the highest photosynthesis rate, transpiration rate, stomatal conductance and maximum efficiency of photosystem II and the lowest leaf temperature. In both times Orum cultivar had the lowest photosynthesis rate and also transpiration rate, stomatal conductance and maximum efficiency of photosystem II and the highest leaf temperature. Water deficit in 7 and 21 days after anthesis significantly increased leaf temperatures and significantly reduced Sub-stomatal CO2, stomatal conductance, transpiration rate and maximum efficiency of photosystem II. Pishgam cultivar had the highest remobilization in two water regimes. The Relative contribution of stem reserve in grain yield in all evaluated cultivars was less than 30%.

The application of post-anthesis water deficit stress through decreasing stomatal conductance and consequently reducing transpiration rate significantly increased leaf temperature and decreased grain yield. Therefore, it seems that considering the ease of measuring leaf temperature in comparison with other physiological traits examined in this study and not destroying the method of measuring it, this trait can be considered as a very suitable indicator for investigation. Then with further study, it is possible to estimate the grain yield loss in wheat cultivars using this indicator in environmental drought stress conditions. Application of post-anthesis water deficit on average caused 30% reduction in grain yield in spring and facultative growth type cultivars. Regarding to the high probability of occurrence of drought stress at the end of the growing season of wheat in the studied area and most of Iran's agricultural areas, therefore, further studies on the recognition of the mechanisms of coping with post-anthesis drought stress conditions and the breeding of wheat cultivars in this regard, are necessary.