Photosynthetic Metabolism and Antioxidant System of Spike and Flag Leaf of Bread Wheat under Drought Stress

IntroductionDrought stress and consequent lack of available water for plants is one of the main causes of accumulation of reactive oxygen species (ROS) in various organs of plants, which is effective in reducing the yield of cereals such as wheat. The antioxidant system, which contains various enzymes and genes, is responsible for removing and detoxifying plants from ROS. Unfortunately, genes responding to drought stress and their enzymatic activities associated with spike and flag leaf of wheat have received less attention. Therefore, in the present study, photosynthetic parameters, key enzymes of the antioxidant system, and expression analysis of some genes involved in this system under field capacity (FC) and drought stress (DS) conditions in spike and flag leaves of wheat were investigated.Materials and MethodsThe present study was performed in Azadshahr Research Center, Golestan Province, Iran in 2021. The wheat cultivar used in this study was Qaboos. This cultivar was cultivated under two experimental conditions of field capacity and drought stress in a randomized complete block design with three replications in the field. In the present study, differences in net photosynthesis (PN) rate, transpiration (E), stomatal conductance (gs), chlorophyll (Chl), relative water content (RWC), and the activity of APX, DHAR, MDHAR, and GR enzymes involved in the antioxidant system of flag leaf and spike of wheat were examined under field capacity and drought stress conditions. Transcription levels of APX, DHAR, MDHAR, and GR genes were measured using qRT-PCR under drought stress and field capacity conditions. Flag leaf and spike of Wheat for photosynthetic parameters and enzymatic activities at 0, 5, 10, 15, 20, and 25 and relative gene expression at 0, 1, 3, 5, 10, 15, 20, and 25 days after anthesis (DAA) were harvested. The recorded data were analyzed by analysis of variance and all analyzes were performed by SPSS software. Mean differences were compared using Duncan's multiple range test at 5% probability level.Results and DiscussionIn general, the spike had a better capacity to maintain net photosynthesis, transpiration, stomatal conductance, chlorophyll, and relative water content under drought stress conditions. This superiority reached its peak on the fifth day after anthesis. The enzymatic activity of the antioxidant system was normal under field capacity conditions. However, with the application of drought stress, APX, DHAR, MDHAR, and GR enzymes were affected in both spike and flag leaf of wheat and their activity increased, which indicates more activity of the antioxidant system in spikes and flag leaf of wheat for controls the level of ROS. After more days of grain filling, the activity of enzymes increased, and finally, after the plant entered the senescence stage, the activity of enzymes decreased to some extent. With drought stress, the antioxidant system in spike, especially from the 10th day after anthesis onwards, had a better performance. However, enzymatic activity in flag leaves under drought stress was not significantly different from field capacity conditions. Transcriptional levels of APX, DHAR, MDHAR, and GR genes associated with the antioxidant system increased under drought stress compared to field capacity conditions. It seems that the increase in enzymatic activity simultaneously with the increase in the level of transcription of the genes of the antioxidant system in the spikes and flag leaves of wheat is to counteract the oxidative damage.ConclusionIn the present study, spikes of wheat played a more important role in responding to water deficiency through the antioxidant system than its flag leaf. The results of this study showed a high potential of the spike compared to flag leaf in the face of drought stress and this results in future breeding programs in wheat can play a key role in selecting drought-resistant cultivars with high yield.