A combination of stomata deregulation and a distinctive modulation of amino acid metabolism are associated with enhanced tolerance of wheat varieties to transient drought

Introduction: Mediterranean winter crops are commonly and increasingly exposed to irregular rainfall and high temperatures, which lead to transient drought events of different degrees, adversely affecting growth and yield. Hence, exploring the diverse degrees of tolerance to drought existing in the crop and the molecular strategies behind it is pivotal for the development of ad hoc breeding programs. Objective: We investigated the physiological and metabolic response of six commercial wheat cultivars to transient water stress at the tillering and grain-filling stages. Methods: Drought experiments in lysimeters were set up at two developmental stages including six wheat cultivars. Newly expanded youngest leaves and flag leaves were sampled during the drought and following recovery. Metabolite profiles were generated using a GC–MS based protocol. Data on transpiration were continually acquired by measuring the weight variation of pots using electronic temperature compensated load cells. Results: The tillering stage in wheat is more sensitive to droughts than the grain filling stage. The former stage was characterized by pronounced metabolic alterations also during recovery from the drought, and plants exhibited reduced transpiration. Notably, cultivars varied considerably in their susceptibility to drought. Exceptionally only in cv Zahir was transpiration not reduced at tillering. During recovery, the transpiration rate of Yuval and Zahir was not significantly affected, while except Ruta the other varieties maintained lower values. At grain-filling, a moderate decrease in transpiration in response to drought was evident in Bar-Nir, Yuval and Zahir varieties as compared with the stronger response of Gedera, Galil and Ruta. The transpiration trend during recovery remained lower than the control plants, particularly in Gedera and Zahir, while it reached higher values than control plants in Yuval and Ruta varieties. Metabolite profiling of leaves across cultivars showed varietal specific trends of response. Particularly during tillering, amino acid metabolism was differentially regulated across cultivars. For instance, Ruta and Zahir exhibited major changes in central carbon nitrogen metabolism during stress response, accumulating large amounts of proline and threonine during tillering, while in Bar-Nir a general decrease in relative amino acid content was noted. Changes in stress related GABA were common to Galil, Ruta, Yuval and Zahir. Desiccation related raffinose family oligosaccharides were mostly associated with a later stage of grain-filling and recovery stages of response. Conclusion: The results indicate the occurrence of stage-dependent metabolic diversification along with a physiological response during transient droughts among wheat cultivars. It can be concluded that the most tolerant cultivar was Zahir, where a combination of stomatal closure deregulation and a significant accumulation rate of stress-related metabolites were evident.