Current state of the problem of water relations in plants under water deficit

The review presents current literature data on the mechanisms maintaining plant water balance or those providing for tolerance to its disturbance. We consider the processes enabling the changes in the transpiration rate under water deficit due to changes in stomatal conductivity and the changes in the rate of leaf growth, as well as the role of hydraulic and hormonal (ABA, ethylene, cytokinins) signals in their regulation. Factors involved in the improvement of water use by the regulation of stomatal movements are also regarded, e.g., transcription factors, kinases, GTP-binding proteins, aquaporins participating in CO2 transfer. Negative consequences of stomata closure induced by the disturbances in gas exchange, ROS generation, and accelerated senescence and the ways of their overcoming (with the involvement of antioxidants and cytokinins as factors of senescence delay) are discussed as well. The great attention is paid to the mechanisms maintaining plant growth and transpiration under water deficit due to the optimization of water uptake (modulation of hydraulic conductivity and relative activation of root growth). It is emphasized that the role of ABA in adaptation to water deficit is not limited only to stomatal closure but also concerns the regulation of root growth and assimilate inflow to reproductive organs. Dual significance of this hormone in the growth regulation is considered: direct inhibitory and mediated stimulatory action (via normalization of water relations). The contradictory data about changes in aquaporin capacity for water transfer and their role in the changes of hydraulic conductivity under water deficit are discussed. Apparently, this contradiction may be related to specific features of water transport in various plant species (relative contribution of apoplastic and symplastic pathways) and also to the effects of such factors as an increase in the hydraulic resistance of the apoplast due to the depositions of lignin and suberin, vessel cavitation, and changes in their anatomy on hydraulic conductivity under water deficit.