Water transport and regulation of water balance in plants: water stress in crop plants in humid climate regions

Most of the crop plants we use in food production are angiosperms, the plant clade that has evolved high drought tolerance. Nevertheless, in semi-arid regions and their surrounding areas, where much of the world’s crops are produced, limited soil water availability significantly constrains crop production. Even when crop plants are grown in moist soil in humid climate regions, their stomata close in response to intense transpiration. This reduces photosynthesis in the late morning to afternoon − observed as an afternoon decline of leaf gas exchange − and reduces productivity as a consequence. Moreover, even in moist soil, leaves and other organs of crop plants can occasionally experience severe dehydration and die due to catastrophic xylem embolism under intense transpiration. Such events in wet fields are expected to become more remarkable due to the rise in atmospheric vapor pressure deficit caused by global warming. This review summarizes our current understanding of the regulation of water balance via stomata, and of water transport in roots, stems, and leaves. Examples of leaf gas exchange performance and severe plant dehydration actually observed in wet fields under intense transpiration are presented. These examples characterize water transport properties and regulation of water balance of plants in humid climate regions associated with the afternoon decline of leaf gas exchange, and plant dehydration under intense transpiration. Future research aimed at mitigation of water stress in crop plants growing in humid regions is proposed.