Stomatal responses to drought stress

Stomatal closure is the first reaction to drought stress in most plants. Stomatal closure prevents water loss from transpirational pathways. Stomatal closure is more closely related to soil moisture content than leaf water status, and it is mainly controlled by chemical signals such as abscisic acid (ABA) production in dehydrating roots. This process may be referred to hydropassive closure. It could also be metabolically dependent and engage processes and result in ion flux inversion, causing stomatal opening. This process requires ions and metabolites in a process known as hydroactive closure and it seems that the process is mostly regulated by ABA content in plants. This is brought about by regulating a sophisticated cascade of biochemical events that involve the ABA complex formation. As well as the ABA, other signaling molecules and important elements are interfered with in changing stomatal status. The lower amount of nitrogen increases stomatal sensitivity to drought. On the other hand, low phosphorous decreases cytokine in levels of plants and can increase stomatal sensitivity to drought. Decrease in the number of stomata is the another morphological change in response to drought stress. Achievement of lower amount of transpiration and water loss reduction is a good tradeoff for survival of plants under variable environments, and CO2 assimilation and net photosynthesis are decreased due to reduced stomatal opening. Stomata can completely be closed under severe drought stress, which is closely dependent on plant species, so tolerant species control status of their stomata to allow carbon fixation and photosynthesis as well as improving water use efficiency. In this chapter, stomatal responses to drought stress condition are discussed, and effect of drought stress on stomatal opening and its morphological changes under this condition are considered.