Stomatal closure in oxygen-stressed plants

Stomatal closure of plants in flooded soil, as determined by leaf gas exchange, has been recognized since 1973, and for simple hypoxia since 1975. At least 58 species have been shown to close stomata with hy-poxic or flooded conditions. Various fac-tors interact to affect the relationship be-tween rhizosphere oxygen availability, as measured by soil oxygen diffusion rate (ODR), and degree of stomatal closure. These factors include root temperature, species, plant growth stage, plant mineral nutrition, and duration and nature of hy-poxia. Soil water content, bulk density, and temperature also influence ODR. Ab-scisic acid accumulation in leaves appears to induce stomatal closure, as a stress re-sponse to root hypoxia, through its effect on potassium ion regulation of guard cell turgor. Stomatal closure generally persists well beyond actual soil hypoxia. Photosyn-thesis is reduced by root hypoxia, both by reduction of leaf gas exchange and by a lowering of the photosynthetic rate at a given leaf gas exchange rate. This phenom-enon deserves greater attention in evalu-ating and modelling of crop response to soil hypoxia and as a sensitive indicator of se-verity of soil hypoxic stress. Many deleterious effects of waterlogging on plants have been recognized for centuries (Clem-ents 1921). As early as 1853 (Boussignault and Lewy 1853) soil oxygen's specific role in main-taining plant vigor was recognized. By the 1950's, some plant physiological flooding re-sponses were known to resemble drought re-sponses (Kramer and Jackson 1954). A com-monly noted expression of this syndrome was the rapid wilting of susceptible crops such as tobacco (Nicotiana tabacum) and tomato (Ly-copersicon esculentum). As instrumentation has become available to monitor stomatal gas exchange, it has become apparent that stomatal closure is an important component of whole-plant stress response t