SOIL-OXYGEN EFFECTS ON STOMATAL RESPONSE

Abstract

Several greenhouse studies were reviewed and compared on the effects of low soil oxygen (O2) levels upon plant response. The studies are discussed with particular reference to stomatal closure. Both indirect and direct observations of stomatal closure at low soil O2 diffusion rates (ODR) were compiled. Plant response was similar over a wide range of species, including tomato (Lycopersicon esculentum), sunflower (Helianthus sp.), jojoba (Simmondsia chinensis), cotton (Gossypium hirsutum and G. barbadense), and wheat (Triticum aestivum). Elimination of soil O2 from the profile can cause stomatal closure even at optimum matric potentials. When ODR dropped below 20 .times. 10-8 g .cntdot. cm-2 s-1, leaf diffuse resistance rose as an exponential function in all the species observed. In some of the studies, photosynthesis and water use were observed to drop when ODR fell below 20 .times. 10-8 g .cntdot. cm-2 s-1. Whereas stomatal response to low soil O2 levels was highly consistent, the effect of low soil O2 on plant water potential was not. Apparently, theories pointing to increased root resistance resulting from low soil O2 as the cause of stomatal closure do not fully explain some of the observed data. Stomatal response under the described conditions may not be entirely a passive mechanical response.