Leaf Chlorosis and Stomatal Aperture *

Summary Under conditions which encourage stomatal opening, nitrogen deficiency (induced in wheat, sugar beets, maize and groundnuts) and iron deficiency (induced in groundnuts) markedly reduce stomatal aperture, as evaluated by means of a porometer. The effect of these deficiencies on stomatal behaviour seems to be mediated through their effect on leaf chlorophyll content. Foliar applications of urea to chlorotic leaves of nitrogen‐deficient maize restore both normal chlorophyll content and normal stomatal behaviour to the treated leaves, indicating that both variables are related on a localized, rather than a systemic level. In groundnuts the relation between chlorophyll content and stomatal aperture is the same, regardless of whether chlorosis was caused by nitrogen or iron deficiency. Chlorotic leaves had a lower apparent water saturation deficit than green leaves.