Interactions between temperature, drought and stomatal opening in legumes

The effect of heat and drought on stomatal behaviour of 2–4-week-old legumes, bean (Phaseolus vulgaris L.) and red clover (Trifolium pratense), was investigated. Drought stress was induced by complete water deficit or with polyethylene glycol (PEG), and abscisic acid (ABA) was applied to mimic plant drought response. Heat stress was simulated by water bath (leaf segments) and infrared and halogen lighting (whole plants). Various experimental conditions were studied: high temperature alone or combined with drought, and low or high photosynthetically active radiation (PAR). Stomatal opening was either measured directly or determined using thermal imaging as a proxy. When water was not limiting, stomata opened in darkness under heat stress. At high PAR, drought and moderate heat caused increased leaf temperatures and temperature oscillations (±3–4 °C), attributed to the opening and closing of stomata. At low PAR, heat led to leaf temperature oscillations in control plants, whereas the application of drought caused stomatal closure, increasing leaf temperature to 39 °C. Stomatal opening occurred under high temperatures, despite the presence of the drought-induced hormone ABA, and was maintained into a recovery period at room temperature for 30 min. This study helps to illustrate stomatal plasticity and the interplay between leaf gas-exchange and maintaining favourable metabolic conditions (water status and temperature) within the leaf. Knowledge of how legumes are affected by two environmental stresses, heat and drought, expected to occur simultaneously with greater frequency in the future, is important in determining overall plant survival strategies.