The effect of leaf movement on leaf gas exchange and instantaneous leaf water-use efficiency for leguminous species

Environmental stresses, especially in pastures, cause yield reduction. Therefore, growing species which endure stressful conditions is important. The leguminous species due to their heliotropic functionality could reduce these stresses. Solar tracking leaf movements in some plant families are one of the important ecological adaptations. The leguminous species during the leaf movements regulate the incident radiation and leaf water potential. In favourite environmental conditions leaves are diaheliotropic and intercept more radiation, while during the stress conditions they move away, paraheliotropic, from the sun incident radiation. Therefore, the aim of this research is to evaluate the effect of bean leaf movements at heat stress condition on instantaneous leaf water-use efficiency (WUEi), stomatal conductance and leaf temperature. Potted plants of bush bean (Phaseolus vulgaris L. var. Provider) at the stage of the second extended trifoliate leaf, which were grown in Chicot sandy loam soil under well-watered condition in greenhouse, were chosen for this research. One plant was used to measure the pulvinus water status and pulvinus bending by using a beta-ray gauging (BRG) meter using a point source of thallium-204 (204Tl). Leaf gas exchange measurements took place at air temperature interval of 33-42oC by a steady-state LI-6200 photosynthesis system. A Copper-constantan thermocouple was used to monitor pulvinus temperature. Pulvinus bending followed the daily circadian rhythm. Quadratic significant correlations were found between leaf-incident angle and stomatal conductance (R2 = 0.54; p < 0.01), and photosynthesis rate (R2 = 0.84; p < 0.01). By reduction of leaf-incident angle and increase of air temperature WUEi reduced. The leaf temperature remained bellow air temperature and was a significant function of it (r = 0.92; p < 0.01). In conclusion, air thermal stress causes leaf paraheliotropic movement, and regulate leaf temperature and leaf water loses.