Solar Tracking: Light Avoidance Induced by Water Stress in Leaves of Kidney Bean Seedlings in the Field1

Abstract

Solar tracking by unifoliolate leaves of small red kidney bean (Phaseolus vulgaris L.) seedlings was studied in the field in the summer, using a simple solar‐foliar inclinometer to measure the angle between the leaf surface and the sun's rays. Changes in leaf orientation increased light interception in the morning and late afternoon, while reducing light interception at midday. Light avoidance occurred even in well‐watered plants, but was more marked for plants growing in drier soil. Light avoidance was also observed under diffused but directional light in a growth chamber, where changes in leaf angle were proportional to changes in shoot water potential factors. Temperature differences between leaves of tracking plants in well‐watered soil and those in drier soil were small due to the compensating change in light interception. Holding leaves horizontal increased leaf temperatures in the hottest part of the day slightly above those of tracking leaves for well‐watered plants, but considerably more for plants with lower water potentials. When leaves were held horizontal, water potentials were lower at midday and in early afternoon than they were for tracking leaves. Photosynthesis was enhanced at midday in both wet and dry treatments for leaves held horizontal. At 1600 h horizontal leaves of plants in both treatments, and tracking leaves of plants in the dry treatment, photosynthesized at a low rate, although tracking leaves intercepted more light. Tracking leaves of plants in the wet treatment assimilated at a substantially higher rate.