Midday depression in net photosynthesis and stomatal conductance in Yucca glauca : Relative contributions of leaf temperature and leaf-to-air water vapor concentration difference.

Yucca glauca in the Colorado shortgrass prairie undergoes a pronounced midday depression in net photosynthesis and stomatal conductance under summer field conditions. This phenomenon can be duplicated in the laboratory using potted plants by simulating a typical summer daily pattern of leaf temperature and leaf-to-air water vapor concentration difference (Δw). The decrease in photosynthetic rate appears to be due primarily to high leaf temperatures, while the decrease in stomatal conductance can be attributed mainly to high Δw values.