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. Stomatal conductance also decreases when leaf temperatures exceed a critical threshold value, even when Δw is artificially maintained at a constant level. The threshold temperature is commonly attained for leaves in situ, but only after substantial stomatal closure has already occurred as a result of high Δw values.The photosynthetic temperature optimum and threshold temperature which promotes stomatal closure increases substantially as the growing season progresses. As a result, the midday depression in photosynthesis occurs at higher temperatures in mid-summer than in late spring. Preliminary evidence suggests that the photosynthetic temperature optimum closely follows the naturally-occurring morning leaf temperatures, while the threshold temperature for stomatal closure matches afternoon leaf temperatures.