Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf,Hyptis emoryi

Abstract

The temperature and water relations of sun versus shade leaves of Hyptis emoryi Torr. were evaluated from field measurements made in late summer. Throughout most of the day sun leaves had higher temperatures and higher resistances to water vapour diffusion, but lower transpiration rates and lower stem water potentials, than did shade leaves. Leaf absorptivity to solar irradiation was less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shade leaves (0.56). For both leaf types the stomatal resistance increased as the water vapour concentration drop from the leaf to the air increased. Energy balance equations were used together with the measured temperature dependence of photosynthesis to predict the effect of variations in leaf absorptivity, length, and resistance on net photosynthesis. The influence of leaf dimorphism on whole plants was determined by calculating daily photosynthesis and transpiration for plants with various percentages of sun and shade leaves. A hypothetical plant with all sun leaves in the sun had about twice the photosynthesis and half the transpiration ratio as did plants with sun leaves in the shade or shade leaves in the sun or shade. Plants with both sun and shade leaves had the highest predicted photosynthesis per unit ground area. The possible adaptive significance of the seasonal variation in sun and shade leaf percentages observed for individual H. emoryi bushes is discussed in terms of water economy and photosynthesi