Photosynthetic Rates of Sun versus Shade Leaves of Hyptis emoryi Torr.

Leaves on a bush of Hyptis emoryi Torr. varied in length from less than 1 cm when development occurred in full sunlight (e.g. 40 Mjoules m(-2)) to over 7 cm when the total daily solar irradiance was less than 3 Mjoules m(-2). The 1-cm sun leaves were 3-fold higher than the 7-cm shade leaves in chlorophyll per unit area, mesophyll thickness, and the internal to external leaf area ratio (A(mes)/A). The higher A(mes)/A caused a 1.2-cm leaf to have a 3-fold lower CO(2) liquid phase resistance than did a 7.1-cm leaf. Large thin shade leaves captured photosynthetically active radiation effectively (less than 7% passed through), but were not adapted to full sunlight. Specifically, when a 6.9-cm leaf was placed at 910 w m(-2) for 30 min, its temperature exceeded that of the air by nearly 8 C. For the common daytime air temperatures above 30 C for this desert shrub, large shade leaves would have temperatures far in excess of that optimum for photosynthesis for H. emoryi, 29 to 32 C.