LEAF ORIENTATION, RADIATION INTERCEPTION, AND NOCTURNAL ACIDITY INCREASES BY THE CAM PLANT AGAVE DESERTI (AGAVACEAE)

Abstract

Both field measurements and a computer model were used to study the interception of photosynthetically active radiation (PAR) by Agave deserti (Engelm.), a desert CAM plant with a basal rosette of massive opaque leaves. PAR interception was determined in the winter and the summer for upper and lower leaf surfaces on a plant with about 60 leaves. Total daily PAR on the leaf surfaces was approximately 10 mol m‐2 for a winter day and 20 mol m‐2 for a summer day. For a PAR of 15 mol m‐2, the nocturnal increase in acidity was about 0.6 mol m‐2 for both leaf surfaces and various leaf orientations, except for the oldest most horizontal leaves where the increase was less than half as large. The acidity increase measured in the field was 90% saturated at 25 mol m‐2. Thus, daytime PAR in the desert is often limiting for the nocturnal acidity increase, especially for the lower leaf surfaces. Simulated tilting of the plant by 55° so that the vertical axis pointed to the sun at solar noon on a winter day increased the PAR incident on the upper surfaces of the leaves, but did not affect the total nocturnal increase in acidity by the whole plant. Although simulated removal of alternate leaves increased the PAR per unit leaf area for the remaining leaves, it reduced the total increase in nocturnal acidity of the whole plant by 31%. PAR interception by plants on slopes facing steeply north, east, or west was substantially reduced compared to the horizontal. Thus, the model proved to be quite useful for quantifying the relation between leaf orientation, PAR interception, and nocturnal increases in acidity by A. deserti, and it indicated that the lower frequency of plants on north‐ compared to south‐facing slopes was due to PAR limitations.