Influences of Photosynthetically Active Radiation on Cladode Orientation, Stem Tilting, and Height of Cacti

Abstract

Stem orientation and morphology were investigated for 14 species of cacti in Chile, Ecuador, Mexico, and the United States. The interception of photosynthetically active radiation (PAR) was specifically considered for cladodes (flattened stems) of platyopuntias, for tilted cylindrical stems, and in the presence of surrounding vegetation. Terminal cladodes of Opuntia chlorotica had a statistically significant tendency to face north—south at specific sites in the Mojave desert and east—west in the Sonoran desert. There was also a tendency for cladodes of three varieties of O. echios in the Galapagos Islands, Ecuador, to face east—west, while three times more cladodes of O. stricta var. dillenii faced within 15° of east—west than within 15° of north—south in Florida. In all cases, the orientation increased PAR absorption at times of the year favorable for growth. The longitudinal axes of ceroid cacti with multiple stems were vertical in both the northern (Carnegiea gigantea, Lemaireocereus thurberi, and Lophocereus schottii) and southern (Trichocereus chilensis) hemisphere. However, the longitudinal axes of barrel—shaped cacti with single cylindrical stems tended to tilt southward in the northern hemisphere (Ferocactus acanthodes, F. covillei, F. viridescens, and F. wislizenii) and northward in the southern hemisphere (Copiapoa cinerea var. columna—alba, C. lembckei, and Eriosyce ceratistes). Although equatorial tilting can increase the temperature of the apical region for certain cacti in the wintertime, for other species it has relatively little influence of minimum or maximum stem temperatures. Also, it tends to reduce the total amount of PAR intercepted by the sides of stem. Hence, equatorial tilting may not be of adaptive advantage for all cacti. The maximum stem height of O. echios varied 2—fold in a series of communities extending over 16 km on Santa Cruz island in the Galapagos Islands, and the maximum height of T. chilensis varied 4—fold over 12 km in central Chile. Changes in stem height were positively correlated with changes in the height of the surrounding vegetation (r ° .98) and negatively correlated with PAR in the vertical planes of the stems at 2 m (r ° —.96). The diurnal change in tissue acidity of these Crassulacean—acid—metabolism (CAM) plants was 90% light—saturated for a total daily PAR of about 20 mol of quanta.m—2.d—1 which is approximately the average PAR that unshaded vertical stem would receive on clear days. Hence, the increase in height of these cacti accompanying increases in height of the surrounding vegetation helps to insure that they receive adequate PAR. In fact, for these and other cacti, PAR is often limiting for CO2 uptake, even in the high—radiation environment of deserts.