Orientation, Par Interception, and Nocturnal Acidity Increases for Terminal Cladodes of a Widely Cultivated Cactus, Opuntia ficus-indica

Terminal vertical cladodes (flattened stems) of Opuntia ficus-indica growing in widely separated locations were nonrandomly oriented. On plantations at 33°S latitude in Chile individual cladodes tended to orient in the same direction as the planted cladodes on which they developed. However, after 2 years unshaded new cladodes tended to face east-west. Terminal cladodes also tended to face east-west for irrigated O. ficus-indica in California (at 34°N) and in Israel (32 to 33°N), but cladodes developing in the winter tended to face north-south. Except for the residual effect of initial planting direction, the observed patterns tended to maximize the interception of photosynthetically active radiation (PAR). Specifically, east-west cladode orientation would maximize PAR interception, except for cladodes developing near the winter solstice at latitudes more than 27° from the equator. Nocturnal acidity increases and hence productivity would generally be light-limited, since the nocturnal increase in acidity was 90% saturated for a total daytime PAR of 24 mol m–2 day–1 and the PAR received on vertical surfaces is usually less than this. Topographical features can modify the orientation patterns, since at a site where PAR was considerably blocked by surrounding mountains the maximal nocturnal acidity increases and peak in cladode orientation occurred 20° from facing east–west. Laboratory studies showed that developing cladodes oriented toward a horizontal light and were rotated an average of 16° in a direction that increased PAR interception compared to the cladodes on which they developed. Such phototropic responses, the higher productivity of favorably oriented cladodes, and the tendency to orient similarly to the underlying cladode presumably accounts for the overall orientation patterns observed, where up to four times more cladodes may face in a particular direction than at right angles to it.