Ecophysiological consequences of differences in plant size: abscisic acid relationships in the epiphytic orchid Dimerandra emarginata

Changes in abscisic acid (ABA) content and several water relations paramete rs were studied in the epiphytic orchid Dimerandra emarginata in the tropic al lowlands of Barro Colorado Island, Panama. Similar to previous observati ons of size dependency of physiological parameters in vascular epiphytes, w e found significant differences in ABA accumulation (both in terms of tempo ral patterns and amount) in small and large plants under drought stress. Th e highest values of ABA content in leaves, stems, and roots were found in t he smaller specimens, reaching the levels of poikilohydric resurrection pla nts. Substantial accumulation of ABA occurred after stomatal closure, indic ating other functions of ABA than stomatal regulation in this bark epiphyte .