Water stress in the epiphytic orchid, Dimerandra emarginata (G. Meyer) Hoehne

The epiphytic orchid, Dimerandra emarginata, occurs in habitats characterized by high light intensities and very variable water supply. Long-term observations of the water status indicate that this species experienced at least moderate water stress over most of the year. Well-watered, it showed high rates of net CO₂ uptake (A, 7 μmol m^-2 s^-1), high transpiration rates (E, 1.4 mmol m^-2 s^-1), and little nocturnal acidification. Drought induced crassulacean acid metabolism activity and resulted in a strong decline of both A and E. The same conditions also led to chronic photoinhibition as indicated by a predawn ratio of variable to maximum fluorescence (F_v/F_m) of 0.6 after 9 days of drought. Diurnal changes in fluorescence characteristics were much more pronounced. Midday ratios of dark adapted F_v/F_m were as low as 0.35, but recovered in the afternoon and were probably mostly due to non-photochemical quenching of photosynthesis. Measured water losses of leaves and stems were compared to plant water content (PWC). In large plants, about 32% of PWC was transpired on a single day under well-watered conditions. Model calculations indicate that transpiration per unit leaf area should be strongly dependent upon plant size in this species. Comparisons of calculated daily water loss, PWC and the length of drought periods suggest that D. emarginata can only survive a dry season by drawing on the water reserves of older stems and by an efficient uptake of the occasional rain.