Abstract
Effects of water stress on gas exchange characteristics were investigated in a crassulacean acid metabolism plant, Dendrobium Ekapol cv. Panda when water was withheld from pot. The diurnal CO2 exchange rate, phosphoenolpyruvate carboxylase activity, malate content, leaf water potential and relative water content were measured at O, 3, 7, 14, 21 days from the beginning of treatment. The data showed that the diurnal CO2 exchange of D. Ekapol was rapidly changed after starting of treatment, that is, CO2 uptake of phase 4 disappeared and those of phases 1 and 2 decreased by half. In addition even though plants were exposed to such a long series of water stress, the CO2 exchange rate maintained a positive value. It was indicated that D. Ekapol had strong tolerance to drought as a crassulacean acid metabolism plant. The CO2 balances of each phase were decreased quickly when leaf water potential was decreased frim -0.38 to - 0.4MPa, it was recognized that the response of CO2 exchange was sensitive to a slight change of leaf water potential. Non-stomatal factors in CO2 fixation were not damaged by water stress due to phosphoenolpyruvate carboxylase activity was increased. The close correlation was observed between leaf conductance and CO2 exchange rate in each phase, therefore, it was suggested that CO2 exchange under water stress was controlled more by stomata than biochemistry in mesophyll cell.
