抄録
デンドロビウム (Dendrobium Ekapol cv. Panda)を用い, 水ストレスが個葉のガス交換に及ぼす影響を調べた. Crassulacean Acid Metabolism型CO_2交換は, 水ストレスを与えると速やかに変化し, まず明期の後半に生じるCO_2吸収が減少した. 次いで暗期と明期始めのCO_2吸収も減少したが, 葉の水ポテンシャルが-0.6MPa以下でも正のCO_2交換速度を維持した. 各PhaseのCO_2収支量は, 水ポテンシャルが-0.38MPa以下に低下すると急激に減少し, デンドロビウムのCO_2交換速度が低下する水ポテンシャルの閾値は-0.38MPa付近にあるものと考えられた. ホスホエノールピルビン酸カルボキシラーゼの活性は, 水ストレスを与えると増加し, CO_2吸収の減少と対応しなかった. 一方, 水ストレス条件下の葉の拡散伝導度とCO_2吸収速度との間には, いずれのPhaseにおいても高い正の相関関係が認められ, 本研究で認められた水ストレス下のCO_2吸収は, 主に気孔を含むガス拡散系により律速されていると考えた.
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 CO_2 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 CO_2 exchange of D. Ekapol was rapidly changed after starting of treatment, that is, CO_2 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 CO_2 exchange rate maintained a positive value. It was indicated that D. Ekapol had strong tolerance to drought as a crassulacean acid metabolism plant. The CO_2 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 CO_2 exchange was sensitive to a slight change of leaf water potential. Non-stomatal factors in CO_2 fixation were not damaged by water stress due to phosphoenolpyruvate carboxylase activity was increased. The close correlation was observed between leaf conductance and CO_2 exchange rate in each phase, therefore, it was suggested that CO_2 exchange under water stress was controlled more by stomata than biochemistry in mesophyll cell.
