【摘要】 土三七的CO₃交换曲线在六月中旬以前呈C₃型的“方波”形,白天能大量吸收CO₂,夜间有CO₂放出,含酸量无显著的昼夜波动。夏季的其余时间,夜间有净的CO₂吸收（阶段Ⅰ）,早晨有CO₂吸收峰（阶段Ⅱ）,随后有光下的脱羧作用（阶段Ⅲ）,并有明显的昼夜酸波动,呈CAM型,但无阶段Ⅳ。CAM季节出现前,水分胁迫可使成年叶子转变为CAM型,所需时间2～6天,再经浇灌（5 h）可恢复到原来的C₃型,表明诱导的CAM是可逆的。更多还原

【Abstract】 The seasonal shifts of photosynthesis under natural conditions and the inductionof CAM by drought treatment in Sedum aizoon, a widely distributed plant in China,were investigated in Xian. The plants were irrigated every day throughout theperiod of study. Before the second ten days of June, the diurnal CO₂ exchangecurve assumed a C₃‘square-wave’pattern, the plants took up CO₂ largely by dayand released CO₂ at night. No obvious diurnal fluctuations in acid content of themature leaves were observed. During the rest of time in summer, however, netCO₂ uptake was observed at night （phase I） and a CO₂ uptake peak appeared inthe morning （phase II）; furthermore, decarboxylation was observed in ’the light（phase III）, and a clear diurnal acid fluctuation was shown. It indicated thatthe plant assumed a CAM pattern; however, no phase IV was observed. Before the normal season of CAM appearance, mature leaves could be turnedinto CAM type by drought treatment lasting for 2-6 days. After reirrigation,the plants returned to the original C₃ pattern within 5 h. These results indicatedthat the induced CAM was reversible.更多还原