Occurrence of Inducible Crassulacean Acid Metabolism in Leaves ofTalimum triangulare(Portulacaceae)

In this paper we report for the first time the occurrence of an inducible weak CAM in leaves of Talinwn triangulare (Jacq.) Willd. This plant is a terrestrial perennial deciduous herb with woody stems and succulent leaves which grows under full exposure and in the shade in northern Venezuela. Plants grown in a greenhouse (‘sun’ plants) and a growth cabinet (‘shade’ plants) with daily irrigation showed CO2 uptake only during the daytime (maximum rate, 4·0 μmol m−2 s−1) and a small acid accumulation during the night (6·0 μmol H+g−1 FW). Twenty-four hours after cessation of irrigation, no CO2 exchange was observed during part of the night. Dark fixation reached a maximum (1·0 μmol CO2 m−2 s−1, 100 μmol H+ g−1 FW) on day 9 of drought. By day 30 almost no gas exchange was observed, while acid accumulation was still 10 μmol H+ g−1 FW. Rewatering reverted the pattern of CO2 exchange to that of a C3 plant within 24 h. Daytime and night-time phosphoenolpyruvate carboxylase activity increased up to 100% (shade) and 62% (sun) of control values after 10 and 15 d of drought, respectively. Light compensation point and saturating irradiance were similar in well-watered sun and shade plants, values being characteristic of sun plants. CAM seems to be important for the tolerance of plants of this species to moderately prolonged (up to 2 months) periods of drought in conditions of full exposure as well as shade, and also for regaining high photosynthetic rates shortly after irrigation.