OCCURRENCE OF INDUCIBLE CRASSULACEAN ACID METABOLISM IN LEAVES OF TALINUM-TRIANGULARE (PORTULACACEAE)

In this paper we report for the first time the occurrence of an inducible weak CAM in leaves of Talinum 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-mu-mol m-2 s-1) and a small acid accumulation during the night (6.0-mu-mol 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-mu-mol CO2 m-2 s-1, 100-mu-mol H+ g-1 FW) on day 9 of drought. By day 30 almost no gas exchange was observed, while acid accumulation was still 10-mu-mol 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.