Abstract
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, 40ftmol m~2 s') and a small acid accumulation during the night (60 f±mol H+ g1 FW). Twenty-four hours after cessation of irrigation, no CO 2 exchange was observed during part of the night. Dark fixation reached a maximum (1 0 ^mol CO2 m ~2 s', 100 /imol H + g' FW) on day 9 of drought. By day 30 almost no gas exchange was observed, while acid accumulation was still lO^mol H+ g1 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.
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