Water loss and malate fluctuations during the day for plants in the southern Namib desert

The daily course of transpiration for 12 different plants growing in the southern Namib desert was investigated. Sclerophyllous species with C₃ photosynthesis were the most effective water savers followed by succulents exhibiting CAM, while C₃ pathway-succulents lose as much water as malacophyllous species. The different species showed either one or two peaked patterns of both transpiration and leaf conductance. With C₃ plants the most common pattern is a single morning peak in leaf conductance followed by decreases in conductance over the remainder on the day. With CAM succulents leaf conductance is high in the morning, shows a pronounced midday depression and increases in the early afternoon when the malate pool is depleted, but malate consumption did not start earlier than 4 h after dawn. Seven of nine investigated C₃ plants had rather high contents of malate, up to 180 μmol·g^-1 dry matter. In these plants the malate content increased with increasing leaf conductance and disappeared when leaf conductance declined in the evening, indicating that malate was synthesized during photosynthesis.