The shift from C3 to CAM was investigated as a function of both leaf and plant age in well‐watered and salt‐stressed (300 mM NaCl solution) plants of Mesembryanthemum crystallinum. Initiation of a night‐time accumulation of malic acid, the decisive criterion of CAM, was followed in plants that were continuously stressed at different points in their life cycle. The deinducibility of CAM was examined after the release from stress by extensively rinsing the potting soil with de‐mineralized water.
Our results show that in M. crystallinum CAM is under strict developmental control, since CAM appeared only when a certain stage of development of the whole plant was reached. CAM was not present in any plant before this threshold, which was the same in salt‐stressed as well as in well‐watered plants. The metabolic shift coincided with the change from the seedling to the juvenile growth phase, and not with that from vegetative to reproductive growth, represented by the start of branching. The latter is timed to the end of extension growth.
In well‐watered plants, after this decisive point in development, a weak nighttime accumulation of malic acid could be measured (≅ 0.05 mol kgDW−1) in the oldest, mature leaves but not in young, developing ones. This “CAM capacity” gradually increased up to 0.2 mol kgDE−1 with further plant ageing. Leaf senescence, characterized by wilting and yellowing, diminished the CAM activity. In mature leaves salt stress drastically enhanced the magnitude of diurnal fluctuation in malic acid content.
Removal of salt stress did not deinduce CAM activity, but diminished the amplitude of malic acid oscillations to some extent in those plants which had been stressed from early in their life cycle. In these plants, salt stress delayed plant development and growth thus retarding the life cycle. Well‐watered plants, for example, branched about three weeks earlier than those that had been stressed continuously from one week after germination. After removal of stress a quasi‐preserved earlier developmental stage in relation to the control plants determined the weaker CAM expression.
Protea acaulos, a prostrate fynbos shrub, often experiences very low air humidity at leaf temperatures over 10 "C higher than mean air temperature. We determined to what degree this particular microclimate influenced photosynthetic performance, leaf conductance and water relations of non-irrigated and trickleirrigated plants. Measurements were made at the end of the dry summer season in the sand plain lowland fynbos on the west coast of South Africa. Independent of water supply, plants showed a pronounced midday depression of gas exchange. While in non-irrigated plants leaf water potential dropped to -2.0MPa around noon, it never fell below -1 .O MPa in irrigated plants. On the other hand minimum pressure potential was similar in irrigated and non-irrigated plants. The latter showed higher turgor after rain, due to osmotic acclimation, which resulted from a reduction in maximum water volume. The main osmoticum was 1,5-anhydro-D-glucitol. Leaf temperature, directly or via the vapour pressure deficit between leaf and air (Awl, rather than plant water status, was the determinant of the midday depression of gas exchange. High Aw caused stomata1 closure during times of saturating light, thus limiting photosynthetic C 0 2 uptake and availability and enhancing the susceptibility for photoinhibition. This, as well as high leaf temperature per se, decreased the efficiency of photochemistry of photosystem 11. Initial fluorescence remained constant until temperatures exceeded 35 "C, above which changes
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.