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Mesembryanthemum crystallinum L., an inducible crassulacean acid metabolism (CAM) plant, was grown for approximately 5 weeks following germination in well-watered, non-saline soil in a controlled-environment chamber. During this time, plants were characterized by C 3 photosynthetic carbon metabolism. After the initial 5 weeks, CAM was induced by a combination of high soil salinity and reduced soil water content. One group of plants was allowed to engage in CAM by being continuously exposed to normal CO 2 -containing air (about 350-400 ppm). A second group of plants was deprived of ambient CO 2 each night (12 h dark period) until completion of their life cycle, thereby minimizing potential carbon gain via dark CO 2 fixation. The capacity to express CAM under conditions of drought and salinity stress markedly improved reproductive success: plants kept in normal CO 2 -containing air produced about 10 times more seeds than plants kept in CO 2 -free air during dark periods. Seeds from plants deprived of ambient CO 2 overnight had more negative δ 13 C values than seeds from plants kept in normal air.

The carbon isotope compositions of samples of Kalanchoë species collected at the natural stands in Madagascar were determined. The results suggest that all species of the genus Kalanchoë are capable of crassulacean acid metabolism. The observed δ 13 C values cover the whole range from -10 to -30‰. This high diversity of the δ 13 C values was found among the species of the genus as well as, in certain cases, within a single species. This suggest that the CAM patterns in Kalanchoë are generally very flexible. The δ 13 C values show a clear correlation with the climate of the habitats from where the samples derived. Values indicative of CO 2 fixation taking place exclusively during the night were found in the dry regions of Madagascar, whereas δ 13 C values indicative of mixed CO 2 fixation during night and day or of CO 2 fixation entirely during the day are distributed in the humid zones.

Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C 3 plant) and Amaranthus retroflexus L (C 4 ) on the one hand and of the grasses Avena sativa L (C 3 ) and Panicum miliaceum L. (C 4 ) on the other hand were maintained in a standard soil with different ground water tables. After 12 weeks the length, dry weight and nitrogen-content of the aboveground and belowground parts of the plants, and in addition the carbon-and ash-content and the δ 13 C value of the aboveground parts were determined. It turned out that the length and the dry weight of the shoots of the C 3 species showed on increasing tendency with increased water supply, while the values of the C 4 species were drastically diminished at the highest water level only. The roots showed in most cases an increased length and dry weight at drier conditions, more pronounced in the C 4 than in the C 3 species. The nitrogen content of the shoots was mostly higher in the shoots of the C 3 plants and in the roots of the C 4 plants; it changed in a non-regular manner with variations in water supply. Since the carbon content did not change markedly, the C/N ratio was variable. There was a slight tendency for a higher carbon content and mostly also for a higher C/N-ratio in the shoots of C 4 plants. The δ 13 C values of both C 3 as C 4 plants were in general not at all influenced by the water supply; they were fixed genetically. The ash content of the analyzed species did not show a clear relationship to the type of photosynthetic CO 2 -fixation or to the water regime.The influence of light intensity was studied with mixed cultures of all four plant species, again with different water supply. There was a strong effect of light intensity on the competitive behaviour of the C 3 and C 4 plants under modified water conditions. The wild C 3 plant Atriplex hortensis was most successful under conditions of relatively low light intensity and high water availability, while the cultivated 'artificial' species Avena sativa showed much less differences between full-light grown and shadow plants. The C 4 plant Amaranthus retroflexus is most successful under competitive conditions at high water stress in full light. The C 4 grass Panicum miliaceum showed maximum shoot growth in light, but was successful under competitive conditions especially also with good water supply. The light intensity had no effect on the δ 13 C values. - There was no indication that the soil-type as such has a distinct influence on the success of C 3 or C 4 plants in mixed cultures.