Winter and summertime active desert annual species were grown at different temperatures to assess their capacity for photosynthetic acclimation. Thermal stability of photosynthesis was determined from responses of chlorophyll fluorescence to increased temperature. Photosynthesis in winter ephemerals grown at 28°C/21°C became unstable close to 41°C in contrast to the summer annuals which were stable up to about 46°C. Growth at higher temperature (43°C/32°C) resulted in increases in thermal stability of 5 to 7C for the winter annuals and 3 to 4C for the summer annuals, showing that temperature can provide the primary stimulus for acclimation of the photosynthetic apparatus. The magnitude of these changes was very similar to the range of field values observed for the respective floras, indicating that the thermal acclimation response under field conditions was qualitatively similar to that occurring under controlled growth conditions. Perennial species, co-existing with these annuals in the desert, were on average more thermostable. The cacti were exceptionally heat stable, the threshold for fluorescence increase aver- aging 55°C.
Both cool climate winter annuals and warm climate summer annuals, like perennial species (4, 9, 12), have the capacity to adjust the thermal stability of their chloroplast membranes to withstand the effects of increasing temperature. The extent of this capacity differs between the two annual floras although the maximum thermal stability reached by either type was similar. The major difference appeared to lie in the lower thermal stability of winter-active species when grown at low temperatures. Within each group of plants there was very good agreement between the acclimation induced in the laboratory and the range of thermal stability observed in the field. Consequently, much of the variation between these groups of species in the field can be attributed to direct effects of temperature on the acclimation mechanism. The response is not likely to be linear with temperature since winter annuals grown at either 20°C or 28°C had similar stabilities (data not shown). We have observed that temperatures approach- ing 40°C stimulate acclimation, but further work is necessary to determine plant reaction to intermediate temperatures. A much greater range of temperatures occurs during the growth of the winter annuals and perennials, and the ability of these plants to adjust to temperatures as the seasons change is probably more significant to their success than for the summer annuals which appear to have reduced capacity for acclimation.