Tolerance of Photosynthesis to High Temperature in Desert Plants

Abstract

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 averaging 55°C.Unlike some perennial evergreen species of the desert such as Atriplex lentiformis and Larrea divaricata, which have the capacity for photosynthetic adjustment to changing temperatures (9, 12), the desert annuals have been considered to be 'opportunistic' and, with perhaps the exception ofthe C4 summer annuals, lacking in specific physiological adaptations to cope with high temperature stress (10). This long standing notion of an avoidance strategy, in large part derived from observations of the rapidity with which the ephemerals can complete a life cycle following germination, has been reinforced by the demonstration of exceptionally high rates of photosynthesis among certain winter annuals (8,19

An ecological and evolutionary analysis of photosynthetic thermotolerance using the temperature-dependent increase in fluorescence

“…However, in the field, desert species did have greater realized photosynthetic thermotolerance on aver age than coastal species. Our study concurs with other studies indicating that plant species have a large capacity for plastic acclima tion of photosynthetic thermotolerance (Seemann et al 1979(Seemann et al , 1986Berry and Bjorkman 1980;Raison et al 1982;Downton et al 1984). Seven out of eight species were significantly different for T S20 and five out of eight were significantly different for T 50 and T between max common environment and field measurements.…”

“…The mag nitude of plasticity within a species was greater than the magnitude of evolved differences between the coastal and desert congeners. Others have reported even greater plasticity (Seemann et al 1979(Seemann et al , 1986Raison et al 1982;Downton et al 1984). If we were able to acclimate plants to a number of different common environments or make measurements in the field on several occasions, we may have found an even greater range of photosynthetic ther mal acclimation.…”

“…These observations have led to the hypothesis that species with warm climate distribu tions should have greater intrinsic photosynthetic thermotolerance and thus the temperature-dependent in crease in fluorescence should occur at higher tempera tures (Smillie and Nott 1979;Bilger et al 1984). While there has been considerable work on the photosynthetic thermotolerance of desert plants utilizing the temperature-dependent increase in fluorescence (Seemann et al 1979(Seemann et al , 1986Downton et al 1984) few comparative studies have been conducted across contrasting habitats or climates (but see Smillie and Nott 1979;Berry and Bjorkman 1980). As a result, the question still remains whether there is a correlation between photosynthetic thermotolerance and the temperature regime within a species' distribution.…”

“…Previous studies have shown that the temperature-depen dent increase in fluorescence is correlated with a number of physiological factors including the decline in photo synthetic capacity (Schreiber and Berry 1977;Seemann et al 1979Seemann et al , 1986Downton et al 1984) and the onset of irreversible tissue damage following high temperature stress (Bilger et al 1984). These observations have led to the hypothesis that species with warm climate distribu tions should have greater intrinsic photosynthetic thermotolerance and thus the temperature-dependent in crease in fluorescence should occur at higher tempera tures (Smillie and Nott 1979;Bilger et al 1984).…”

“…Most studies in volve a limited set of species from either heterogeneous environments or a single common environment. Evolu tionary studies concerning photosynthetic thermotoler ance are further complicated by the fact that a variety of environmental factors including plant water status (Seemann et al 1979(Seemann et al , 1986Havaux 1992;Valladares and Pearcy 1997), growth temperature (Schreiber and Berry 1977;Seemann et al 1979Seemann et al , 1986Downton et al 1984), soil salinity (Larcher et al 1990), and light levels (Schreiber and Berry 1977;Weis 1982;Havaux and Strasser 1992) can affect the plastic state of photosynthe sis. In addition, photosynthetic acclimation can occur on the scale of minutes to hours in response to moderately elevated temperatures (Havaux 1993), and comparable leaves from different individuals of a single species in the same environment can also exhibit considerable vari ation (Fig.…”