Seasonal variations and acclimation potential of the thermostability of photochemistry in four Mediterranean conifers

Thermostability of photosynthesis was studied in four Mediterranean conifer species growing in southern France, namely Cedrus atlantica and Pinus nigra growing usually on mid elevation areas, and Cupressus sempervirens and Pinus halepensis from coastal areas. Chlorophyll a fluorescence was used and lead to two indices assessing the degree of thermostability of the photosynthetic apparatus: the critical temperature at ground fluorescence breakpoint (T-c) and the temperature threshold inducing a 15% decrease in photochemical efficiency (T-15). The two indices were correlated and yielded similar rankings among species, although mean values of T-15 were 6.5 degreesC lower than that of T-c. Values of T-c were in the range 44 to 52 degreesC and clear interspecific differences were detected. C. atlantica consistently displayed higher Tc than the other species (1-1.5 degreesC difference during a seasonal time course). Among the three other species (C. sempervirens, P. nigra and P. halepensis), the differences were smaller and not always significant. T-c also displayed a large intraspecific plasticity, with: (i) a seasonal time-course showing significant increases during summer and lower values during Spring and Autumn; and (h) large responses to ambient temperatures, with 5-6 degreesC increases in response to a gradual rise of temperature from 10 to 35 degreesC. The amplitude of the rise was of the same magnitude in all species. Therefore records of thermostability of photosynthesis, whatever the parameter used (T-c or T-15) need to take into account the large plasticity in this parameter when comparing species or genotypes. The degree of plasticity in response to given changes in micro-environment could be an important functional trait for the tolerance to environmental stresses.