INFLUENCE OF ELEVATED CO2 AND TEMPERATURE ON THE PHOTOSYNTHESIS AND RESPIRATION OF WHITE CLOVER DEPENDENT ON N2 FIXATION

Single clonal plants of white clover (Tnfohum repens L) grown from explants in a Perlite rooting medium, and dependent for nitrogen on N2fixation in root nodules, were grown for several weeks in controlled environments which provided two regimes of CO2and temperature 23/18 °C day/mght temperatures at 680 /imol mol-1CO2(C680), and 20/15 °C day/mght temperatures at 340//mol mol-1CO, (C340) After 3-4 weeks of growth, when the plants were acclimated to the environmental regimes, leaf and whole-plant photosynthesis and respiration were measured using conventional infra-red gas analysis techniques Elevated CO2and temperature increased rates of photosynthesis of young, fully expanded leaves at the growth irradiance by 17-29%, despite decreased stomatal conductances and transpiration rates Water use efficiency (mol CO2mol H2O-1) was also significantly increased Plants acclimated to elevated CO2and temperature exhibited rates of leaf photosynthesis very similar to those of C340 leaves ‘instantaneously’ exposed to the C680 regime However, leaves developed in the C680 regime photosynthesised less rapidly than C340 leaves when both were exposed to a normal CO2and temperature environment In measurements where irradiance was varied, the enhancement of photosynthesis in elevated CO2at 23 °C increased gradually from approx 10% at 100 /imol m-1s-1to > 27% at 1170 /imol n-1s-1In parallel, water use efficiency increased by 20-40% at 315 /imol m-1s-1to approx 100% at the highest irradiance Elevated CO2and temperature increased whole-plant photosynthesis by > 40%, when expressed in terms of shoot surface area or shoot weight No effects of elevated CO, and temperature on rate of tissue respiration, either during growth or measurement, were established for single leaves or for whole plants Dependence on N, fixation in root nodules appeared to have no detrimental effect on photosynthetic performance in elevated CO, and temperature. © 1992 Annals of Botany Company.