Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Abstract

Single clonal plants of white clover (Trifolium repens L) grown from explants in a Perlite rooting medium, and dependent for nitrogen on N2 fixation in root nodules, were grown for several weeks in controlled environments which provided two regimes of CO2, and temperature 23/18 °C day/night temperatures at 680 μmol mol−1 CO2, (C680), and 20/15 °C day/night temperatures at 340 μmol mol−1 CO2 (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 CO2 and 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 CO2 mol H2O−1) was also significantly increased Plants acclimated to elevated CO2, and 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 CO2, and temperature environment In measurements where irradiance was varied, the enhancement of photosynthesis in elevated CO2 at 23 °C increased gradually from approx 10 % at 100 μmol m−1 s−1 to > 27 % at 1170 μmol m−2 s−1 In parallel, water use efficiency increased by 20–40 % at 315 μmol m−2 s−1 In parallel, water use efficiency increased by 20–40 % at 315 μmol m−2 s−1 In parallel, water use efficiency increased by 20–40 % at 315 μmol m−2 s−1 In parallel, water use efficiency increased by 20–40 % at 315 μmol m−2 s−1 to approx 100 % at the highest irradiance Elevated CO2, and temperature increased whole-plant photosynthesis by > 40 %, when expressed in terms of shoot surface area or shoot weight No effects of elevated CO2 and temperature on rate of tissue respiration, either during growth or measurement, were established for single leaves or for whole plants Dependence on N2, fixation in root nodules appeared to have no detrimental effect on photosynthetic performance in elevated CO2, and temperature