Will rising CO2 protect plants from the midday sun?: A study of photoinhibition of Quercus myrtifolia in a scrub-oak community in two seasons

Over a large part of the photoperiod, light energy absorbed by upper canopy leaves saturates photosynthesis and exceeds the energetic requirements for light-saturated linear electron flow through photosystem II (J(PSII)), so that photoinhibition results. From a theoretical consideration of the response of light-saturated photosynthesis to elevated atmospheric CO2 partial pressure (pCO(2)) it may be predicted that, where light-saturated photosynthesis is Rubisco-limited, an increase in pCO(2) will stimulate J(PSII). Therefore, the proportion of absorbed quanta dissipated photochemically will increase and the potential for photoinhibition of photosynthesis will decrease. This was tested by measuring modulated chlorophyll a fluorescence from Quercus myrtifolia Willd. growing in the field in open-top chambers, at either current ambient or elevated (ambient +35 Pa) pCO(2) on Merritt Island, Florida, USA. During spring and summer, light-saturated photosynthesis at current ambient pCO(2) was Rubisco-limited. Consistent with theoretical prediction, J(PSII) was increased and photoinhibition decreased by elevated pCO(2) in spring. In the summer, when growth had largely ceased, an acclimatory decrease in the maximum Ribulose 1,5 bisphosphate saturated carboxylation capacity (V-c (max)) removed the stimulation of J(PSII) seen in the spring, and photoinhibition was increased in elevated pCO(2). It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated pCO(2) on J(PSII) and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated pCO(2) in a predictable manner.