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 (pCO₂) it may be predicted that, where light‐saturated photosynthesis is Rubisco‐limited, an increase in pCO₂ 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) pCO₂ on Merritt Island, Florida, USA. During spring and summer, light‐saturated photosynthesis at current ambient pCO₂ was Rubisco‐limited. Consistent with theoretical prediction, J_PSII was increased and photoinhibition decreased by elevated pCO₂ 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 pCO₂. It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated pCO₂ on J_PSII and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated pCO₂ in a predictable manner.