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 CO 2 partial pressure ( p CO 2 ) it may be predicted that, where light-saturated photosynthesis is Rubisco-limited, an increase in p CO 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) p CO 2 on Merritt Island, Florida, USA. During spring and summer, light-saturated photosynthesis at current ambient p CO 2 was Rubisco-limited. Consistent with theoretical prediction, J PSII was increased and photoinhibition decreased by elevated p CO 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 p CO 2 . It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated p CO 2 on J PSII and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated p CO 2 in a predictable manner.