Use of an analytical model to study limitations on net photosynthesis in Arbutus unedo under field conditions

From field gas-exchange measurements on Arbutus unedo growing in Portugal, parameter values necessary to apply an analytical, physiologicallybased model of C ₃ photosynthesis were obtained. The model successfully simulated measured diurnal photosynthetic responses in Arbutus during periods without water stress, under both natural and CO₂-saturating conditions. The model was used to analyze those factors limiting primary productivity during each of the experimental days. Due to a large investment in ribulose bisphosphate (RuBP) regeneration capacity, irradiance was rarely limiting, even during cloudy periods, but the limitation imposed by stomatal conductance was quite large, averaging over 30%. The fact that experimental leaves were maintained in a horizontal position is at least partially responsible for these results. Possible other reasons for this apparent excess of RuBP regeneration capacity visa-vis RuBP carboxylase-oxygenase concentration are discussed.