We explored the effects, on photosynthesis in cowpea (Vigna unguiculata), of high temperature and light — environmental stresses that often co-occur under field conditions. We observed contrasting responses in the light and carbon assimilatory reactions, whereby in high temperature, the light reactions were stimulated while CO2 assimilation was substantially reduced. There were two striking observations. First, the primary quinone acceptor (QA), a measure of the regulatory balance of the light reactions, became more oxidized with increasing temperature, suggesting increased electron sink capacity, despite the reduced CO2 fixation. Second, a strong, O2-dependent inactivation of assimilation capacity, consistent with down-regulation of rubisco under these conditions, a phenomenon that has not been previously reported. The dependence of these effects on CO2, O2 and light led us to conclude that both photorespiration and an alternative electron acceptor, supported increased electron flow, and thus provided photoprotection, under these conditions. Further experiments showed that the increased electron flow was maintained by rapid rates of PSII repair, particularly at combined high light and temperature. Overall, the results suggest that photodamage to the light reactions can be avoided under high light and temperatures by increasing electron sink strength, even when assimilation is strongly suppressed.