Calvin cycle activity in fruit and the effect of heat stress

Abstract

As green fruit mature the permeability of the outer epidermis decreases. Consequently, gas exchange with the outside air becomes more restricted and it is unclear whether or not maturing fruit continue to fix carbon dioxide (CO 2) photosynthetically, possibly utilizing accumulated internal CO 2. To examine this, Calvin cycle activity in fruit was investigated by chlorophyll a fluorescence quenching in vivo, the fluorescence emission from the fruit surface being measured with a modulated fluorometer. Fruit of 15 species were examined and all showed evidence of Calvin cycle activity as indicated by relaxation of chlorophyll fluorescence quenching. Allowing for the differences in chlorophyll content, the activity in fruit was comparable with that in leaves. When exposed to CO 2-free air, tomato leaves and discs of avocado peel quickly lost activity, but loss of activity was slow in intact fruit, indicating that most of the CO 2 fixed photosynthetically in fruit was derived from CO 2 accumulated within the fruit. Calvin cycle activity in fruit was especially sensitive to heat, more so than either photosynthetic electron transfer activity or photophosphorylation. Optical monitoring of Calvin cycle activity by fluorescence quenching thus has the potential to detect early symptoms of heat stress in fruit, for instance, as the result of post-harvest heat treatments to disinfest fruit of insects.