INFLUENCE OF ENVIRONMENTAL AND PLANT FACTORS ON CANOPY PHOTOSYNTHESISAND TRANSPIRATION OF APPLE-TREES

We estimated carbon and water flows, canopy conductance and the assimi lation/transpiration ratio of fruiting and non-fruiting apple trees gr own in the field, from daily gas exchange measurements taken during th e summer with a whole-canopy enclosure device. The relationships betwe en photosynthetic and transpirational responses and environmental cond itions were also investigated, as well as the role of canopy conductan ce in controlling carbon dioxide and water vapor exchange. Light-satur ated net photosynthetic rates, which were higher for the fruiting cano py than for the non-fruiting canopy, showed a general decrease in the afternoon, particularly for the non-fruiting canopy, compared with rat es in the morning. When light was not limiting, the afternoon decrease in net photosynthesis appeared to be regulated more by non-stomatal f actors than by changes in canopy conductance. Canopy conductance, whic h was higher for the fruiting canopy than for the non-fruiting canopy, may actively regulate photosynthetic activity and may also be modulat ed by feedback control in response to assimilation capacity. We conclu de that adjustments in canopy conductance, which were partially depend ent on the vegetative-reproductive status of the tree, control the equ ilibrium between photosynthesis and transpiration. We also demonstrate d that whole-canopy chambers can be used to estimate photosynthetic an d transpirational responses thereby overcoming the difficulty of scali ng these physiological responses from the leaf to the whole-canopy lev el.