Canopy photosynthesis and respiration of kiwifruit (Actinidia deliciosa var. deliciosa) vines growing in the field

Net CO(2) assimilation (A) for canopies of kiwifruit (Actinidia deliciosa var. deliciosa) vines enclosed in a whole-canopy cuvette was measured continuously for three periods of 15-20 days during late summer, near Hamilton, New Zealand (latitude 38.2 degrees S). Canopy A showed an asymptotic response to incident radiation (PAR), saturating at about 1300 micromol m(-2) s(-1) for one vine and about 800 micromol m(-2) s(-1) for two other vines. Radiation interception at low solar angles and low leaf area apparently limited the response of A to PAR. Radiation saturated rates of A were 25-30 micromol CO(2) m(-2) s(-1) for one vine, and 12-18 micromol CO(2) m(-2) s(-1) for two other vines. At any PAR, canopy A was often lower in the afternoon than in the morning. Canopy respiration averaged 8.9 micromol CO(2) m(-2) s(-1) at 12 degrees C, but increased only 24-34% over the range 7-17 degrees C. Net daily C gains for the whole canopy, calculated as the temporal integral of A, ranged from -0.8 g C m(-2) for a cloudy day (PAR </= 450 micromol m(-2) s(-1)) to 6.2 g C m(-2) for a day with intermittent cloud. Measurements of gas exchange for single leaves in a leaf cuvette were used to examine the basis of the whole-canopy responses to environmental variables. Partial stomatal closure due to water stress apparently contributed to the afternoon depressions in A, although non-stomatal limitations may also have been involved.