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

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Abstract
Net CO2 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° S). Canopy A showed an asymptotic response to incident radiation (PAR), saturating at about 1300 μmol m−2 s−1 for one vine and about 800 μmol 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 μmol CO2 m−2 s−1 for one vine, and 12–18 μmol CO2 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 μmol CO2 m−2 s−1 at 12 °C, but increased only 24–34% over the range 7–17 °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 μmol 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.

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