THE IMPACT OF CANOPY GROWTH AND TEMPORAL CHANGES IN RADIATION ON THE DYNAMICS OF CANOPY CARBON ASSIMILATION FOR KIWIFRUIT (ACTINIDIA-DELICIOSA) VINES DURING SPRING

Diel trends of canopy CO2 assimilation (A) for four kiwifruit (Actinidia deliciosa) vines during spring were measured during a 78-day period from 12 days after bud burst until the summer solstice. The canopy of each vine was enclosed in an open-system gas exchange cuvette, and a computer-controlled sampling and logging system enabled multiplexed analysis of gas exchange rates for each vine. The canopy leaf area, estimated on five occasions, increased on average from 0.25 m2 m-2 at 15 days after bud burst to 2.39 m2 m-2 at 85 days after bud burst. Inter-vine differences in leaf area were consistent throughout the measurement period. Diurnal integrals of quantum flux density (Q), measured above the canopy within the cuvette, varied from 9 to 56 mumol m-2 day-1, and did not show a clear tendency to increase with increasing daylength towards the summer solstice. Mean daily air temperatures within the cuvettes tended to increase during the measurment period, from 12-14-degrees-C soon after bud burst to 15-21-degrees-C near the summer solstice. Diel integrals of canopy A tended to increase during the measurment period, from ca -0.7 mumol CO2 m-2 day-1 soon after bud burst to as high as 1.4 mumol CO2 m-2 day-1 near the summer solstice. Inter-vine differences at any stage of the season were related to differences in leaf area. The temporal trend for increasing diel integrals of canopy A during spring followed the increasing leaf area, although large day-to-day differences at any stage of the season could be associated with concomitantly varying diurnal integrals of Q. The temporal trend of increasing canopy A was almost entirely due to increasing day-time A, as night-time A (negative) changed relatively little during the measurement period. Asymptotic exponential curves were used to describe the relationship between instantaneous Q and canopy A at three stages of the season. These relationships indicated that the initial response of canopy A to increasing incident Q (the apparent quantum yield) was 0.03, 0.07 and 0.08 mol CO2 mol-1 Q at ca 35, 55 and 85 days after bud burst, respectively. The quantum saturated rate of canopy A at any stage, however, responded significantly to inter-vine differences in leaf area, ranging from 9.6 to 11.9 mumol CO2 m-2 s-1 at ca 35 days after bud burst, 25.7-29.7 mumol CO2 m-2 s-1 at ca 55 days after bud burst, and 29.0-37.8 mumol CO2 m-2 s-1 at 85 days after bud burst. The temporal changes and inter-vine differences at any stage in the quantum saturated rate of canopy A could be related to canopy leaf area.