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 (Actinid ia 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 an alysis of gas exchange rates for each vine. The canopy leaf area, esti mated on five occasions, increased on average from 0.25 m2 m-2 at 15 d ays after bud burst to 2.39 m2 m-2 at 85 days after bud burst. Inter-v ine differences in leaf area were consistent throughout the measuremen t period. Diurnal integrals of quantum flux density (Q), measured abov e 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 daylengt h towards the summer solstice. Mean daily air temperatures within the cuvettes tended to increase during the measurment period, from 12-14-d egrees-C soon after bud burst to 15-21-degrees-C near the summer solst ice. Diel integrals of canopy A tended to increase during the measurme nt 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 i ncreasing canopy A was almost entirely due to increasing day-time A, a s night-time A (negative) changed relatively little during the measure ment period. Asymptotic exponential curves were used to describe the r elationship between instantaneous Q and canopy A at three stages of th e season. These relationships indicated that the initial response of c anopy 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 bur st, respectively. The quantum saturated rate of canopy A at any stage, however, responded significantly to inter-vine differences in leaf ar ea, 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 saturat ed rate of canopy A could be related to canopy leaf area.