OBSERVATIONS OF DIURNAL DECLINE OF PHOTOSYNTHETIC GAS-EXCHANGE IN KIWIFRUIT AND THE EFFECT OF EXTERNAL CO2 CONCENTRATION

The prevalence of diurnal decline of photosynthesis in field-grown kiw ifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var . deliciosa 'Hayward') and the effects of elevated CO2 concentration d uring decline were studied. During the seasonal period from soon after fruit set to harvest, marked diurnal reductions of photosynthesis rat e were found that could not be correlated with levels of photosyntheti cally active radiation (PAR), temperature, and transpiration. Declines of photosynthesis were observed only on clear days characterised by b enign environmental conditions other than sustained irradiance at satu rating or near saturating levels. Elevation of CO2 concentration to 20 0 mu mol/mol above ambient during photosynthesis decline overcame the decline effect, allowing photosynthesis to track irradiance levels thr oughout the day. Possible mechanisms generating the diurnal decline an d the alleviation by elevation of CO2 concentration are discussed.