THE ACQUISITION AND UTILIZATION OF CARBON IN EARLY SPRING BY KIWIFRUIT SHOOTS

Measurements of net photosynthetic rate (at 1450 mu mol m(-2) s(-1) ph otosynthetically active radiation) of leaves, of leaf and stem respira tion, and of shoot growth of potentially-fruiting laterals on kiwifrui t (Actinidia deliciosa) were used to estimate weekly shoot carbon bala nces over the first 10 weeks of shoot growth (budburst to anthesis). C onsistent differences in the rate of shoot elongation, of internode ex pansion and of increase in basal diameter were found among shoots. Fas ter-growing (long) shoots acquired carbon by photosynthesis al a faste r rate even in the first few weeks after budburst, but the amount of c arbon required to sustain this growth resulted in shoot carbon deficit s which were approx. seven times greater than those of the slower-grow ing (short) shoots. It was estimated that the transition from shoot ca rbon deficit to carbon surplus occurred 3-4 weeks after budburst, irre spective of shoot growth rare. As a result of subsequent rapid increas es in shoot photosynthetic rate, long shoots had a shoot carbon surplu s of 44 g C week(-1) in the week before anthesis, approx. three times that of the short shoots. Defoliation (66%) of shoots 1 week after bud burst, and subsequent removal of later-emerging leaves to maintain the level of defoliation, had the effect of slowing shoot growth in the c arbon deficit period, particularly for the long shoots. However, the d uration of shoot expansion in the defoliated shoots was longer, result ing ultimately in shoots which were longer than the control shoots. Li nkages among early carbon balance dynamics of shoots, shoot length at anthesis, and fruit growth are discussed. (C) 1997 Annals of Botany Co mpany.