The Acquisition and Utilization of Carbon in Early Spring by Kiwifruit Shoots

Abstract

Measurements of net photosynthetic rate (at 1450μmol m-2s-1photosynthetically active radiation) of leaves, of leaf and stem respiration, and of shoot growth of potentially-fruiting laterals on kiwifruit (Actinidia deliciosa) were used to estimate weekly shoot carbon balances over the first 10 weeks of shoot growth (budburst to anthesis). Consistent differences in the rate of shoot elongation, of internode expansion and of increase in basal diameter were found among shoots. Faster-growing (long) shoots acquired carbon by photosynthesis at a faster rate even in the first few weeks after budburst, but the amount of carbon required to sustain this growth resulted in shoot carbon deficits which were approx. seven times greater than those of the slower-growing (short) shoots. It was estimated that the transition from shoot carbon deficit to carbon surplus occurred 3–4 weeks after budburst, irrespective of shoot growth rate. As a result of subsequent rapid increases in shoot photosynthetic rate, long shoots had a shoot carbon surplus of 4.4 g C week-1in the week before anthesis, approx. three times that of the short shoots. Defoliation (66%) of shoots 1 week after budburst, and subsequent removal of later-emerging leaves to maintain the level of defoliation, had the effect of slowing shoot growth in the carbon deficit period, particularly for the long shoots. However, the duration of shoot expansion in the defoliated shoots was longer, resulting ultimately in shoots which were longer than the control shoots. Linkages among early carbon balance dynamics of shoots, shoot length at anthesis, and fruit growth are discussed.