Competition between fruit and vegetative growth in Hayward kiwifruit

Fruit dry matter content (DM) of kiwifruit, defined as the fruit dry weight (DW) expressed as a percentage of the fresh weight (FW), is used as an indicator of eating quality. A high DM at harvest results in increased consumer satisfaction. To determine the role of carbohydrate availability and sink competition in altering fruit size and DM at commercial harvest, we used an experimental model system based on phloem-girdled shoots to manipulate the number of source leaves, fruit and vegetative sinks on a shoot of 'Hayward' kiwifruit. Commercially produced kiwifruit fruits typically weigh 95-115 g and have a DM of 14-17 units (this is a percentage but the % sign has not been shown, to reduce confusion with changes in DM expressed as a % change). With our experimental system, this corresponded to the fruit growth observed with two to three leaves when there was no vegetative competition. With four source leaves and a single fruit, competition from a vegetative regrowth reduced fruit fresh weight by 28% and dry weight by 39%. With two fruit, and no vegetative sink, between-fruit competition had less severe effects on the fruit and dry weight. Doubling the number of fruit on a girdled shoot reduced average fruit fresh weight by 15%, dry weight by 23% and DM by 12%. With a high source supply (seven leaves with no regrowth competition) fruit growth was very high with a fresh weight of 174 g and 18.4 DM, substantially above that of commercial production.