Preharvest practices affecting postharvest quality of 'Hayward' kiwifruit : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Physiology and Horticultural Science at Massey University, New Zealand
Repeat purchase of kiwifruit is primarily driven by consumer judgement of internal fruit quality attributes, including those affected by dry matter concentration (DMC) and mineral composition in fruit. This research investigated mechanisms affecting carbohydrate, mineral and water accumulation in 'Hayward' kiwifruit (Actinidia deliciosa), and related these to specific management practices. Canopy manipulation through pruning and treatments such as artificial pollination, defoliation, girdling, thinning and application of the auxin transport inhibitor TIBA, may affect fruit DMC and mineral composition. Leaf photosynthesis and fruit dry matter concentrations (DMC) started to decline as leaf area index values increased above 3-4. In addition to reducing competition for carbohydrates between vegetative and reproductive growth, leader pruning probably increased DMCs of fruit in the leader zone by improving light interception. Photosynthesis was not affected by crop loads between 20- 60 fruit m-2, but was consistently higher on non-terminating (long) shoots than on terminating (short) shoots, as were fruit DMCs. Differences in photosynthetic rate of leaves on these two shoot types were attributed to differences in shoot exposure to the sun, and also to the greater demand for carbohydrate within long shoots. Leaves subtending fruit may increase Ca, and to a lesser extent Mg, flow into fruit, however their accumulation was not affected by leaves outside the fruiting shoot. Xylem sap Ca and Mg concentrations were higher in shoots with a high rather than a low leaf: fruit (L:F) ratio and this may, at least partially, relate to the increase in shoot transpiration that occurs as shoot L:F ratios increase. Within vine variation in fruit Ca concentrations may reflect variations in xylem sap flow rates and Ca concentrations of xylem sap reaching fruit. Calcium translocation may occur independently of ion movement in the transpiration stream. Timing and extent of vascular differentiation in flower and fruitlet pedicels, possibly regulated by auxin, may influence fruit Ca accumulation. It is likely that early differentiation of vascular tissue in flower and fruitlet pedicels influenced cell division and subsequent (carbohydrate) sink strength of fruit by determining availability of carbohydrate for partitioning into cell walls. While growers have the potential to induce minor changes in fruit DMC, further increases will depend on the separation of carbohydrate and water accumulation. Further research is required to elucidate the mechanisms regulating phloem transport and unloading of sucrose in kiwifruit.