Consumers are concerned about the quality of the avocados they purchase, since fruit on the retail shelf can contain rots and physiological disorders. Previous studies have shown that high fruit calcium (Ca) and magnesium (Mg), and low potassium (K), concentrations are related to less internal disorders and rots. Several pre-harvest factors such as rootstocks and pruning systems can reduce rots and disorders, and these may exert their effect partly through improving fruit mineral concentrations. This thesis examined the scion/rootstock characteristics that might affect the movement of cations from the soil to the roots, and from the rootstock to the scion, as well as the effect of leaf to fruit ratio on distribution to the fruit and the partitioning of cations between the fruit tissues. Four seedling rootstock cultivars (Velvick, Hass, Duke 7 and Fuerte) were grown from 6 weeks of age under adequate and marginal Ca nutrition in inert growing medium in the glasshouse to determine the potential of the rootstock to affect cation uptake. There were no significant cultivar effects on total root and suberised root Ca concentrations. However, there was a strong trend for higher Ca concentrations in Velvick leaves compared with Hass, Duke 7 and Fuerte leaves. In contrast, Velvick had significantly lower leaf K and higher root K concentrations, compared with the other cultivars. As a result, Velvick had the lowest ratios of K/Ca, Mg/Ca and (K+Mg)/Ca in the leaves. This suggests that Velvick may be better able to increase Ca and decrease K concentrations in the leaves, which could be related to the higher Ca concentrations in fruit from Hass scions on Velvick rootstock trees.
Submitted in the fulfilment of the requirements of the degree of Doctor of Philosophy, University of the Sunshine Coast, 2015.