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Fruit composition, growth, and water relations of Braeburn apples under reduced plant water status : this thesis is presented in partial fulfilment of the requirements of the degree of Doctor of Philosophy in Horticultural Science at Massey University, Palmerston North, New Zealand
Water plays a major role in the physiological processes of plants. Effective irrigation relies on a comprehensive understanding of the impact of water on plant processes. As water becomes an increasingly scarce resource, the impact of reduced plant water status on crop performance (quality and yield) requires investigation. The effects of reduced plant water status on fruit composition, growth, and water relations were therefore studied using both field-grown and container-grown 'Braeburn' apple trees. Vegetative growth and carbon assimilation were also measured. Plant water deficit was imposed at various times during the growing season. The treatments were: control, which was fully irrigated during the experimental period, entire-season deficit, deficit irrigation from 55 days after full bloom (DAFB) until final fruit harvest (183 DAFB), early-season deficit (from 55 - 100 DAFB) followed by rewatering, and late-season deficit (from approximately 105 DAFB until final harvest). Reduced leaf water potential developed in all deficit irrigated trees during the stress period. Only the entire-season deficit irrigation treatment resulted in a significant reduction in vegetative growth as measured by total leaf area, shoot growth, and trunk growth. Return bloom was reduced under an early-, but not late-season deficit. Photosynthesis was generally reduced in water deficit treatments, as was stomatal conductance. Only an entire-season deficit irrigation reduced individual fruit weight. Fruit soluble solids and sugar concentration were generally increased under deficit irrigation treatments. However, upon rewatering of the early-season deficit trees, the values again became the same as controls. Fruit mineral concentration did not show consistent differences between treatments and the incidence of storage disorders was low in all treatments and unaffected by deficit irrigation. Early-season water deficit lowered both fruit water potential and osmotic potential. Despite turgor maintenance within the fruit during the stress period, growth was reduced at this time. A late-season water deficit did not modify fruit water relations. It appears that 'Braeburn' fruit are resilient to periodic water deficit during the season, and that water conservation is possible with limited impact on total crop yield. Additionally, a late-season deficit may even enhance some fruit quality attributes, such as increased total soluble solids. An early-season deficit reduced return bloom and must therefore be used with caution. An entire-season water deficit is not recommended due to the reduction in fruit size.