Partial rootzone drying in apple and in processing tomato : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Institute of Natural Resources, Massey University, Palmerston North, New Zealand
New water saving irrigation strategies need to be explored and partial rootzone drying (PRD) is such a strategy as it involves irrigating only part of the rootzone with the complement left to dry to a pre-determined level. In other deficit irrigation (DI) methods the entire rootzone is irrigated with less water than evapotranspiration. I focussed on PRD for its effects on apple and on processing tomato. For apple three field experiments were done, two on 'Pacific RoseTM' in Manawatu and one on 'Royal Gala' in Hawke's Bay. In all three, leaf water potential (ψleaf) was similar between PRD and commercially irrigated (CI) treatments and so were yield and fruit quality. However, 'Pacific RoseTM' PRD fruit in one experiment had lower water loss in storage than did CI fruit. For 'Royal Gala', PRD fruit quality was improved in terms of flesh firmness and total soluble solids concentration. In all apple experiments PRD trees received only 50% of water given to CI trees. I recommend PRD as a feasible irrigation strategy for apples in New Zealand, but suggest further research for drier areas. 'Petopride' tomato was studied in six glasshouse experiments. Depending on the experiment, PRD irrigation was shifted to the previously-unwatered rootzone on the basis of volumetric soil water content, on a daily basis, and on intervals of 2, 4, and 6 days. Maintenance of ψleaf, photosynthetic rate, stomatal conductance, yield, and fruit quality in PRD depended on the extent of soil drying. Irrigation use efficiency was almost twice higher in PRD plants than in CI plants. Blossom-end rot was higher in some of the PRD treatments, but in an especially-designed experiment I found out that PRD per se could not be the cause. From an experiment involving the measurement of root water potential, I concluded that water does not move from the wet roots to dry roots during PRD. I found that the tomato fruit, which is normally a stronger sink than vegetative parts, becomes a weaker sink during water stress. I recommend PRD for processing tomato, but with a suitable irrigation frequency to avoid lowering the midday ψleaf to a value of less than -1.2 MPa. This necessitates field trials in various environmental conditions.