Water stress and apple fruit quality : a thesis presented in partial fulfilment of the requirements for the degree of Master of Horticultural Science at Massey University

Regulated Deficit Initiation (RDI) has been used successfully in dry climates to control vegetative growth of fruit trees during the early part of the growing season without seriously disadvantaging fruit growth or quality. This project was undertaken as part of a study to investigate the feasibility and practicality of using the RDI concept in a humid fruit growing environment using Royal Gala apple trees. Treatments consisted of a lucerne cover crop, black polyethylene undertree covers and a within-row herbicide strip which is the normal commercial practice in New Zealand orchards. A full irrigation treatment (FI) was used on half the experimental trees and an RDI treatment was used on the other half of the trees. The RDI treatment consisted of withholding water until 105 days after full bloom, then using a full irrigation for the remainder of the season. Integration with depth of the soil moisture content (θ) (measured with a neutron probe) at the commencement of the experiment revealed 230 mm of water was stored in the top 900 mm of soil. Full irrigation resulted in θ increasing, with storage of about 250 mm 83 days after bloom and remaining at this level for the remainder of the season. The storage in the RDI treatments decreased in a linear manner until 58 days after bloom, after which it remained constant until initiation was started 105 days after bloom. The lucerne RDI (LRDI) treatment had a lower storage (105 mm) during this constant period than plastic RDI (PRDI), or herbicide (HRDI) treatments which both had a storage of 130 mm. The amount of water in the soil at this time for LRDI and for PRDI/HRDI was 42% and 52% of the total available capacity. Immediately after irrigation commenced on the RDI treatments, profile water storage returned to the values of the FI treatments remaining at these values for the rest of the season. Removal of water from the soil profile was not uniform. It appeared that lucerne removed moisture from the upper horizons first, before extracting it from the zone below 500 mm. Leaf water potentials (ψₑ) were lower in RDI treatments, when measured at midday (but not pre-dawn), than in FI treatments. Leaves from LRDI trees had lower ψₑ values than did leaves from PRDI and HRDI trees. Rate of fruit growth was reduced in all RDI treatments during the early part of the season, but returned to the same value as FI fruit once irrigation was resumed, except for LRDI fruit which did not attain the same growth rate of FI fruit. There were less large fruit and more small fruit from LRDI treatments than from other treatments where no significant effects on fruit size were measured. Fruit from RDI treatments were firmer, less mature and contained more soluble solids at harvest than FI fruit; some of these differences were maintained through 12 weeks storage at 4°C. There was no consistent effect of irrigation or cover treatment on fruit colour, mineral content or disease incidence at harvest or after storage. Vegetative growth, measured as pruning weights and the increment in trunk diameter, was significantly reduced by RDI treatments with LRDI causing the greatest reduction in pruning weight and PRDI inducing the smallest trunk diameter increase. The combined lucerne cover crop and RDI treatment was the most successful method found for reducing soil moisture in a humid climate. It also resulted in the greatest amount of stress being induced in these trees, reducing both vegetative growth - a desirable effect, but also fruit growth, a commercially undesirable effect. It is suggested that different methods of managing lucerne, or the use of less successful water extracting plants will need to be evaluated before recommending a successful, yet practical method of using the RDI concept in apple orchards growing in humid environments