Postharvest Deficit Irrigation in Peach and Pear

Rebecca Bruce and Ian Goodwin,
Department of Primary Industries
Tatura, Victoria, Australia
Email: rebecca.bruce@dpi.vic.gov.au

Introduction: Drought, reduced water allocations and high water prices are likely to remain long-term issues for Australian fruit growers. Fruit growers need to make every drop count. Already many growers have converted to micro-irrigation and have adopted good irrigation scheduling techniques. In some situations growers are using regulated deficit irrigation (RDI) during the slow growth period to increase water use efficiency. But what if you could significantly reduce water consumption with no negative effects on the following season’s fruit yield or quality?

Figure 1: Commercial block of T204 peaches where the trial is set up.

Previous Research:

There has been significant research into the application of RDI and the benefits associated with a subsequent reduction in vegetative growth, increased water savings and yield maintenance. The effects of severe water deficit on yield and quality during the slow fruit growth period have been well documented, and the implications of imposing deficit irrigation throughout the season have previously been established for many fruit crops. Considerably less research has been published on the effects of post harvest deficit irrigation. Significant tree growth occurs during the postharvest period, and the potential to save a substantial amount of water without productivity loss or a reduction in fruit quality exists, particularly in early and mid-season maturing varieties.

In peach, studies in California have shown that 4 years of moderate to severe postharvest water stress imposed on an early maturing variety led to no reduction in the next season’s yield or fruit size, and no decline in tree health (Larson et al., 1988; Johnson & Handley, 2000). Any increase in double fruit or deep sutures was counteracted with a single autumn irrigation to alleviate stress during flower development. In a subsequent trial, applying moderate water stress to peach trees after harvest increased flower density and fruit set in the next year, with commercial thinning required to maintain fruit size. Postharvest vegetative growth was also reduced. Moderate deficit irrigation postharvest resulted in significant water savings without affecting fruit yield or time to maturity. In pear, the response is similar. As in all pome and stone fruits, flower organs in the reproductive buds develop during the postharvest period and severe water stress during development may reduce productivity in the next season. A moderate level of water stress applied postharvest to pears has previously resulted in a higher crop yield compared to trees exposed to a high or low water stress environment (Naor et al., 2006). In this case, both low water stress and severe water stress appeared to slow flower organ development, leading to reduced flowering density and lower fruit set. Investigating Postharvest Irrigation Impacts: These previous trials have indicated that significant water savings (up to 50 %) may be made postharvest on early maturing peach varieties with minimal effect being placed on the long term health of the tree, or the following season’s yield or quality. The aim of our trial is to investigate the impact of reducing postharvest irrigation on tree water use and stress response of mid-season maturing peach and pear, and how this may affect the following season’s fruit yield and quality. The current trial is sited on two commercial orchards in the Goulburn Valley; in a block of Williams Bon Chretien (WBC) pears and a block of T204 peaches (Figure 1) utilising the existing microjet irrigation infrastructure and following the irrigation schedule of the growers. Each site has five irrigation treatments, replicated four times, with buffer trees and rows to prevent interference between treatments. Irrigation volume is controlled by replacing the existing microjet emitters with similar sprays of appropriate flow rates. In the peach block, irrigation is being applied at 50, 75, 100, 160 and 200 % of the grower’s normal irrigation volume. Figure 3: Pressure chamber for monitoring tree water stress Figure 3: Pressure chamber for monitoring tree water stre The pear block is similar, with 0, 50, 100, 160 and 200 % of the scheduled irrigation volume being applied. Irrigation levels in excess of those normally applied by the grower were chosen as both orchards are already minimising their postharvest irrigation applications to preserve water and reduce costs.

Figure 2: Gbug data logger recording soil moisture at 30, 60 and 120 cm depth taken by the gypsum blocks.

Investigating Postharvest Irrigation Impacts: Measurements of Water Stress and Productivity: Gypsum blocks were installed in the soil at the pear site at depths of 30, 60 and 120 cm to monitor soil moisture for comparison with tree water stress results (Figure 2). Tree water stress is being monitored weekly (from harvest until leaf fall) in two ways. Leaf and stem water potential is being measured using a pressure chamber which applies air pressure to a detached leaf until water appears at the cut surface of the petiole. A high value of pressure indicates that the tree is under water stress (Figure 3). The other water stress measurement being used is leaf conductance, measured using a porometer. This instrument measures the rate of plant leaf transpiration, calculated by the time it takes for the leaf being measured to release enough water vapour to change the relative humidity within the instrument's measuring chamber. In this case, a lower number indicates a lower leaf conductance and that the tree is under water stress.		Figure 4: A thermal camera mounted underneath a blimp is being used to measure canopy temperature.

Other measurements to be taken over the postharvest period will include tree canopy temperature as an additional tree water stress measurement (Figure 4), canopy foliage cover to indicate canopy size and health, post-leaf fall flower bud dry weight, number of buds per lateral, lateral diameter as a measure of lateral strength and the degree of autumn flowering. In the following season, measurements will be taken to determine the impact of the five irrigation levels on yield and fruit quality. Some of these include flower number, fruit number, fruit size at harvest and evidence of fruit disorders.

References
Johnson, R.S. & Handley, D.F. 2000, ‘Using water stress to control vegetative growth and productivity of temperate fruit trees’, HortScience, 35(6), pp. 1048-1050.

Larson, K.D., DeJong, T.M. & Johnson, R.S. 1988, ‘Physiological and growth responses of mature peach trees to post-harvest water stress’, Journal of the American Society for Horticultural Science, 113(3), pp. 296-300.