Trial To Improve Fruit Quality

Mandarin growers should benefit with a saving in irrigation water usage and the production of better quality fruit from a new trial being undertaken by HortResearch Kerikeri staff.

Research associates Mark Astill and Phil Prendergast are working with mandarin growers in Kerikeri trailing the use of deficit irrigation. Ground covers intercept the rain allowing control of the amount of water reaching the trees with the aim of producing fruit with the right flavour, size and firmness for the lucrative Japanese market.

Not only do these ground covers stop the rain; they also provide reflective light to increase the photosynthesis available for fruit growth. The more light, the higher the fruit sugar levels.

Mark Astill has even been heard to complain that he has been sunburnt under his arms from the reflection. But in a more serious vein he explains: "We need to find a measure that the growers can use to work out when trees need irrigation."

The study will apply different amount of irrigation to the mandarin trees and examine the influence of soil moisture on plant and fruit growth, and then on the maturity and quality of the fruit.

To help with the research heat-pulse sensors have been installed in the trunks of trees to monitor transpiration rates. Electronic probes are used to measure soil and to see where the plant is extracting the water from in the soil profile. This is monitored every two days.

A met station has also been installed in the research orchard to relate tree water use and fruit growth to the orchard microclimate. On a hot day a tree will use 15 litres of water. On cloudy days the water use drops to about 5 litres a day.

Regular measurement of the water status of the plant has been taken throughout the summer. Fruit measurements are being made on 100 fruit each from stressed and from well-watered trees. Then once a week, the fruit volume is measured. When the juice sacks have developed the brix ratios will also be monitored. Finally at harvest the total sugars and sugar/acid ratios of the fruit will be measured

Although this is the first season of the trial, researchers are confident that the outcome will be a saving in irrigation and the improved fruit quality.
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