Improving citrus quality with regulated deficit irrigation (CT17000)
What’s it all about?
International research indicates that a regulated deficit irrigation approach can be applied during citrus maturation and ripening stages to enhance fruit sugar content, while saving irrigation water – however there are currently no practical recommendations for its implementation in improving fruit quality on-farm.
This project is using irrigation trials to develop a practical irrigation deficit method. It will ultimately produce guidelines allowing Australian growers to adopt smart, innovative agronomic practices that can deliver improved fruit quality – with a particular focus on enhancing sugar content for fruit to be exported to Asian markets, where there is a preference for sweeter citrus.
Trials for six irrigation treatments continued at the Dareton site in the third year of the program.
The team monitored electronically for soil moisture and stress, with data collected weekly for sugar (0Brix), acid, and sugar-to-acid ratio to establish sweetness profiles throughout the growing season. Fruit quality, size distribution and measurement were collected at harvest, assessing irrigation effects on fruit growth.
Heavy rain events during the stress treatment period impacted some results and saw variation across treatments. The team found that stress imposed very early in fruit development increased sugar levels which were then carried through to harvest in all three varieties (M7, Houghton, Lane).
The team also analysed 61 years of historical rainfall data to determine that March and April are safest for successful regulated deficit irrigation treatments.
It was decided to carry the experimental program into a further year, with the fourth-year program to utilise the six irrigation treatments commencing in March 2021.
The team continue to provide updates and engage with industry. In April 2021, project progress was shared with 80 growers at a field day held at the Dareton Research Institute as a part of the Citrus Australia and NSW Department of Primary Industries roadshow.
Six irrigation treatments are currently on trial at the New South Wales Department of Primary Industries’ Dareton Research Institute.
The second year of work involved six irrigation treatments applied to an established orchard of 10-year-old navel orange trees. The site included early-maturing M7, mid-maturing Houghton and late-maturing Lane navel varieties grafted onto two rootstock, Poncirus trifoliate and Troyer citrange.
In this second season of trials, the focus was on adjusting the deficit irrigation treatments to enhance sugar levels without significantly reducing fruit size.
Data was collected throughout the season to establish a sweetness profile, with weekly recording of sugar (0Brix), acid, and sugar to acid ratio. The effects of soil moisture on fruit quality, juice percentage and fruit size were determined.
There were no significant effects on juice percentage for Houghton and Lane varieties compared to control, however there was a slight reduction in juice for the early maturing M7 navels across some trial timings. Fruit size distribution at harvest was affected by the irrigation treatments, particularly in the Houghton and Lane varieties at certain stages.
The Year two results will feed into ongoing project investigations, due to be complete in 2021.
The first-year findings of the trials are summarised in this article Early results for deficit irrigation trials that appeared in Australian Citrus News, Autumn 2019.
Read how Leeton grower Dean Morris of Moricom Orchards has applied his own “gut feel” for irrigation deficit in his crops in this article in Sunraysia Daily, Feb 2019.
Initial project work has seen trial trees and rootstocks subjected to an irrigation deficit (50 per cent normal irrigation rates) for 60 days across three different parts of the growing season (Feb-April, March-May and April-June). The trees were ten-year-old navels of different varieties, including the early maturing M7 navel, mid maturing Houghton navel, and late maturing Lane late navel. Rootstocks involved were Poncirus trifoliata, Troyer citrange, Citrus volkameriana, Swingle citrumelo and Citrus marcophylla.
Soil moisture was monitored and measurements made of fruit sugar, sugar-acid ratio, fruit quality, juice, yield and fruit size distribution. The results were compared against control trees not subject to irrigation stress.
For some varieties, fruit subjected to deficit irrigation in February/March and March/April differed from fully irrigated controls, including increased sweetness and BrimA values (acid value) – but the regimen also reduced the size of fruit at harvest.
These initial findings suggest that lower levels of irrigation will increase sweetness of fruit, but more water is required to maintain yield. The next round of trials will repeat the experiment with drought stress set at 75 per cent of normal irrigation.
Growers in the Riverina and Perth heard about the trials from researchers at meetings during the year.
This project is a strategic levy investment in the Hort Innovation Citrus Fund