The end of summer is around the corner, days are getting shorter and shorter. Citrus growers in the northern hemisphere are approaching a special time in the season. Growers aiming for high quality fruits, are now preparing to start deficit irrigation (also known as Regulated Deficit Irrigation or RDI) aimed to increase sugar level in the fruits, also known as Brix.
Many markets pay premium price for those fruits containing high Brix degree. Brix level can often be a ripping millstone. If a certain required Brix level is not achieved, packing houses, processing factories and market distributers will refuse to accept the fruits. Hence, achieving higher Brix as soon as possible is key for achieving farm profitability.
The Brix challenge
Dr. Ido Gardi, head of Phytech's Agronomic Research, explains the plant-based approach leading to a new way to optimize Brix. "To achieve high accumulation of high energy compounds such as sugars, the plant must channel its energy reservoir towards the fruits on the account of other parts. But how can a grower affect and maximize this internal biological mechanism of the tree? Can we help trigger this process and help the plant allocate its energy towards producing sweeter fruits?"
A trunk sensor (Dendrometer)
"Without this clear understanding, growers are left with traditional practices or common beliefs and most importantly, without a way to know beforehand if the fruits are 'on the right way' to high Brix level"
Different tactics to manage Brix
Over the years, there has been an effort to achieve higher sugar content by selecting specially breaded varieties or rootstocks for new planting.
Once planted, several agrotechnical methodologies can be applied in order to meet the brix potential such as fertilization (Dichio, Xiloyannis, Sofo, & Montanaro, 2007; GI Moss, 1974), spray of growth regulators (Agustí, Martinez-Fuentes, & Mesejo, 2002; Goldschmidt, 1999) or deficit irrigation.
Deficit irrigation – In search for a protocol
From all the above, Regulated Deficit Irrigation (RDI) seems a relatively safe and not expensive approach. What makes RDI a good candidate for achieving the sweet prize?
Dr. Gardi: "The trunk or stem is the one single point which represents the whole plant. Hence, measuring the trunk growth, which is correlated to vegetative processes in the plant, can be a useful method to monitor carbon allocation shift between vegetative and reproductive (fruits) organs in the plant. This can be a helpful feedback for any practice aimed to shift carbon towards the fruits as well as an indication of whether plant productivity is compromised"
First step: establishing the relation between trunk growth and Brix levels
Over the last few seasons, we performed random fruit sampling, selective fruit sampling and harvest data analysis. In more than 500 citrus blocks located in United States, Israel and Australia over multiple seasons, a strong and significant negative correlation can be found between the monthly growth during the autumn months and the brix level. However, upon strong water stress which even translates to reduction in fruits' growth, Brix levels were not strongly depended on trunk growth.
Figure 1: Late summer trunk growth of four trees yielded fruits with different brix levels
Bottom Line: On a happy tree, when trunk growth is restrained during the autumn, higher Brix levels can be expected
The results validated that dendrometers can be an effective monitoring tool for trunk growth and can assist in estimating sugar accumulation in the fruits for either fine tuning harvest date or predicting fruit quality. The immediate feedback it provides, may allow controlled intervention while maintaining tree health.
Second step: Developing Dynamic Plant-Based Regulated Deficit Irrigation (RDI) Protocol - An
Being able to estimate Brix level prior to harvest can help the grower improve market positioning and better prepare for harvest. The next step is optimizing this process. How, and when a grower should act in order to increase brix level at harvest?
Dr. Gardi: "By stressing the tree and limiting its sugar production, the tree will shift energy towards the fruits on the account of vegetative growth. This process is made visible by our dendrometers and fruit sensors, which allow the growers to track in real time the effect of RDI on the trunk growth rate and the fruit size".
Being already a common practice for many fruit growers, RDI could be a relatively safe and simple method to limit the tree water resources. However, relying on rules of thumb or eye sight might result in over stressing the trees and damaging their future productivity and health. Without a constant monitoring tool and a gold standard to achieve, the effectiveness of an RDI treatment can be observed only post-harvest.
By using Phytech's monitoring system during the correct time of the season, growers can determine the RDI intensity in order to increase and optimize brix levels at harvest. We aim to restrain the trunk growth on the one hand and maintain fruit growth on the other. We want to restrain tree’s resources but maintain its productivity.
Below you can see 4 examples from Phytech's web app, demonstrating how growers can work with the tree and fruit signals to adapt their irrigation:
1) This block has a negative growth rate and fruit growth is starting to drop. Recommendation: Maintain weekly amount but change to more frequent and constant irrigation:
2) A good example of a successful RDI application. In this block, we can see that the grower managed to reduce trunk growth without affecting fruit growth. Maintaining the topsoil wet with the same irrigation coefficient at a constant frequency.
3) Trees in this block are growing too much: fruit growth rate is also increasing. Recommended action: Reduce irrigation amount to restrain growth and improve fruit quality.
4) Trunk growth has been restrained for about 4 weeks now. Recommended action: Switch from RDI to regular irrigation to optimize fruit size:
"A Dynamic approach means we're looking at real-time signals from the trees and its fruits. We're consulting "him" directly and based on this we apply irrigation to optimize Brix levels", explains Dr. Gardi, "Since we're connected directly to the trunk and the fruit, We're not fixed on a pre-determined RDI plan that might miss the target. The goal is to restrain trees' autumn growth to trigger the energy allocation to the fruit. We reduce the irrigation amount (maintaining frequency) to slow down the trunk growth until it flattens, as long as the fruit size continues to increase. After that 'switch" we can safely stop the Dynamic RDI treatment, go back to regular irrigation and help the tree recover as light days are shortening".
Below you can find the results from our trials establishing the effectiveness of the Plant-Dynamic RDI method:
Grapefruit Gan Shmuel (Israel)
A plant-guided deficit irrigation trial was performed in Israel. The trail was aimed to test if deficit irrigation during the end of summer can stop trunk growth. As mentioned, lower trunk growth was linked to higher Brix levels, so the trial aimed to improve Brix levels by stopping the trunk while reducing irrigation. The methodology was to gradually reduce irrigation duration until trunk growth declined (not negative yet) and the fruit growth is not affected. Once the fruit growth starts to slow down the irrigation treatment is terminated, and duration will be back to normal.
Irrigation duration was adjusted once a week based on the previous week growth. After one month the fruits started to slow down on the treatment rows, therefore the deficit treatment has been stopped. From the end of the irrigation treatment and every 10-30 days since then Brix levels were measured.
The results in figure 10 show consistent and higher Brix levels on the treatment rows with no significant fruit size difference and no substantial fruit drop. The deference in juice content between the treated and the control rows was not significant (48.5 ±0.4%). These results support our hypothesis, in which, deficit irrigation can slow trunk growth which in fact reduce the vegetative growth in the autumn (a desirable outcome anyway). The reduction of autumn growth is translating to more energy being directed to the fruits and more sugars. Trunk and fruit sensors were crucial in monitoring and guiding the deficit irrigation to a level where no change in fruit size or significant stress was observed and therefore no effect on next season yield should be expected.
Oranges Mildura (Australia)
Guided deficit irrigation trial was performed in a large citrus farm near the city of Mildura, Australia. In one orange block (Cara-Cara) two rows were selected in the middle of the block. Automatic, remote-controlled valves were installed at the head of each row. Using these valves an RDI treatment was preformed based on the abovementioned methodology. The two treatment rows as well as tow control rows were monitored by Phytech’s dendrometers and fruit sensors.
During the course of February-March 2022 the RDI treated rows were irrigated at approximate 50% deficit compered to the controlled rows. A preliminary fruits quality test was made at April’s 24 (two weeks after the treatment was terminated). The RDI treated rows presented a higher brix (10.8%) and higher acid concentration (2.4%) compared to the control (10.3% and 1.7% respectfully).
Figure 5: Brix measurements during August to February from RDI treated and control rows in Gan Shmuel (Israel) grapefruit block