By Fernando Alferez and Divya Aryal
Like the work being done with Hamlin sweet orange, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers are studying the effect of brassinosteroids (BRs) on the internal maturation of Valencia sweet orange fruits. The conditions of the treatments are the same as in Hamlin: A treatment of 186 milliliters of BR (homobrassinolide 0.1%) per 100 gallons water is applied just once at the right time of fruit maturation. This concentration has been determined after several dose-response trials. One gallon of this solution per tree is enough to fully cover the canopy.
The only difference with Hamlin treatments is the time of application, as the window of efficacy for the Valencia oranges is different. Plant growth regulators (PGRs) usually have very narrow windows of efficacy that depend on the physiological status of the plant organ to be treated (in this case, the fruit). BRs have reported effects on influencing the maturation of different fruits, but the research on citrus has been scarce to date, so there is no clear guidance on when to apply BRs to increase internal maturation in different varieties. Targeting the fruit at the right maturation stage will produce the desired effect of the PGR.
The goal is to increase Brix and the sugar-to-acid ratio in the juice. The approach for the past two seasons has been to treat independent plots of mature fruit-bearing Valencia trees with the dosage mentioned above. Every independent plot receives only one treatment, and treatments are performed two weeks apart in each independent plot from early January to mid-March (in other words, from the time the fruit starts to slightly change color). Then, the change in Brix and titratable acidity is measured two weeks after every treatment, in order to determine the sugar-to-acid ratio at each date.
Last year, researchers started to see an increase in Brix with applications performed in mid-February, and this trend continued well into March. Applying BRs before February did not make any difference in Brix or in the sugar-to-acid ratio. However, starting in mid-February, there was a mild increase in Brix. By March, Brix in fruit from BR-treated trees was 9.4 as compared to 8.5 in non-treated controls.
There was also a more pronounced decrease in titratable acidity after BR treatment as compared to controls. As a result, the sugar-to-acid ratio in the BR-treated fruit was 10.5 as compared to 7.9 in the controls.
As was previously noticed in Hamlin, trees with variable health also vary in their response to this PGR. For this reason, this year researchers are assaying these treatments on trees that are being treated with oxytetracycline. It is expected that better tree health will result in better BR response.
Acknowledgement: This research is supported with funding from the Citrus Research and Development Foundation (grants 22-003 and 24-001).
Fernando Alferez is an associate professor, and Divya Aryal is a graduate Ph.D. student — both at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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