By Fernando Alferez and Divya Aryal
Brassinosteroids (BRs) are a class of plant growth regulators with several effects on plant growth and development. They engage in crosstalk with other hormones like auxin, gibberellins, ethylene and abscisic acid, influencing all plant growth and development aspects including fruit maturation, but the timing of this effect appears to be short and tightly regulated.
University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) work in the last few years has shown that BR treatment may influence internal maturity in Hamlin and Valencia sweet oranges if performed at the right time. A treatment of 186 milliliters of BR (0.1%)/100 gallons water applied just once at the right time of fruit maturation has the most consistent results. For mature trees 7 years or older, 1 gallon per tree of the prepared solution is typically used for full coverage.
To achieve significant results, there are two variables to consider. One is tree health status. The other is the fruit maturation stage, which will determine the right timing. Regarding tree health status, trees in better shape respond better to the treatments. Using a tree health scale from 0 (healthy looking trees with a dense canopy) to 5 (abundant limb dieback, scarce canopy with small leaves and blotchy mottle symptoms), better and more consistent results were seen in trees ranking 1, 2 or 3. Since last season, researchers have also been treating with BRs trees that were previously injected with oxytetracycline. In this case, there are more consistent results in Brix increase and in sugar-to-acid ratio, as most trees are at stage 1 thanks to canopy recovery.
In Hamlin, significant differences in Brix have been seen when treatments are performed in a time window between mid-November and mid-December, with harvesting occurring at least two weeks after treatment. In these conditions, increase in Brix is close to 1 degree. In Valencia, significant results are achieved when treatments are performed around mid-March, with harvesting occurring at least two weeks later as well.
In both cases, the increase in ratio is even more pronounced. The ratio in Hamlin treated with BRs is 12 as compared to 9.8 in the control. In Valencia, the ratio is 12.3 as compared to 10.7 in the control. These time windows coincide with fruit peel at a stage of mature-green to turning yellow, when carotenoid synthesis is fully activated in peel. Since peel maturation occurs later than pulp maturation in citrus, this means that pulp maturation has already started. BRs seem to have the most effect once the natural process of maturation is already underway.
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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