By Fernando Alferez, Christopher Vincent and Tripti Vashisth
Brassinosteroids (BR), a relatively newly discovered class of plant hormone, regulate several developmental and physiological processes in fruit crops such as grapes, pears and cherries and in some non-edible plants. BR also induce disease resistance against different pathogens in a great number of crop plants.
A form of BR (epibrassinolide) was shown to reduce bacterial titer and alleviate symptoms of greening in HLB-affected 2-year-old citrus trees in a study performed in Cuba. In response to these findings, a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) research project was initiated in December 2017 to assess effects of homobrassinolide (HBr), a form of BR, on 6- to 8-year-old HLB-affected Valencia trees on Swingle rootstock.
Treatments performed biweekly were: water (control), 0.01 micrometer (μM) HBr (low concentration, 18.6 milliliters/100 gallons) and 1 μM HBr (high concentration, 186 milliliters/100 gallons). The experiment was performed in the central and southern Florida citrus-producing regions to ensure that results were applicable statewide. After 10 months, no reduction in bacteria population was found. However, some interesting physiological effects were found that are indicative of better tree health after HBr treatments. These effects included an earlier blooming date by 10 days, acceleration of fruit maturation and higher fruit yield.
To assess whether these effects can be sustained over time or be enhanced, treatments in the same plots have continued. Two-year-old trees with the same HBr concentrations and water (control) are also being treated on a biweekly basis. Younger trees are being treated now because the study performed in Cuba was on 2-year-old trees, and younger trees could not be included in the 2017 study. Tree age may play some role in the effect of HBr. A summary of findings during the second season of research are as follows:
NO BACTERIA REDUCTION
In mature trees, assessment of HLB bacteria population has continued every three months by cycle threshold (Ct) method using quantitative real-time polymerase chain reaction assay. This method is approved by the U.S. Department of Agriculture Animal and Plant Health Inspection Service. To date, all trees remain HLB positive, with readings in the lower 20s for Ct values. No significant differences in bacteria between control trees and trees that received HBr were observed.
ACCELERATED BLOOM, REDUCED HLB SYMPTOMS
Like last year, both HBr treatments advanced bloom in mature trees by 10 days as compared to the non-treated trees. In addition, flowering was more intense and uniform than in non-treated trees. In 2-year-old trees, these effects were enhanced, and HBr-treated trees bloomed even earlier. First open flowers were observed in high HBr-treated trees 14 days before the non-treated trees.
In both locations, mature trees treated with HBr looked noticeably healthier. Canopies were denser, and leaves were larger with a darker-green color. Canopy volume measurements in mature trees showed significant differences when compared to control trees. At the lower HBr dose, there was an 18 percent increase in canopy area. The higher dosage increased canopy area by 7 percent.
In 2-year-old HBr-treated trees, no leaf symptoms (blotchy mottle or yellow leaf veins) of HLB were found. Also, in younger trees, canopies were denser than in controls, and treated trees had more new flushes and more upright, water-sprout-type new branches, which are indicative of vigorous growth (Figure 1). Similar to what was observed last season, chlorophyll content was higher in both mature trees and 2-year-old trees.
LESS FRUIT DROP
Fruit detachment force (FDF) was measured before (December), during (early March) and after (late March) blooming. Like last season, FDF was higher at higher HBr concentrations, and the natural decrease in FDF due to blooming was avoided by HBr treatment.
This year, fruit drop was counted when it became apparent after petal fall (late March). In both absolute (number of dropped fruit) and relative (percentage of dropped fruit from the total crop) measurements, non-treated control trees dropped more fruit than HBr-treated trees (Figure 2).
GREATER YIELD AND QUALITY
Like last year, an increase in yield after HBr treatment was found. The low rate of HBr resulted in a 21 percent yield increase, whereas the higher rate resulted in a 10 percent yield increase when compared to the untreated trees.
Internal fruit quality was greatly advanced. By mid-March, in non-treated controls, the soluble solids content/titratable acidity ratio was 9.53. In HBr-treated fruit, the ratio was 17.5 for the low rate and 17.5 for the high rate. These values are more typical during the late season (June) for Valencia. This opens the possibility of starting the harvesting season earlier, thus avoiding preharvest fruit drop.
In summary, for the second year, research has documented some positive effects of brassinosteroids on tree health and fruit quality of Valencia oranges. In addition, results have shown that brassinosteroids promote more vigorous growth in younger trees.
Acknowledgements: This research was supported by state legislative funding for the UF/IFAS Citrus Research Initiative. The HBr used in this study was a gift from Repar, Inc. The authors thank the HLB Detection Laboratory at the UF/IFAS Southwest Florida Research and Education Center for diligent analysis of samples.
Fernando Alferez, Christopher Vincent and Tripti Vashisth are UF/IFAS assistant professors.