By Alessandro Franco, Faisal Shahzad and Tripti Vashisth
Gibberellic acid (GA) is one of the most commonly used plant growth regulators which affects flowering, fruit and vegetative growth. Multiple monthly GA foliar applications have reduced preharvest fruit drop and increased yield in huanglongbing (HLB)-affected sweet oranges. The potential impact of these findings is huge for commercial citrus production. However, there are a few concerns with the recommended application regime.
Despite the positive effects, GA applications are a high-cost investment for growers. The estimated cost is at least $50 per acre. For this reason, it becomes crucial to fine-tune GA-application strategies for the optimal response of citrus trees with minimal investment. There are some questions about the efficacy of day applications versus night applications. Anecdotal observations suggest that at night, GA efficacy is higher than daytime use (since trees at night have a longer time to absorb GA) and therefore, some growers have considered nighttime GA application. Consequently, some growers have also considered reducing the number of applications to one or two if completed at night.
However, night applications can be complicated and expensive. In the present study, the application of exogenous GA to the growth and development of citrus plants was investigated with the foliar supply in the morning (GA AM, applied at 9 a.m.) or in the evening (GA PM, applied at 9 p.m.). Results showed that consecutive GA foliar application every three months improved the number of leaves by more than 70% in the morning (AM) application. With the evening application, the increase was around 50% over the control.
Moreover, in leaves after four days from the third GA application, the superoxide dismutase and the peroxidase activities (the enzymes related to oxidative stress mitigation) were 30% higher in both GA PM and GA AM conditions in comparison to control trees. Additionally, the application of GA influences in a non-univocal way the phytohormones content with a rise of JA, JA-ILE and OPDA amount in the GA AM and an increase of IAA-Asp, SA and tZR content in the GA PM. Taken together, these results further suggest that evening applications are not better than morning applications. In addition, multiple applications of GA are needed for the desired vegetative growth effects.
Alessandro Franco is a postdoctoral research associate, Faisal Shahzad is a horticultural sciences graduate student, and Tripti Vashisth is an associate professor — all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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