Huanglongbing (HLB), also known as citrus greening, is a devastating disease of citrus and has severely impacted the citrus industry in Florida, causing substantial economic losses. With no known cure, management practices just aim to mitigate the harmful effects on the tree. Recent studies have shown that the 5 monthly foliar applications (September-January) of gibberellic acid (GA) can reduce preharvest fruit drop, improve fruit set, and enhance tree health. The potential impact of these findings is huge for commercial citrus production; however, growers are looking for alternatives to foliar application as foliar sprays are highly weather dependent and expensive, currently an air blast sprayer application can range anywhere from $30 to $50 per acre. Therefore, it becomes critical to find alternatives to foliar GA application. Supply of GA to the plants through irrigation could be a valid alterative instead of foliar application. This study explores the feasibility of GA application through irrigation as an alternative to foliar sprays. Specifically, we investigated GA uptake efficiency through citrus roots, its effects on overall plant growth, and transcriptomic changes in leaves and roots. Preliminary results indicate that both foliar (GF) and root applications (GR) induced tree responses, altering the hormone profiles in leaves and roots. Fifteen days post application, GR showed significantly higher concentration of GA₁ (950 ng/g fresh weight (FW)), GA₃ (450 ng/g FW), and GA8(112 ng/g FW) in the roots whereas these hormones were undetectable in the GF or untreated controlled. In the leaves, GA3 was significantly higher (210 ng/g FW) in GF but undetectable in untreated and GR. At 30 days post treatment, an enhanced leaf growth response was observed in GF trees with a mean new growth of 35 leaves as compared to 20.2 in GR and 17.8 in untreated control. Similarly, a 60-day hydroponic study demonstrated that root GA application significantly increased leaf production (p = 0.04). Bud growth was also significantly higher in root-treated plants compared to controls at both 10 days (p = 0.01) and 20 days (p = 0.01) post-application These preliminary findings suggest that root-applied GA does not mimic foliar application effects but offers some benefits for plant growth. An in-depth, long-term analysis is underway to validate the potential of GA delivery via irrigation.
