Plant growth regulators (PGRs) could give citrus growers another tool for managing vigorous trees, reducing pruning needs and potentially improving production efficiency, according to a review of decades of citrus research.
The review — “Managing Citrus Canopy Growth for Higher Productivity: A Review of Plant Growth Regulator (PGR) Applications” — was published Sept. 8 in Horticulturae. The authors are Kare Mahmud and Dave Monks of the New South Wales Department of Primary Industries and Regional Development in Australia and Paul Cronje of Stellenbosch University in South Africa.
The researchers found that PGRs that inhibit gibberellin, a plant hormone that promotes shoot elongation, can consistently slow citrus shoot growth and reduce canopy expansion. Paclobutrazol (PBZ) and uniconazole were the most consistently effective compounds reviewed. Prohexadione-calcium also can suppress flush growth, but its effects are shorter lived.
For growers, one of the biggest potential benefits of PGRs is reduced pruning. Smaller, slower-growing canopies can require less frequent and less intensive pruning, potentially reducing labor costs. The review notes that some orchards have been able to extend pruning intervals. A Spanish study found workers harvested a PBZ-treated block about 15% faster because fruit were easier to reach.
Importantly, reducing vegetative growth does not necessarily mean reducing yield. Across numerous studies, appropriately treated trees generally maintained or improved yield efficiency.
PGRs also can increase flowering and may help moderate alternate bearing, particularly in mandarins. However, heavy crop loads can limit these flowering benefits, and excessive flowering may require fruit thinning to prevent small fruit.
The researchers caution that PGRs are not a one-size-fits-all solution. Responses vary with cultivar, rootstock, tree condition, crop load, timing, rate and environmental conditions. Overapplication can cause excessive dwarfing, reduced leaf growth or other problems, particularly with persistent products such as PBZ.
For citrus growers, the key takeaway is that PGRs could become a useful component of canopy-management programs, particularly in high-density, vigorous orchards where pruning and labor are significant expenses. The review emphasizes that PGRs should be integrated with pruning, irrigation and nutrition programs and used only according to applicable product registrations and regulations.
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