Fresh Oranges on a tree.

The Role of Plant Growth Regulators in Citrus

Josh McGillPGRs

Multi-county citrus Extension agent Ajia Paolillo discussed the role of plant growth regulators (PGRs) in groves in a recent Extension newsletter. She cited University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) horticulturists Tripti Vashisth and Fernando Alferez as sources.

plant growth regulators

Here’s a summary of Paolillo’s article:

Gibberellic acid (GA) is one of the PGRs used in citrus. It promotes growth and development in plants, including shoot elongation, vegetative growth, seed germination and fruit set. GA is also involved in the timing of fruit and leaf senescence and can be used to delay this in citrus.

Recent studies have shown that repeated monthly applications of GA during the months of September to January have a positive effect on Valencia oranges. The rate in which GA is metabolized by the plant has indicated that the effects from a single application are not long lasting. Therefore, repeated monthly applications are needed to achieve positive results.

Trees sprayed with GA have a denser canopy than untreated trees due to an increase in vegetative growth. Denser canopies contribute to the health of the tree by providing more leaf surface for photosynthesis.

GA also reduces the number of flowers, thus conserving resources. The flowers that are produced are generally leafy. Flowers that are produced on leafy shoots, known as leafy inflorescences, are more likely to set and hold fruit until maturity.

GA can also reduce preharvest fruit drop by delaying fruit senescence or aging. In GA-treated trees, the fruit peel stays greener longer, indicating reduced ethylene levels. Ethylene, another plant hormone, is responsible for fruit ripening and abscission. This delay in ethylene production allows the peel integrity to stay stronger and the fruit to hold at the abscission zone tighter.

It has been shown that fruit size increased with GA applications. This is important because fruit that were most likely to drop preharvest were small. So, the larger the fruit, the less likely it is to drop.

Auxins are a hormone needed for plant growth, including cell elongation and root growth. Auxins can also be used to suppress abscission. The auxin called 2,4-D is used in citrus, typically as an herbicide. However, some formulations of 2,4-D can be applied to the tree to achieve larger fruit sizes and reduce preharvest fruit drop.

Brassinosteroids are another type of PGR that have shown some promise in current trials to prolong tree health after individual protective cover removal, and to improve fruit quality.

Research studies are being conducted to determine proper timing, concentrations and other factors to maximize these positive effects when using different types of PGRs.

Read more from Vashisth and Alferez about the benefits of GA and 2,4-D in citrus.

Source: UF/IFAS

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