Biochemical Biopesticides: Plant Growth Regulators

Plant Growth Regulators (PGRs) include a category of both natural and manufactured versions of natural substances that affect major physiological functions of plants. PGRs can promote, inhibit, or modify the physiological traits of a range of fruit, vegetable, ornamental and agronomic crops.

Once considered a rather esoteric practice, use of PGRs in mainstream agriculture has steadily increased as their benefits have become better understood by growers. From a crop production perspective, PGRs are used to maximize productivity and quality, improve consistency in production, and overcome genetic and abiotic limitations to plant productivity.

There are five major classes of natural plant hormones. Under each class there are a number of PGR products that play specific and very important roles in optimizing crop yield and quality.

• Gibberellins play a role in cell elongation, stem growth, bolting in long-day plants, induction of seed germination, enzyme production during germination, fruit setting and growth, and induction of maleness in dioecious flowers.
• Cytokinins are involved in growth promotion through cell division, fruit abscission, growth of lateral buds, leaf expansion, delay of leaf senescence, enhancement of stomatal opening, and chloroplast development.
• Abscisic acid can influence water relations via stomatal closure, inhibit shoot growth, inhibit alpha-amylase synthesis in seeds affecting dormancy, increase fruit coloring, and promote leaf and fruit abscission.
• Ethylene can stimulate lateral growth in stems, stimulate defense responses to injury, break dormancy, abscise leaves, flowers and fruit, induce flowering, and stimulate fruit ripening.
• Auxins can promote growth through cell enlargement, release apical dominance, and influence rooting, leaf senescence, flowering, fruit set and growth, fruit ripening, and leaf and fruit abscission.

In addition to the major five classes of plant hormones, there are other plant compounds with suggested PGR activity that are involved with cell division, maturation, cell enlargement, plant defenses and plant resistance. These compounds include a variety of secondary metabolites produced by plants and some plant-associated microbes. Examples include polyamines, brassinosteroids, jasmonates, salicylic acid and signal peptides.

There are a variety of concrete examples of how PGRs meet the needs of growers in the fresh produce market. These include:

Improved Fruit Quality
Gibberellins reduce russet on apples. Gibberellic acid improves firmness in cherries, and abscisic acid improves red color on red grapes. Ethylene is used to ripen fruit crops.

Improved Yield
Gibberellins increase fruit set on citrus and many other crops. Gibberellins increase leaf expansion on vegetable crops and cytokinins increase berry size on table grapes. And, ethylene modulation by aminoethoxyvinylglycine increases nut set on walnuts.

Overcoming Genetic Limitations
Gibberellic acid improves seed germination on dwarf rice varieties and increases berry size on seedless table grapes. Gibberellins and cytokinins can also increase fruit size in small apple varieties.

Reduce Labor Costs
Cytokinins and auxins induce thinning of flowers and fruits. Ethylene modulation by aminoethoxyvinylglycine allows growers to manage the timing of fruit maturity and, thus, the harvest process.

Expand Post Harvest Life
Ethylene modulation by aminoethoxyvinylglycine and ethylene receptor blockage by 1-methylcyclopropene can enhance fruit storageability. And, gibberellic acid extends the green life of bananas during transit and storage.