Onion development is affected by photoperiod and temperature, and these stimuli effect processes at the cellular level. Investigations with onion has dealt with the states of rest, dormancy, and the breaking of dormancy since these have economic importance. However, since economic returns to the producer is also effected by marketability of bulbs, it is important to understand how factors affect bulb sizing.

Several classes of plant growth regulators affect physiological activities of the onion plant. Abscisic acid has been reported to induce senescence on onion plants and prolong bulb dormancy. Gibberellic acid has been proposed as the anti-bulbing hormone because it stabilizes microtubules in cells. If microtubules remain intact, cells do not increase in size and bulbs are not formed. Indole-acetic acid and kinetin promoted leaf growth under laboratory conditions. Kinetin also affects carbohydrate metabolism, stimulates bulb growth, and increases bulb marketability after 4.5 months in storage. It has been suggested that jasmonic acid that initiates bulb formation. Maleic acid hydrazide has been used for many years as an anti-sprouting agent when applied prior to harvest.

There is minimal knowledge about how exogenous applications of growth regulators affect development and sizing of bulbs. Combinations of plant growth regulators have been marketed for use in onion production with the claim that the producer will benefit from increased grade and quality. These products are proprietary and concentrations of individual components or combinations of components are not readily available. It is not clear if these materials actually affect onion bulb development and sizing. Since the use of these materials on onions is relatively new there is little information on what amounts of which growth regulators are needed, or when they should be applied, to affect developing onion plants. The objective of this study was to determine if concentration, and timing of application, of several growth regulators effected development of onion plants.

Four to six seeds of the yellow short-day onion, cv. TX1015Y were sown in potting soil in pots in a glass greenhouse on 3 Oct. 2000. Plants were thinned to one per pot approximately 2 weeks after sowing. Plants were exposed to no more than 10 h of light from fluorescent bulbs. Temperatures were set at 24°C day/ 15°C night. Five-hundred milliliters of a soluble fertilizer (Peters Professional All Purpose Plant Food) was applied at full strength at 2-week intervals to each pot beginning 1 month after sowing. The fertilizer supplied 20% of N, 8.8% of P, and 16.6% of K per unit.

All growth regulators employed were obtained from Sigma (St. Louis). Most of the concentrations for the plant growth regulators

Two concentrations of the growth regulators abscisic acid, gibberellic acid, indole-acetic acid, jasmonic acid, kinetin, and maleic acid hydrazide, and water controls, were applied at the 7- and 20- leaf stages to the middle of the leaf whorl in greenhouse grown onion plants. Bulbs were harvested 25 April, 2001. Although leaf and bulb weights were lighter, and bulb diameters were smaller, from plants treated with growth regulators applied at the 7- leaf stage than those from plants treated at the 20-leaf stage. Bulbs produced on plants treated with water were the same size, or larger, than those produced on plants treated with individual growth regulators.

It is difficult to transpose the results of a greenhouse trial to field conditions. However, these results indicate that these exogenously applied growth regulators did not increase bulb size.