RECENT ADVANCES IN CHEMICAL GROWTH REGULATION OF ORNAMENTAL PLANTS
Five groups of native growth regulators are now recognized: auxins, gibberellins, cytokinins, abscisic acid and similar inhibitors and ethylene.
This was the first phytohormone identified. So far only one auxin, indole acetic acid, is generally accepted as a native hormone, although a few other compounds have been also suggested. In addition to IAA, many synthetic compounds are known to act as auxins. They have been used and known for many years, and no new development in their practical use has been reported recently. The most common use of auxins in ornamental horticulture is in rooting cuttings.
The availability of gibberellins is quite the opposite as that with auxins. About 25 native gibberellins have been isolated and identified so far, but only one of them, GA3, is commercially available. A few other gibberellins are available for research purposes. In some cases it has been shown that gibberellins other than GA3 are active where GA3 is not active or only slightly so. This is the case with promotion of flowering in several plants with GA7 or GA9, and with growth enhancement by GA4. Recently I.C.I. reported that GA4 and GA7 will soon be placed on the market for large scale trials.
Purified gibberellin is too expensive at present to consider its commercial utilization, despite the positive results obtained with GA in promotion of flowering and growth. The reason for the high cost of GA is that gibberellins are produced by fermentation, much like antibiotics. The purification procedure is very expensive, but it is essential for antibiotics and for gibberellins used in research. This is not the case for the practical use of GA. A new unrefined gibberellin has been introduced recently by Pfizer Co. and was tried successfully on many acres of sugarcane fields in Hawaii (1). This crude material was found to be as active as the pure gibberellin, while its cost is greatly reduced.
The most pronounced effect of GA is the stimulation of growth. One should however be careful in applying GA for growth enhancement. The effect of gibberellin on the shoot is obvious, but the effect on root growth