Effects of Gibberellic Acid on Growth and Development of Plants of Various Genera and Species

1. Growth responses of forty-nine kinds of plants were studied when gibberellic acid, a naturally occurring plant regulator, was applied to them in various ways. The plants included representatives of various genera, species, varieties, and hybrids of agronomic, forestry, or horticultural interest. Gibberellic acid is a growth substance produced by the fungus Gibberella fujikuroi (Saw.) Wr. (conidial stage Fusarium moniliforme Sheld.), a pathogen of rice. The chemical was applied in lanolin or as an aqueous spray, dip, or soak. Treated parts of plants included stems, leaves, vegetative buds, flowers, fruits, and roots. 2. Most kinds of plants studied responded in several ways to gibberellic acid, the most obvious of these responses being stem elongation. Marked differences in the responsiveness of different genera, however, were noted. Pinto bean plants, for example, responded when sprayed with a mixture containing 0.01-p.p.m. concentration of the acid, while Virginia pine, white pine, and white spruce responded only slightly or not at all to much larger amounts of the chemical. No important differences were observed in responsiveness either among the different species of the genera studied or among the different varieties and hybrids. 3. Greatest stem elongation resulted when the chemical was applied to stems that had just begun to elongate. Treatment of 0.75-inch-long flower stems of Better Times roses (table 1) with a 1% gibberellic acid-lanolin paste mixture resulted in their average increase of 57% in length over the controls during a 3-week period following treatment. The longer, treated stems were thicker than the controls, and the flower buds produced on them were similar in quality to the controls. Non-dormant but unsprouted potato tubers failed to respond to the gibberellic acid. Treatment of young potato stems as they emerged from the soil, however, resulted in a very marked response. 4. The following responses by different parts of plants resulted from the application of gibberellic acid. Stems: Once stem elongation had been initiated, most of the treated plants developed longer internodes than in the controls, including internodes above the treated portion of the plant. Some treated plants developed very thin, threadlike stems; in others, these elongated stems thickened as the plant matured and finally reached a greater diameter than those of control plants (fig. 1). Leaves: Most treated plants developed leaves that were temporarily lighter green than those of controls. Some leaves were smaller and more spatulate while others were broader than were those of untreated plants. Petioles of some treated plants, including young celery, were longer and broader than those of control plants. Vegetative buds: The main stems of treated plants first elongated without apparent elongation of the stems in the lateral buds. The rate of elongation of the main stem then decreased. Simultaneously, lateral buds of several kinds of plants, including citrus and snapdragon, elongated in a bushy type of growth. Stems in terminal buds of oak and maple that had experienced a rest period but had not yet begun to grow were induced to elongate 1-2 weeks sooner than did untreated ones. Flower buds: There was no evidence that gibberellic acid could induce plants to initiate flower primordia. Accelerated stem elongation was, however, sometimes accompanied by relatively rapid flower development compared with that of untreated plants. Treated pepper plants, on the other hand, remained vegetative longer and flowered 30 days later than did controls. Elongation of seed stalks of beet plants was stimulated, but it is not known at this time whether seed production was accelerated. Fruits: Gibberellic acid treatment did not increase the number of fruits that developed when tomato and bean plants were sprayed with it at the time of flowering, nor did the acid hasten the rate of development of bean pods. Gibberellic acid did not accelerate the rate at which immature banana fruits ripened. Roots: Fresh- and dry-weight measurements of roots showed that rapid and prolonged stem elongation of the treated plants was associated with lesser root growth.