Studies on the effects of hormones and their interactions on apical dominance of dwarf bean

Abstract

Hormones (IAA, GA₃ and K) singly or in combinations in lanolin at concentrations (0.01%, 0.1% and 1%) were applied to decapitated stems, debladed petioles, or directly to the buds of dwarf bean (Phaseolus vulgaris c.v. Canadian Wonder) under four different conditions of light (4300 lux, 13000 lux, 17200 lux and 20400 lux) and nutrition (water, ½ normal nutrient solution, normal nutrient solution and twice normal nutrient solution). 1. Indoleacetic acid at all concentrations, whether applied to decapitated stems, debladed petioles, or directly to the buds consistently, but not significantly, reduced the growth of main stems. 2. Although applications of IAA often reduced growth of laterals, the reduction was slight and seldom significant. 3. Kinetin generally had no significant effects on the growth of stems or laterals. 4. Gibberellic acid increased growth of both main stem and laterals in the axils of trifoliate leaved regardless of site of application and its effect was more pronounced an 1% than at lower concentrations. In contrast, it seldom increased growth of laterals in the axils of primary leaves and even on one occasion reduced it significantly. 5. The lateral growth-promoting effect of GA₃ was to some extent modified by light or nutrition, thus, the only instance where GA₃ appreciably and significantly promoted the growth of laterals in the axils of primary leaves was a higher light intensity (20400 lux), whereas its effect in promoting the growth of laterals in the axils of trifoliate leaves was consistently high at all light levels except 17200 lux. 6. The interaction between IAA and GA₃ either to promote or inhibit the growth of laterals depended on nutrient conditions. Thus, addition of IAA to GA₃ significantly decreased the effect of GA₃ in promoting growth of laterals in the axil of trifoliate leaf under lower nutrient conditions, whereas, addition of IAA slightly, but not significantly increased the growth-promoting effects of GA₃ on these laterals under high nutrient condition. The effect of GA₃ along with IAA on the growth of laterals in the axils of primary leaves was quite different from that exerted on the growth of laterals in the axils of trifoliate leaves. Thus, under lower nutrient conditions, addition of IAA to GA₃ significantly increased the promoting effect of GA₃ on the growth of laterals in the axils of primary leaves, and the situation was slightly but not significantly reversed under high nutrient conditions. 7. The effect of light on the interaction between IAA and GA₃ was less than that of nutrition. Under low light intensity particularly 4300 lux, GA₃ interacted with IAA to induce a small increase in growth of laterals, but the situation was reversed under other light conditions. 8. Gibberellic acid always interacted with K to promote lateral bud growth; this effect was far greater than GA₃ or K alone and was more pronounced on the growth of laterals in the axils of trifoliate leaves and 1 per cent concentration. But this effect was frequently nullified by IAA. 9. There was no significant interaction between IAA and K either to promote or inhibit lateral growth. 10. Site of application had some influences on the effect of hormone treatments in certain instances. Thus, no hormone treatment significantly affected the growth of laterals in the axils of primary leaves when applied directly to the buds, whereas, some treatments (e.g. GA₃ alone or along with IAA or K) frequently affected the growth of these laterals when applied to decapitated stem or debladed petioles. The effect of hormones applied to decapitated stem and debladed petioles were not consistently different. 11. Fresh weight of roots and tops were also measured, but no consistent results from any treatment were obtained.