Effects of root applications of gibberellic acid on photosynthesis and growth in C₃ and C₄ plants

The effects of root applications of gibberellic acid (GA₃) on growth and photosynthesis of 12 species of plants including C₃ monocots (Triticum aestivum L., wheat, Hordeum vulgare L., barley and Avena sativa L., oat), C₃ dicots (Vigna radiata L., mung bean, Cucurbita moschata L., squash and Capsicum annuum L., pepper), C₄ monocots (Zea mays L., corn, Sorghum vulgare L., sorghum and Panicum ramosum L., millet) and C₄ dicots (Amaranthus retroflexus L., pigweed, Kochia scoparis L., kochia and Gomphrena celosoides L., gomphrena) were evaluated. Relative growth rates (RGR) of barley, oat, squash, pepper, corn, sorghum, millet, pigweed and kochia were increased above the control by 12.7%, 9.9%, 11.3%, 10.7%, 19.2% 10.1%, 11.5%, 16.4% and 32.7% respectively, four days following optimum GA₃ treatments. There was no effect of GA₃ on RGR in wheat, mung bean, and gomphrena. Gibberellic acid decreased the chlorophyll content expressed on an area basis by 20.0%, 13.9%, 20.9%, 17.1%, 11.9% and 28.0% in barley, squash, pepper, sorghum, pigweed and kochia, respectively, while that of oat, wheat, mung bean, corn, millet and gomphrena remained unchanged. When photosynthetic rates were expressed per mg of chlorophyll, it showed that GA₃ could stimulate photosynthesis in barley, squash, pepper, sorghum, millet, pigweed and kochia by 20.4%, 20.6%, 16.5%, 17.4%, 10.4%, 24.2%, and 29.4%; while there was no effect in oat, wheat, mung bean, corn and gomphrena. An increase in leaf blade area and/or length of sheath were observed in GA₃ treated plants of oat, barley, mung bean, squash, pepper, corn, sorghum, millet and kochia. The transpiration rate remained unchanged following GA₃ treatment in all 12 species.