Effect of prohexadione calcium on growth components and endogenous gibberellins contents of rice (Oryza sativa L.)

Prohexadione calcium (Pro-Ca) is a novel plant growth regulator that inhibits the late stages of gibberellins (GAs) biosynthesis in plants, thus reducing the vegetative growth of the plant. Pro-Ca was applied to field-grown rice cultivar Ilpumbyeo to evaluate its effect on growth and yield components and endogenous GAs, when applied at different rates and timings. This study illustrated that Pro-Ca application decreased the internode length, stem length, panicle length, shoots fresh weight and stem lodging index, while stem breaking force was increased in all treatments. The effect of Pro-Ca was more pronounced in 20 ppm concentration than 10 ppm. The number of panicles m^-2, ripened grain ratio and 1000 grain weight and milled rice yield parameters, increased with the application of Pro-Ca, though the number of grains per panicle decreased insignificantly. The endogenous bioactive GA₁ contents significantly decreased with the application of Pro-Ca, as the GA biosynthesis was interrupted at later stages, when GA₂₀ conversion to GA₁ was blocked, thus drastically reducing GA₁ and GA₈ levels, but only insignificant changes were observed for intermediates from GA₁₂ to GA₂₀. The levels of the endogenous GAs in rice shoots were measured by GC-MS-SIM using ²H₂-labelled GAs as internal standards. Present results also illustrated that early application of Pro-Ca was more effective than its late application for effective control of different growth parameters and thus reducing lodging in rice plants.