Effects of a Plant-Growth Regulator, Prohexadione-Calcium (BX-112), on the Endogenous Levels of Gibberellins in Rice

The effects of the plant-growth regulator, prohexadione-calcium, on the levels of the endoge-nous gibberellins in rice shoots were measured by GC-SIM using 2H-labeled gibberellins as inter-nal standards. The compound was applied at the 4-leaf stage and shoots were harvested 5 and 12 days after treatment. Plant length was reduced to 78 % and 66%, respectively, relative to control by application of the compound at 1 and 30 mg m~2, when plant length was measured 12 days after treatment. The level of GA, was reduced to 36 % and 18%, respectively, relative to control in the treated plants. The levels of GA19and GA20 increased but that of GA44 was reduced in the treated plants. The level of GA53 was unchanged. These results suggest that primary mode of ac-tion of the compound in vivo is the inhibition of the 3/?-hydroxylation of GA20 to GA | and fur-ther support the hypothesis that GA,, not GA,9 nor GA20 is active in promoting shoot elongation in rice. Key words: BX-112 — Gibberellin biosynthesis — Oryza sativa — Plant-growth regulator — Pro-hexadione-calcium. In rice seedlings, the growth retardation caused by pro-hexadione-calcium (calcium 3,5-dioxo-4-propionylcyclo-hexanecarboxylic acid) was reversed by the application o