Effect of Gibberellin Biosynthesis Inhibitors on Native Gibberellin Content, Growth and Floral Initiation in Sorghum...

CCC, uniconazol, ancymidol, prohexadione-calcium (BX-112), and CGA 163'935, which represent three groups of gibberellin (GA) biosynthesis inhibitors, were applied as a soil drench to Sorghum bicolor cultivars 58M (phyB-1, phytochrome B-deficient mutant) and 90M (phyB-2, equivalent phenotypically to wild type, PHYB, except for small differences in flowering dates). The inhibitors that block steps before GA12 (CCC, uniconazol, and ancymidol) lowered the concentrations of all endogenous early-C13alpha-hydroxylation pathway GAs found in sorghum: GA12, GA53, GA44, GA19, GA20, GA1, and GA8. In contrast, the inhibitors that block the conversion of GA20 --> GA1, (CGA 163'935 and BX-112) drastically reduced GA1 and GA8 levels, but they either did not change or caused accumulation of intermediates from GA12 to GA20. Combinations of pre-GA12 inhibitors and GA3 plus GA1 strongly reduced GAs other than GA1 and GA3. Each of these compounds inhibited shoot growth in both cultivars and delayed floral initiation in 90M. Floral initiation of 58M was also delayed by CCC, uniconazol, and ancymidol but not by CGA 163'935 and BX-112. This separation of shoot elongation from floral initiation in sorghum is novel. Both inhibition of shoot growth and delayed floral initiation were almost completely relieved by a mixture of GA3 and GA1 in both 58M and 90M. This observation, plus the much lower levels of endogenous GA3 than of GA1 observed in these experiments, implies that GA1 is the major endogenous GA active in shoot elongation. CGA 163'935 and BX-112 also failed to promote tillering in 58M, whereas inhibitors active before GA12 did so. The possibility that the GA20 --> GA1 inhibitors fail to block flowering and promote tillering in 58M because biosynthetic intermediates between GA12 and GA20 accumulate and/or because 58M is altered in GA metabolism in this same region of the biosynthetic pathway is discussed. Key Words. Gibberellin biosynthesis inhibitors-Phytochrome B-Flowering-Sorghum