Genetic Regulation of Development in Sorghum bicolor: IV. GA(3) Hastens Floral Differentiation but Not Floral Development under Nonfavorable Photoperiods.

Sorghum bicolor (L.) Moench lines with genetic differences in photoperiod requirement were planted in the field near Plainview, Texas (about 34 degrees northern latitude) around June 1 and treated with gibberellic acid (GA(3)) solutions applied in the apical leaf whorl. GA(3) hastened the date of floral differentiation (initiation). The greatest responses to GA(3) were by 90M and 100M, the latest of the genotypes, for which floral initiation dates were hastened an average of 19.5 and 21.7 days, respectively, for the 4 years beginning in 1980. There were very small differences in dates of anthesis between control and GA(3)-treated plants. Microscopic examination of apical meristems collected between the date of floral initiation of GA(3)-treated plants and the later date of initiation of control plants revealed: (a) several morphological characteristics of floral differentiation in the apical meristem of treated plants, (b) consistent occurrence of vegetative morphology in control plants, (c) a few meristems from GA(3)-treated plants that appeared to be regressing in floral development and thus possibly exhibiting dedifferentiation. Dedifferentiation of prepanicle primordia into leaves would explain the observed equal or greater number of leaves in GA(3)-treated plants rather than the expected smaller number. It is apparent that the presence of a morphological differentiated floral meristem in sorghum does not drive subsequent floral development in the absence of inductive photoperiods. This further suggests that initial floral differentiation and subsequent floral development may be controlled separately in sorghum.