Photoperiod control of gibberellin levels and flowering in sorghum

Regulation of rhythmic peaks in levels of endogenous gibberellins (GAs) by photoperiod was studied in the short-day monocot sorghum (Sorghum bicolor [L.] Moench). Comparisons were made between three maturity (hla) genotypes: 58M (Ma(1)Ma(1), Ma(2)Ma(2), phyB-1phyB-1, and Ma(4)Ma(4) [a phytochrome B null mutant]); 90M (Ma(1)Ma(1), Ma(2)Ma(2), phyB-2phyB-2, and Ma(4)Ma(4)); and 100M (Ma(1)Ma(1), Ma(2)Ma(2), PHYBPHYB, and Ma(4)Ma(4)). Plants were grown for 14 d under 10-, 14-, 16-, 18-, and 20-h photoperiods, and GA levels were assayed by gas chromatography-mass spectrometry every 3 h for 24 h. Under inductive 10-h photoperiods, the peak of GA(20) and GA(1) levels in 90M and 100M was shifted from midday, observed earlier with 12-h photoperiods, to an early morning peak, and flowering was hastened. In addition, the early morning peaks in levels of GA(20) and GA(1) in 58M under conditions allowing early flowering (10-, 12-, and 14-h photoperiods) were shifted to midday by noninductive (18- and 20-h) photoperiods, and flowering was delayed. These results are consistent with the possibility that the diurnal rhythm of CA levels plays a role in floral initiation and may be one way by which the absence of phytochrome B causes early flowering in 58M under most photoperiods.