The Effects of Daylength and Treatment with Gibberellic Acid on Spikelet Initiation and Development in Clipper Barley

The effects of photoperiod, light quality and a single applicationof gibberellic acid (GA₃) on the development of the main-stemapex in Clipper barley are reported. In 16 and 24 h days spikeletinitiation was rapid but extended over a short period whereasin 8 h photoperiods both spikelet initiation and developmentwere slower but occurred for a much longer time. Initiationalways stopped when the anther primordia were clearly visiblein the most advanced spikelet. Daylength extensions with lowintensity incandescent light were most effective when they followedrather than preceeded the 8 h period of high light intensity.Plants grown in 8 h high intensity followed by 8 h low intensitylight initiated spikelets almost as rapidly as those grown in16 h high intensity light. Thus, the effects of daylength onspikelet production were primarily mediated through photoperiodicallycontrolled processes rather than through photosynthesis andassimilate supply. The effects of applied GA₃ were long livedand greatest in short days where the rates of both spikeletinitiation and development were promoted. The parallels betweenthe effects of long days and GA₃ treatment are discussed togetherwith possible reasons for the cessation of initiation and thelong duration of the GA₃ effect. daylength, gibberellic acid, spikelet initiation, Hordeum vulgare L., barley, main-stem apex, primordia