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

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Abstract
The effects of photoperiod, light quality and a single application of gibberellic acid (GA3) on the development of the main-stem apex in Clipper barley are reported. In 16 and 24 h days spikelet initiation was rapid but extended over a short period whereas in 8 h photoperiods both spikelet initiation and development were slower but occurred for a much longer time. Initiation always stopped when the anther primordia were clearly visible in the most advanced spikelet. Daylength extensions with low intensity incandescent light were most effective when they followed rather than preceeded the 8 h period of high light intensity. Plants grown in 8 h high intensity followed by 8 h low intensity light initiated spikelets almost as rapidly as those grown in 16 h high intensity light. Thus, the effects of daylength on spikelet production were primarily mediated through photoperiodically controlled processes rather than through photosynthesis and assimilate supply. The effects of applied GA3 were long lived and greatest in short days where the rates of both spikelet initiation and development were promoted. The parallels between the effects of long days and GA3 treatment are discussed together with possible reasons for the cessation of initiation and the long duration of the GA3 effect.

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