The effects of light and gibberellic acid on development of the mainstem apex of barley (Hordeum vulgare L.)

• Main Author: Cottrell, Joan Elizabeth
• Published: University of Edinburgh 1980
• Subjects: 571.2
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.645026

The major objective of this project was to obtain detailed information regarding the effect of daylength on the development of the mainstem apex in barley and to determine whether its effects are exerted through a change in the levels of endogenous plant growth regulators, particularly gibberellins. In long days, spikelet initiation in the cultivar Clipper was rapid but extended over a short period. In short days, initiation was slower, as was the rate of spikelet development, but occurred over a much longer period. It appeared that spikelet initiation stopped following the appearance of stamen initials, and large numbers of spikelets resulted when development was slow in relation to initiation. Applied GA3 had its greatest effect in short days where treatment led to an increase in the rate of initiation and a reduction in the final number of spikelets. In long days, GA did not affect the rate of initiation but reduced final spikelet number. In both daylengths, reductions in spikelet number were associated with increased rates of development in relation to initiation. The effects of applied GA3 on spikelet initiation and development were long lived and it is proposed that this was the result of an autocatalytic effect of applied GA on the endogenous synthesis of gibberellins. It is suggested that the spikelets are sources of gibberellin which is transported upwards to accelerate the development of the younger spikelets and to bring about the osbserved trend towards the synchronization of spikelet development along the ear. Promotion of the development of the basal spikelets by exogenous GA3 may therefore have increased the source of endogenous gibberellins so that the rate of development was enhanced long after the original application. On this interpretation the levels of gibberellins in the developing ear should rise between the beginning and end of spikelet initiation. Estimates using the barley endosperm bioassay indicated that, at least in long days, more gibberellin was present in the ear at stamen initiation than double ridge formation. During normal development, the apical dome was initially small, elongated to a maximum, then decreased in size until initiation ceased, after which it elongated slightly then died. In short days, the apical dome of GA3-treated plants continued to elongate by an increase in cell number and reached massive proportions. This extension was followed by renewed production of spikelet primordia close to the tip of the dome. Death of the apical dome is not therefore an inevitable consequence of the cessation of initiation. There is some evidence to suggest that the most advanced spikelets produced inhibitors which prevented the initiation of new spikelets and may eventually have risen to prevent the growth of dome as well. Continued extension of the dome in the GA3-treated plants suggests that inhibitor may not have reached sufficiently high concentrations in the dome to curtail growth.