The Growth of the Shoot Apex and the Apical Dome of Barley During Ear Initiation

Abstract

The growth of the floral main shoot apex of spring barley was studied during the period of ear initiation (that is, from initiation of the collar primordium until maximum primordium number was attained). While floral primordia were being initiated the relative length growth rate of the shoot apex was low. After maximum primordium number there was about a twofold increase in relative length growth rate. Estimates of the volume, fresh and dry weight of the floral apex indicated that the relative weight growth rate was also low at first and increased after maximum primordium number. The rates of growth and the size at initiation of the floral primordia was affected by their position on the floral shoot apex. The relative volume growth rate increased acropetally from the first initiated (collar) primordium. The collar was the smallest and each subsequently-initiated primordium increased in length. The diameter of the newly-initiated primordium also increased until more than half the primordia had been initiated and then it declined. The apical dome increased in both length and diameter and both were at a maximum at the time of the double-ridge stage and then both measurements declined. Length and diameter were at a minimum at maximum primordium number. Subsequently there was an increase in the length of the dome, after which both the dome and some of the last formed, distal primordia died. The period of spikelet initiation therefore is a stage during which the relative growth rate of the floral shoot apex is low, there are changes in the size of the dome and the primordia show a progression of increasing relative growth rates acropetally along the shoot apex. These changes produce the embryo ear in which the most advanced spikelets are in the lower mid-part of the ear. Changes in size of the apical dome prior to maximum primordium number may be related to the subsequent death of spikelet primordia and therefore also to grain number in the mature ear.