The Effects of Vernalization on the Growth of the Wheat Shoot Apex

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Abstract
He effect of vernalization on the growth of the wheat shoot apex was examined by comparing three genetic lines of Chinese Spring (CS) wheat having strong [CS (Hope 5D)], medium (CS Euploid), or no [CS (Hope 5A)] vernalization requirement. The mean volume of the apical dome increased gradually in all lines, and then the apical dome enlarged rapidly as its relative growth rate (RGR) increased prior to double ridge formation. Phytomer volume at initiation remained constant, so that the ratio of phytomer to apical dome at primordium initiation decreased in successive plastochrons. In CS Euploid and in unvernalized CS (Hope 5D), the RGR of the apical dome tended to decrease at least until initiation of the collar primordium. The rate of primordium initiation at double ridge formation increased in proportion to the RGR of the apex at that time; i.e. it increased greatly in CS (Hope 5A) and vernalized CS (Hope 5D), less so in CS Euploid, but no increase was observed in unvernalized CS (Hope 5D). The time of formation of double ridges seemed to be independent of the growth rate or size of the apical dome. The number of tillers present at ear emergence was inversely proportional to vernalization requirement and was reduced by vernalization. Vernalization resulted in a decrease in the RGR of the newly-initiated leaf primordia in relation to the RGR of the apical dome and the axial part of the phytomer. Transfer of plants from long to short days at various times during growth showed that vernalization increased the number of labile primordia which could develop into either leaf, collar or spikelet. Vernalization therefore seems to alter the ability of the apex to respond to subsequent photoperiod rather than to affect its growth directly.