Leaf Growth and Apex Development of Perennial Ryegrass During Winter and Spring

The relationships between rates of leaf extension, leaf appearance,and primordia production on the apex were studied in the fieldduring winter and spring in established swards of four contrastingperennial ryegrass lines (Aurora, Melle, a hybrid selectionfrom a cross between Aurora and Melle and S.24). All four linesshowed an increase in leaf extension rates which commenced whenspikelet primordia were first initiated at the apex. This wassome time after vernalization requirements had been satisfied.In early-flowering lines the stimulus to leaf growth rates occurredearlier than in late-flowering lines. Maximum leaf growth ratesoccurred about the time of double ridge formation, in the middleof the period of spikelet primordia production. The rate andduration of the period of spikelet primordia production variedbetween lines. By the time of flowering, leaf growth rates declinedto values recorded for vegetative plants in the winter. Leafappearance rates followed a similar pattern to leaf growth rates,although the increase in leaf appearance rate was less thanin primordia production or leaf growth rates. Hybrids from across between early- and late-flowering lines showed early enhancementof leaf extension rates due to early initiation of spikeletprimordia production. These high rates of leaf growth were maintainedfor longer, compared with the early-flowering line, as the durationof spikelet production was longer. This illustrates a mechanismfor combining early spring growth with lateness of floweringin ryegrass breeding programmes Leaf growth, apex development, vernalization, initiation, spikelet primordia production, flowering, thermal time, Lolium perenne, perennial ryegrass, spring growth