Abstract
Understanding how management practices apply selective pressures that shape adaptive traits in forage grass is essential for managing genetic resources and breeding improved cultivars. The defoliation regime, for instance, induces a genotypic differentiation in plant height and heading. To explore these changes, plant height, heading date, and their relationship were studied in a nursery experiment among populations of perennial ryegrass (Lolium perenne L.) that had experienced different selection pressures: the cultivar ‘Clerpin’ from the original seed lot and its derived population from a pasture intensively grazed, 23 commercial cultivars, and 86 families coming from a divergent and recurrent mass selection for leaf length. A controlled experiment was performed on two populations contrasted on both plant height and heading date to compare their phyllochron, leaf elongation rate, leaf elongation duration, tiller number, and biomass per plant. Dissections were performed to monitor apex development and stem elongation. Genetic differentiations in plant height and heading date were found as a result of grazing pressure, commercial breeding, and selection for leaf length. Heading date was negatively correlated to plant height. Delayed flowering and reduction in plant stature resulted from a reduction in both leaf and stem elongation rate and were associated with a delay in the production of double ridge at the apex level. Selecting for a short plant resulted in an overall decrease in both vegetative and reproductive plant growth. Genetic differentiation in heading date in response to defoliation regime could be interpreted as an indirect effect of an adaptive differentiation in plant height.