EVIDENCE FROM THE USE OF ISOZYME MARKERS OF COMPETITION IN SWARDS BETWEEN SHORT-LEAVED AND LONG-LEAVED PERENNIAL RYEGRASS

Intraspecific competition in Lolium perenne was studied using a replac ement series of a short-leaved and a long-leaved genotype that was man aged under different cutting frequencies and nitrogen supplies. Leaf l ength, tiller density and yield were recorded 6 months after sowing. S ince the two competitors were labelled at the PGI/2 locus, it was poss ible to identify unambiguously the origin of tillers in the mixtures a nd to estimate the average length of lamina of each genotype. The diff erence in leaf length between the two competitors was maintained under particular types of management and generated contrasting competitive abilities leading to evolution of the genotypic composition of the mix tures. Thus, under infrequent cutting, competition for light resulted in the domination of the long-leaved genotype and in a concomitant int ragenotypic selection for leaf length. Supplying nitrogen fertilizer e mphasized the phenotypic plasticity of the genotypes, particularly of the dominated one. Frequent cutting reduced competition for light and the change towards the short-leaved genotype was far less rapid. The y ielding and competitive abilities of ryegrass genotypes and the implic ations for breeding are briefly discussed.