VARIATION WITHIN WHITE CLOVER (TRIFOLIUM-REPENS L) FOR PHENOTYPIC PLASTICITY OF MORPHOLOGICAL AND YIELD RELATED CHARACTERS, INDUCED BY PHOSPHORUS SUPPLY

The range of variation for phenotypic plasticity for morphological and yield related characters, caused by varying the level of phosphorus (P) supply, was determined both across and within cultivars of white clover (Trifolium repens L.). Analysis of variance was used as a qualitative measure of phenotypic plasticity (i.e. significance of P effect) and to determine differential phenotypic plasticity (i.e. significance of genotype x P interaction). Quadratic P response curves were calculated and plasticity measured quantitively as maximum slope. While all plant characters measured exhibited plasticity, yield related characters were generally more plastic than characters associated with plant morphology when compared using a scale independent test based on F-ratios. Internode length and leaf size were the most plastic morphological characters and it is suggested that they are chiefly responsible for the resultant yield response of white clover to increased phosphate fertiliser supply. There was highly significant variation for plasticity among genotypes for all plant characters measured, although within cultivars, significance of variation for plasticity among genotypes varied with cultivar. Magnitude of plasticity of genotype was not closely related to origin of cultivar, plant type, growth or breeding history. Many characteristics varied independently for plasticity. The large variation observed for phenotype plasticity would suggest that breeding for an increase or decrease in plasticity of characters is an achievable option.