DISTRIBUTION OF MINERAL NUTRIENT FROM NODAL ROOTS OF TRIFOLIUM-REPENS- GENOTYPIC VARIATION IN INTRAPLANT ALLOCATION OF P-32 AND CA-45

To assess genotypic variability in nutrient supply of shoot branches, the distribution of P-32 and Ca-45 exported from a source nodal root ( 24-h uptake period) was measured within a genotype of a large-leaved ( Kopu) and a small-leaved (Tahora) cultivar of Trifolium repens. Source -sink relationships of plants were modified by root severance, defolia tion, and shade treatments. In control plants of both genotypes distri bution of P-32 and Ca-45 closely followed the pathways that could be p redicted from the known phyllotactic constraints on the vascular syste m. As such there was little allocation of radioisotopes (3.1% and 2.5% of exported P-32 and Ca-45, respectively) from the source root to bra nches on the opposite side of the parent axis (far-side branches). How ever, genotypic differences in nutrient allocation were apparent, when treatments were imposed to alter intra-plant source-sink relationship s. In the large-leaved genotype, the imposed treatments had minor effe cts on the allocation to far-side branches; whereas, in the small-leav ed genotype, root severance and defoliation treatments increased later al transport to far-side branches to 30% (P-32) and 10% (Ca-45) of exp orted radioisotopes. Genotypes with low (8-9) and high (12-13) numbers of vascular bundles were selected from within the large-leaved cultiv ar. Distribution of P-32 was then measured after plants had been pre-t reated by removal of all far-side roots two days prior to labelling. G enotypes with low vascular bundle number allocated 20% and those with high vascular bundle number 3.2% of exported P-32 to far-side branches . It was concluded (1) that genotypic variation exists within T. repen s for potential to alter intraplant allocation of mineral nutrients in response to treatments that modify source-sink relationships within p lants; and (2) that this variation is correlated with differences amon g genotypes in the organisation of the vasculature of their stolons.