Distribution of mineral nutrient from nodal roots of Trifolium repens: Genotypic variation in intra‐plant allocation of 32P and 45Ca

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Abstract
To assess genotypic variability in nutrient supply of shoot branches, the distribution of 32P and 45Ca 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, defoliation, and shade treatments. In control plants of both genotypes distribution of 32P and 45Ca closely followed the pathways that could be predicted from the known phyllotactic constraints on the vascular system. As such there was little allocation of radioisotopes (3.1% and 2.5% of exported 32P and 45Ca, respectively) from the source root to branches on the apposite side of the parent axis (far‐side branches). However, genotypic differences in nutrient allocation were apparent, when treatments were imposed to alter intra‐plant source‐sink relationships. In the large‐leaved genotype, the imposed treatments had minor effects on the allocation to far‐side branches: whereas, in the small‐leaved genotype, root severance and defoliation treatments increased lateral transport to far‐side branches to 30% (32P) and 10% (45Ca) of exported radioisotopes. Genotypes with low (8–9) and high (12–13) numbers of vascular bundles were selected from within the large‐leaved cultivar. Distribution of 32P was then measured after plants had been pre‐treated by removal of all far‐side roots two days prior to labelling. Genotypes with low vascular bundle number allocated 20% and those with high vascular bundle number 3.2% of exported 32P to far‐side branches. It was concluded (1) that genotypic variation exists within T. repens for potential to alter intra‐plant allocation of mineral nutrients, in response to treatments that modify source‐sink relationships within plants; and (2) that this variation is correlated with differences among genotypes in the organisation of the vasculature of their stolons.