Phloem and Xylem Transport in the Supply of Minerals to a Developing Legume (Lupinus albus L.) Fruit

Abstract

The economy of carbon, nitrogen, water and mineral elements in fruits of Lupinus albus L. was studied by measuring accumulation of these quantities in the developing fruit and estimating its transpirational losses and CO₂ exchanges. Combining this information with data on levels of mineral elements in the xylem sap and phloem sap supplying the fruit, it was possible to test whether transport based on mass inflow through xylem and phloem would account for the observed intake of elements. A model of transport based on water and carbon intake suggested that vascular intake during the fruit's life matched the recorded increment for minerals to within ± 15 per cent for N, Na, Zn, Fe and Cu, and to within ± 23 per cent for P, K and S. However, estimated vascular intake of Ca, Mg and Mn accounted for less than one–third of the recorded intake by the fruit, inadequacy of vascular intake being especially great early in growth. Transport in phloem accounted for more than 80 per cent of the fruit's vascular intake of C, N and S, and 70–80 per cent of its P, K, Mg and Zn. Xylem contributed 68 per cent of the vascular input of Ca, 59 per cent of the Na, and 34–38 per cent of the Fe, Mn and Cu. Enclosure and darkening of fruits reduced levels of Ca and Fe but increased levels of N, P, K and Zn in fruit dry matter relative to unenclosed, illuminated fruits. Results were related to previous observations on fruit functioning.