RETRANSLOCATION OF BORON IN BROCCOLI AND LUPIN DURING EARLY REPRODUCTIVE GROWTH

The objective of the present study was to determine if boron (B) retra nslocation depends on plant-B status and external-B supply. The stable B-10 isotope was supplied to the root system of broccoli (Brassica ol eracea var. italica Plenck cv. Commander) and lupin (Lupinus albus L. cv. Ultra) plants to provide a quantitative picture of B distribution during early reproductive development. Regardless of the B regime (i.e . continuous supply with luxury, sufficient or deficient B; transfer a t inflorescence emergence from either a luxury- or sufficient-B supply to a deficient one) and whether LOB was acquired before or during inf lorescence development, a significant proportion of the B recovered in broccoli florets and lupin fruit was LOB enriched. B acquired during inflorescence development was an important source of B for reproductiv e structures, but the relative importance of B acquired before and aft er inflorescence emergence appeared to be species dependent. The occur rence of B retranslocation was not dependent upon the induction of B d eficiency. The concentrations of B in phloem exudates (0.38 to 0.03 mM ) were 4- to 23-fold those in xylem sap, and more similar to the conce ntrations in the reproductive structures (0.86 to 0.07 mM) than those in source leaves (2.4 to 0.19 mM). The decreasing acropetal gradient o f tissue-B concentrations with luxury-B supply declined dramatically o r was reversed in plants grown with sufficient or deficient B. The dat a are consistent with B being a phloem-mobile element, and suggest tha t newly acquired B is particularly important during the early reproduc tive growth of plants.