Xylem-to-phloem transfer of boron in broccoli and lupin during early reproductive growth

The aim of this study was to test the hypothesis that newly-acquired boron (B) undergoes rapid xylem-to-phloem transfer in plants with restricted mobi lity. Analysis of the element accumulation and water usage by shoots of int act broccoli (Brassica oleracea var, italica Plenck cv. Commander) and lupi n (Lupinus albus L. cv. Ultra) plants provided with a non-deficient supply of B, revealed that the concentration of various mineral elements (K, P, Mg , Ca, B, Fe, Zn, Mo, Cu, Mn) in xylem sap of intact plants ranged from 0.3 mu M to 3.5 mM, with B being present at 2.9-3.5 mu M. For each element assa yed, the concentration was higher in phloem exudate (1.6 mu M to 91 mM) tha n in xylem sap; B was present at about 0.4 mM. Intact broccoli and lupin pl ants or detached transpiring broccoli shoots were supplied simultaneously w ith enriched B-10, strontium (a xylem marker) and rubidium (a xylem/phloem marker) during early reproductive growth. The contents of these three compo unds were determined in foliage and florets or fruits as a function of time (i.e. up to 12 h and 4 days for broccoli and lupin plants, respectively), and the content in florets or fruits was expressed as a percent of the tota l recovered. In general, the percent recovery of both 10B and rubidium in f lorets or fruits was similar and markedly greater than that for strontium, even at the earliest harvest times (within 2 h for broccoli and 1 day for l upin). The data indicate that in plants with restricted B mobility, B is su pplied to sink tissues in the phloem, and the extent of B xylem-to-phloem t ransfer is closely determined by current uptake.