No abstract

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 mobility. Analysis of the element accumulation and water usage by shoots of intact broccoli (Brassica oleracea var. italica Plenck cv. Commander) and lupin (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 µM to 3.5 mM, with B being present at 2.9‐3.5 µM. For each element assayed, the concentration was higher in phloem exudate (1.6 µM to 91 mM) than in xylem sap; B was present at about 0.4 mM. Intact broccoli and lupin plants or detached transpiring broccoli shoots were supplied simultaneously with enriched 10B, strontium (a xylem marker) and rubidium (a xylem/phloem marker) during early reproductive growth. The contents of these three compounds 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 total recovered. In general, the percent recovery of both 10B and rubidium in florets 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 lupin). The data indicate that in plants with restricted B mobility, B is supplied to sink tissues in the phloem, and the extent of B xylem‐to‐phloem transfer is closely determined by current uptake.

Boron (B) is a micronutrient essential for the normal growth of monocots, dicots. conifers, ferns and several diatom species. Boron deficiency causes many anatomical, physiological and biochemical changes, making it difficult to identify a primary role for it: however, evidence does indicate that B is involved at the membrane level. Whatever the role(s). it likely involves the complexation of B with compounds containing cis‐hydroxyl groups. Boron deficiency in crops is more widespread than deficiency of any other micronutrient. Nutritional disorders in vegetables include brown heart in rutabaga, turnip and radish roots, and hollow stem in cauliflower and broccoli. The occurrence of these disorders even when B is in ample supply suggests that they are physiological in nature and related to the mobility of B in the plant. The distribution of B is related to the loss of water from shoot organs, suggesting that it is primarily xylem‐mobile with limited retranslocation in phloem. However, research has shown that B is present in the phloem, albeit at low concentration, and that it is generally retranslocated in the phloem to satisfy the demands of sink organs that do not readily transpire. Further progress into the mechanism(s) of B retranslocation will be facilitated by insights into the role and metabolism of B in plants.