Evidence of phloem boron transport in response to interrupted boron supply in white lupin (Lupinus albus L. cv. Kiev Mutant) at the reproductive stage.

The present study investigates whether previously acquired boron (B) in mature leaves in white lupin can be retranslocated into the rapidly growing young reproductive organs, in response to short-term (3 d) interrupted B supply. In a preliminary experiment with white lupin in soil culture, B concentrations in phloem exudates remained at 300-500 microM, which were substantially higher than those in the xylem sap (10-30 microM). The high ratios of B concentrations in phloem exudates to those in the xylem sap were close to values published for potassium in lupin plants. To differentiate 'old' B in the shoot from 'new' B in the root, an experiment was carried out in which the plants were first supplied with 20 microM (11)B (99.34% by weight) in nutrient solution for 48 d after germination (DAG) until early flowering and then transferred into either 0.2 microM or 20 microM (10)B (99.47% by weight) for 3 d. Regardless of the (10)B treatments, significant levels of (11)B were found in the phloem exudates (200-300 microM in 20 microM (10)B and 430 microM in 0.2 microM (10)B treatment) and xylem sap over the three days even without (11)B supply to the root. In response to the 0.2 microM (10)B treatment, the translocation of previously acquired (11)B in the young (the uppermost three leaves), matured, and old leaves was enhanced, coinciding with the rise of (11)B in the xylem sap (to >15 microM) and phloem exudates (430 microM). The evidence supports the hypothesis that previously acquired B in the shoot was recirculated to the root via the phloem, transferred into the xylem in the root, and transported in the xylem to the shoot. In addition, some previously acquired (11)B in the leaves may have been translocated into the rapidly growing inflorescence. Phloem B transport resulted in the continued net increment of (11)B in the flowers over 3 d without (11)B supply. However, it is still uncertain whether the amount of B available for recirculation is adequate to support reproductive growth until seed maturation.