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

Abstract

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 μM, which were substantially higher than those in the xylem sap (10–30 μM). 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 μM ¹¹B (99.34% by weight) in nutrient solution for 48 d after germination (DAG) until early flowering and then transferred into either 0.2 μM or 20 μM ¹⁰B (99.47% by weight) for 3 d. Regardless of the ¹⁰B treatments, significant levels of ¹¹B were found in the phloem exudates (200–300 μM in 20 μM ¹⁰B and 430 μM in 0.2 μM ¹⁰B treatment) and xylem sap over the three days even without ¹¹B supply to the root. In response to the 0.2 μM ¹⁰B treatment, the translocation of previously acquired ¹¹B in the young (the uppermost three leaves), matured, and old leaves was enhanced, coinciding with the rise of ¹¹B in the xylem sap (to >15 μM) and phloem exudates (430 μM). 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 ¹¹B in the leaves may have been translocated into the rapidly growing inflorescence. Phloem B transport resulted in the continued net increment of ¹¹B in the flowers over 3 d without ¹¹B supply. However, it is still uncertain whether the amount of B available for recirculation is adequate to support reproductive growth until seed maturation.