Influence of Ca²⁺ and pH on the Stability of Different Boron Fractions in Intact Roots of Vicia faba L.

Abstract

An experimental setup was developed which allowed the detection of different boron fractions in intact plant roots. The B fractions were water-soluble B (WSB), BaCl₂-soluble B (BaSB) and the respective insoluble fractions (WIB and BalB). In root tips of V. faba seedlings, WSB represents approx. 4%, BaSB 9% of B_tot. BaSB consists of (at least) two compounds, the larger of which (65%) interacts with phosphate and is considered a soluble B complex. Within 2 h of B deficiency, WSB and a large part of BaSB disappear, suggesting that they may be involved in early deficiency reactions in root tips. Concerning insoluble fractions, there was no decline in WIB within the first 6 h of B deficiency. Within the same time frame, BalB dropped to 35% of the initial B content, indicating a stabilizing effect of Ca on B complexes under in vivo conditions. The concentration of WSB in root tips was lower at pH 3.6 as compared to pH 6.0, indicating that changes in apoplastic pH rapidly alter the equilibrium between free and bound B. WIB was more tightly bound in older root parts, as indicated by a maximum loss of 34% of B_tot after 24 h of continuous B removal, as compared to 99.5% in root tips. Our results indicate that different fractions of B (additional to B-RGII) are affected in the early stages of B deficiency and that Ca is not only a stabilizing agent for B complexes but is also necessary to bind soluble B complexes in the apoplast of intact roots.

Key words

Boron - calcium - pH - Vicia faba L. - boron complexes