Effects of boron starvation on boron compartmentation, and possibly hormone-mediated elongation growth and apical dominance of pea (Pisum sativum) plants

Two experiments were carried out to study the effects of boron (B) deficien cy on 7-day-old pea plants for 6 or 9 days under controlled growth chamber conditions. Growth and apical dominance (AD) of the plants and their B conc entration and compartmentation were followed throughout the starvation peri od. Additionally, auxin (indoleacetic acid, IAA) concentration in the shoot apex and polar transport from it were measured along with the cytokinin (C K) concentration in the shoot apex and the roots. The results demonstrate t hat during a 6-day B-deficiency period, B concentration in the water-insolu ble residue of the roots was very stable and could not easily be reduced. I n contrast, B concentration in the cell sap fraction was very sensitive to external B supply. Twelve hours after transferring the plants from B-suffic ient to B-deficient solutions, the B concentration in root cell sap decline d to half the concentration of the control plants. In addition, B concentra tion in the new aerial plant parts, which developed after the onset of the B-deficiency treatment, was extremely low. A decline in elongation growth c ould be observed as soon as about 4 days after the imposition of B deficien cy. This preceded the first measurable growth of lateral buds (release from AD). Before the onset of these morphological changes, there was a consider able decline in CK concentration, accompanied by a dramatic decrease in IAA export out of the shoot apex, a decline in IAA concentration in the shoot apex and the roots and a reduced capacity for polar IAA-transport. These ch anges are discussed as possible reasons for the observed reduction in elong ation growth and AD. These hormonal changes themselves are possibly the res ult of the decreased symplasmic B concentration, which in turn may be respo nsible for the reduced concentration in apical CKs, A sequence of events, w hich may be causally related, is suggested to explain the effects of B defi ciency on the growth and AD of pea plants.