Mechanisms Of Lateral Bud Growth Induced By Boron Deficiency In Pea Plants

Apical dominance is a common phenomenon in higher plants. Plant hormones play important roles in apical dominance. Boron is one of the essential nutrients for plants and B deficiency induces a series of physiological changes, including the release of lateral bud. In the present study, the mechanism about the B deficiency-induced apical dominance was studied in pea plants (Pisum sativum L. cv. Lisa). In addition, the roles of auxin and cytokinin in apical dominance were tested under both control and B deficient conditions. Further, the interaction of auxin and cytokinin was investigated with traditional decapitation method. The main results were as followed:1. Compared with the control, B deficiency reduced the auxin and cytokinins concentrations in the shoot apex, indole-3-acetic acid (IAA) export from the shoot apex, and the 3^H-IAA transport capacity in the newly formed internode of pea plants, which resulted in the release of lateral bud growth. Re-supply B to the shoot apex of B deficient pea plants reversed these changes and restored the apical dominance, which suggested that B could affect auxin and cytokinin synthesis or metabolism. On the contrary, re-supply N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU, a synthetic cytokinin) to the shoot apex of B deficient pea plants had no effect on auxin and cytokinins concentrations, as well as IAA export from the shoot apex, even together with gibberellic acid (GA₃). As a result, re-supply CPPU could not decrease the lateral bud growth to the control level. These results demonstrated that only CPPU supply could not reverse the apical dominance induced by B deficiency.2. CPPU application to the 2nd node of control plants gradually stimulated the lateral bud growth, manifested by increasing the number, length and fresh weight of the lateral buds. Moreover, CPPU application increased the IAA concentration in the 2nd node, and decreased it in the 2nd internode as well as IAA export from the shoot apex of pea plants, reflecting the release of apical dominance. These processes conformed to the 'auxin autoinhibition theory', which believes that IAA transport is determinable for dominanted and dominant branches in apical dominance. CPPU application to the 2nd node of B deficient pea plants could not sustain the continuous growth of lateral bud, and had no influence on the IAA concentrations in the 2nd node, 2nd internode and shoot apex, as well as IAA export from the shoot apex.3. Applying B to the 2nd node of B deficient pea plants increased the IAA concentration and export from the shoot apex, and resulted in the establishment of apical dominance. This may imply that most of supplied B had been translocated to the shoot apex, which in turn enhanced the apical dominance. These results also suggested that nutrient supply to the lateral bud alone could not stimulate its growth, that was, 'nutrient hypothesis' was not enough to explain apical dominance. On the contrary, applying both B and CPPU to the 2^nd node of B deficient pea plants obviously increased the lateral bud growth, which suggested that the stimulatory effect of CPPU on lateral bud growth could occur under enough B supply condition.4. Both wild type and mutant tomato plants were cultivated under 0, 0.5 and 1 μmol·L^-1 B supply,respectively. After 7 days, plants in 0 μmol·L^-1 B supply obviously decreased the 3^H-IAA transport capacity in the stems in both wild type and mutant plants compared with 0.5 and 1 μmol·L^-1 B supplied plants. In addition, the 3^H-IAA transport capacity in mutant plants was always lower than that in wild type at the same B supply level, respectively. In pea plants, even after direct application of IAA to the shoot apex of B deficient plants, 3^H-IAA transport capacity in the internode still could not be restored to the level of control plants. These results showed that B directly involved in the polar auxin transport process.5. Under control condition, decapitation resulted in the rapid increase of cytokinins concentrations in the shoot and the release of lateral bud of pea pl...