Apical dominance in Phaseolus vulgaris: effect of the nitrogen supply

Abstract

Nitrogen is one of the most effective of the many factors that can influence the degree of apical dominance, but the mechanism involved has not been determined. In experiments with Phaseolus vulgaris, grown in sand culture under controlled conditions, increasing the nitrogen concentration in the nutrient solution from 52.5 to 315 mg/L released the bud at the primary leaf node from inhibition. Precise measurements (± 1 μm) with a strain gauge transducer showed that the bud growth response was initiated ca. 3 h after increasing the N supply. The increase in the total N content of the buds of the high-N plants was significantly correlated with their uptake of water. Increases in the amino-N content of the buds of the high-N plants over the first 24 h of the treatment period were correlated with significant increases in their total water potential and pressure potential, but their solute potential was unaffected. The absence of any reduction in their solute potential was attributed to the dilution effect of the increased uptake of water. These results are consistent with the hypothesis that N initiates the release of buds from apical dominance by an osmotic effect that increases their capacity to compete for water with the parent shoot. Key words: apical dominance, nitrogen, Phaseolus, water potential.