Root phenolic accumulation and loss of autoregulation of root nodule formation in Bambara groundnut (Vigna subterranea) following boron nutrition and cotyledon excision

Abstract

Effects of boron nutrition and cotyledon removal on nodulation and N ₂ fixation were investigated on Bambara groundnut (Vigna subterranea L. Verdc.) grown in the glasshouse. Nodule number was siginificantly lower in boron-deprived plants relative to those receiving 90 µM boron. As a result, the amount of N fixed was markedly reduced in the zero-boron control plants. Phenolic content, measured relative to the molar concentration of daidzein, an active phenolic nod gene inducer from this species, was significantly lower in roots of boron-deprived plants. However, with provision of 90 µM boron, plants significantly increased root accumulation of total phenolics by 37%. HPLC analysis of root extracts showed that the increase in phenolics was due to greater tissue accumulation of the nod-gene-inducing daidzein, following the supply of 90 µM boron. The correlation between nodulation performance and increase in tissue phenolics, especially daidzein, suggests that the increase in nodulation and N ₂ fixation with boron supply (90 µM) to intact plants was triggered by the greater accumulation of the inducer molecule as a result of improved boron nutrition. However, removal of seedling cotyledons also affected the number of nodules formed per plant. Excision of cotyledons resulted in significantly reduced photosynthetic leaf area, followed by a marked increase in the number of nodules per plant due to loss of autoregulation of nodule formation. Boron supply and cotyledon removal both increased nodule formation in Bambara groundnut plants, but via different mechanisms.