Effects of salicylic acid on the development and root nodulation of soybean seedlings

Salicylic acid (SA) is recognized as an endogenous regulator of plant metabolism, mainly involved in induction of systemic acquired resistance (SAR). Exogenous SA can also induce a SAR reaction and SAR gene expression. Excessive SAR-related activity can be an overall cost to the plant in terms of energy and materials expended unnecessarily. Elicitation of plant defence responses might also block beneficial plant-microbe interactions and result in negative effects on plant growth. The objective of this study was to investigate the effects of SA concentration (5, 1, 0.5, 0.1 and 0 mM) on soybean seedling growth and nodulation by watering of soybean seedling roots or soaking of seedling leaves with SA solutions. It was found that 5 mM SA had negative effects on soybean seedling development, but other concentrations of SA did not affect the development of soybean seedlings. In addition, there were no negative effects on seedling development due to SA soaking of seedling leaves. Soybean seedling growth in sterile soil was reduced due to repressed nitrogen uptake following addition of 5 mM SA, indicating that some concentrations of SA can alter the N nutrition of seedlings. A model is presented that ties SA to nodule formation and plant growth.