The Effect of Plant Water Potential on Nitrogen Fixation of Soybeans

Abstract

Field experiments at two sites, Elora and Arkell,were conducted to investigate the effect of plant water potential on the nitrogen fixation of two soybean cultivars, Vansoy and Anoka. The treatments were Vansoy and Anoka both under irrigated and non-irrigated conditions. The acetylene reduction technique was employed to estimate nitrogen fixation of nodules and measurements of plant water potential were made using the pressure chamber technique. Nitrogen fixation and plant water potential measurements were made at various stages during the entire growing period of the soybeans, and the results showed that water stress which was related to low plant water potential caused a reduction in nodule numbers, fresh weight and dry weight. There was evidence that moderate moisture stress mainly affected the amount of nodule tissue formed and not nitrogen fixing efficiency whereas severe stress led to reduction in both nodule mass and efficiency of fixation. The total seasonal amount of nitrogen fixed by Vansoy and Anoka was reduced substantially by water stress while high plant water potential resulting from irrigation led to large amounts of nitrogen fixed. Regression analysis of nitrogen fixed with plant water potential indicated that reduction in plant water potential was accompanied by reduction in the amount of nitrogen fixed, thus showing a relationship between plant water potential and nitrogen fixation. Irrigation substantially increased seed yields of Vansoy and Anoka at Arkell, but there was no significant increase in either variety at Elora.