Soybean root morphological and anatomical traits associated with acclimation to flooding

Soybean [Glycine max (L.) Merr.] acclimation to flooding has been investigated at biochemical and physiological levels, but longterm acclimation to flooding may be associated with morphological changes. Supplementing legumes with nitrate ameliorates flooding stress, compared with plants dependent on N-2 fixation. Two experiments evaluated morphological and anatomical changes induced by flooding for plants relying on N2 fixation or supplemented with nitrate. An additional experiment assessed the role of aerenchyma in flooding acclimation by blocking aerenchyma formation with silver. Flooding soybean for 21 d with nutrient solution without N increased biomass allocation to roots. No aerenchyma was observed in roots of nonflooded plants; however, it was abundant in roots of flooded plants. Porosity measurements of root and stem-base tissues indicated that 10 to 15% of the volume was gas filled in flooded plants, while gas-filled volume was negligible in the nonflooded controls. Flooding in the presence of nitrate decreased aerenchyma and increased adventitious roots, compared with plants flooded with nutrient solution without N. Increased growth under flooding of nitrate-supplemented plants, therefore, was not directly associated with increased root porosity. For flooded plants dependent on N-2 fixation, silver prevented aerenchyma and adventitious root development and decreased biomass accumulation and N-2 fixation by approximately one-half compared with plants flooded in the absence of silver. These results indicate that acclimation to flooding in soybean involves preferential allocation of photosynthates to development of porous adventitious roots and that aerenchyma formation For soybean relying on N-2 fixation is critical for acclimation to flooding.