Regresar

Outstanding adaptation of N<inf>2</inf>-fixing Sesbania sericea to flooded soils is not mediated by symbiosis with arbuscular mycorrhizal fungi

Abstract:

Flowering Sesbania sericea (Willd.) Link sampled from seasonally-flooded soils in the field displayed a 3-fold greater aerial biomass and root nodulation than individuals grown in nearby dry soils. Flooded plants (FP) showed high N2-fixation rates as apparent from their active leaf ureide catabolism and the high ureide content in nodules surrounded by an aerenchymatous phellem. Field-grown FP also exhibited high leaf Mn, P and chlorophyll but low carotenoid contents, suggesting non-stressful mineral nutrition and photosynthetic conditions. However, sufficient P in growing FP was not ascribed to an effective arbuscular mycorrhiza (AM) symbiosis. This was indicated by the age-related progressive decrease of root AM fungal colonization, and by the absence of AM arbuscules from the juvenile stages of growth. Aeration of flooded soils under greenhouse controlled conditions hindered growth, nodulation, leaf P content, and N2-fixation in rhizobia (Rh) and AM co-inoculated (Rh + AM) FP ruling out anoxia as the key factor underlying the inefficiency of the AM symbiosis in FP. Negative effects of aeration on FP were not counteracted by the addition of 2 mM KNO3 that inhibited nodulation, nor by the presence of the aerenchymatous phellem in the aerated roots. On the other hand, addition of 2 mM P enhanced the aerial mass of aerated and non-aerated FP, while inhibiting the colonization of root by AM. Analyses of rhizospheric fluorescein diacetate hydrolysis, urease, alkaline phosphatase and dehydrogenase enzymatic activities did not reveal marked differences in the soil microbial status between aerated and non-aerated FP, neither Rh + AM inoculated nor when fertilized by N and P. Overall results suggest a key role of AM only in seedling and juveniles providing sufficient P for the onset of N2-fixation.