Development of the nodulated soybean plant after flooding of the root system with different sources of nitrogen

Abstract

0 alagamento leva a hipoxia, urn estresse ao qual a fixacao simbiotica de N2 e muito sensivel. Estudou-se a resposta da planta de soja nodulada ao alagamento do sistema radicular por 21 dias, na presenca e na ausencia de NO 3- e NH4+ no meio. Avaliaram-se parametros de crescimento da planta e componentes nitrogenados organicos transportados a parte aerea atraves do xilema. A inundacao diminuiu drasticamente a fixacao de N2, independentemente da sua fonte, como indicaram os niveis de ureideos na seiva do xilema. Na ausencia de fonte de N, o desenvolvimento foi fortemente reduzido durante o alagamento e seu aciimulo na parte aerea foi pequeno. 0 alagamento na presenca de 5 mM de NO 3- ou NH4+ proporcionou acumulacao de N total na parte aerea, mas somente o NO 3- promoveu aumento na materia seca total, estatura e area foliar das plantas acima do tratamento inundado sem N. Entretanto, seu acumulo na parte aerea dos tratamentos inundados com NO 3- e NH4± foi menor que a testemunha nao inundada. 0 incremento na materia seca total, estatura e area foliar das plantas do tratamento corn NO 3- foi similar ao da testemunha nao inundada. Os dados demonstram claramente o efeito benefico do NO 3- durante urn periodo prolongado de alagamento do sistema radicular nodulado da soja.

Flooding leads to hypoxia, a stress to which symbiotic N2 fixation is especially sensitive. The response of fully nodulated soybean plants to a 21-day period of flooding was studied by measurements of growth parameters and xylem transport of organic nitrogenous components to the shoot, in the presence and absence of NO 3- and NH4+ in the medium. Flooding was found to seriously impair N2 fixation, irrespective of the N source, as indicated by strongly reduced xylem ureide levels. In the absence of a source of N, growth was strongly reduced during flooding while accumulation of N in the shoot was virtually abolished. Flooding in the presence of 5 mM NO3- or NH4+ led to the accumulation of total N in the shoot but only NO 3- promoted increases in total dry matter, plant height and leaf area above that found in the absence of N. The accumulation of N, however, was lower than that of the non-flooded control for both NO 3- and NH4+. The increases in total dry matter, plant height and leaf area with NO 3- was as high as those of the non-flooded control. These data clearly show the beneficial effects of NO 3- during a prolonged period of flooding of the nodulated root system of soybean.