Fixacao Simbiotica Do Nitrogenio Na Soja Glycine Max L Merril

Author : S. M. de ALENCAR
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (79 download)

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Download or read book Distribuição dos compostos nitrogenados exportados pelos nódulos de soja (Glycine max), através do uso de traçadores enriquecidos com 14C. written by S. M. de ALENCAR and published by . This book was released on 1997 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: O nitrogenio e o nutriente mais limitante para o crescimento das plantas, especialmente em sistemas agricolas. As plantas normalmente adquirem nitrogenio do solo em uma forma inorganica. Na ausencia de um suprimento adequado de nitrogenio do solo, algumas especies de leguminosas sao capazes de formar uma associacao simbiotica com bacterias fixadoras de nitrogenio conhecidas como rizobio. Atraves desta simbiose, a planta e capaz de obter parte ou todo o nitrogenio requerido para seu crescimento. O crescimento e o rendimento de leguminosas produtoras de graos, dependentes da fixacao biologica de nitrogenio, sao o resultado da distribuicao e utilizacao dos compostos dependentes das interacoes entre a planta e a estirpe de rizobio, provenientes da associacao simbiotica. Para a soja, a eficiencia da fixacao simbiotica tem sido relacionada com a proporcao de ureidos na seiva xilematica e a particao destes para os frutos e graos. Tendo em vista a importancia desta cultura para o Brasil, tem-se buscado um melhor entendimento da fisiologia desta cultura, principalmente no que diz respeito a assimilacao, transporte e distribuicao dos compostos nitrogenados oriundos da fixacao simbiotica. Visando o estudo da distribuicao dos compostos nitrogenados provenientes da associacao simbiotica, em um dos periodos mais criticos do ciclo desta cultura, a fase de enchimento de graos, fez-se uso de compostos nitrogenados enriquecidos com o radiosotopo 14C. Os compostos 14C-ASN, 14C-ASP, 14C-GLN e 14C-GLU foram adquiridos comercialmente, enquanto que a 14C-ALN foi obtida atraves do enriquecimento por meio de sintese quimica. Foram feitas modificacoes do processo de sintese de ALN, a partir da condensacao do ester etilico do acido dicloroacetico e ureia, com aumentos significativos do rendimento. Os compostos nitrogenados enriquecidos com 14C foram aplicados atraves da via fluxo transpiracional. As plantas supridas com ALN mostraram uma intensa marcacao nas folhas mais novas, com pouca marcacao nas folhas mais velhas. A ASN e a GLN apresentaram uma menor intensidade de marcacao nas regioes internervais das folhas mais novas e nas folhas mais idosas, quando comparados com aquelas das folhas intermediarias. Este padrao de marcacao sugere, assim, que o 14C nao chega em grandes quantidades nas folhas mas novas quando se fornece os compostos nitrogenados na forma de GLN e ASN. Os aminoacidos dicarboxilicos (14C-GLU e 14C-ASP) mostraram forte marcacao nas folhas mais novas quando comparados com as amidas correspondentes. Nos frutos em inicio da formacao, a porcentagem de recuperacao da ALN marcada nas vagens das plantas, nao diferiu estatisticamente das amidas e aminoacidos, apesar das taxas de transferencias, de todos os compostos nitrogenados, diminuirem com a fase de maturacoa, assim mesmo, os ureidos mostraram-se superiores, demonstrando sua importancia para a nutricao nitrogenada das vagens. A distribuicao, dos compostos nitrogenados enriquecidos com 14C, nos graos foi semelhante para todos os compostos testados. Os graos em formacao, mostraram uma alta procentagem de recuperacao para a amida asparagina, declinando bruscamenete com o amadurecimento. J a ALN, mostrou uma distribuicao similar em todas as fases de desenvolvimento dos graos. Estes resultados corroboram a hipotese de que os ureidos sao capazes de manter um adequado suprimento de nitrogenio, necessario ao desenvolvimento das vagens e sementes, resultando em melhores indices de colheita. Nitrogen is the most limiting nutrient to plant growth, especially in agricultural systems. Plants normally acquire nitrogen from the soil in inorganic form. In the absence of an adequate supply of available soil nitrogen, certain leguminous species are capable of forming a symbiotic association with nitrogen fixing microorganism. Through this symbiosis, the plant is able to obtain part or all of the nitrogen required for plant growth. Growth and seed yields of grain legumes depending on biological nitrogen fixation are the result of interactions between plant cultivars and rhizobium strains, which affect the intake, distribution, and utilization of nitrogen compounds produced in the nodules. For soybean, biological nitrogen fixation efficiency has been related to ureide contents in xylem sap and, the partitioning of these compounds for pod and seed production. Due to the importance of this crop to Brazil, research efforts have been directed to the comprehensive understand of the physiology of this plant, mainly respect to the assimilation, transport and distribution of the nitrogeneous compounds provenient from the symbiotic fixation. The study of the distribution of nitrogen compounds exported from the nodules, in one of the most critical periods of the life cycle of this crops that is grains development was achieved by the use of 14C enriched tracers. The compounds 14C-ASN, 14C-ASP, 14C-GLN and 14C-GLU were commercially purchased, while 14C-ALN was obtained through chemical synthesis and labelling. Modifications were made in the process of ALN synthesis, starting from the condensation of the diethoxyaceti acid ethyl ester and urea, with significant increases on the yield. The 14C labeled nitrogen compounds were applied through the excised shoots of uniforms plants. The plants supplied with ALN showed intense high densities of labeling in young leaves, with much less retention in older leaves. The ASN and GLN reaveled less retention of label in interveinal regions of young leaves and old leaves, when compared with middle leaves. This pattern suggests that 14C do not arrive in any great amounts in the young leaves of the meristematic regions when GLN and ASN are supplied. The dicarboxylic amino acids (14C-GLU and 14C-ASP) showed heavier labeling in regions of young leaves than their corresponding amides. During pod development period, the percentage of recovery of ALN in pods walls did not differ statistically of that showed by amides and amino acids, in spite of the rates of transfers. The transference of nitrogenous compounds studied, decreased with pod maturation, even so, the ureides showed a somehow superior rates of transference, demonstrating its importance to the nitrogen nutrition of the pods. The distribution of the nitrogenous compounds labeled with 14C to the grains was similar for all the compounds tested. Developing grains showed a percentually higher recovery when asparagine was supplied declining abruptly with the maturation. ALN however showed a homogenous distribution for all the phases of grain development. These results seem to corroborate the hypothesis that ureides are capable of maintaining an appropriate supply of nitrogen, during the development of the pods and seeds, resulting in better harvest indexes than other forms of nitrogenous compounds.