Acumulación de componentes nitrogenados y reducción de carbohidratos en la fijación de nitrógeno de plantas de Vigna unguiculata (L.) Walp sometidas a estrés hídrico y rehidratadas

In this study nodulated cowpea plants were exposed to a progressive drought-stress followed by rewatering in orderto establish mechanisms related to the nodule sensitivity to N2 fixation. Water stress caused a prominent decreasein the dry matter of leaf and nodule in contrast to the root growth that was stimulated. The drought-stressed plantsshowed a great decline in the ureide concentration of xylem sap whilst the concentration of the nodule leghaemoglobindecreased slightly. The stressed-nodules had significant accumulation of soluble carbohydrates in parallel to intenseincrease in the concentration of ureide, total amino acid and proline. After two days rewatering, the leaf growthshowed a significant enhancement whereas in nodules a poor recovery was observed. In addition, the nodule N2 fixationcapacity did not improve after rehydration whereas the sap ureide experienced only a slight recovery and surprisinglythe content of nodule soluble protein continued to drop. In contrast, a significant recovery in the nodule leghaemoglobincontent was observed. Rewatering greatly induced depletion of both sucrose and total soluble sugarcontents of nodules that were associated to a great decrease in the concentration of ureide, proline and amino acids,especially in leaves. Conversely the poor recovery of cowpea nodules after rehydration was apparently related to a carbohydrateshortage.A fin de establecer los mecanismos relacionados con la capacidad de los nódulos para fijar nitrógeno atmosférico, se expusieron plantas de Vigna unguiculata (L.) Walp noduladas a un estrés progresivo de sequía seguido de rehidratación. La sequía produjo una importante disminución en la materia seca de las hojas y de los nódulos, y en cambio estimuló el crecimiento de las raíces; por otro lado, hubo una gran disminución en la concentración de ureidos de la savia del xilema. Los nódulos sometidos a estrés presentaron una acumulación significativa de carbohidratos solubles, junto con un intenso aumento de la concentración de ureidos, aminoácidos totales y prolina, disminuyendo levemente la concentración de leghemoglobina. Tras dos días de rehidratación, el crecimiento de las hojas aumentó significativamente, pero tanto los ureidos de la savia como los nódulos sólo se recuperaron de forma leve, y no aumentó su capacidad de fijación de N2. (...

Bioprospecção de rizobactérias promotoras de crescimento em Brachiaria brizantha Bioprospection of plant growth promoter rhizobacteria in Brachiaria brizantha

Objetivou-se selecionar e caracterizar bactérias do gênero Bacillus em rizosfera de Brachiaria brizantha como promotoras de crescimento de plantas e correlacionar os diferentes critérios de seleção das bactérias utilizados em laboratório e casa-de-vegetação. Após coleta de amostras de plantas em três municípios da região oeste de São Paulo, foram realizados o isolamento de bactérias da rizosfera e posterior caracterização fenotípica quanto ao potencial de colonização de raízes de braquiária, antagonismo a fungos fitopatogênicos, produção de auxinas, fosfatases e proteases, além da caracterização genotípica utilizando a técnica de PCR-RAPD. Como avaliação final dos isolados bacterianos foi realizado experimento em casa-de-vegetação com inoculação de sementes e cultivo de Braquiaria brizantha com efetivação de três cortes, durante 180 dias. O experimento foi distribuído em delineamento inteiramente casualizado com quatro repetições. Dos 45 isolados de Bacillus sp. avaliados, sete foram competentes para colonização radicular e três (1A, 2B e 6A) foram capazes de promover o crescimento de Brachiaria brizantha, na média de três cortes efetuados na cultura. A avaliação da produção de auxinas e fosfatases foi relevante no protocolo de seleção dos isolados de Bacillus sp. como promotores de crescimento de plantas. A produção de proteases foi detectada em todos os isolados avaliados, mas não houve correlação desta característica com o crescimento das plantas.<br>This study proposed to select and characterize bacteria of the genus Bacillus on rhizosphere of Brachiaria brizantha as plant growth promoters and to correlate the different selection criteria of the bacteria utilized in laboratories and greenhouse. After collecting samples of plants in three municipalities of Sao Paulo, bacteria were isolated from the rhizosphere and subsequent phenotypic characterization was carried out, regarding the potential for colonization of the roots of pasture, antagonism to pathogenic fungi, production of auxins, phosphatases and proteases in addition to the genotypic characterization using PCR-RAPD. As final evaluation of Bacillus sp. isolates, a greenhouse experiment was performed with seed inoculation and cultivation of Brachiaria brizantha with three harvests, during 180 days. The experiment was distributed in completely randomized design with four replications. Of the 45 isolates of Bacillus sp. evaluated, seven were able to root colonization and three (1A, 2B and 6A) were able to promote the growth of Brachiaria brizantha, on the average of three harvests. The evaluation of auxin and phosphatase production was important in the selection protocol of Bacillus sp. to promote plant growth. Protease production was detected in all isolates, but there was no correlation of this characteristic with plant growth