Transporte de nitrogenio e metabolismo da aspargina em soja (Glycine Max L.) sob deficiencia na assimilação do nitrogenio

AUTOR(ES)

Juliana Domingues Lima

DATA DE PUBLICAÇÃO

2002

RESUMO

Experiments with soybean plants and other legumes were conducted under greenhouse conditions. Both nodulated and non-nodulated plants were found to present substantially increased aspartic acid levels and lower asparagine in the xylem bleeding sap under conditions that limit nitrogen assimilation. In soybean, the response was reversed when optimum conditions for nitrogen assimilation were restored. Non-nodulated plants, grown in pots with vermiculite using nitrate as sole source of nitrogen, presented this phenomenon when transferred to N-deficient water-culture. Subsequent return to nitrate reversed the effect. Nodulated plants grown in vermiculite with N-deficient nutrient solution and totally dependent on N2-fixation as a source of nitrogen presented the same response when transferred to aerated water -culture. In this case xylem ureide levels also fell substantially suggesting, along with other evidence, that nitrogen fixation was impaired under these conditions. On return of plants to vermiculite, both the Asp/Asn ratios and ureide levels returned to initial values. During growth and nodulation of soybean plants cultivated in vermiculite with N-deficient nutrient solution, the phenomenon was observed over a short period corresponding to the "N hunger" phase. This phase is characterised by transient yellowing of the leaves and occurs when reserves are depleted and nodulation is not yet sufficiently developed to maintain the plant with fixed nitrogen. Following the above-described treatments, an analysis of free amino acids in the phloem, roots and nodules also revealed some increase in the Asp/ Asn ratios but these were much less pronounced than those found for the xylem. Asparagine synthetase activity was investigated in the root system in an attempt to elucidate the factors underlying the phenomenon. Activity in the nodules was found to fall during treatments which elevated the Asp/Asn ratio such as transfer of nodulated plants to water-culture and the fall in activity reversed on return of the plants to vermiculite when ratios returned to normal. However, measurement of asparagine synthetase activity in the roots was unsuccessful. Evidently, in qualitative terms, nitrogen transport in the xylem appears to be very sensitive to the nitrogen status of the plant

ASSUNTO(S)

nitrogenio soja asparaginase

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