No abstract

No abstract

Three rice (Oryza sativa L.) varieties: Caiapó (bred for upland crop), Piauí (a landrace), and IAC-47 (an improved variety) were used to study the effects of nitrogen (N) suppression in plant metabolism. Nitrogen suppression increased root growth in Caiapó plants. This growth was simultaneous with increases in hydrogen (H + )-pyrophosphatase (H + -PPase) and vacuolar H + -ATPase (V-H + -ATPase) activities in the roots of Piauí plants. The SDS-PAGE shows 73, 67, and 53 kDa fractions proteins accumulating in the tonoplast what agrees with the observed activities of H + -PPases and V-H + -ATPases. Piauí and IAC-47 varieties exhibited different responses from one another at the beginning of N-Stress: there was an increase in P-H + -ATPase activity for Piauí and a decrease for IAC-47. Piauí plants had a higher content of nitrate (NO 3 − ) in roots and sheaths, notwithstanding it mobilized less of this NO 3 − -N during this growth period than did IAC-47. These results suggest that Piauí plants are better adapted to nutrient-poor environments.

RESUMODuas cultivares de arroz, Piauí (tradicional) e IAC-47 (melhorada), foram colocadas em solução nutritiva contendo 20 mg de N-NO 3 -L -1 até 32 dias após a germinação (DAG). Depois disso, um grupo de plantas passou a receber 200 mg de N-NO 3 -L -1 , enquanto o outro continuou recebendo 20 mg de N-NO 3 -L -1 até os 42 DAG. De 42 até os 56 DAG, todas as plantas receberam 5 mg de N-NO 3 -L -1 até o fim do ciclo. Esses tratamentos foram empregados visando simular o fluxo sazonal de nitrato existente nos solos de regiões tropicais. As plantas foram coletadas aos 133 DAG e separadas em folha bandeira, folha 2, colmos e raízes. Foram determinados os teores de N na forma de aminoácidos, nitrato e amônio, açúcares solúveis, atividades da nitrato redutase (NR) e glutamina sintetase (GS). O fluxo sazonal de nitrato influenciou significativamente a absorção e o uso de N. As variedades estudadas foram responsivas de maneira diferenciada em relação ao acúmulo e à translocação de N, quando submetidas ao fluxo sazonal de nitrato. A cultivar Piauí parece acumular mais NO 3 -em seus tecidos na fase inicial do seu ciclo, podendo permitir sua posterior translocação. A folha bandeira e folha 2 parecem atuar como local preferencial de acúmulo de N, sendo este acúmulo mais intenso na cultivar Piauí.Palavras-chave: N-amino, enzimas de assimilação de nitrogênio, absorção de nitrato, Oryza sativa L., açúcares solúveis, variedades tradicionais. ABSTRACT EFFECT OF NITRATE AVAILABILITY IN NUTRIENT SOLUTION ON NITROGEN UPTAKE AND ENZIMATIC ACTIVITY OF TWO RICE CULTIVARSTwo rice cultivars, Piaui (a landrace) and IAC-47 (an improved variety), were grown in nutrient solution containing 20 mg nitrate (N-NO 3 -) L -1 up to 32 days after germination (DAG). After this, a group of plants received 200 mg N-NO 3 -L -1 , while the other was kept at 20 mg N-NO 3 -L -1 up to 42 DAG. From 42 until 56 DAG, all plants received 5 mg N-NO 3 -L -1 until the end of the cycle. These treatments were applied with the objective to simulate nitrate flush in tropical regions. Plants were collected at 133 DAG and separated in to leaf flag, leaf 2, stem and roots. N contents in amino acids, nitrate and ammonium forms, soluble sugars, nitrate reductase (NR) and glutamine synthetase (GS) enzymatic activities were determined. The evaluated varieties replay in different manner about accumulate and translocation nitrate. The variety Piaui seems to accumulate more NO 3 -in the tissues at the initial phases of its cycle, allowing subsequent remobilization. Flag leaf and leaf 2 seem to act as preferential sites of N accumulation, being this accumulation more intense in the landrace variety.

Symbiotic biological nitrogen fixation (BNF) is a complex process that involves rhizobia, a diverse group of α and β-proteobacteria bacteria, and legume species. Benefits provided by BNF associated with legume trees in tropical environments include improvements to efficiency of nitrogen (N) use, increase of soil carbon sequestration, stabilization of soil organic matter, decrease of soil penetration resistance, and improvement of soil fertility. All these benefits make BNF a crucial ecosystem service to the sustainability of tropical agriculture. Due to the importance of this ecological process and the high diversity of rhizobia, these bacteria have been extensively characterized worldwide. Currently, over 400 species of rhizobia are known, distributed into seven families. In the humid tropics, Leucaena leucocephala, Acacia mangium, Gliricidia sepium, and Clitoria fairchildiana are four of the most common species used by family farmers to create sustainable agricultural systems. These four legumes perform symbiosis with different groups of rhizobia. Exploring BNF could help to enable sustainable intensification of agriculture in the humid tropics, mainly because it can increase N use efficiency in an environment where N is a limiting factor to plant growth.