N-partitioning, nitrate reductase and glutamine synthetase activities in two contrasting varieties of maize

N-partitioning, nitrate reductase and glutamine 249 N-partitioning, nitrate reductase and glutamine synthetase activities in two contrasting varieties of maize(1) Altair Toledo Machado(2), Ladaslav Sodek(3) and Mânlio Silvestre Fernandes (4) Abstract – In order to identify useful parameters for maize genetic breeding programs aiming at a more efficient use of N, two maize varieties of contrasting N efficiency, Sol da Manhã NF (efficient) and Catetão (inefficient) were compared. Experiments were carried out under field and greenhouse conditions, at low and high N levels. The parameters analysed included total and relative plant and grain N content, biomass and the activities of nitrate reductase and glutamine synthetase in different parts of the plant. It was found that the translocation efficiency of N and photoassimilates to the developing seeds and the source-sink relations were significantly different for the two varieties. N content of the whole plant and grain, cob weight and the relative ear dry weight were useful parameters for characterizing the variety Sol da Manhã NF as to its efficient use of N. Enzymes activity of glutamine synthetase (transferase reaction) and nitrate reductase did not differ among the varieties. Index terms: nutrient transport, nitrogen, grain, source sink relations, plant breeding. Partição de nitrogênio e atividade das enzimas nitrato redutase e glutamina sintetase em duas cultivares contrastantes de milho Resumo – Com o objetivo de identificar parâmetros que possam ser utilizados em programas de melhoramento genético em milho para uso eficiente de N, duas cultivares de milho contrastantes quanto ao uso deste nutriente, Sol da Manhã NF (eficiente) e Catetão (não-eficiente), foram avaliadas em dois experimentos conduzidos no campo e em casa de vegetação, respectivamente, sob nível baixo e alto de N. Os caracteres avaliados foram: teor e conteúdo de N em diferentes partes da planta; massa seca; peso dos grãos e de diferentes partes da planta; biomassa, e atividade das enzimas nitrato redutase e glutamina sintetase. O mecanismo de translocação de N e de fotoassimilados para os grãos e a relação fonte/dreno foram importantes para diferenciar a cultivar eficiente da não-eficiente. Conteúdo de N nos grãos e total das plantas, peso do sabugo e relação peso de espiga/matéria seca foram importantes para caracterizar a cultivar Sol da Manhã NF eficiente no uso do N. A atividade das enzimas glutamina sintetase (reação da transferase) e nitrato redutase não foram eficientes para discriminar as cultivares em estudo. Termos para indexação: transporte de nutrientes, nitrogênio, grãos, relação fonte-dreno, melhoramento de plantas. (1) Accepted for publication on April 26, 2000. (2) Embrapa-Centro Nacional de Pesquisa de Agrobiologia, Caixa Postal 74505, CEP 23851-970 Seropédica, RJ, Brazil. E-mail: (3) Universidade Estadual de Campinas, Dep. de Fisiologia Vege- tal, Cidade Universitária Zeferino Vaz, CEP 13081-971 Campinas, SP, Brazil. E-mail: (4) Universidade Federal Rural do Rio de Janeiro, Dep. de Solos, BR 465, km 47, CEP 23851-970 Seropédica, RJ, Brazil. E-mail: Introduction Nitrogen (N) is the most expensive nutrient and the one required in largest quantity by the majority of crops, especially maize. Although present at high levels in soils, the amounts of N in mineral forms are generally low. It is commonly observed that in natural ecosystems a continued loss of N occurs not only through its removal by plants but also by leaching, volatization and denitrification. It has been estimated Pesq. agropec. bras., Brasília, v. 36, n. 2, p. 249-256, fev. 2001 250 A. T. Machado et al. that only 40% to 50% of the N applied as fertilizer is absorbed by the maize crop under tropical conditions (Peoples et al., 1995). The availability of N to plants in tropical climates is also impaired by environmental stress such as drought, waterloging, low fertility soils, aluminium, among others (Magalhães & Fernandes, 1993; Machado & Magalhães, 1995). Variations in N availability can affect plant development and grain production in maize (McCullough et al., 1994; Uhart & Andrade, 1995b). The effect of N availability on grain yields of maize can be assessed by physiological components such as the interception and efficient use of radiation and the partitioning of N to reproductive organs (Uhart & Andrade, 1995b). Parameters such as leaf area index, longevity of the leaf canopy and the efficient use of light in maize are all increased by N (Muchow & Davis, 1988). It is also known that both deficiency and excess N affect the partitioning of assimilates between vegetative and reproductive organs (Donald & Hamblin, 1976). N deficiency affects the supply of assimilates to the ear mainly through reductions in leaf area, photosynthetic activity and light absorption (Lemcoff & Loomis, 1986; Uhart & Andrade, 1995b). The flow and remobilization of carbon (C) and N to the kernels during the grain filling period depends on the source-sink relationship, which is influenced by genotype-environment interactions, that among others may be altered by management factors such as planting date, population density, nutrients and water (Uhart & Andrade, 1995a). Furthermore, stress conditions can undoubtedly act directly on availability, assimilation and metabolism of N, interfering with the activity of enzymes of N metabolism and probably the catabolism of amino acids, proteins and other nitrogenous compounds (Machado et al., 1992; Magalhães & Fernandes, 1993). Genetic improvement trials, where tolerance to abiotic and biotic stresses are focused, can result in the selection of genotypes that are more efficient to scavenge and use N under stress conditions (Tollenaar et al., 1991; McCullough et al., 1994). The selection of genotypes with a more efficient mechanism of N uptake and metabolism is a strategy aimed at increasing N utilization efficiency of the maize crop. Several trials for efficient use of N unPesq. agropec. bras., Brasília, v. 36, n. 2, p. 249-256, fev. 2001 der conditions of low N availability have been carried out with maize (Thiraporn et al., 1987; Eghball & Maranville, 1991; Machado et al., 1992). In order to characterize and select genotypes for efficient use of N, several authors have used physiological and biochemical parameters, such as high nitrate in leaves (Molaretti et al., 1987), increased nitrate reductase activity (Cregan & Berkum, 1984; Feil et al., 1993), glutamine synthetase activities (Machado et al., 1992; Magalhães et al., 1993; Machado & Magalhães, 1995), or increased mobilization of N from leaves and stems to the kernels (Eghball & Maranville, 1991). The objective of this study was to identify morphological, physiological and biochemical parameters that could be used to help genetic improvement programs to get maize genotypes more efficient in the use of N. Material and Methods Plant material The two maize varieties used in this study, So (...truncated)