Altair Toledo Machado

Participatory maize breeding for low nitrogen tolerance

The local maize variety Sol da Manhã has a broad genetic background. It was identified in 1985 in a participatory evaluation trial as being suitable for cultivation under low soil fertility conditions in Sol da Manhã, situated in Seropédica, State of Rio de Janeiro, Brazil. The variety was then improved for 6 selection cycles by the formal breeding sector at the experimental station of Embrapa Agrobiologia. There were three cycles of mass selection, one selection cycle between and within half-sib families, one selection cycle within full-sib families, and one selection cycle between and within S1 families. Thereafter, the variety was improved in collaboration with the informal sector, viz. a group of farmers of the agricultural community of Sol da Manhã for 6 cycles of mass selection. The variety was evaluated in 1994 in field trials for grain yield and nitrogen use efficiency. Sol da Manhã can be characterised as efficient in nitrogen use, under both favourable and unfavourable growing conditions.

Variabilidade entre genótipos de milho para eficiência no uso de fósforo

Exploiting genetic variability in maize for phosphorus (P) efficiency is one of the viable strategies to turn around the element deficiency in tropical and subtropical regions. In order to verify differences among maize genotypes on phosphorus utilization and grain yield, two experiments were conduted. First, eighteen corn genotypes (ten local varieties, six improved varieties, and two hybrids) were evaluated in two soil P levels (10 and 100 mg.kg -1 de P), under greenhouse conditions. The second experiment, five local varieties and one improved variety, and one hybrid were evaluated under field conditions at crescent levels of P fertilization (0; 8.7; 17.5; 35; 70, and 140 kg.ha -1 de P). Genetic variability was detected for P efficiency at greenhouse conditions. The local varieties (Argentino, Amarelao, Caiano de Sobralia, Asteca, Quarentao and Caiano de Alegre), the improved ones (BR 106 and BR 107), and the hybrid P 6875 were the genotypes with the highest efficiency indexes. Under field conditions, the effects of P addition in grain production and P content in the grains were observed. The genotypes exhibited different levels in response to increased nutrient availability. The local varieties Cravinho and Carioca, and the hybrid P 6875 were the most responsive, besides presenting the highest grain yield in the absence of P fertilization. Nitroflint (improved variety) and Catetao (local variety) presented the lower response to P addition and the lower grain yield without P addition.

Inoculação de Azospirillum amazonense em dois genótipos de milho sob diferentes regimes de nitrogênio

The adaptability of maize genotypes to environments where nutrients are not readily available may be related to an adaptation to the predominance of the soil nitric and ammoniacal forms of N and to the association with beneficial microorganisms such as diazotrohpic bacteria and/or plant growth promoters. The objective of this study was to evaluate the response of two maize intervarietal hybrids to different nitrogen doses and forms as well as the effect of inoculation with A. amazonense. The experiment was carried out in a greenhouse with pots filled with vermiculite and Hoagland nutrient solution. A three-factor randomized complete block design was used with treatments arranged in a factorial scheme represented by: H1 (Carioca x Eldorado) and H2 (Palha Roxa ES x Sol da Manha) intervarietal hybrids; plants inoculated or not with A. amazonense; and three nitrogen proportions and doses: 126 mg week-1 of N (75 % NH4+ : 25 % NO3-); 126 mg week-1 of N (25 % NH4+ : 75 % NO3-); and 12.6 mg week-1 of N (50 % NH4+ : 50 % NO3-). After 25 days of growth the plant roots and shoots were separated for the determination of dry matter production, total N and P content, nitrate reductase and glutamine synthetase activity and total soluble sugars. The H1 hybrid, considered more efficient in preliminary field evaluations, produced greater dry matter and was more efficient in N and P utilization. In plants that received the highest N dose, independently of the predominance of the ammoniacal or nitric forms, the N (roots and shoots) and P (shoots) accumulation as well as the P utilization index were higher. Under the predominance of the ammonium-N plants grew more and the glutamine synthetase activity was increased, while the concentration of root total soluble sugars was lowered. The inoculation with A. amazonense resulted in higher root dry matter production and N accumulation.

ÍNDICES DE EFICIÊNCIA DE VARIEDADES LOCAIS E MELHORADAS DE MILHO AO FÓSFORO

Local and improved corn varieties were evaluated to phosphorus (P) efficiency in nutrient solution and field. In nutrient solution, the experiment was set up at Instituto Agronomico, Campinas, Sao Paulo State, Brazil, during 1995-1996, in a randomized complete block design, with 5 replicates, in a factorial with three P concentrations (0.0645; 0.129 e 0.258 mmol.L-1) and ten maize varieties. In the field, the experiment was set up at the research center of Embrapa Agrobiologia, Seropedica, Rio de Janeiro State, Brazil, during 1998-1999, using a randomized complete block design, with 6 replicates, and 44 kg.ha-1 of P (100 kg.ha-1 of P2O5). The varieties used were Caiano de Sobralia, Carioca, Antigo Maya, Catetao, Pedra Dourada, Nitrodente, Sol da Manha (BRS Sol da Manha), BR 105, BR 106 and BR 107. Dry matter and, or grain yield, P content and P efficiency index were determined in plants of both experiments. The varieties Nitrodente and BR 106 showed the highest shoot dry matter in nutrient solution and the highest grain yield and efficiency index at field conditions. Catetao, with the lowest grain yield in field, showed also low shoot and root dry matter production in nutrient solution. The results in field and nutrient solution were also comparable for Pedra Dourada, BR 105, BR 107, Antigo Maya and Sol da Manha (intermediary production). The exceptions were the varieties Carioca and Caiano, with inferior performance in nutrient solution and high grain yields in the field.