Article

Bases morfofisiológicas para maior tolerância dos híbridos modernos de milho a altas densidades de plantas

Bragantia (Impact Factor: 0.53). 08/2002; 61(2). DOI: 10.1590/S0006-87052002000200003
Source: DOAJ

ABSTRACT

O lançamento de híbridos de milho tolerantes ao aumento da densidade de plantas contribuiu para o incremento do potencial produtivo da cultura na segunda metade do século XX. Objetiva-se com esta revisão de literatura discutir características morfológicas, fisiológicas, fenológicas e alométricas que contribuíram para maior adaptação do milho a elevadas densidades de plantas. Os processos de seleção utilizados pelos melhoristas minimizaram a natureza protândrica da planta, reduzindo o tamanho do pendão. Isso propiciou desenvolvimento alométrico mais equilibrado entre as inflorescências masculina e feminina, limitou a esterilidade feminina e favoreceu a sincronia entre antese e espigamento. O ideotipo de planta compacto dos híbridos modernos, caracterizado pela presença de plantas baixas, com menor número de folhas e folhas eretas, melhorou a qualidade da luz no interior do dossel, contribuindo para reduzir a dominância apical do pendão sobre as espigas. A menor produção de fitomassa reduziu a competição intra-específica e aumentou a eficiência de uso dos fatores ambientais, disponibilizando mais carboidratos para atender às diferentes demandas da planta na fase reprodutiva. O maior equilíbrio nas relações entre fonte e dreno contribuiu para retardar a senescência foliar, resultando em maior absorção de nutrientes e maior eficiência de uso do nitrogênio. O desenvolvimento de híbridos com menor estatura e espigas mais próximas do solo reduziu a quantidade de plantas acamadas e quebradas. A compreensão das bases morfofisiológicas responsáveis pela maior tolerância do milho à competição intra-específica auxiliará melhoristas e fisiologistas a maximizar a eficiência do arranjo de plantas para alcançar altos rendimentos.

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    • "A time series between hybrids and varieties from 1930 to 2001 highlighted a tendency of reduction of height of ear insertion of about three centimeters per decade (Duvick, 2005). Plants that show reduced stature and height of ear insertion can take advantage of the use of nitrogen fertilizers more efficiently, because the plant's center of gravity is more balanced, reducing lodging and stem breakage and favoring the absorption and translocation of nutrients to grain filling (Sangoi et al., 2002). Regarding GCA of group I (females) for the characters GY, ED and EL (Table 2), it was found that lines 1, 3, 4, 5, 6, 13 and 15 had positive GCA estimates in the average of the five environments, indicating tendency of increasing additive genetic contribution to these characters in the crosses in which these lines took part. "

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    • "O aumento da população de plantas proporciona a dominância apical, resultante da maior relação de luz no espectro vermelho distante/vermelho e da menor fotooxidação de auxina no ponto de crescimento das plantas, e que prioriza o transporte de substâncias para o pendão, em detrimento das espigas (SAnGOI et al., 2002). Tal mudança fisiológica, juntamente com a maior competição intra-específica por recursos do meio seria, a princípio, a responsável pela redução do conteúdo de açúcares totais em milho doce, devido à redução da disponibilidade de fotoassimilados por planta e espigas, em uma determinada área (ARGEnTA; SILVA; SAnGOI, 2001). "
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    • "The earliness characteristic has been sought together with shorter plants and low ear insertion, because these plants can withstand better high plant populations without damage from root and stalk lodging (PEIXOTO et al., 1997). The smaller demand in heat units for flowering of the contemporary hybrids of Southern Brazil favors the production of plants with low ear insertion height (SANGOI et al., 2002). With this, the center of gravity of the plant is better balanced and its sustainability is increased. "
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