Genetic improvement of sunflower in Argentina between 1930 and 1995 II. Phenological development, growth and source-sink relationship

Historic sets of sunflower (Helianthus annuus L.) cultivars released in Arg entina between 1930 and 1995 were compared in two field experiments. Breedi ng and selection shortened cycle length: most of the reduction came from a reduction in time to anthesis, which together with a stable time from anthe sis to maturity increased the proportion of the growing cycle given to grai n growth. Shoot biomass at maturity was unrelated to year of cultivar relea se but a greater portion was produced after anthesis. The ratio between lea f area index duration from anthesis to maturity and grain number was taken as a measure of source-to-sink ratio; this ratio was smaller in hybrids tha n in open-pollinated cultivars. Variation in both the amount of oil per gra in and grain mass were small in comparison with the variation in source-to- sink ratio indicating that additional carbon may have attenuated the effect s of less leaf area duration per grain. Owing: to the relative stability of the amount of oil per grain and the reduction in grain mass, the grain oil concentration increased despite the drop in source relative to sink. Furth er yield improvement will mostly depend upon the increase in, the supply of substrates to fill the grain provided grain size is not restricted by even ts occurring before anthesis. (C) 1999 Elsevier Science B.V. All rights res erved.