CONICET | Buscador de Institutos y Recursos Humanos

1. Abstract The effect of pre and post-anthesis shading (to 20% of incident radiation) on pericarp development, cotyledon cell number and seed growth dynamic of fruits from three positions in the capitulum (peripheral, mid and central) of two sunflower (Helianthus annuus L.) cultivars were studied at two locations. At bBoth shading treatments reduced grain yield per plant. With pre-anthesis shading, plants produced the same number of filled fruits than control ones but reduced their individual fruit weight. Conversely, post-anthesis shading reduced the number of filled fruits but not their individual weight. Both shading treatments reduced pericarp weight and fruit volume. Pre-anthesis shading reduced seed weight and cotyledon cell number of fruits in the three position on the capitulum while post-anthesis shading only affected these variables in the central ones. Seed growth dynamics was analized by fitting seed dry weight data to sigmoidal functions. Then the duration of lag phase and seed growth rate (SGR) and the effective filling period (EFP) were calculated. Pre-anthesis shading reduced EFP of peripheral fruits, SGR of mid fruits and SGR and EFP of central ones while post-anthesis shading increased the duration of the lag phase of mid and internal fruits. SGR depended not only on the cotyledon cell number fixed during the cell division phase of seed development but also on the supply of assimilate to the cotyledons during the lineal phase of seed growth. Fruit weight heterogeneity within the capitulum was not modified by shading treatments and it was associated with differences between fruit positions in cotyledons cells number (except between mid and central fruits in pre-anthesis shading) and SGR, (except perypheral and mid fruits in post-anthesis shading). Key words. Helianthus annuus, cell number, seed development, shading, sunflower, yield components