Growth and development of sunflower fruits under shade during pre and early post-anthesis period

The effect of pre and post-anthesis shading (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.) genotypes were studied at two locations.Both shading treatments reduced pericarp weight, fruit volume and total yield per plant. Plants shaded during pre-anthesis maintained the number of filled fruits but reduced their individual weight and cotyledon cell number in the three positions on the capitulum. In contrast, post-anthesis shading reduced the number of filled fruits but their individual weight and cotyledon cell number were reduced only in the central fruits.Sigmoidal functions were fitted to seed growth data to estimate the duration of lag phase, the seed growth rate (SGR) and the effective filling period (EFP). 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.The hierarchy of fruit growth between positions within the capitulum was not modified by shading treatments and it was associated with differences, among fruit positions, in cotyledons cells number (except between mid and central fruits in pre-anthesis shading) and SGR (except peripheral and mid fruits in post-anthesis shading).SGR not only depended on the cotyledons cell number, which was fixed during the cell division phase of seed development, but it was also sensitive to environmental conditions during the linear phase of growth.