Grain number in sunflower as affected by shading during floret growth, anthesis and grain setting

The effects of shading (to 20% of incident radiation) on grain number (GN) in sunflower were examined by shading the crop in each of the six separate 10-day intervals running from the end of floret differentiation through to early grain growth (roughly, from mid-anthesis minus 15 days to mid-anthesis plus 26 days). An ontogenic index, based on floret morphology, male and female floret organ development, and early embryo development was formulated and used to describe the timing of shading and to categorize floret responses at three positions (periphery, mid-section, centre) on the head. Floret number per head was not affected by treatment, but shading reduced grain set (and, consequently, GN) significantly (P < 0.05) in all shading treatments. Florets in the peripheral position on the head showed no change in grain set in response to any shading, grain set of florets in the mid-section of the head radius were affected by immediate post-anthesis shading, and grain set of florets in the central position on the head were affected by most treatments, and particularly so when shading took place immediately after anthesis (globular proembryo stage). Observations of anatomical sections of florets sampled after anthesis from all three positions on the head suggested that fertilization was normal, and that the loss of grain set evoked by shading was due to embryo abortion. There was some evidence of impairment of pollen functionality due to shading, but this involved slower pollen germination and pollen tube growth rather than a complete lack of viability. Shading during the floret growth phase prior to anthesis reduced the unit mass of the resulting grain at physiological maturity. Final unit grain mass was correlated with floret mass at anthesis in both peripheral and mid-section grains. (C) 2003 Elsevier B.V. All rights reserved.