You may need use tissue culture methods

Solid initiation medium containing half-strength MS medium,

vitamins of Morel and Wetmore with B-12 and a mixture of amino

acids, 120 g/l sucrose, pH at 5.9, plus NAA and BAP at 0.5 g/l

• Liquid embryo transfer medium – Monnier’s macro and micro,

vitamins of Morel and Wetmore with a mixture of amino acids,

15g/l sucrose, pH at 5.9

you can refere to USDA projects

In vitro germination ....i.e. embryo rescue....may help

You have in the sunflower head diferents time of seed growing. Of course the 3  to 5 rows far from the center are the oldest.  I had good results using the embrios of this rows once the yelow leaves are ready to fall. You must take out the shell, planting in bags in greenhouse and you will see that those embrios start to emerge. You can gain aproximately 40 days in advance of the harvest, I am  talking in tropical conditions