Maturing sunflowers do not follow the sun

The final part of my sunflower tracking experiment has confirmed that once the sunflower head (at least on the variety that is planted in our fields) has formed, it will discontinue its tracking of the sun and remain (mostly) static in one direction.

This photograph (taken 2nd July 2009) shows the majority of the sunflower heads facing pretty much towards where the sun rises.

I have no idea why their behaviour changes so much at this point. I can speculate that the once fleshy tissue of the stem is hardening and cannot react as it initially did or that that the sunflower only needs a large dose of ultra violet light in the development stage.

The photo (click to enlarge) is over exposed but I wanted to demonstrate that the sunflowers were indeed facing the opposite direction to the sun.

Do you have any thoughts?