Personally, I think your suspicion is correct, that it's been a bit too cold is why you're not seeing an uptake of the boron.

I would assume the boron is still available for uptake.  Even if you get rain before the wheat has a chance to take it in through the leaves, I would think the boron would be able to enter through the root system with enough rain.

Will be interesting to see the results of your tissue tests, yet-to-come.


gordon