It uses those starch statoliths to determine which direction it is going to send up its cotyledons. IIRC they did some trick with the starch statoliths to confuse the plant, and it sent shoots the wrong way. This gravotropism is also why NASA scientists were interested in sprouting seeds in zero gravity.
If you have a hypothesis about infra-red detection, we need an experiment, and data to support that; then find your mechanism and the signaling pathways. Or if the work has already been done you need a good reference to it.
Gravotropism has the hypothesis, the experiment, the data that supports it, as well as a known mechanism. I am unaware of the exact signaling pathways, but no doubt some sort of cascade of phosphorylations or some such modifying proteins, transcribing different genes, etc. Plant cells are often interconnected via cytoplasm, so signals are regional and often need no specific inter-cellular mechanism.
So, going by the rest of your response, even a seed is 'intelligent'.
It senses heat,light,nutrient,water, and as you have justly defended, gravity.
Gravity detection may be(or is) the leading sensation used by the seed until it reaches the surface. Does it remain the leading 'sense' after that?