How do plants ascertain their starch reserves? How do they measure time? What mechanism enables them to do math?

They had good teachers; they learned from their parents.

That -- and complex biochemistry

Here's a guess for the first question - I assume that any photosynthate not used immediately for current functions like growth etc. would be stored until the plant ran out of storage space, if that latter would ever actually happen. Perhaps @admmad can help with this one.

They measure time as far as daylength is concerned with a pigment in the leaves called phytochrome that accumulates and degrades in response to duration of red light wavelengths (which is why incandescent light bulbs at low intensity work for greenhouse photoperiod manipulation even though they are not efficient for photosynthesis).

Not a perfect or complete article but some of the responses are mentioned here:
http://garden.org/ideas/view/g...

BeKind said:How do plants ascertain their starch reserves?

Plants make sugar in their leaves. That is called photosynthate. How much of the photosynthate is divided among different possible uses is called photosynthate partitioning or carbon partitioning. Some plants may store some of the photosynthate as starch but other plants store photosynthate as other carbohydrates, proteins or oils. The general term for determining how all the possible materials (carbohydrates, proteins, oils, etc.) are divided among different possible uses is called assimilate partitioning.

How photosynthate is divided between growth and storage reserves is very complex. What is known about the process is described in chapter 4, Photosynthate Partitioning by N. G. Halford, pages 67-82, in the book Plant Developmental Biology – Biotechnological Perspectives: Volume 2 edited by E‐C. Pua and M.R. Davey and published in 2010 by Springer-Verlag.

I have always said we have some very smart and knowledgeable people on this forum. This thread proves it. Thank you for sharing your vast knowledge.