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).
