I see what you mean GpsFrontier. The link doesn't actually specify root exudation occurs only at night as I was supposing.
I tried to post something prior to that link but it didn't make it online for some reason. I had asked what/how do the roots get thier share of the starches and sugars (the plants "food") which are produced by the leaves ? I am making an assumption here that there MUST be some flow from leaves to the roots via the phloem of the starches and sugers.
What you say about transpiration ceasing when dark even when it's hot air surrounding the plant, makes sense. Thefore I will make another assumption here, transpiration occurs when water and
nutrients are being converted to starches and sugars. Without light, the clorophyl bearing parts of the plant cannot convert the nutrients and so (here comes another assumption) water and nutrients stop flowing from the roots to the leaves up through the xylem.
I'm wondering if this time when transpiration has ceased (in darkness) is when the roots get fed and perhaps do most of their exudation regardless of species. The link did mention that root exudation is cyclic.