Let me start by saying thanks for the link, I have printed the 17 pages so I can go through it in more detail in the next few days.
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Does it seem possible that root exudate happens only when there is no transpiration ?
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Though I am not really sure what you mean by this question, because so far that's not what I have read. From what I understand they suggested that compounds secreted by plant roots were done mostly at night. My guess is that's so the compounds secreted by plant roots will have more time to do their job (in the soil), before the plants start sucking up water/moisture in the morning. But I also did not read anything that suggested that was the only time it would/could occur (just that it was most obvious it had at that time). But that also would need to be taken into perspective because not all plants will exudate their particular compounds/chemicals the same way and for the same reasons, weather it be from the roots, stems, leaves or flowers. So in short, it would be specific to the particular plant in question.
Also
transpiration is smiler to a person sweating, or a dog panting. Both are ways to cool the body, even a plants body. When the human body stops sweating, it's already dehydrated. It doesn't want to secrete any more sweet in order to conserve moisture. Not sure a plant is sophisticated enough to know when to conserve moisture that way (or they wouldn't wilt so fast). But it stands to reason if the plant is lacking in water/moisture it would
NOT want to secrete any unnecessary fluids in survival mode.
During the night time the plants wont be transpiring (breathing), thus shutting down the uptake of water and
nutrients. But this action is dew to the lack of light and not related to temperature (in general). I say this basically because plants don't uptake water during the dark periods, even when it's 100 degrees at night, or they would wilt with the
pump shut off for extended periods of time at night. In fact I can see this as being the perfect time for the roots to secrete exudate's, because the moisture needed to do this wouldn't need to be diverted from the plants other vile functions.