I would think that nutrient uptake happens at all times of the day, as it is not dependent on light (though I am not a plant biologist by education, so take it with a grain of salt).
From:Basically I think this is what you want to know:
- PO4 is taken up in the day and night, but much more is absorbed in the light. PO4 is also preferentially taken up by shoots and roots.
- Ca and K are both absorbed in light and darkness but more during the dark.
- Mg is absorbed almost equally in light and darkness.
Phosphate and nitrate uptake thread:
Phosphate uptake in aquatic plants paper:
http://ir.ihb.ac.cn/bitstream/34200...ersed macrophyte Myriophyllum spicatum L..pdf