I think plant cells uptake nutrients throughout the day through cell transports; however, they only store glucose during the day as it is produced during photosynthesis and consumed at night during respiration. I'm sure they will still store nurtients even if they use it at night, as there is a greater concentration in the water column. Just a thought.
20G heavily planted tank with Eco Complete
Pressurized CO2, Eheim Ecco 2236, 1 55W and 1 36W bulb