MGRox said:

Something else possibly to consider, though I'm just adding this for awareness. I'm not trying to solve anything here.

At any rate. By products from reduction of nitrate in plant tissues Does compete for CO2 uptake in plants. CO2 and Nitrate Reductase pathways are interconnected to some degree. In cases where Nitrate nitrogen is the only source and CO2 is low or atmospheric; then CO2 uptake will be reduced by as much as 30%. Conversely, elevated levels of CO2 will lower the plants ability to form nitrate reductase and thus lower its' ability to assimilate nitrate nitrogen. This does not occur with NH4 as the nitrogen source as it does not rely on nor is it invovled with Nitrate reductase.

It is possible maybe (if your feeding Nitrate nitrogen only) that your CO2 levels are negatively impacting your plants ability to uptake nitrate. If this is the case; you can eliminate this via; A. lowering CO2, or B. use other forms of nitrogen to make up your total N (e.g. NH4 or NO2).

EDIT: I should also note that this interconnection and response occurs BEFORE any other plant signals can happen. E.G response from low or excess nitrate, etc.

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