Chief, I just like looking at the big picture. In hydroponics, nutrient availability tends to drive transpiration, not CO2. For plants in wild, hydrology would drive nutrient availability. CO2 is a likely an indicator not the cause of transpiration changes.
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2008.02595.x/full
Interestingly, plants sequester CO2 in the soil based on the overall growing conditions. You could theoretically modify plants or crops to increase CO2 sequestering for a variety of growing conditions. Which would mean that the huge change in agricultural in the past few hundred years could have a larger impact on atmospheric levels of CO2 than expected.