Spending a bit of time looking at some more papers on rate of oxygen uptake at the roots (root respiration):
Some notes that I found interesting from the following paper, Veen, B. W. (1988). Influence of oxygen deficiency on growth and function of plant roots. Plant and Soil, 111(2), 259–266. doi:10.1007/bf02139950. veen1988.pdf (557.4 KB)
The most striking effect of reduction of oxygen supply to the roots was the increase in alkalinization of the nutrient solution by increased HCO3 production of the roots.
Reduction of the oxygen supply to 20% of the maximal respiration rate caused death of the root tips of tomato, reduction of the water uptake rate and hence wilting of the plant.
And, their experimental maximal uptake numbers:
This one looks interesting, too: Changhoo CHUN, Tadashi TAKAKURA, Rate of Root Respiration of Lettuce under Various Dissolved Oxygen Concentrations in Hydroponics, Environment Control in Biology, 1994, Volume 32, Issue 2, Pages 125-135, Released June 22, 2010, Rate of Root Respiration of Lettuce under Various Dissolved Oxygen Concentrations in Hydroponics. 32_125.pdf (1.1 MB)
In hydroponic systems, aeration is usually performed in storage tanks. Bubbling, agitation or other oxygen-enrichment facilities are installed to maintain the DO concentration near the saturation concentration. However, the rapid decrease in the DO concentration as shown in Fig. 3 suggests that only raise the initial DO concentration are insufficient in providing enough oxygen for the whole growing period. Though the nutrient solution is oxygen-rich in the storage tanks, its concentration may decrease while the nutrient fluid passes through the root respiration. This could explain the nonuniform DO concentrations in the NFT beds.