Development of a fluorescent tracer technique to evaluate mixing in plant root culture

Liquid mixing times were measured in shake-flask root cultures based on dispersion of a concentrated riboflavin solution. Microplate fluorimetry allowed rapid quantification of small sample volumes and permitted mixing analysis in small vessels. An increase in mixing time from 2 min at a tissue concentration of ~150 g fresh wt/l to beyond 1 h at ~200 g fresh wt/l, as well as Fourier analysis of oscillator, mixing profiles, provided evidence for severe mixing impairment. Comparison of these mixing times to biological O₂ demand suggested that high density, shake-flask root cultures are severely O₂ limited due to impaired bulk liquid mixing.