The rate of change of top projected leaf area (TPLA) of lettuce (Lactuca sativa L.) seedlings was determined with machine vision technology. Differences of TPLA between control and treatment plants were detectable with this technique within 48 hours from the onset of an imposed nutrient stress. The nutrient stress treatments were 0%, 50%, 150% of the control (100%). There were no differences for the 50% and 150% treatments compared to the control plants, even after a 6-day observation period. However, the 0% treatment caused different TPLA expansion within 48 hours and required a recovery period of 3 or 4 days after being returned to normal EC levels before again attaining prestressed growth rates.