Improving the measurement of gas exchange in closed systems

Effects of pressure changes and CO2 accumulation on gas exchange measurements were quantified for closed systems by measuring gas percentages (% of total number of moles) and total pressure in closed cuvettes containing pears. A range of O-2 partial pressures was used in combination with 0 and 5 kPa CO2. The change in total pressure over time depended on the gas exchange rate of pears and the ratio between CO2 production and O-2 consumption. The influence of various factors that affect pressure, like sampling, temperature and humidity is discussed. When these factors are taken into account, pressure readings will improve the expression of gas exchange rates in absolute terms. Gas exchange rates were also measured in closed cuvettes with pears at normoxic conditions. The corresponding respiration rates at cellular level were estimated. The measured CO2 production rate decreased over time. This could partly be explained by accumulation of intercellular CO2. The increase in intercellular CO2 was calculated using the skin permeance of the pears. Dissolving of CO2 in the water phase of the fruit could cause the respiratory CO2 production to be significantly underestimated in closed systems. The O-2 consumption rate was shown to be a more accurate measure of respiration rate due to the much lower solubility of O-2 compared to CO2. (C) 2001 Elsevier Science B.V. All rights reserved.