Calculating Air Leakage Rates in Controlled-Environment Chambers Containing Plants
- B. Acock * and
- M. C. Acock
Air leakage rates from semi-closed plant growth chambers under elevated C02 concentration ([CO2]) treatments must be known in order to calculate plant CO2 exchange (CE). It is usually assumed that air leakage rates cannot be measured with plants in the chambers, so air leakage rates are measured before and after each experiment, and assumed to remain constant or vary linearly over time. Variability in leakage rate will diminish the accuracy of calculating CE. A method for calculating air leakage rate at night in the presence of respiring plants was devised. Simultaneous equations for three situations were developed: (i) when respired C02 = CO2 leaked from chamber; (ii) when [CO2] is maintained at a set level overnight by injection; and (iii) when respired CO2 > CO2 leaked from chamber. All equations were based on two principles: (i) at night, the change in mass of C02 within the chamber results from a loss of C02 by leakage and a gain by respiration; and (ii) leakage of CO2 from the chamber is proportional to the difference between chamber [CO2] and ambient [CO2]. Respiration rate is assumed constant during the measurement period. Results are reported for the situation when chamber [CO2] reaches an equilibrium overnight (respired C02 = CO2 leaked). Air leakage rate was found to have a coefficient of variability as high as 26.2% over eight daily measurements. Errors in calculating CE depend on the magnitude of the leakage rate, its variability, [CO2], and photosynthetic activity. A leakage rate that varied by 10% from two air changes h−1 in the 800 μL L−1 [CO2] treatment chamber could have given a 5% error in calculating CE during a cloudless day and a 14% error during a day with complete cloud cover. Using these procedures, variability in leakage rate can be checked automatically each day, with plants in the chamber, and without monitoring any gas except CO2.
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