120 Produce Degradation: Reaction Pathways and their Prevention
5.2.1.1 How to Measure Respiration Rates
Respiration of plant tissue is measured in millimoles of O 2 consumed or CO 2
produced per kilogram of tissue per hour. Other units may be used for respiration
rate, such as milliliters or milligrams of gas per kilogram per hour. Because the
volume of gases is dependent on temperature, the unit measure of millimoles per
kilogram per hour should be preferred, especially when the effect of temperature on
respiration rate is considered.
The respiratory quotient (RQ) is the ratio RRCO2/RRO2 when respiration rates
are measured in millimoles or milliliters per kilogram per hour. Whereas respiration
rates of different plant tissues, assessed under air, vary a lot, RQ ranges from 0.8 to
1.3 [4] depending on the nature of the catabolite. RQ is equal to 1 if the catabolic
substrates are carbohydrates. Catabolism of lipids results in a lower RQ and, con-
versely, in catabolism of organic acid the RQ is higher than unity. It should be
pointed out that RQ measured in a closed system method yields an abnormally low
RQ, especially at low temperature, because of the dissolution of CO 2 produced in
the plant tissue during the equilibration of the system. High RQ is also the sign of
a switch to anaerobic catabolism. For example, a fermentative respiratory pathway
displays an RQ higher than 2.
Techniques for measurement of respiration rates were reviewed recently by
Fonseca et al. [8], who detailed three main methods: (1) the closed or static system,
(2) the flowing or flushed system, and (3) the permeable system.
In the closed system, the plant tissue is placed in a gas-tight container fitted with
a sampling port (a silicone septum, for example). The weight of the plant tissue and
volume of the container are known. The initial atmosphere is usually normal air but
can be actively modified at closing of the container using an atmosphere generator.
Using gas chromatography or any sensitive gas analyzer, changes in O 2 and CO 2
within the containers are monitored for several hours at regular time intervals. The
respiration rate of the commodity is proportional to the absolute value of the slope
of the O 2 depletion and CO 2 production curves. The closed system is simple and
fast, but it does not allow measurements under stable atmospheric conditions and
the internal pressure within the container changes if RQ deviates from unity.
The flushing technique overcomes this drawback. A continuous flow of air or
any gas or mixture of gases (flow rate must be constant and accurately measured)
ensures the permanent renewal of the atmosphere in the containers. Comparison of
gas composition at the entry and exhaust points permits calculation of the respiration
rate over a long duration. However, this technique is time-consuming and requires
a considerable amount of gas.
The permeable system consists of placing the plant tissue in a flexible pack of
known volume, permeability, and diffusing area. The pack is sealed and stored at a
constant temperature. The O 2 and CO 2 respiration rates of the plant tissue are
proportional to the steady-state concentrations, respectively. As described by Fonseca
et al. [8], all of these systems were submitted to modifications to increase their
reliability and accuracy.
The last method to determine respiration rate is based on a controlled atmosphere
system in which O 2 input and CO 2 scrubbed are accurately measured. The respirometer