A CUVETTE GLASSHOUSE FOR SPECIFYING THE ENVIRONMENTAL REQUIREMENTS OF GLASSHOUSE CROPS
The net rate of carbon dioxide uptake by plants under different environmental regimes and over short time intervals may be determined in controlled environment chambers frequently referred to as cuvettes. BOURDEAU and WOODWELL (1965) reviewed a number of different methods in which the chambers were part of either open, closed or mixed systems. In an open system air is passed through the chamber and gas analyser at a constant flow rate and is then exhausted to the outside air. The product of flow rate and difference in the carbon dioxide concentration of the incoming and outgoing air of the chamber yields the rate of carbon dioxide exchange. A closed system depends upon the re-circulation of air within the enclosed volume of the chamber and analyser complex. If the system is gas-tight the rate of change of the carbon dioxide concentration provides a measure of the rate of carbon dioxide exchange. The size of chamber is dictated by the type and nature of the plant material under investigation and an appreciation of the inherent engineering problems. There is however a serious risk with both systems that measurements of carbon dioxide exchange may be rendered worthless as a result of undetected leaks. The risk may be especially serious in the case of enclosures made large enough to contain an array of plants. KOLLER and SAMISH (1964) described a method whereby a carbon dioxide gas analyser is used as a null point indicator so that the injection rate of a mixture of carbon dioxide and air into the chamber is a measure of the rate of carbon dioxide assimilation. The pressure in the plant enclosure and ancillary apparatus is kept above atmospheric so that any leaks are in an outward direction. This means that leakage downstream of the plant enclosure is accounted for by the injection of an equivalent volume of the carbon dioxide-air mixture and it is not necessary to use a tracer gas. However, leakage upstream of the enclosure will result in an error that cannot be estimated.
LAKE (1966) suggested the use of the nitrous oxide tracer gas method (ANON. 1957) for measuring the variable and often high ventilation or leakage rates of a small cuvette glasshouse. It would then be possible to measure the exchange of water vapour and carbon dioxide of an array of plants under natural flux densities of radiation A method previously outlined by LAKE, BROWNE and BOWMAN (in press) has now been further developed so that the net carbon dioxide assimilation of an array of plants growing in a small glasshouse can be measured with improved accuracy over periods as short as 30 minutes or less. During this time, the solar radiation, atmospheric concentration of carbon dioxide, air temperature