Photosynthesis measurements must be made within a few seconds after closing the leaf chamber: and the operator has limited control over environmental conditions within the chamber. • Once the leaf is sealed in the chamber, CO₂ concentration in the leaf chamber- is continually decreasing. • Consequently, if the leaf has a high photosynthetic rate, resulting in a rapid reduction of the chamber CO₂ concentration, measurements must be made quickly to avoid the possibility of a direct effect of low CO₂ concentration on photosynthesis.