The Supply of Air to Leaves in Assimilation Chambers

Factors affecting the supply of air to leaves enclosed in assimilationchambers have been studied using models and leaves of appleand plum. The rate of CO₂ assimilation of a leaf does not increaselinearly with increasing rate of air flow. Explanations of thisand other observations are put forward in terms of the boundary-layerconcept and diffusion paths. Comparisons are made with free-airconditions, and it is concluded that air should be suppliedat the same rate (ml/min) for leaves of different size in thesame chamber. Three cup designs were also studied. A methodis described for making valid comparisons of assimilation ratesper unit area by extrapolation to an infinitely high rate ofair flow. Measurements of different sized leaves with chambersand cups can be compared in this way. Rates expressed per unitarea can be correctly compared only if the resistance to diffusionoutside the leaf is negligible, or under special conditions.Assimilation rates measured with rates of air flow giving CO₂availability equivalent to that occurring in static free airshould be expressed per unit perimeter. Methods of correctionof assimilation rates for the depletion of carbon dioxide fromthe air stream are considered.