Air pressure in clamp-on leaf chambers: a neglected issue in gas exchange measurements.

Air pressure in leaf chambers is thought to affect gas exchange measurements through changes in partial pressure of the air components. However, other effects may come into play when homobaric leaves are measured in which internal lateral gas flow may occur. When there was no pressure difference between the leaf chamber and ambient air (DeltaP=0), it was found in previous work that lateral CO(2) diffusion could affect measurements performed with clamp-on leaf chambers. On the other hand, overpressure (DeltaP>0) in leaf chambers has been reported to minimize artefacts possibly caused by leaks in chamber sealing. In the present work, net CO(2) exchange rates (NCER) were measured under different DeltaP values (0.0-3.0 kPa) on heterobaric and homobaric leaves. In heterobaric leaves which have internal barriers for lateral gas movement, changes in DeltaP had no significant effect on NCER. For homobaric leaves, effects of DeltaP>0 on measured NCER were significant, obviously due to lateral gas flux inside the leaf mesophyll. The magnitude of the effect was largely defined by stomatal conductance; when stomata were widely open, the impact of DeltaP on measured NCER was up to 7 mumol CO(2) m(-2) s(-1) kPa(-1). Since many other factors are also involved, neither DeltaP=0 nor DeltaP>0 was found to be the 'one-size fits all' solution to avoid erroneous effects of lateral gas transport on measurements with clamp-on leaf chambers.