Air pressure in clamp-on leaf chambers: a neglected issue in gas exchange measurements
2006
Jahnke, Siegfried | Pieruschka, Roland
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 (ΔP=0), it was found in previous work that lateral CO₂ diffusion could affect measurements performed with clamp-on leaf chambers. On the other hand, overpressure (ΔP>0) in leaf chambers has been reported to minimize artefacts possibly caused by leaks in chamber sealing. In the present work, net CO₂ exchange rates (NCER) were measured under different ΔP values (0.0-3.0 kPa) on heterobaric and homobaric leaves. In heterobaric leaves which have internal barriers for lateral gas movement, changes in ΔP had no significant effect on NCER. For homobaric leaves, effects of ΔP>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 ΔP on measured NCER was up to 7 μmol CO₂ m⁻² s⁻¹ kPa⁻¹. Since many other factors are also involved, neither ΔP=0 nor ΔP>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.
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