Overestimation of respiration rates in commercially available clamp-on leaf chambers. Complications with measurement of net photosynthesis

The possible interference when measuring gas exchange with respiratory CO2 produced under the gasket of commercially available clamp-on leaf chambers was investigated. Two of these chambers were compared with a leaf chamber that accommodated an entire leaf without clamping it under a gasket. An overestimation of dark respiration rate (R-D) by 55% was found with Plantago major leaves, a species with homobaric leaves that have high resistance for lateral gaseous transport. The percentage was similar in the heterobaric Ficus benjamina, but was 32% in the highly porous homobaric Nicotiana tabacum. Net photosynthetic rate at low photon flux density was underestimated by 35% in the clamp-on chamber. However, the gasket effect was not detectable at light saturation because the error was small in comparison with the high photosynthetic rates. Estimation of respiration in the light (R-L) in Nicotiana as derived from CO2 exchange at low CO2 concentrations was complicated by three factors. The inward diffusion of respiratory CO2 from under the gasket was added to a diffusion of CO2 from outside through the gasket material and through the leaf, which produced an even larger error in R-L in comparison with R-D at ambient CO2. These errors are significant for estimations of carbon gain at whole plant and canopy level and also at the leaf level when photosynthetic rates are low. Possible improvements in gasket design and corrections of CO2 exchange measurements for the gasket effect are discussed.