Major diffusion leaks of clamp‐on leaf cuvettes still unaccounted: how erroneous are the estimates of Farquhar et al. model parameters?

Abstract

Estimates of leaf gas-exchange characteristics using standard clamp-on leaf chambers are prone to errors because of diffusion leaks. While some consideration has been given to CO2 diffusion leaks, potential water vapour diffusion leaks through chamber gaskets have been neglected. We estimated diffusion leaks of two clamp-on Li-Cor LI-6400 (LiCor, Inc., Lincoln, NE, USA) leaf chambers with polymer foam gaskets and enclosing either 2 or 6 cm 2 leaf area, and conducted a sensitivity analysis of the diffusion leak effects on Farquhar et al. photosynthesis model parameters -the maximum carboxylase activity of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) (Vcmax), capacity for photosynthetic electron transport (Jmax) and nonphotorespiratory respiration rate in light (Rd). In addition, net assimilation rate (An) versus intercellular CO2 (Ci) responses were measured in leaves of Mediterranean evergreen species Quercus ilex L. enclosing the whole leaf chamber in a polyvinyl fluoride bag flushed with the exhaust air of leaf chamber, thereby effectively reducing the CO2 and water vapour gradients between ambient air and leaf chamber. For the empty chambers, average diffusion leak for CO2, K CO2 , (molar flow rate corresponding to unit CO2 mole fraction difference) was ca. 0.40 mmol s -1 . K CO2 increased ca. 50% if a dead leaf was clamped between the leaf chamber. Average diffusion leak for H2O was ca. 5-to 10-fold larger than the diffusion leak for CO2. Sensitivity analyses demonstrated that the consequence of a CO2 diffusion leak was apparent enhancement of An at high CO2 mole fraction and reduction at lower CO2 mole fraction, and overall compression of Ci range. As the result of these modifications, Farquhar et al. model parameters were overestimated. The degree of overestimation increased in the order of Vcmax < Jmax < Rd, and was larger for smaller chambers and for leaves with lower photosynthetic capacity, leading to overestimation of all three parameters by 70-290% for 2 cm 2 , and by 10-60% for 6 cm 2 chamber. Significant diffusion corrections (5-36%) were even required for leaves with high photosynthetic capacity measured in largest chamber. Water vapour diffusion leaks further enhanced the overestimation of model parameters. For small chambers and low photosynthetic capacities, apparent Ci was simulated to decrease with increasing An because of simultaneous CO2 and H2O diffusion leaks. Measurements in low photosynthetic capacity Quercus ilex leaves enclosed in 2 cm 2 leaf chamber exhibited negative apparent Ci values at highest An. For the same leaves measured with the entire leaf chamber enclosed in the polyvinyl fluoride bag, Ci and An increased monotonically. While the measurements without the bag could be corrected for diffusion leaks, the required correction in An and transpiration rates was 100-500%, and there was large uncertainty in Farquhar et al. model parameters derived from 'corrected' An/Ci response curves because of uncertainties in true diffusion leaks. These data demonstrate th...

Tracing Photosynthetic Response Curves with Internal CO₂ Measured Directly

“…The applicability of the system for A Ci curve measurement was tested in the sunflower (Helianthus annuus L.) leaf. We took special care to minimize bulk air flow through the leaf (Lauer and Boyer, 1992;Boyer and Kawamitsu, 2011), diffusion leaks (Flexas et al, 2007;Rodeghiero et al, 2007), and changes in the CO2 gradient across the leaf mesophyll (Mott and O'Leary, 1984;Parkhurst et al, 1988;Parkhurst and Mott, 1990) that can be important for the measurement.…”

Mixed-power scaling of whole-plant respiration from seedlings to giant trees

“…Furthermore, most studies of plant metabolic scaling have been based on the aboveground parts and have ignored respiration of the roots, which is very difficult to measure (23). Furthermore, the flow of dissolved CO 2 in the sap brings into question the accuracy of estimating whole-tree respiration based on measuring small portions of large trees using the standard clamp-on chambers that are commercially available (16,(27)(28)(29)(30).…”

“…Furthermore, most studies of plant metabolic scaling have been based on the aboveground parts and have ignored respiration of the roots, which is very difficult to measure (23). Furthermore, the flow of dissolved CO 2 in the sap brings into question the accuracy of estimating whole-tree respiration based on measuring small portions of large trees using the standard clamp-on chambers that are commercially available (16,(27)(28)(29)(30).Given the methodological difficulties inherent in whole-plant physiological studies, it is not surprising that there have thus far been no empirical measurements of complete whole-plant respiration, including roots, that have a reasonable sample size and encompass a wide range of plant sizes (2,22,26,(31)(32)(33)(34). Accurate and efficient methods using whole-plant chambers to measure respiration rates have recently been developed (5,18,19,(24)(25)(26) and can be applied to assess metabolic scaling (Fig.…”

Day and night respiration of three tree species in a temperate forest of northeastern China

“…Some studies have proposed that the overestimation of RLL is caused by the CO2 acting as an inhibitor of certain enzymes (Brooks & Farquhar 1985, Kirschbaum & Farquhar 1987, Tjoelker et al 2001. However, a large number of studies suggest that leaf respiration changes with CO2 possibly due to small leaks or diffusion in the gas exchange measurement systems (Amthor et al 2001, Pons & Welschen 2002, Shapiro et al 2004, Flexas et al 2007, Rodeghiero et al 2007. Therefore, in this study RLL was corrected based on the method suggested by Li-Cor Inc (2004) to compensate the CO2 leakage in the Laisk method.…”

“…Therefore, the respiration rate measured at low CO2 concentration was generally overestimated. A correction for leakage was necessary because it is hard to avoid CO2 exchange or leakage between the infrared gas analyzer (IRGA) chamber and the surrounding air (Pons & Welschen 2002, Shapiro et al 2004, Flexas et al 2007, Rodeghiero et al 2007). We corrected for CO2 leakage by diffusion in the Laisk method based on the following equation (eqn.…”