Carbon dioxide, a natural acidic gas, may be ušed for in vivo titration of the buffers in the compartments of cells that contain carbonic anhydíase in intact leaves. A gas systém for the measurement of the CO2 capacity (total inorganic carbon content) of leaves under different extemal CO2 concentrations has been developed. The systém consists of a leaf chamber and of two open (flow-through) channels, one for pre- loading the leaf with CO2 and the other for measurement of the desolubilized CO2. CO2 concentration in the loading channel may be adjusted from 0.03 to 20 %, while CO2 free air is flowing in the burst (desolubilization) channel. The leaf chamber (4.3 X 4.3 X 0.3 cm^) is switched either into the loading channel or into the burst channel. After loading the leaf with high CO2 in the dark the .chamber is switched into the burst channel and the desolubilized CO2 is measured. Preliminary measurements show that the CO2 capacity of a sunflower leaf decreases when the CO2 concentration is increased from 0.03 to 15 %, because of saturation of buffers. The systém can be ušed for direct, non-destructive measurements of pH and buffer capacity in leaf chloroplasts in order to investigate the influence of pH on photosynthesis, the operation of proton pumps and other pH-stabilising mechanisms.
The impact of a heterogeneous distribution of actinic light within a leaf chamber for photosynthetic measurements by gas exchange on the photosynthesis-irradiance relationship was investigated. High-resolution light distributions were measured over the area of a commercially available clamp-on leaf chamber equipped with build-in red and blue LEDs, as well as over the area of a custom-made leaf chamber with external light source, using a low-cost digital camera and freely available software. The impact of the measured heterogeneity on the photosynthesis-irradiance response curve was calculated for two realistic scenarios. When the average light intensity over the leaf chamber area was estimated accurately, heterogeneity had minor effects on the photosynthesis-irradiance response curve. However, when the irradiance was measured in the chamber centre, which is common practice, and assumed to be homogeneous, for both leaf chambers the photosynthesis-irradiance response curve was subject to considerable error and led to serious underestimation of the light-limited quantum yield of photosynthesis. Additionally, mixed light sources with different heterogeneity patterns per light source, such as in the clamp-on leaf chamber, potentially increase errors due to heterogeneous physiological responses to light spectrum. High-resolution quantification of the leaf-chamber light distribution enables calculation of the correct average light intensity and already resolves the most pressing problems associated with heterogeneity. To exclude any light-distribution related errors in
gas-exchange measurements a leaf chamber and actinic irradiance source design with a homogeneous light distribution is an absolute requirement. and S. W. Hogewoning ...[et al.].