A computer-operated routine of gas exchange and optical measurements to diagnose photosynthetic apparatus in leaves

Photosynthesis is a complex process whose rate is affected by many biochemical and biophysical factors. Fortunately, it is possible to determine, or at least estimate, many of the most important parameters using a combination of optical methods and gas transient analyses. We describe here a computer-operated routine that has been developed to make detailed assessments of photosynthesis at a comprehensive level. The routine comprised the following measurements: steady-state light and CO2 response curves of net CO2 assimilation at 21 and 2 kPa O-2 ; transients from limiting to different saturating CO2 concentrations at 2 kPa O-2 ; post-illumination CO2 fixation transient; dark-light induction of O-2 evolution; O-2 yield from one saturating single-turnover flash; chlorophyll fluorescence F (0) , F (s) and F (m) during the light and CO2 response curves; leaf transmission at 820 nm (P700(+) ) during the light and CO2 response curves; post-illumination re-reduction time of P700(+) . The routine was executed on a two-channel fast-response gas exchange measurement system (A. Laisk and V. Oja: Dynamic Gas Exchange of Leaf Photosynthesis. CSIRO, Canberra, Australia). Thirty-six intrinsic characteristics of the photosynthetic machinery were derived, including quantum yield of CO2 fixation (Y (CO2) ), time constant of P700 re-reduction (tau '), relative optical cross-sections of PSII and PSI antennae (a (II) , a (I) ), PSII and PSI density per leaf area unit, plastoquinone pool, total mesophyll resistance, mesophyll diffusion resistance, V (m) , K (m) (CO2 ) and CO2 /O-2 specificity of Rubisco, RuBP pool at CO2 limitation (assimilatory charge). An example of the routine and calculations are shown for one leaf and data are presented for leaves of 8-year-old-trees of two birch clones growing in Suonenjoki Forest Research Station, Finland, during summer 2000. Parameters Y (CO2) , basic tau ', a (II) , a (I) , K (m) (CO2 ) and K (s) varied little in different leaves [relative standard deviation (RSD) < 7%], other parameters scattered widely (RSD typically 10-40%). It is concluded that the little scattered parameters are determined by basic physico-chemical properties of the photosynthetic machinery whereas the widely scattered parameters are adjusting to growth conditions. The proposed non-destructive routine is suitable for diagnosing the photosynthetic machinery of leaves and may be applied in plant ecophysiology and in genetic engineering of plants.