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 O₂; transients from limiting to different saturating CO₂ concentrations at 2 kPa O₂; post‐illumination CO₂ fixation transient; dark–light induction of O₂ evolution; O₂ yield from one saturating single‐turnover flash; chlorophyll fluorescence F₀, 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 (τ′), 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/O₂ 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 τ′, 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.