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

2002, Plant, Cell and Environment

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 CO 2 response curves of net CO 2 assimilation at 21 and 2 kPa O 2 ; transients from limiting to different saturating CO 2 concentrations at 2 kPa O 2 ; post-illumination CO 2 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 CO 2 response curves; leaf transmission at 820 nm (P700 + ) during the light and CO 2 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 CO 2 fixation ( Y CO2 ), time constant of P700 re-reduction ( τ τ τ 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 (CO 2 ) and CO 2 /O 2 specificity of Rubisco, RuBP pool at CO 2 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 τ τ τ ′ ′ ′ ′ , a II , a I , K m (CO 2 ) 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 condi-