DISCRIMINATING AND CONTINUOUS MEASUREMENT OF PHOTOSYNTHESIS AND RESPIRATION BY MONITORING ¹³CO₂ AND ¹²CO₂ AS TRACERS

6CO₂+12H₂O &rarrow; C₆H₁₂O₆+6O₂+6H₂O

6¹³CO₂+12H₂O &rarrow; ¹³C₆H₁₂O₆+6O₂+6H₂O

6CO₂+12H₂¹⁸O &rarrow; C₆H₁₂O₆+6¹⁸O¹⁸O+6H₂O

Assuming that respiratory consumption of the new born carbon substrate fixed by photosynthesis is negligible during the measurement, the photosynthetic CO₂ consumption VPCO₂ and the respiratory CO₂ production VRCO₂ are measured according to the estimation (1) or (2),

(1) for closed method,

VPCO₂ = k(V₀ - V·.t) {F¹³CO₂+(F¹²CO₂/F¹³CO₂)F¹³CO₂}

VRCO₂ = k(V₀ - V·.t){F¹²CO₂-(F¹²CO₂/F¹³CO₂)F¹³CO₂}

(2) for open method,

VPCO₂ = k V_E { (F_I¹³CO₂-F_E¹³CO₂)+ (F¹²CO₂/F¹³CO₂)(F_I¹³CO₂-F_E¹³CO₂)}

VRCO₂ = k V_E { (F_I¹²CO₂-F_E¹²CO₂)-(F¹²CO₂/F¹³CO₂)(F_I¹³CO₂-F_E¹³CO₂)}

where V₀ is initial volume of growth chamber including attached flexible bag, F_ICO₂ is the inlet or initial gas concentration of CO₂ and F_ECO₂ is the ambient gas concentration of CO₂ in the chamber, V and V_E are the sampling rate of mass spectrometer and the ventilation rate of the growth chamber respectively, k is the STPD conversion factor = {273(P_B-PH2O)/760(273+tE)}, tE(°C) is the ambient gas temperature. In the closed method, the gas container of the growth chamber is circulated, resulting FECO₂ is varied according to the balance of consumption and production of CO₂, while in the open method V_E is controlled to keep FECO₂ at a constant value.

Both (1) and (2) methods were examined and evaluated on the measurements of komatsuna and maize.

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