Gross efficiency ignoring respiration

Green plants contain carbon in a reduced state, mainly as carbohydrates. The chemical energy stored in these compounds is released when they are metabolised in the living plant, or when the plant biomass is burned or otherwise oxidised to C02 and H2O. Since plant biomass is mainly comprised of D-glucose polymers, the energy

released when biomass is oxidised to C02 and H20 is roughly equal to the enthalpy of synthesis of D-glucose from COz and H20. Under normal atmospheric conditions, we can write this reaction as

where C6H1206 is D-glucose. For this reaction, AH = 467 kJ and AG = 496 kJ at 298 K; the standard values are AH0 = 467 kJ and AGO = 480 kJ (Bolton, 1979). Neglecting for the moment the inevitable loss of some biomass in a living plant by the process of respiration, we can define the photosynthetic energy (enthalpy) storage efficiency %s of a green plant, acting as a photoconverter of solar energy, as

To calculate 77ps for sunlight of a given spectral distribution, Schneider (1973) and Bolton (1979) rewrote eq. 1.7 as

where N A is the Avogrado constant and, for the wavelength band h to h + dh, j: is the incident solar spectral photon flux (photons m-2 s-' nm-I), E: (W m-2 nm-l) is the incident solar spectral irradiance ah is the spectral absorptivity (fraction of light absorbed by the plant), 41. is the quantum yield for the production of 0 2 or consumption of COz, AH is the enthalpy of the photosynthesis reaction, and h,,,in and A,,, are the minimum and maximum wavelengths that effect the reaction,. Using experimental data at 10 nm intervals for a k , j ; , E: and 4, Bolton (1979) calculated rps from eq. 1.8 and obtained a value of (9.2 k 0.8)%. This is the upper bound on the gross efficiency of energy storage in a healthy growing leaf, ignoring respiration. In using experimental values of ah and 4, it takes account of the dip in ah values in the green and 4 values towards the red. If 4 were to maintain the 'ideal' value of 0.125 (or 8 photons per O2 molecule evolved) from 360 nm to 700 nm and then drop off as the experimental values from 700 nm to 720 nm, the gross efficiency would rise even higher, to 13.3%.