Quantum Yield of Photosynthesis

The question here is in regard to the number of light quanta which are needed to form a molecule of O2. From our scheme for noncyclic electron 

---

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

Krause, G.H. Laasch, H. (1987). Energy-dependent chlorophyll fluorescence quenching in chloroplasts correlated with quantum yield of photosynthesis. Zeitschrift fiir Naturforschung, 42, 581. ... 

Chlorophyll a fluorescence was measured with a dual-modulated fluorometer (Photosystem Instruments, Trtilek et al. 1997). Minimum (Fa) and maximum fluorescence (Fm) was recorded after dark adaptation (5 min at 4°C, sufficient to attain stabilization of the fluorescence signal for all light regimes). The maximum quantum yield of photosynthesis FvIFm was calculated as (Fm-F0)/Fm (Krause and Weis 1991). 

The maximum quantum yield of photosynthesis (FvIFm) varied with daily dynamics in light intensity in both the iron-limited and iron-replete cells (Fig. 1A). Maximum values were recorded in the morning, followed by decreasing values towards the middle of the light period. In replete cells, the midday depression was almost 50% of the morning values FvIFm decreased by about 30% under iron limitation. The FJFm recovered towards the end of... 

The result of such a process would be that two electrons are cycled twice through the PQ, and the ratio of H /e between PS II and PS I would be higher than one. This, if definitively confirmed, would be of great importance from the point of view of understanding the coupling of electron transport to the synthesis of ATP, and of the quantum yield of photosynthesis (see discussion under photophosphorylation). 

McCree KJ The action spectrum, absorptance and quantum yield of photosynthesis in crop plants, Agric Meteorol 9(3) 191—216, 1971. 

Thus, electron tunneling appears to be capable of providing simultaneously conditions for a high quantum yield of charge separation and a high efficiency of the multistage process of photosynthesis as a whole. 

Green, R. M., Z. S. Kolber, D. G. Swift, N. W. Tindale, and P. G. Falkowski. 1994. Physiological limitation of phytoplankton photosynthesis in the eastern equatorial Pacific determined from variability in the quantum yield of fluorescence. Limnology and Oceanography 39 1061-1074. 

Fig. 1 Fluorescence characteristics of photosynthesis determined over the day for iron-limited (open symbols) and iron-replete cells (closed symbols), plotted alongside irradiance (photons m 2 s dashed line). Fluorescence dynamics are expressed in percentages, taking the maximum morning values as reference (i.e. 100% n = 3). (A) Maximum quantum yield of fluorescence (Fv/Fm). (B) Minimum fluorescence (/< ). (C) Maximum fluorescence (Fm)...

Fig. 14. Measured quantum yield of oxygen evolution in Chlorella plotted against wavelength of excitation light with out and with supplementary light at 650 nm. Figure source Emerson, Chalmers and Cederstrand (1957) Some factors influencing the long-wave limit of photosynthesis. Proc Nat Acad Sci, USA 43 137.

R Emerson and CM Lewis (1943) The dependence of the quantum yield of Chlorella photosynthesis on wave length of light. Am J Bot 30 165-178... 

Because of the short lifetime, the quantum yield of fluorescence emission from this state is very low (< 10" ) in spite of the strongly allowed nature of the transition and its correspondingly large radiative rate constant ( 10 s )- Surprisingly, this state is important as an energy donor in photosynthesis (Trautman et al., 1990 Ricci et al., 1996). 

---