Photosynthesis quantum yield

The enhancement effect" (Emerson s 2nd effect), i. e. the increase of photosynthesis quantum yield in the far-red spectral region at simultaneous illumination of chlorophyll-containing cells with shorter-wave light, has been drawing attention of researchers for almost 30 years. 

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. ... 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

A potentially more serious problem than that of low efficiency is the requirement that the photochemical absorber operate without any significant side reactions. For example, if the quantum yield for side reactions were 1%, then after only 100 cycles the concentration of the absorber would have decreased to 37% of its original concentration. It is interesting that in photosynthesis each chlorophyll molecule processes at least 10 photons in its lifetime in a leaf. This means that the quantum yield for reactions leading to the degradation of chlorophyll must be less than 10-5 ... 

Measurement of Quantum Yield, Quantum Requirement, and Energetic Efficiency of the 02-Evolving System of Photosynthesis by a Simple Dye Reaction 127... 

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. 

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). 

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)...

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... 

Determination of such a quantum yield indicates how efficiently the photochemistry is performed versus all other deexcitation processes. For instance, the photochemical reactions of photosynthesis can be decreased by environmental stresses such as freezing temperatures, excessive light, and drought, leading to a decrease in < Photochem (Eq. 5.10), an important assessment in both the laboratory and in ecophysiological field studies. 

Figure 8-19. Idealized hyperbolic relationship between the photosynthetic photon flux incident on the upper leaf surface and the net C02 uptake rate for a C3 plant. The intercept on the ordinate (y-axis) indicates the net COz flux by respiration in the dark (-1 pmol m-2 s 1), the intercept on the dashed line indicates the light compensation point (a PPF of 15 pmol m 2s l), the essentially linear initial slope (37co2 ppf) indicates the quantum yield (Eq. 4.16) for photosynthesis [(5 - 0 pmol m 2 s l)/(115 -15 pmol m-2 s l) = 0.05 mol C02/mol PPF], and the maximum Jco2reached asymptotically at high PPF indicates the light-saturated net C02 uptake rate (about 12 (xrnol m-2 s l often designated AmaK or Amax). Here the quantum yield is based on incident photons, but more appropriately it should be based on absorbed photons.

At 30°C and for an absorbed PPF up to about 100 jimol m-2 s-1, leaves of C3 and C4 plants can have a similar quantum yield (approximately 0.053 mol CCh/mol photons at an of 325 pmol mol-1 Fig. 8-20 Ehleringer and Bjorkman, 1977 see Chapter 4, Section 4.4B for a definition of quantum yield). As the temperature is raised, however, photorespiration increases relative to photosynthesis, so the quantum yield declines for C3 plants but is essentially unchanged for C4 plants. On the other hand, lowering the ambient O2 level raises the quantum yield for C3 (photorespiring) plants because the oxygenase activity of Rubisco (see Fig. 8-13) is then suppressed such changes have little effect on C4 plants until the 02 level falls below about 2%, where mitochondrial respiration is affected. 

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). 

As anticipated, some non-essential elements may respond to biological redox gradients in a quite similar manner as Fe does because there need not be any direct transformation during photosynthesis europium which both undergoes ready, high-quantum yield photoreductions when irradiated (X > 340 nm) together with alcohols, sugars or amino acids (Horvath and Stevenson 1992, see below), and the redox potential of... 

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