A. Theoretical and Actual Values
It is well known that according to the Calvin-Benson cycle in photosynthesis, assimilating one molecule
of CO 2 into carbohydrate requires 2NADPH and 3ATP. The production of 2NADPH is the result of trans-
porting four electrons from 2H 2 O to 2NADPalong an electron transport chain (Z-scheme). Because the
chain includes two photosystems in series, two photons are needed for one electron transport. Thus, at
least eight photons are required for the production of 2NADPH. Therefore, the maximal or theoretical
quantum yield for photosynthetic carbon assimilation is 0.125 mole CO 2 /mole photons. With respect to
the amount of ATP produced through PSP coupled with the photosynthetic electron transport mentioned
above, i.e., the number of molecules of ATP produced by coupling with the transport of two electrons or
the evolution of one-half molecule O 2 , or P/O ratio, there are several values, namely 1, 1.33, and 2 in dif-
ferent laboratories [58]. If P/O is lower than 1.5 or part of the ATP produced by PSP is used in the biosyn-
thesis of compounds other than carbohydrates, cyclic or pseudocyclic PSP is required to meet the demand
of carbohydrate synthesis for ATP. In such cases, the quantum requirement (the reciprocal of quantum
yield) must be higher than 8. An accurate value of the minimum quantum requirement of photosynthetic
carbon assimilation, in fact, is still uncertain. Values in a range of 8–12 are acceptable to most scientists
in the area [59]. The uncertainty may be related to the complex regulatory mechanisms of photosynthesis
and the variability of environmental conditions.
It should be noted that the values of 8–12 for minimum quantum requirements are obtained under the
most suitable conditions. If an inevitable loss such as photorespiration occurs, the value will be about 17
[60]. However, such a value is seldom obtained under field conditions. It is often much higher than 25
even if under normal conditions without any environmental stress [61]. The causes leading to the differ-
ence between theoretical and actual values of the quantum requirement are worth studying.
B. Factors Affecting Quantum Yield
- Environmental Factors
Emerson and Lewis [62] showed that the values of quantum yield were related to the quality of light. A
high quantum yield was measured at red light around 680 nm. The quantum yields of sun and shade leaves
grown under different light intensities were similar, although there was a significant difference in light-
saturated photosynthetic rate between them [63,64]. At 21% O 2 and a temperature range of 15–35°C the
quantum yield decreased gradually with temperature increase in C 3 plants but not in C 4 plants [60,65].
Water deficiency and excessive water or flooding could lead to a decline in quantum yield [66,67]. After
several rainy days, the photosynthetic quantum efficiency became lower in spinach leaves [68]. The rea-
son may be that the reduction of NADPis severely hindered in swollen chloroplasts under hypotonic
conditions [69]. Decreasing O 2 concentration or increasing CO 2 concentration in air could increase quan-
tum yield in C 3 plants but not in C 4 plants [60,70]. We found that reduced atmospheric pressure had an
adverse effect on photosynthetic quantum efficiency [71]. The difference in apparent quantum yield cal-
culated on the basis of incident photon flux density under different nitrogen nutrition levels could be at-
tributed to decreased light absorption induced by a low nitrogen level [72]. Phosphate deficiency in nu-
trient solution could lead to a declined quantum yield in spinach leaves [73]. This may be due to decreased
excitation energy transport from antenna pigments to PSII reaction centers and enhanced excitation en-
ergy dissipation as heat under phosphate deficiency conditions [74].
- Plant Factors
Among all internal factors, photorespiration has the most significant effect on quantum yield. The effects
of air temperature and CO 2 or O 2 concentration on quantum yield mentioned earlier, in fact, are related
to the changes in photorespiratory rate caused by these factors. In normal air and at 20–25°C, the quan-
tum yields of C 3 and C 4 plants were similar. However, when the air temperature was over 30°C, the quan-
tum yield in C 4 plants was slightly higher than that in C 3 plants [75]. When photorespiration was inhib-
ited by high CO 2 and/or low O 2 , C 4 plants had about 30% lower quantum yields than C 3 plants because
they used two additional ATP molecules in the C 4 pathway for fixation of one molecule of CO 2 to form
carbohydrate [76]. So C 4 plants are not more efficient than C 3 plants in weak light. Quantum yield was
lower in younger leaves than in mature leaves [77,78]. This may be because more ATP is used in the
biosynthesis of components other than carbohydrates in younger leaves growing luxuriantly. The chloro-
826 XU AND SHEN