phyll content of leaves affects only the apparent quantum yield calculated on the basis of the quantity of
light incident on the leaf surface but not that based on the quantity of light absorbed by leaves [79]. When
ATP was insufficient or NADPH was excessive, cyclic PSP would be enhanced, leading to a decline in
quantum yield [80]. The dark respiratory rate does not affect the quantum yield under conditions in which
dark respiratory rate is constant.
C. Diurnal Variation of Quantum Yield
In field studies we found that the apparent quantum yield of photosynthetic carbon assimilation often dis-
played a significant midday decline in many C 3 plants such as soybean and wheat but not in C 4 plants such
as maize and sorghum on clear days [81]. It was deduced that photoinhibition may be a cause of the mid-
day decline of the photosynthetic efficiency [61]. The molecular mechanism of photoinhibition is still not
fully clear. For more than a decade photoinhibition has been considered almost synonymous with photo-
damage to the photosynthetic apparatus [82], mainly the loss of D1 protein, a central component of the
PSII reaction center complex. However, no evident change in D1 protein content in the leaves of sweet
vibrium, wheat, and soybean was observed when photoinhibition occurred in strong light [83–85]. These
results indicate that under normal conditions without other environmental stress photoinhibition is a re-
flection of enhanced operation of protective mechanisms rather than a result of damage to the photosyn-
thetic apparatus [86]. DTT (dithiothreitol), an inhibitor of the xanthophyll cycle, could exacerbate pho-
toinhibition and result in a substantial loss of D1 protein in wheat leaves after exposure to midday strong
light, indicating that the xanthophyll cycle–dependent heat dissipation plays an important protective role
against photodamage to the photosynthetic apparatus in strong light [84]. Nevertheless, our studies also
demonstrated that the xanthophyll cycle–dependent heat dissipation is a predominantly protective mech-
anism only in some plant species such as wheat and barley but not in other plant species such as soybean
and cotton [85,87,88]. In soybean leaves, the predominantly protective mechanisms is likely the re-
versibly inactivated PSII reaction center–dependent heat dissipation [85,87,88]. The mechanisms of re-
versible inactivation and heat dissipation of PSII reaction centers are still not clear. There have been ex-
perimental results showing that the reversible inactivation is related to the dissociation of light-harvesting
complex II (LHCII) from the PSII reaction center complex [89].
In addition to photoinhibition, enhanced photorespiration is another cause of the midday decline in
the photosynthetic efficiency of C 3 plants [90]. For a long time photorespiration has been considered a
wasteful process. Many efforts have been made to eliminate it, but no success has been reported. Exten-
sive screening programs involving several species (wheat, barley, oats, soybean, potato, tall fescue) failed
to identify genotypes with a low CO 2 compensation point [91]. Attempts to select C 3 plants with low rates
of photorespiration and high rates of net photosynthesis have had little success. Some mutant genotypes
of tobacco with increased productivity have been selected at low CO 2 concentrations, but this increased
productivity is related to a greater leaf area per plant and higher photosynthetic rates rather than reduced
photorespiratory rate or CO 2 compensation point or improved Rubisco properties [92]. It is likely that it
is not simply a wasteful process but a protective one for plants, at least under some stress conditions. Our
study has demonstrated that it can protect the photosynthetic apparatus against photodamage through ac-
celerating phosphate recycling during photosynthesis [93].
IV. GREEN REVOLUTION
As mentioned before, the economic yield of crop is a function of photosynthetic production, respiration
consumption, and harvest index. Thus, it is related not only to leaf photosynthetic performance but also
to plant type and canopy structure. Dwarfing the stem of a crop may lead to higher yield through in-
creasing the harvest index. Erect leaves are also favorable for an increase in crop yield because the leaves
of middle and bottom layers in the canopy may receive more light energy, thus improving the light use
efficiency of the canopy [94]. The breeding of high-yielding varieties with dwarf stems and erect leaves
brought about a great revolution in agriculture.
A. First Green Revolution
In the 1950s and 1960s, some agricultural scientists developed a package of high-yielding crop varieties
and agricultural management techniques. The package brought about an unprecedented boom (more than
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