For simplicity, among the three major C4 subtypes, we
will consider only the NADP–Malic Enzyme (ME) subtype,
which includes all the major C4 crops, such as
maize, sorghum, and sugar cane, and is considered to
be the most efficient [7]. Here, five ATP and two
NADPH are required to assimilate one CO2 since, in
addition to the energy requirement of the Calvin cycle,
two additional ATP are required for the phosphorylation
of pyruvate to phosphoenol pyruvate. In effect these two
extra ATP represent a light-energy driven pump, which
transports CO2 from the outer mesophyll and concentrates
it at the site of Rubisco in the inner photosynthetic
bundle sheath. The 10-fold higher concentration of
CO2 around Rubisco in C4 leaves, eliminates, or minimizes
photorespiration [8]. In C3 photosynthesis, we have
shown that translocation of protons in the reduction of the
two NADPH required to fix one CO2 is just sufficient to
generate the three ATP also required. In C4, requiring
two additional ATP, then even at maximum efficiency,
proton translocation resulting from the reduction of
NADPH in noncyclic electron transport will be
inadequate. Cyclic electron transport returns electrons
from PSI back to the cytochrome b6/f complex resulting in
the translocation of two protons per photon into the
thylakoid lumen [9
,10]. In order for the cyclic flow to
provide the additional 8 protons needed to synthesize the
2 additional ATP, absorption of 4 additional photons at
PSI for a minimum total of 12 will be required per CO2
assimilated in C4 photosynthesis. Following our reasoning
above for C3 photosynthesis, the minimum energy
loss in electron transport and synthesis of carbohydrate in
C4 photosynthesis, between reaction center primary
photochemistry and carbohydrate production is
1 (477/2052); this amounts to a loss of 28.7% of the
energy contained in the original incident solar radiation
(Figure 2). Therefore, the maximal energy conversion
efficiency (ec) of C4 photosynthesis, prior to respiration, is
8.5%