PHOTOSYNTHESIS - Energy in Nature and Society - page 68

green light, as well as minor forms of inactive absorption
of blue and red, total at least 10%, reducing the photo-
synthetic efficiency maximum to around 11%. An ideal
leaf exposed at a 90 angle to direct sunlight would then
reduce about 250 mg CO 2 /dm 2 h and synthesize 170
mg/dm 2 of new carbohydrates, converting the radiant
energy to chemical bonds with a power density of 8
mW/m 2 . With an average leaf weight of 2.85 g/dm 2 ,
the metabolic intensity would be about 280 mW/g.
During a sunny day this leaf, absorbing the total of 250
kJ/dm 2 , would produce about 1.7 g of new photosyn-
thate. A field of such ideal leaves would fix daily 1.7 t/
ha of phytomass, and where growth could continue for
the whole year, 620 t/ha would be added.
Actual short-term increments of new phytomass are at
best 50%, more likely just 33% of these rates. The top
seasonal or annual additions are between 20% and 25%
of the ideal rates, and long-term, large-scale averages are
merely 10% and all the way down to just 2% of the best
hypothetical performance. The two main reasons for
these disparities are the respiration costs and the inevit-
able losses that go with rapid rates of photosynthetic
reactions. In order to conserve as much light as possible
during the limited hours of intensive insolation, the rates
must be quite fast, but this rapidity results in two kinds of
considerable inefficiencies. Unless the plant's enzymes
can keep up with the radiation flux coming into the
excited pigments, the absorbed energy will be reradiated
as heat. Utilization must be immediate because the chlo-
rophyll molecules cannot store sunlight. Only at very low
light intensities, when radiation would be the only factor
limiting the rate of the terrestrial photosynthesis, is there
such a perfect match.
Rapid photosynthesis maximizes growth rates and
hence improves the chances of early survival and compet-

itive maturation, but it is paid for by large irreversible
losses that lower the photosynthetic efficiency to roughly
8%-9%. Fixed carbon dissipated in metabolic processes
and in the maintenance of the photosynthetic system
and its supporting structures is highly variable, ranging
from less than 20% in high-yielding crops to virtually
100% in old-growth forests. With respiratory losses at
40%-50%, the peak plant growth efficiencies would be
around 5%. Only at this point do theoretical calculations
and actual performance meet; the highest recorded val-
ues of net photosynthesis (for highly productive plants
under optimum conditions during short periods of time)
are between 4% and 5% (see fig. 1.7). For most plants,
even the 4% efficiency is impossible because their poten-
tial performance is limited by environmental factors

(Nemani et al. 2003).
The two most widespread limiting factors in
carbon fixation are the availability of water, dominant
on some 40% of land, and temperature, the strongest
productivity-limiting factor on a third of land (fig. 3.4).
Photosynthesis is impossible without an extremely lop-
sided trade-off between CO 2 and H 2 O. The difference
between internal (inside the leaves) and external water
vapor is 2 OM higher than the difference between exter-
nal and internal CO 2 levels. As a result, C 3 plants need
900-1200 mol, and some up to 4000 mol of H 2 O, to
fix 1 mol of CO 2 , whereas C 4 plants can manage with
400-500 mol H 2 O/mol of CO 2 fixed, and the rates in
CAM species are as low 50, with typical values between
70-150 mol H 2 O/mol CO 2 , losses of 1 OM lower
than in C 3 plants. As the kinetic energies of enzyme mol-
ecules and substrates increase with rising temperatures,
thermochemical reactions proceed faster, and conversion
efficiencies eventually reach maximum rates and then
decline as higher temperatures denature the enzymes.