The energy losses so far computed have been derived
from the known, well-understood and conserved mechanisms
of photosynthesis. By contrast there is no quantitatively
defined mechanism of linkage between
photosynthetic rate and mitochondrial respiration. This
final loss (Figure 2) can therefore only be an estimate
based on empirical measurements. Measured ratios of
respiratory CO2 loss as a fraction of photosynthetic CO2
uptake for major crops vary from 30% to 60% [18,19]. It
has been shown that there is heritable variation in this
ratio within crop species, and selection for a lower ratio in
the case of ryegrass (Lolium perenne) resulted apparently in
higher yielding lines [20]. We assume 30% here as the
minimum respiratory loss that might be achieved without
otherwise adversely affecting plant growth. This
represents a loss of the original incident solar energy of
1.9% (C3) and 2.5% (C4), with the result that the maximum
conversion efficiencies of solar radiation into biomass
ec are 4.6% (C3) and 6.0% (C4) at 30 8C.