Efficiencies achieved in wild and cultivated crops

The time-average energy-storage efficiency of green-plant photosynthesis is much lower than the instantaneous maximum values of -5% calculated in the previous section, for a number of obvious reasons. For example, the growing season is limited, the plant canopy does not intercept all incident sunlight, light levels may be too low or too high for maximum photosynthetic efficiency, and plant growth may be inhibited by factors such as water or thermal stress.
 

The global energy-storage efficiency of photosynthesis can be calculated from knowledge of net primary production (NPP), the mass of carbon fixed annually by photosynthesis. Global NPP is -100 GtC yr-I, about half on land and half in the oceans (see Section 6.5). The molar free energy of the photosynthesis reaction (eq. 1.6) is 496 kJ mol-I, which corresponds to a specific energy-storage capacity of 41.3 kJ g-’ fixed carbon. Global NPP of 100 GtC yr-’ thus corresponds to -4 x 10” J of chemical energy stored in photosynthetic biomass per year. J yr-’. Thus the net efficiency of photosynthesis averaged over Earth’s surface (land and oceans) is -0.15%. About half of the incoming solar energy is in the PAR range of 360-720 nm, so the energy efficiency of PAR utilisation is -0.3%. Modest as these efficiencies are, the amount of energy stored annually by photosynthesis is about ten times greater than current world energy consumption.
 

Agriculture-the cultivation of plants for food-arose in the Fertile Crescent about 10,500 years ago. Traditional methods of plant breeding by the selection and crossing of species with favourable characteristics (for example, hardiness and plant size) have hugely improved food crop yields since then. Energy -storage efficiencies of 0.5-1.0% on an annual basis are typical in modern food crops, and short-term yields can be as high as -4%. Cd plants, with their modified C02 fixation pathway, have considerably higher efficiencies than C3 plants, especially in tropical and subtropical areas where their growth rate is less likely to saturate under high light levels. In future, global warming may extend the geographic range of some crops and trees to higher latitudes, and increased CO1 levels may increase growth rates.

It is not widely appreciated that traditional plant-breeding techniques were effectively an imprecise form of genetic engineering: plants that have favourable characteristics and are therefore selected for breeding have favourable genotypes, which are therefore selectively replicated in future generations of plants. About half of past improvements in yields of rice, wheat and maize is due to genetic inputs. Recently, plant-breeding methods have been extended by two new ‘genetic’ techniques: tissue culture, which allows the crossing of favourable genotypes at cellular level to form new cultivars, and genetic modification, which involves the incorporation of individual genes directly into plant genomes. Despite the current outcry about GMOs (genetically modified organisms), a significant part of future improvement is likely to come from transgenically improved plants. Denis Murphy describes the enormous potential of ’agbiotech’-the application of genetic techniques to improve food and non-food crop traits and yields in Chapter 13.