The surface of the Earth is 510,072,000 km2 which on average receives 170 Wm-2 which is equal to 7500 times the world’s energy use of 450 EJ with a total of

3,375,0 EJ. In temperate zones, the amount of energy reaching the Earth’s surface is about 1.3 kW m-1 but only 5% of this energy is converted into carbohydrates by photosynthesis (Fig. 8.29). Any improvement in the efficiency of photosynthesis would have a considerable effect on crop and biofuel production. The rate-limiting steps in the fixation of carbon dioxide by photosynthesis are ribulose-1,5-bisphosphate car­boxylase (rubisco), regeneration of ribulose bisphosphate and the metabolism of triose phosphates. In addition, photosynthetic organisms stop growing and fixing carbon dioxide at light intensities lower than typical levels at midday in equatorial regions. The typical light intensity is about 2000 pmole m-2 s-1, whereas light saturation of photosynthesis occurs at 200 pmole m-2 s-1. Above the saturating light intensity inhibi­tion of photosynthesis, known as photoinhibition, occurs (Fig. 8.30). Improvements in the three stages of carbon fixation, the increase in the light saturation values and

f 196

Reflected and ^ transmitted 8%

Metabolism 19%

Fig. 8.29. The amount of energy converted into carbohydrates during photosynthesis.

reduction in photoinhibition would all be of value in increasing crop yields. During photosynthesis, leaves must also dissipate heat and improvements in heat tolerance would also be useful.