Photosynthesis is the source of our food and fiber. Increasing
world population, economic development, and diminishing
land resources forecast that a doubling of productivity is critical
in meeting agricultural demand before the end of this century. A
starting point for evaluating the global potential to meet this
goal is establishing the maximum efficiency of photosynthetic
solar energy conversion. The potential efficiency of each step
of the photosynthetic process from light capture to
carbohydrate synthesis is examined. This reveals the maximum
conversion efficiency of solar energy to biomass is 4.6% for C3
photosynthesis at 30 8C and today’s 380 ppm atmospheric
[CO2], but 6% for C4 photosynthesis. This advantage over C3
will disappear as atmospheric [CO2] nears 700 ppm.