Enhancing Photosynthesis

Abstract

The industrialization of cereal production in India about 50 years ago was associated with dramatic increases in yields worldwide. As a consequence, there was a reduction in famine. The worldwide impact was called the “Green Revolution,” for which Norman Borlaug was awarded the Nobel Peace Prize in 1970. The introduction of dwarfing genes into cereals enabled the use of greater amounts of fertilizer, which resulted in substantial yield increases. The dwarf stature also led to a larger fraction of crop biomass being allocated to grain, the harvested product. A concerted effort by plant breeders and agronomists has driven continuous improvement in cereal yields. World population has grown from 3 billion in 1960 to nearly 7 billion in 2010, increasing demand for food, animal feed, and bioenergy. With world population projected to pass 9 billion by 2050, a further increase in yield of at least 28% is necessary. Crops will also have to be adjusted for climate change, and production may be limited by the supply of irrigation water and loss of arable land. Reducing the gap between average farm yield and potential yield (that obtained with best management practice in the absence of water, nutrient, pest, and disease limitations) will enable yield increases to continue for some time. However, there is a growing realization that to sustain increases in potential yield, biomass production needs to be increased. Biomass is produced by photosynthesis. Once a crop has been adapted to fully exploit the interception of sunlight and allocate the maximum amount of biomass to grain, further increases in yield will require increasing biomass by enhancing photosynthesis. The year 2011 marks the 50th anniversary for the Nobel Prize in Chemistry awarded to Melvin Calvin for the elucidation of the photosynthetic carbon reduction cycle. A large body of knowledge has been accumulated about photosynthesis, which can be used to formulate strategies for enhancing photosynthesis, the concern of this Focus Issue. Fifteen Update articles in this issue target basic research covering diverse aspects of photosynthesis. These range from initial light capture, electron transport and photophosphorylation, carbon dioxide fixation, and metabolic pathways, to integration among organelles and transport from the subcellular to the whole-plant level. One aspect of enhancing photosynthesis will be the need to adjust other properties, such as leaf anatomy, to ensure that the biochemistry works efficiently. It is also important to consider the performance of photosynthesis at the level of the crop canopy. The Update articles identify many opportunities and targets where photosynthesis could be enhanced. Perhaps the clearest candidate for improvement is Rubisco, the enzyme used by all plants to ultimately fix carbon dioxide. This enzyme catalyzes both carboxylation and oxygenation reactions with the five-carbon sugar ribulose bisphosphate. The oxygenase reaction leads to the release of one-half a carbon dioxide and consumes ATP and NADPH, which could otherwise be used productively to fix more carbon. Photosynthesis can be increased by raising the carbon dioxide concentration surrounding Rubisco, either by CO2-concentrating mechanisms used by C4 plants, cyanobacteria, and algae, or by enriching the atmosphere, which is happening as a result of the human use of fossil fuels. There is abundant evidence demonstrating that photosynthetic rates can be increased in the short term in response to increasing atmospheric CO2 concentrations. Plant growth depends on how newly fixed carbon is invested. To translate enhanced photosynthesis into greater plant growth and crop yield, a great deal of research is required, but this should be attainable if sufficient resources are invested to support the research effort by the plant science research community. Recent increases in yield differ widely among crops; to a large extent, this reflects how much money has been invested into each crop. We thank the contributors for their insights and hope that this Focus Issue draws attention to the many possibilities that should be explored.