Submitted to: Yields of Farmed Species: Constraints and Opportunities In the 21st Century
Publication Type: Book / Chapter
Publication Acceptance Date: 8/1/2004
Publication Date: 10/1/2005
Citation: Long, S.P., Zhu, X., Naidu, S.L., Raines, C.A., Ort, D.R. Limits to efficiencies of primary production, constraints and opportunities. In: Sylvester, B.R., Wiseman, J. Yields of Farmed Species: Constraints and Opportunities In the 21st Century. Nottingham, United Kingdom. Nottingham University Press. p. 319-333. Interpretive Summary: Does improving crop photosynthesis have a future role in extending the impressive rate of increase in the yield of farmed species that occurred in the last decades of the 20th Century? In this chapter we identify clear opportunities in which crop yield could be increased by manipulating or tailoring specific photosynthetic processes. It is likely many of these could be achieved within a 10 year time frame. The analysis conducted here demonstrates that engineering of specifically target aspects of the photosynthetic process would substantially increase carbon uptake by crop canopies in the field. This analysis is an important to understanding the molecular basis for the limitation of yield by photosynthesis in crop plants and of interest to agricultural researchers working to improve crop production.
Technical Abstract: Genetic improvements in the potential production of the major food crops over the past century were largely achieved through increases in the partitioning of biomass into grain (harvest index, ') and increase in the efficiency of interception of solar radiation ('i). There appears little further capacity for improvement here, leaving the efficiency of conversion of intercepted radiation into biomass ('c) the remaining option. By comparison to ' and 'i, maximum observed 'c appears low. Analysis of the maximum theoretical efficiencies of each step in energy transformation after radiation interception to biomass accumulation suggests that the potential efficiencies are only 0.051 and 0.060 for C3 and C4 crops, respectively. Conversion efficiency depends predominantly on photosynthesis. Elimination of photorespiration by elimination of the oxygenase activity of the primary carboxylase or by engineering C4 photosynthesis into C3 crops would increase the maximum 'c of C3 crops. But there are significant barriers to both. Decreased oxygenase activity carries the apparent penalty of decreased carboxylase capacity, while theoretical analysis shows that the improved efficiency of C4 photosynthesis cannot be achieved by simply introducing the C4 photosynthetic pathway into the mesophyll of C3 plants. Major factors preventing actual crop canopies achieving theoretical 'c are the slow reversal of photoprotection when leaves become shaded and insufficient capacity for leaves to make full use of the available radiation in direct sun. Increasing rates of recovery from photoprotection are shown to increase 'c by ca. 15%, and could be achieved both through exploiting natural variation and transgenic technology. Altered canopy architecture to improve the distribution of radiation between leaves could similarly exploit natural variation and increase 'c by ca. 10%. Substitution of current C3 Rubisco with algal or C4 crop forms could increase 'c by ca. 20%. Finally, over-expression of sedoheptulose-1:5-bisphosphatase or cytochrome b6/f complex could the rate regeneration of the primary CO2 acceptor, allowing an increase in 'c of ca. 10%. These latter four approaches exploit existing genes and technologies, and appear achievable routes to substantial increases in 'c on a 0 ' 10 year time horizon.