Not all plants are equal in the efficiency with which they convert sunlight energy into biomass and ultimately provide food for people. The most efficient plants known are C4 plants, which use carbon dioxide (CO2) and sunlight energy to make four carbon compounds as the initial product of photosynthesis. The majority of the world's plants are C3 plants, which when grown under ideal conditions are about 40% less efficient in their use of sunlight than C4 plants. C4 plants such as tropical grasses and the crop species corn, sorghum and sugarcane remain largely confined to warm temperate environments. These observations have led to the suggestion that the higher efficiency of C4 photosynthesis was inherently limited to warm conditions. Recent work has shown that some Miscanthus species not only survive in cold climates, but that they achieve photosynthetic rates and efficiencies of conversion of light into biomass that exceed those of C3 species. Yet these species are exclusively C4 and are so closely related to tropical sugarcane that they can form fertile hybrids. From the center of diversity of sugarcane, forms classified as Miscanthus have radiated out into high-altitude cold grassland habitats in S.E. Asia; forming a likely progression of cold tolerance within the C4 mechanism. This very large range of cold tolerance within closely related taxa provides a unique opportunity to discover how cold tolerance has evolved within C4 photosynthesis. Taxa from this Miscanthus-Saccharum complex occurring in a range of habitats from tropical to cold temperate, will be characterized for their ability to develop photosynthetically competent leaves at low temperature and to photosynthesize at low temperature. This research will provide a basis for testing a range of competing, but not necessarily exclusive, theories on what normally limits C4 photosynthesis at low temperature. Theories include: that the catalytic capacity of the enzyme Rubisco at low temperature is too low, that the enzyme pyruvate orthophosphate dikinase (PPDK) is unstable at low temperature, and that there is inadequate capacity for protection against inhibition of photosynthesis by excess light. These will be assessed by in vivo and in vitro measurement of activity and quantities, using classical physiological and biochemistry techniques, and at the molecular level investigating gene sequence changes and protein stability, including association with chaperone proteins. Regression analysis will be used to assess which of these characters explains most variation in low temperature tolerance across the taxa of this complex.
This work will also have broader impacts to both educational activities and agriculture. An improved understanding of the physiological and molecular mechanisms associated with the adaptation of C4 photosynthesis to cold conditions may offer novel strategies to extending the growing season or cultivation environments of C4 crops such as corn, sorghum and sugarcane. This project will also emphasize research training for students from the high school, undergraduate and graduate levels, including internship opportunities for minority students that integrate plant biology and new approaches to crop production. Furthermore, research results will be communicated to the public at large through field tours and educational presentations that emphasize environmental change and its impacts to agriculture.
