Exploiting the Productivity of C4 Photosynthesis in Cool Temperate Climates: Mechanisms and Thresholds of Cold Tolerance in Miscanthus, Saccharum, and Spartina pectinat

Perennial grasses that use the C4 photosynthetic pathway are the best choice for terrestrial bioenergy and can help mitigate climate change. Much of the marginal land available for bioenergy occurs in cool temperate climates at high latitudes. To be productive in cool temperate climates, C4 perennial grasses must successfully overwinter the first growing season and produce leaves that tolerate spring chilling and frosts to harness the long photoperiods. Although there is nothing inherently cold sensitive about the C4 pathway, cold tolerant C4 grasses are rare and the additional C4 cycle and architecture introduce more potentially cold sensitive sites. In addition, the C4 cycle imposes a greater restriction in demand for energy and may predispose C 4 grasses to chronic photoinhibition. Miscanthus x giganteus is a highly productive C4 perennial grass in cool temperate climates that is a model of photosynthetic chilling tolerance. Miscanthus is closely related to Saccharum a C4 genus that includes commercial sugarcane and energycane. Although M. x giganteus is chilling tolerant, it shows little tolerance of subzero temperatures with reports of winterkill in some cold climates. Spartina pectinata is a C4 perennial grass native to North America with a distribution up to 61°N latitude. The purpose of this thesis is to compare cold tolerance thresholds across these C4 perennial grasses and test hypotheses about the underlying physiology. First, this thesis demonstrates that M. x giganteus and another triploid Miscanthus hybrid have superior photosynthetic chilling tolerance over other Miscanthus hybrids and Saccharum genotypes. Across these genotypes, chronic photoinhibition or photoinactivation is closely associated with chilling sensitivity of net CO2 assimilation rate. Next, a more detailed investigation comparing M. x giganteus with the most chilling sensitive Miscanthus hybrid reveals Rubisco is the predominant limitation on net CO2 assimilation rate at cooler temperatures. Finally, this thesis compares the overwintering capacity, seasonal rhizome freezing tolerance, and leaf frost tolerance of S. pectinata and M. x giganteus in a first year field plot. Spartina pectinata shows greater overwinter survival and a fall/winter lethal temperature threshold of -24°C compared to -4°C for M. x giganteus, as well as greater spring leaf frost tolerance.