Can the exceptional chilling tolerance of C4 photosynthesis found in Miscanthus x giganteus be exceeded? Screening of a novel Miscanthus Japanese germplasm collection.

* Background and Aims A clone of the hybrid perennial C4 grass Miscanthus x giganteus (Mxg) is known for achieving exceptionally high rates of leaf CO₂ uptake during chilling. This is a requisite of success in the early spring, as is the ability of the leaves to survive occasional frosts. The aim of this study was to search for genotypes with greater potential than Mxg for photosynthesis and frost survival under these conditions. * Methods A total of 864 accessions representing 164 local populations of M. sacchariflorus (Msa), M. sinensis (Msi) and M. tinctorius (Mti) collected across Japan were studied. Accessions whose leaves survived a natural late frost in the field were screened for high maximum photosystem II efficiency (F_v/F_m) following chilling weather, as an indicator of their capacity for light-limited photosynthesis. Those showing the highest F_v/F_m were transferred to a high-light-controlled environment and maintained at chilling temperatures, where they were further screened for their capacities for high-light-limited and light-saturated leaf uptake of CO₂ (Φ_CO2,max and A_sat, respectively). * Key Results For the first time, relatives of Mxg with significantly superior capacities for photosynthesis at chilling temperatures were identified. Msa accession '73/2' developed leaves in the spring that survived night-time frost, and during growth under chilling maintained a statistically significant 79 % higher Φ_O2,max, as a measure of light- limited photosynthesis, and a 70 % higher Asat, as a measure of light-saturated photosynthesis. A second Msa accession, '73/3' also showed significantly higher rates of leaf uptake of CO₂. * Conclusions As remarkable as Mxg has proved in its chilling tolerance of C₄ photosynthesis, this study shows that there is still value and potential in searching for yet more superior tolerance. Msa accession '73/2' shows rates of light-limited and light-saturated photosynthesis at chilling temperatures that are comparable with those of the most cold-tolerant C₃ species. This adds further proof to the thesis that C₄ photosynthesis is not inherently limited to warm climates. [ABSTRACT FROM AUTHOR]