Chilling and frost tolerance in Miscanthus and Saccharum genotypes bred for cool temperate climates

Miscanthus hybrids are leading candidates for bioenergy feedstocks in mid to high latitudes of North America and Eurasia, due to high productivity associated with the C₄ photosynthetic pathway and their tolerance of cooler conditions. However, as C₄ plants, they may lack tolerance of chilling conditions (0-10 °C) and frost, particularly when compared with candidate C₃ crops at high latitudes. In higher latitudes, cold tolerance is particularly important if the feedstock is to utilize fully the long, early-season days of May and June. Here, leaf gas exchange and fluorescence are used to assess chilling tolerance of photosynthesis in five Miscanthus hybrids bred for cold tolerance, a complex Saccharum hybrid (energycane), and an upland sugarcane variety with some chilling tolerance. The chilling treatment consisted of transferring warm-grown plants (25/20 °C day/night growth temperatures) to chilling (12/5 °C) conditions for 1 week, followed by assessing recovery after return to warm temperatures. Chilling tolerance was also evaluated in outdoor, spring-grown Miscanthus genotypes before and after a cold front that was punctuated by a frost event. Miscanthus×giganteus was found to be the most chilling-tolerant genotype based on its ability to maintain a high net CO₂ assimilation rate (A) during chilling, and recover A to a greater degree following a return to warm conditions. This was associated with increasing its capacity for short-term dark-reversible photoprotective processes (θ_REG) and the proportion of open photosystem II reaction centres (qL) while minimizing photoinactivation (θ_NF). Similarly, in the field, M.×giganteus exhibited a significantly greater A and pre-dawn F _v/F_m after the cold front compared with the other chilling-sensitive Miscanthus hybrids.