C-4 bioenergy crops for cool climates, with special emphasis on perennial C-4 grasses

摘要

There is much interest in cultivating C-4 perennial plants in northern climates where there is an abundance of land and a potential large market for biofuels. C-4 feedstocks can exhibit superior yields to C-3 alternatives during the long warm days of summer at high latitude, but their summer success depends on an ability to tolerate deep winter cold, spring frosts, and early growth-season chill. Here, we review cold tolerance limits in C-4 perennial grasses. Dozens of C-4 species are known from high latitudes to 63 degrees N and elevations up to 5200 m, demonstrating that C-4 plants can adapt to cold climates. Of the three leading C-4 grasses being considered for bioenergy production in cold climates-Miscanthus spp., switchgrass (Panicum virgatum), and prairie cordgrass (Spartina pectinata)-all are tolerant of cool temperatures (10-15 degrees C), but only cordgrass tolerates hard spring frosts. All three species overwinter as dormant rhizomes. In the productive Miscanthusxgiganteus hybrids, exposure to temperatures below -3 degrees C to -7 degrees C will kill overwintering rhizomes, while for upland switchgrass and cordgrass, rhizomes survive exposure to temperatures above -20 degrees C to -24 degrees C. Cordgrass emerges earlier than switchgrass and M. giganteus genotypes, but lacks the Miscanthus growth potential once warmer days of late spring arrive. To enable C-4-based bioenergy production in colder climates, breeding priorities should emphasize improved cold tolerance of M. xgiganteus, and enhanced productivity of switchgrass and cordgrass. This should be feasible in the near future, because wild populations of each species exhibit a diverse range of cold tolerance and growth capabilities.