Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2010
Publication Date: 9/1/2010
Citation: Gesch, R.W., Johnson, J.M. 2010. Differential Growth and Carbohydrate Usage in Switchgrass Ecotypes under Suboptimal Temperatures. Crop Science. 50:1988-1996.
Interpretive Summary: Switchgrass is a warm-season grass that uses the C4 mechanism of photosynthesis to absorb atmospheric carbon dioxide and fix it into carbohydrates for growth. Recently, it has gained popularity as a potential biomass crop for making biofuels. Switchgrass is native to the U.S. and is adaptable to a wide range of environments. However, across the Great Plains region of the U.S. where switchgrass is commonly grown, its productivity generally decreases the further north it is grown. Across the Great Plains region, switchgrass plants often have to cope with temperatures during the growing season that are below that which is optimum for growth. Low temperatures generally slow the process of photosynthesis. However, many plant species can increase the amount and activity of certain enzymes involved in photosynthesis to compensate for their slower rate under low temperatures. This way they are able to maintain growth even at sub-optimum temperatures. No one knows if switchgrass has this trait, but if it does, plant breeders could use it to develop new varieties that can produce higher yields in regions prone to low temperatures during the growing season. This study was designed to test several varieties of switchgrass to see if they could photosynthetically acclimate to sub-optimum temperatures. The varieties tested were grown under warm day/night temperatures (32/24 deg C) and then switched to low temperatures 22/14 deg C for several days to determine whether they photosynthetically acclimated. None of the varieties tested showed the ability to photosynthetically acclimate. However, one variety, Alamo, which is considered a lowland type of switchgrass originating from the Southern Plains area of the U.S., showed much greater growth than the other varieties under the low temperature treatment. Alamo also showed higher levels of carbohydrates (i.e., sugars) than the other varieties before and after the temperature switch. Furthermore, the carbohydrate levels of Alamo changed very little after switching to low temperatures; whereas, the other varieties showed large increases compared to before the switch. Results indicate that for Alamo, even though photosynthesis was not upregulated under low temperatures, carbon metabolism was maintained, or at least similar to what it was under the higher temperatures. Plant breeders may want to use this as criteria in switchgrass breeding programs to select varieties for improved growth and yield under low temperatures.
Technical Abstract: Switchgrass (Panicum virgatum L.), a warm-season C4 grass, shows good potential as a bioenergy feedstock and is widely adapted throughout North America, but its productivity tends to decline with increasing latitude. Little is known about whether genetic potential exists in switchgrass to photosynthetically acclimate to suboptimal growth temperature, although such potential could be used to enhance productivity. To explore this potential in switchgrass, two lowland (Alamo and Kanlow) and two upland (Cave in Rock and Sunburst) ecotypes were grown under day/night temperatures of 32/24 deg C and later switched to 22/14 deg C. Photosynthesis, growth, and nonstructural carbohydrates were analyzed before and after temperature switching. Leaf photosynthesis was greater in lowland than upland cultivars at 32/24 deg C. After switching to 22/14 deg C and allowing plants to acclimate, photosynthesis declined with little difference among cultivars. Moreover, photosynthetic capacity among cultivars slightly decreased or remained similar as compared to that before the temperature switch, with no clear ecotype trend. However, after switching to 22/14 deg C, Alamo had the greatest dry matter gain (3.9 g plant-1), which was 69, 82, and 152% greater than that of Sunburst, Kanlow, and Cave in Rock, respectively. Total nonstructural carbohydrates increased at 22/14 deg C, but Alamo showed the least degree of change and no difference in leaf sucrose/starch ratio following the switch to lower temperatures. Results indicate that maintenance of carbon metabolism favoring growth, rather than photosynthetic acclimation, may be a key to enhancing switchgrass productivity under suboptimal growth temperatures.