Photosynthetic Response of Switchgrass Ecotypes to Fluctuating Growth Temperatures.

Switchgrass (Panicum virgatum L.), a warm-season C₄ grass that shows good potential as a bioenergy feedstock and conservation crop, is widely adapted throughout North America, but its productivity tends to decline with increasing latitude. In northern regions where growing season temperatures can fluctuate dramatically, the ability of switchgrass to adjust photosynthetically to maximize carbon assimilation under sub-optimal temperatures may enhance productivity. To explore this potential in switchgrass, four cultivars, two lowland (Alamo and Kanlow) and two upland (Cave in Rock and Sunburst) ecotypes were grown under day/night temperatures of 32/24°C and later switched to 22/14°C for 15 d. Photosynthesis, growth, and nonstructural carbohydrates were analyzed before and after temperature switching. Photosynthesis of the lowland cultivars was slightly greater than that of the upland varieties at 32/24°C. After switching to 22/14°C and allowing the newest expanding leaf to mature, photosynthesis at growth temperature was less than that before the switch and there was no difference among cultivars. Under the lower growth temperatures, photosynthetic capacity among cultivars slightly decreased or remained similar as compared to that before the temperature switch, although Alamo leaves increased in PEPCase activity and Chl content. While exposed to 22/14°C, Alamo had the greatest dry weight gain (3.8 g plant^-1), which was 31, 65, and 153% greater than that of Sunburst, Kanlow, and Cave in Rock, respectively. Soluble sugar and starch contents of Alamo leaves were greater than that of the other cultivars before and after the switch. However, Alamo leaves grown under the lower temperatures contained less soluble sugar and starch than those grown at 32/24°C, whereas the other cultivars responded oppositely. Results indicate that efficient assimilate usage for growth rather than photosynthetic acclimation under sub-optimal growth temperatures may be a key to enhancing switchgrass productivity.