Leaf‐Level Physiology, Biomass, and Reproduction of Phytolacca americana under Conditions of Elevated Carbon Dioxide and Increased Nocturnal Temperature

Rising atmospheric CO2 and increasing air temperatures are predicted to increase future plant growth, but plant responses to increasing temperatures could be complicated by the fact that nocturnal temperatures may increase more than diurnal temperatures. The C3 forb Phytolacca americana L. (Phytolaccacea) was grown under either ambient (370 mmol mol 1) or elevated (740 mmol mol 1) CO2 in either of two nocturnal temperature treatments (26 /20 C or 26 /24 C day/night). We predicted that elevated CO2 would increase photosynthetic rate and enhance plant biomass, while elevated nocturnal temperature would increase dark respiration rate and decrease biomass. Thus, increased nocturnal temperature was predicted to diminish the generally positive effects of elevated CO2 on plant growth. Plants grown under elevated CO2 responded as expected, with 69% greater photosynthetic rate and 35% larger whole-plant biomass for the first part of the growing season. Contrary to the predictions, however, increased nocturnal temperature had no negative effect on respiration rate or biomass. In fact, plants grown at higher nocturnal temperatures flowered 1.5 d earlier and exhibited a 32% increase in biomass allocation to reproduction. Thus, higher nocturnal temperatures did not diminish the generally positive effects of elevated CO2 on P. americana growth. Instead, these results indicate that elevated CO2 and increasing nocturnal temperatures of the future could have a neutral or even positive effect on P. americana population growth.