The Combined Effects of Increasing Temperature and CO2 on the Growth of C3 and C4 Annual Species

With CO2 levels predicted to rise in the future, several recent experiments have investigated the effects of increasing CO2 on plant growth and development. Studies on cool weather  C3 annual plants have demonstrated enhanced photosynthesis under elevated CO2 when other environmental factors remain constant. The ability to increase growth under elevated CO2 could be very beneficial for C3 crops, enhancing productivity and increasing yield. Warm weather C4 annual plants have generally been less responsive to increases in atmospheric CO2 levels, demonstrating similar levels for photosynthesis at various levels of CO2. However, studies have shown that increases in both temperature and CO2, a more realistic scenario, demonstrate an ecological advantage for C4 plants as the advantages of C3 plants decline with an increase in temperature. Rising CO2 is also expected to significantly affect the reproductive structures of both C3 and C4 plants, due to high temperature shocks during fertilization, inhibiting vital growth stages. Despite increasing literature on the effects of climate change on plants, few studies have examined the impacts of increased temperature and elevated CO2on weedy plants or grasses. In this study, Lee examined the effects of increased temperature, and increased temperature with elevated CO2on two annual species of C3 and C4 plants, Chenopodium album and Setaria viridis respectively. The author found that elevated temperature significantly                                                                                                                                                                                                                                                                                                                                                                                                                                                                              affects the biomass production in the reproductive stages and this effect may be enhanced for C3 plants. However, the disadvantages of warming are countered in the presence of elevated CO2 in C3 plants.—Taylor Jones
Lee, J.S., 2011. Combined effect of elevated CO2 and temperature on the growth and phenology of two annual C3 and C4 weedy species.  Agriculture, Ecosystems and Environment 140, 484-491.
          Lee assembled three experimental plots subjected to varying condition, the first being a control with ambient CO2and temperature. The second plot (T4) was subjected to a 4°C increase in temperature with ambient CO2 and the third plot (CT4) was subjected to a 4°C increase in temperature along with 1.8 times the ambient CO2level. The plots were rotated to minimize the effects of inadvertent variations in light, air temperature, and CO2 concentration. The biomass of the plants was estimated using the plant size index and was measured at two week intervals throughout the growing period. Leaf area and photosynthesis rates were also recorded. Emerging seedlings and flower number were counted after at least one seedling shoot extended 0.2 cm.
          Throughout this study, Lee found that seedling emergence and flowering times in C3 and C4plants were significantly advanced under T4 and CT4 conditions compared to the control, however the differences between these two conditions was relatively small. The date of emergence of C. albumseedlings was 27.0 and 24.3 days early in T4 and CT4 plots respectively, compared to the control. Also, the length of the flowering time increased significantly in T4 and CT4 scenarios compared to the control in S. viridis, but not in the C3plant. Lee concluded that since most of the differences were between the control and T4 or CT4 plots, plant phenology is likely affected more by the increase of temperature than the elevation of CO2. Also, the seedling emergence time of S. viridiswas more sensitive to increased temperature than that of C. album which could lead to serious implications for population establishment when seeds are competing for resources and space. The author concludes that C4 plants will have an advantage in this scenario due to the increased sensitivity to temperature.
          Throughout the growth stages, the mean temperatures in the plots subjected to elevated temperature remained approximately equal, indicating that the temperatures during critical growth stages were about the same for C3 and C4 plants. The changes in temperature were accounted for by advancing plant phenology. However, these conditions of ideal light and temperature are not realistic and changing plant phenology also affects the solar radiation intensity. Lee determined that accumulated solar radiation decreased by 19% and 16.1% in C3 and C4 plants respectively in T4 conditions and this decrease in solar radiation led to decreases in biomass production. It is likely that these decreases in biomass production would be compensated for in an increased temperature and CO2 environment.
          The effect of elevated CO2seemed to be greater when coupled with increases in temperature, which led to a significant increase in photosynthesis in C3 plants. In this study, elevated CO2 levels and temperature increased the rate of photosynthesis in C3 plants and increased biomass production when compared to ambient conditions. Increased temperature did not significantly affect the biomass production of C4 plants, maintaining their advantage in T4 and CT4 conditions as a result of resistance to increasing temperature and CO2. Based on this study, C3 plants are predicted to have an advantage under future global warming conditions as they can avoid the detrimental effects of high temperatures during the vegetative growth stage by flourishing under increased CO2

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