Carbon assimilation and allocation (¹³C labeling) in a boreal perennial grass (Phalaris arundinacea) subjected to elevated temperature and CO₂ through a growing season

The responses of carbon assimilation and allocation in different plant organs to changed environment depend on the species, their phenology and growth conditions. An integrated experiment was conducted over an entire growing season to understand the effects of elevated growth temperature (ambient+3.5°C) and CO₂ (700μmolmol^-1) on the seasonal photosynthetic capacity, carbon accumulation and allocation (four 21-day periods of ¹³C pulse labeling-to-harvest) in a boreal perennial grass (Phalaris arundinacea) using controlled environmental chambers. Elevated temperature was found to significantly enhance leaf photosynthesis and total carbon accumulation in biomass during the early stages of the growing season, while it also resulted in earlier senescence and lower carbon storage at the final harvest. CO₂ enrichment significantly stimulated photosynthesis and total carbon accumulation over the growing season. The combination of elevated temperature and CO₂ caused a lower total carbon accumulation in biomass at maturity compared to elevated CO₂ alone, indicating that this boreal crop grown under increased temperature could not take advantage of CO₂ enrichment. Elevated temperature significantly increased ¹³C assimilation and allocation to the leaves and the stems during the first labeling period, and resulted in 5-6% higher percentage of carbon allocation in the roots during the final two labeling periods. This did not result in increased carbon storage in the roots, but much less carbon accumulation in the shoots during the latter growing periods. Elevated CO₂ caused a higher ¹³C assimilation in the roots, but did not significantly increase the below-ground ¹³C allocation percentage. This was because of the higher growth of the shoots under high nitrogen availability and CO₂ enrichment. The response of carbon allocation pattern to the combined elevation of temperature and CO₂ was similar to the responses to elevated temperature alone. We conclude that plant growth and carbon assimilation are clearly controlled by phenology in this boreal crop under the climatic treatments, however, seasonal carbon allocation pattern within the plants was not significantly changed by the treatments.