Biomass Allocation and Physiological Responses of Timothy Grass (Phleum pratense) to Future Predicted Levels of Carbon Dioxide and Ozone.

Understanding plant responses to future predicted concentrations of greenhouse gasses, including ozone (O₃) and carbon dioxide (CO₂), will be important to help maintain high levels of agricultural productivity in future climates. Timothy grass (Phleum pratense L.) is an economically important agricultural forage crop that has been previously shown to be sensitive to changes in O₃ and CO₂, whereas limited research has been conducted to investigate interactive effects of these greenhouse gasses on the growth and physiology of Timothy grass at the whole plant level. Therefore the objectives of this study were to quantify the interactive effects of elevated O₃ and CO₂ on biomass accumulation and allocation, and to examine underlying physiological mechanisms associated with changes in growth of Timothy grass from seedling through flowering stages. Plants were fumigated in chambers using two concentrations of O₃ (ambient O₃ of 30 ppb and elevated O₃ of 80 ppb) in combination with two concentrations of carbon dioxide (ambient CO₂ of 400 and elevated CO₂ of 800 ppm) for a total of four treatments.  Destructive harvests were completed every three weeks to measure productivity, including aboveground and belowground biomass accumulation. In addition, changes in photosynthesis rates, stomatal conductance, water use efficiency (WUE), total non-structural carbohydrates (TNC), and antioxidant levels were also assessed.  Elevated O₃ levels resulted in a decrease in plant biomass, whereas elevated CO₂ stimulated biomass and WUE of shoot tissues. Stimulation of biomass growth and TNC in response to elevated CO₂ occurred at the tillering stage leading up to flowering. In general, elevated CO₂ ameliorated the negative impacts of ozone on growth of Timothy, which was associated with reduced stomatal conductance.