Does the Response of Perennial Ryegrass to Elevated CO<sub>2</sub> Concentration Depend on the Form of the Supplied Nitrogen?

Abstract

To test whether different nitrogen form (nitrate or ammonium) in substrate can alter the response to elevated partial pressure of CO2 (pCO2) plants of perennial ryegrass (Lolium perenne cv. Bastion) were grown from seeds in growth chambers under pCO2 of either 35 Pa (ambient, CA) or 70 Pa (elevated, CE) in a hydroponic system (with nutrient and pH control) for 24 d. Nitrogen was supplied as ammonium, nitrate or an equimolar mixture of both N forms. Under CE plants grew faster than their counterparts under CA during the first 14 d but after 23 d of cultivation stimulation disappeared. Despite the strong positive effect of mixed forms of N on plant growth, the beneficial effect of CE was similar to that in the other two N treatments. However, the almost alike final growth response to CE had different underlying mechanisms in different N treatments. Plants supplied with nitrate as a sole source of nitrogen had lower leaf mass ratio but much higher specific leaf area compared to plants supplied with ammonium. The decrease in the content of leaf organic N (per unit of structural dry mass) under CE was found only in leaves of plants supplied with ammonium on day 14. Nevertheless, the available form of N evidently contributes to changes of leaf N content under CE. The high levels of N and non-structural saccharides in plants supplied with ammonium at CE suggest that the CO2 response of these plants was controlled by factors other than amount of available carbon and nitrogen.