Effect of elevated CO2 concentration and nitrate: ammonium ratios on gas exchange and growth of cassava (Manihot esculenta Crantz)

Authors: CRUZ, JAILSON - Embprapa; ALVES, ALFREDO - Embprapa; Lecain, Daniel; ELLIS, DAVID - International Potato Center; Morgan, Jack

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2013
Publication Date: 1/2/2014
Publication URL: http://handle.nal.usda.gov/10113/58881
Citation: Cruz, J., Alves, A., Lecain, D.R., Ellis, D.D., Morgan, J.A. 2014. Effect of elevated CO2 concentration and nitrate: ammonium ratios on gas exchange and growth of cassava (Manihot esculenta Crantz). Plant and Soil. 374:33-43.

Interpretive Summary: Since climate change is expected to have many negative effects on agriculture, it is important to understand how we can optimize plant responses to some aspects of climate change, like rising CO2, to help agriculture adapt to future environmental conditions. Growth of most crop plants are positively affected by rising CO2 concentrations in Earth’s atmosphere, although the magnitude of that growth depends on the supply of essential nutrients like nitrogen (N). Our experiment evaluated the physiological and growth response of cassava to higher CO2 concentrations expected to develop in Earth’s atmosphere near the end of this century. Plants were fertilized with two different forms of N fertilizer, nitrate and ammonium, to determine whether it affected their growth responses to CO2. We unexpectedly discovered that higher CO2 levels helped plants overcome ammonium toxicity, a common plant problem when ammonium is used as the sole N form for fertilization. This finding will help plant scientists develop more effective N fertilization strategies to take advantage of future higher concentrations of CO2 in the atmosphere.

Technical Abstract: This study evaluated how different nitrogen forms affect growth and photosynthetic responses of cassava to CO2 concentration. Cassava was grown in 12-L pots in a greenhouse (30/25o C day / night) at 390 or 750 ppm of CO2. Three nitrogen treatments were applied: (a) 12 mM NO3-, (b) 6 mM NO3- + 6 mM NH4+, and (c) 12 mM NH4+. Thirty-six days after treatments began, plants grown under elevated CO2 and fertilized only with NO3- (750_NO3-) had photosynthetic rates similar to plants grown under 390_NO3-. In addition, photosynthetic rates of plants at 390_NO3- were higher than for plants at 750_NO3- when both were measured at the same CO2 concentration (Ca) of 750 ppm. Further, photosynthetic rates increased as NH4+ increased in the nutrient solution, such that photosynthetic acclimation was reduced for plants fertilized with only NH4+. However, this positive effect of NH4+ was not observed in more advanced growth stages. Plants fertilized with only 12 mM NH4+ eventually exhibited symptoms of toxicity, including yellowing, chlorosis, and wilting on the oldest leaves at both CO2 concentrations, resulting in reduced total dry mass (TDM) by the end of the experiment. The negative effect of NH4+ was lower under elevated CO2 concentrations as plants fertilized with only NH4+ produced 37% more TDM when grown under 750 ppm of CO2 versus 390 ppm of CO2. Our results indicate that cassava will respond with increased biomass accumulation in response to raising atmospheric CO2 levels and that this increasing carbon supply can minimize the negative effect of NH4+ on dry matter accumulation of cassava plants. The challenge is to determine how to manage NH4+ fertilization so that the benefit observed in the initial phase may persist throughout cycle of the crop.