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

Cruz, Jailson L., Alves, Alfredo A.C., LeCain, Daniel R., Ellis, David D., Morgan, Jack A.
Plant and soil 2014 v.374 pp. 33-43
Manihot esculenta, acclimation, ammonium, ammonium compounds, ammonium fertilizers, biomass production, carbon, carbon dioxide, carbon dioxide enrichment, cassava, chlorosis, developmental stages, fertilizer application, gas exchange, greenhouses, leaves, nitrates, nitrogen, nutrient solutions, photosynthesis, toxicity, wilting
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.