Photosynthesis and biomass accumulation in young sugarcane plants grown under increasing ammonium supply in nutrient solution
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The aim of this study was to evaluate the sugarcane responses to varying ammonium:nitrate (NH4+:NO3−) ratio and to reveal how much NH4+ plants can tolerate before showing impairment of photosynthesis and growth. Sugarcane plants were grown in nutrient solution with the following NH4+:NO3− ratios (%): 20:80; 30:70; 40:60; 60:40; 70:30; and 80:20. The lowest photosynthetic rates, stomatal conductance, instantaneous carboxylation efficiency and leaf chlorophyll a content were found in plants supplied with higher than 60% NH4+. The leaf content of chlorophyll b proved to be more sensitive than chlorophyll a and decreases were found from 40% NH4+. We did not observe significant differences in leaf NO3− concentration under varying NH4+:NO3− ratio. However, plants that received 80% NH4+ showed the highest leaf NH4+ concentration and lowest leaf [NO3−]:[NH4+] ratio. The estimated leaf nitrogen content was higher in plants supplied with 20% and 30% NH4+. Taken together, our data revealed that sugarcane plants are sensitive to NH4+, with photosynthesis and plant growth being impaired when NH4+ supply was higher than 30% in nutrient solution. Root biomass was significantly reduced under high NH4+ supply, which explains decreases in stomatal conductance. Besides stomatal limitation, photosynthesis was also limited by low carboxylation efficiency under high NH4+ supply. Apparently, leaf NH4+ concentrations higher than 1.0 µmol g−1 were enough to impair photosynthesis. The balance between [NO3−] and [NH4+] in leaves was more correlated to photosynthesis than either [NO3−] or [NH4+] alone.
KeywordsNitrogen Root growth Saccharum spp. Nitrate
MDP acknowledges the scholarship provided by the São Paulo Research Foundation (FAPESP, Brazil; Grant No. 2017/11279-7). NMS acknowledges the fellowship granted by the National Program of Post-Doctorate (PNPD), Coordination for the Improvement of Higher Education Personnel (Capes, Brazil). ECM, LS and RVR acknowledge the fellowships granted by the National Council for Scientific and Technological Development (CNPq, Brazil).
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