Nitrogen and Water Demands for Maximum Growth of Solanum tuberosum under Doubled CO2: Interaction with Phosphorus Based on the Demands

•Foliar critical N concentration on area basis is more suitable to evaluate N status.•e[CO2] increased N but not water demand for maximum biomass.•N and water demand under e[CO2] depends on P nutrition.•N supply decrease WUE during the early growth stage.•Increased WUE by e[CO2] or high P supply was independent of N. Crop growth promotion utilizing elevated carbon dioxide concentrations (e[CO2]) may be limited by soil nutrient availability. Although numerous studies have suggested the importance of nitrogen (N) for the promotion of growth under e[CO2], N requirement for maximum plant growth is rarely examined. We have found that increase in potato (Solanum tuberosum L.) biomass depends on phosphorus (P) availability under doubled [CO2] conditions. To address whether the N requirement for maximum growth under e[CO2] is dependent on P supply or not in potatoes, we quantified potato growth and water consumption in response to five N supply rates at low P (LP) and high P (HP) conditions. A pot experiment was conducted in controlled-environment chambers with ambient CO2 concentrations (a[CO2]) and an e[CO2] level of double a[CO2]. Foliar critical N concentration per area (critical [N]area), the minimum N requirement for 90% maximum plant growth, was similar (1.43 g m-2) regardless of [CO2] conditions under LP. Under HP, however, the critical [N]area increased under e[CO2] conditions (1.65 g m-2) compared with a[CO2] conditions (1.52 g m-2). Water use did not change with e[CO2] under HP conditions, whereas it decreased with e[CO2] under LP conditions despite the increase in biomass owing to higher water-use efficiency (WUE). Although WUE with e[CO2] or HP was independent of N supply, biomass increment with e[CO2] or HP depended on N supply. We concluded that the N and water required by potato plants under e[CO2] would be dependent on P supply. Although under HP, e[CO2] increased N but not water required to obtain maximum growth during the early growth stage, N demand was unchanged and water demand decreased by e[CO2] under LP conditions, probably owing to growth limited by P availability.