Abstract:  Water and nitrogen (N) are two important factors affecting crop growth and development. The growth and physiological processes of crops are affected by interactions between water and N under drought and poor soil conditions. Therefore, the study on effect of different N fertilization on leaf photosynthetic physiology and root morphology of maize under water stress can provide a scientific theoretical basis for the effective water and N management of maize cultivation. The experiment was carried out in a glasshouse. The soil water treatments included three levels: moderate drought (W1, 40%50% soil water holding capacity (SWHC)), mild drought (W2, 60%70% SWHC) and well-watered (W3, 75%90% SWHC). The N fertilization included three levels: low N (N1, 1.0gpot?1), middle N (N2.5, 2.5gpot?1) and high N (N5, 5.0gpot?1) levels. The maize growth, leaf gas exchange, the photosynthetic CO2 response curve (An/Ci) and light response curve (An/Q), and the root morphology were investigated. The results showed that the physiological responses of maize to N fertilization were different under varied soil water regimes. Compared with well-watered treatment, the root length and root specific surface area increased by 106.39%208.82% and 45.81%105.85%, respectively, under water stress condition, and the root dry weight decreased by 23.94%36.61% under the moderate drought condition. Increasing N dose, especially the high N treatment, significantly decreased the root length and root specific surface area by 41.85%54.10% and 18.68%, respectively, and the root dry weight decreased by 33.75% significantly under low N treatment with soil water stress. Thus, N fertilization aggravated plant water stress in the root zone, leading to reduced root water potential. Consequently, the stomatal conductance (Gs), CO2 and light use efficiency of maize leaf were affected. Both water and N treatments affected CO2 and light response curves, and water treatments showed a more prominent effect. Under the same N treatments, the dark respiration rate (Rd), the maximum net photosynthetic rate (Amax) and the saturation irradiance (Qsat) derived from the photosynthetic light response curve, and the initial carboxylation efficiency (a), the rate of the photorespiration (Rp), the photosynthetic capacity (Amax) and saturation intercellular CO2 concentration(Cisat) under high N level derived from the photosynthetic CO2 response curve decreased with the increase of water stress levels, and the former parameters decreased more significantly. Increasing N dose further decreased these parameters under the moderate drought condition, indicating that N fertilization inhibited plant photosynthetic performance under the moderate drought treatment. The Gs and photosynthesis (An) decreased significantly by 32.37%51.97% and 41.85%56.14%, respectively, under the moderate drought condition, and increasing N supply, especially high N treatment, decreased Gs and An by 35.81% and 30.71%, respectively, under the moderate drought condition compared to low N treatment. Water stress facilitated the root length and root specific surface area, and under water stress increasing N dose did not alleviate the drought stress of plants, while inhibited the root length and root specific surface area, and aggravated water stress. Thus, the root water potential was reduced and leaf Gs was affected. Consequently, the photosynthetic capacity, the CO2 and light use efficiency of maize leaf were decreased. The non-stomatal and stomatal factors inhibited the photosynthesis, resulting in decreased photosynthetic carbon assimilation ability and the accumulation of root biomass.

Key words:  Drought, Stomatal conductance, Water potential, Response curve, Modified rectangular hyperbola model, Root morphology