Nitrogen use efficiency of processing tomato under deficit irrigation in semi-arid climate

Irrigation regime affects crop yield and water use, thus influencing water use efficiency. Irrigation also affects the use efficiency of minerals, mostly nitrogen, applied to the crop through fertilization. In this research the influence of deficit irrigation upon crop nitrogen uptake and nitrogen use efficiency in processing tomato was assessed. Three irrigation treatments (0, 50, 100% evapotranspiration-ETc restoration) were applied to open-field cultivated 'Brigade' processing tomato, under nitrogen (N150, 150 kg ha-1 N) and no nitrogen (N0, 0 kg ha-1 N) fertilization. N content on leaves, stems, and fruits was determined. Leaf chlorophyll content was estimated using SPAD. Full irrigation resulted in the highest N-content, irrespective of N treatment. Plants which did not receive nitrogen by fertilization exhibited overall lower N contents. The asymptotic course of the relationship of SPAD values vs. leaf N content indicated that chlorophyll increased with increasing nitrogen concentrations, but for N values greater than 2.7% chlorophyll content remained constant. Crop N uptake was the highest when crop water requirements were fulfilled by irrigation. Nitrogen use efficiency (NUE), calculated by nitrogen content of total biomass, was gradually reduced by the increasing water supply, and was higher in the unfertilised control, thus revealing a good aptitude of the crop to exploit soil N residual. The overall lowest NUE value corresponded to plants N fertilised under full irrigation indicating that the crop does not benefit from the high rates of nutrient uptake occurred under unrestricted soil water availability, since in these soil conditions it uses less efficiently the mineral applied through fertilisation. Results highlight the possibility to limit the environmental impact of processing tomato crop in semi-arid climate by minimizing N application and reducing to 50% the rate of irrigation, keeping adequate yield levels and allowing a sustainable use of irrigation water, with consequent economic and environmental benefits.