Root Growth, Fruit Yield and Water Use Efficiency of Greenhouse Grown Tomato Under Different Irrigation Regimes and Nitrogen Levels

Soil water and nutrients are the two important factors affecting vegetative growth and the reproduction of greenhouse tomatoes (Solanum lycopersicum L.), but the effect of the interaction between irrigation and nitrogen (N) rates on fruit yield, root characteristics and N uptake have not yet been studied. Tomatoes were irrigated at 100% (W1), 80% (W2), and 60% (W3) of reference crop evapotranspiration (ET0) and N fertilizer was supplied at 240 kg N ha−1 (N240), 180 kg N ha−1 (N180), and 120 kg N ha−1 (N120) under drip fertigation in 2015 and 2016. In 2015, the fruit yield of the irrigation and N rates varied from 54.1 to 75.9 t ha−1. In 2016 (where the total irrigation amount was higher than in 2015), the fruit yield varied from 55.6 to 78.7 t ha−1. The average fruit yield in W1 was 17.8% and 21.4% higher than W3, in 2015 and 2016 respectively. The mean root weight density in N240 was 13.4% and 10.7% lower than N180, in 2015 and 2016, respectively. Averaging the irrigation levels, the total dry-matter production in N180 and N120 was, respectively, 1.4% and 13.2% lower than N240 in 2015. With the application of the irrigation and N fertilizer rates, plant N uptake increased from 28.7 to 94% in 2015 and from 14 to 92.3% in 2016. The water use efficiency (WUE) of the irrigation and N rates varied from 25.4 to 37.2 kg m−3 and from 20.8 to 36 kg m−3 in 2016. The partial factor productivity of the N fertilizer (PFPN) varied from 274.6 to 529.3 kg kg−1 and from 260.1 to 592.1 kg kg−1 in 2016 with the irrigation and N fertilizer rates. We conclude that the effect of irrigation and N rates on fruit yield, dry-matter production and N uptake significantly changes with the root characteristics. Considering the trade-off among the plant N uptake, WUE and PFPN, W2N180 may give a satisfactory fruit yield for greenhouse tomato in north-west China.