Spinach Growth Regulation Due to Interactive Salinity, Water, and Nitrogen Stresses

Salinity, water scarcity, and nitrogen (N) deficiency are the main limiting factors for plant production. However, over-irrigation and excessive N utilization can also limit the growth and biomass yield of leafy vegetables. The aim of this study is to investigate the growth regulation of spinach ( Spinacia oleracea L.) under different levels of irrigation water salinity, water utilization, and nitrogen supply. Combinations of five salinity levels (1, 4, 7, 9, and 11 dS m −1 ) with three levels of water consumption including deficit irrigation (W–), full irrigation (W0) and over-irrigation (W+), and four levels of N supply (0, 50, 75, and 100% of N requirement) were examined under controlled greenhouse conditions. The least plant biomass, evapotranspiration (ET), and nitrogen use efficiency (NUE) were observed when high-saline water was applied in combination of water and N deficiency. However, the osmotic/drought effect is a short-term effect of salinity that can be further exacerbated by the condition W–. With increasing the fertilizer rate up to 150 mg N kg −1 soil, the plant growth traits increased significantly for all salinity levels; however, an excessive N utilization caused no significant increase. Water stress caused a morphological adaptation in the plant by reducing leaf area and ET, resulting in improved water use efficiency (WUE). With increasing salinity, WUE increased then decreased significantly. In other words, both the relative ET and relative yield were reduced with increasing salinity; however, at intermediate salinity levels (4 and 7 dS m –1 ), further decline in the former than in the latter resulted in higher WUE values. The stepwise regression analysis showed that the plant yield was affected mostly by WUE, ET, and leaves number ( R 2 = 0.983***). The findings of this study revealed that the reduced leaf area was the main growth regulation in the spinach plant under stress conditions, which ultimately could enhance WUE.