Synergistic and antagonistic interactions of soil water potential and osmotic potential linked to nitrogen fertilization on spinach traits and water use efficiency

Abstract

Few studies have attempted to investigate the combined effects of soil matric potential (Ψm) and osmotic potential (Ψs) on plant production and water use efficiency (WUE). The purpose of this study is to evaluate the synergistic and antagonistic effects of Ψm and Ψs associated with nitrogen (N) fertilization on spinach (Spinacia oleracea L.) growth traits and WUE. Five levels of salinity (1, 4, 7, 9, 11 dS m−1), three levels of water use amounts including deficit-irrigation (W–), full-irrigation (W0) and over-irrigation (W+), and four levels of N application rates (0, 50, 75, and 100% of N requirement) were applied. A new parameter namely "WUEθ" is suggested based on soil water content (θ). Results showed that water deficit combined with high saline irrigation water caused the most reduction in the plant biomass, leaf area index (LAI) and evapotranspiration (ET). The acquired plant yield coefficient (Ky) of 1.003 indicates that the spinach is a moderately salt-sensitive vegetable. The highest WUE was observed under deficit-irrigation and intermediate salinity conditions, which was attributed to an adaptation mechanism in the plant. However, WUEθ (R2 > 0.98) was a better predictor of the plant yield than WUE (R2 > 0.48). Nitrogen use efficiency (NUE) decreased with increasing salinity and water deficiency. Also, Ψm and Ψs had a synergistic interaction on yield and ET, while their effect on WUE and WUEθ was antagonistic. Salinity greater than drought reduced yield, while both had equal reducing effects on ET. Furthermore, WUE and WUEθ were more influenced by Ψm than Ψs. Research Highlights The new parameter WUEθ is a better predictor of the spinach plant yield than WUE. WUE and WUEθ are more influenced by water potential (Ψm) than osmotic potential (Ψs). Synergistic interaction of Ψm and Ψs on yield and ET, but antagonistic effect on WUE and WUEθ. Salinity greater than drought reduces yield, while both have equal reducing effects on ET. Ψm shows negative effects on yield and ET, but positive effects on WUE and WUEθ.