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

Few studies have attempted to investigate the combined effects of soil matric potential (psi(m)) and osmotic potential (psi(s)) on plant production and water use efficiency (WUE). The purpose of this study is to evaluate the synergistic and antagonistic effects of psi(m) and psi(s) 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 "WUE theta" is suggested based on soil water content (theta). 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 (K-y) 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, WUE theta (R-2 > 0.98) was a better predictor of the plant yield than WUE (R-2 > 0.48). Nitrogen use efficiency (NUE) decreased with increasing salinity and water deficiency. Also, psi(m) and psi(s) had a synergistic interaction on yield and ET, while their effect on WUE and WUE theta was antagonistic. Salinity greater than drought reduced yield, while both had equal reducing effects on ET. Furthermore, WUE and WUE theta were more influenced by psi(m) than psi(s).