Effects of water-nitrogen coupling on soil water and nitrogen, photosynthesis, yield and water use of lettuce under different application rates of poly-γ-glutamic acid

As an environmentally friendly, super water-absorbable agent and fertilizer synergist, the influences of poly-gamma-glutamic acid (gamma-PGA) on soil N and crop photosynthesis are still not well understood. A pot experiment was conducted to evaluate the effects of water-nitrogen coupling on soil water and nitrogen content, photosynthesis, yield and irrigation water use efficiency (IWUE) of lettuce under different gamma-PGA application rates. In this study, there were three factors and four levels: gamma-PGA with four application levels of 0, 1, 2 and 4 g kg(-1); the rates of urea were 0, 0.2, 0.4 and 0.6 g kg(-1); and the irrigation amounts were 40%, 60%, 80% and 100% of the lettuce water requirement (ETc). The results showed that within 8 days after sowing, the soil water and nitrogen (NO3--N and NH4+-N) content increased with increasing gamma-PGA application rates. The amount of irrigation water and nitrogen application had an effect on IWUE, net photosynthetic rate (P-n) and stomatal conductance (G(s)). Lettuce yield was significantly correlated with P-n, G(s) and intercellular CO2 concentration (C-i) by path analysis. Moreover, the P2N2W3 treatment (gamma-PGA 2 g kg(-1), nitrogen 0.2 g kg(-1) and 80% ETc) was the most suitable treatment to obtain the highest lettuce yield and IWUE.