As an environmentally friendly, super water‐absorbable agent and fertilizer synergist, the influences of poly‐γ‐glutamic acid (γ‐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 γ‐PGA application rat...