Graded water supply decouples vegetative growth, reproductive investment, and nitrogen balance in trimorphic Atriplex aucheri

Abstract Background Water availability is a primary driver of plant performance in arid ecosystems, yet its effects on reproductive allocation, tissue stoichiometry, and ion homeostasis remain poorly resolved in heterocarpic desert annuals. Atriplex aucheri Moq. is a trimorphic halophyte of the Junggar Basin that produces three seed types with contrasting dormancy and dispersal. We grew A. aucheri under four graded water regimes (100, 150, 200, and 250 mL day − 1 ) in a greenhouse experiment testing how water supply affects shoot biomass, morph-specific seed production, shoot C: N:P stoichiometry, and inorganic ion composition. Results Increasing water raised shoot dry weight up to 60.6% ( p < 0.001), whereas total seed allocation did not increase proportionally and instead declined along a significant linear trend, decreasing by 35.2% from the 100 mL to the 250 mL treatment (linear contrast p = 0.022). The morphs differed in their responses. Type A seed production peaked at intermediate water (quadratic contrast p = 0.016), type B remained stable, and type C allocation declined monotonically. Shoot nitrogen fell under the two highest water levels ( p < 0.05) and the C: N ratio rose up to 29.8% relative to the 150 mL treatment, consistent with growth-driven nitrogen dilution. Of six ions measured, only sulfate responded to treatment ( p < 0.05), decreasing 45.9% from the 150 mL to the 250 mL level, while sodium stayed dominant (58 to 61% of measured cations). Piecewise structural equation modelling indicated that water influenced reproductive allocation more consistently through a size-mediated pathway (β = 0.74, p < 0.001) than through tissue nitrogen, although the path from shoot dry weight to allocation was not significant. Conclusions Higher water supply promoted vegetative growth without a proportional increase in reproductive investment, altered shoot stoichiometry, and had limited effects on most measured shoot ions. These findings provide insight into the response of Atriplex aucheri to graded water supply under greenhouse conditions.