## 3 Results  
### Growth Trajectory Fitting Based on EDIE  
The ecological mapping population includes 156 full-sib individuals of _Populus eubrnatica_ cultured under salt-free (control) conditions and 117 full-sib individuals cultured under salinity stress (salt). Plants exhibit flexible root phenotypic plasticity by adjusting the biomass allocation of the root system in response to salinity stress, which further affects root system architecture and the underground spatial configuration [45,46]. Here, we choose two representative traits, namely, taproot length and lateral root length, to illustrate the plasticity of root system structure and reveal its genetic potential. As shown in Figure 2a,b, notable differences are visualized between the shape of violins under two cultivation conditions (salt and control). Specifically, the phenotype values of quantitative traits under salt stress are generally lower than those of the control group. The Wilcoxon rank-sum tests confirm these observations, establishing that salt stress significantly impacts root growth (\(p\)-value \(<0.05\)). Figure 2: Analysis of the development of seedlings within a full-sibling group of _Populus euphratica_. Violin plots contrasting taproot (**a**) and lateral root lengths (**b**) under control versus saline environments. Growth curves for both taproot and lateral root length depicted across control (**c**,**d**) versus saline-exposure scenarios (**e**,**f**). Each purple trajectory represents the average growth trend of the corresponding trait, which consists of independent growth (green trajectories), interactive growth with another trait (red trajectories), and environmental effects (yellow trajectories). Here, “average growth” is defined as the arithmetic mean of growth measurements across observations, providing a statistical estimate for the expected growth pattern under each condition. A two-way ANOVA is used to delve into how the experimental treatments--control and salt-stress conditions--affect root length, alongside the potential interaction between these treatments and root types (taproot vs. lateral roots). The analysis (see Supplementary Table S1) highlights significant main effects for both the treatment condition (\(F\)-value = 28.223, \(p\)-value < 0.0001) and root type (\(F\)-value = 25.156, \(p\)-value < 0.0001), underscoring the adverse effects of salinity stress on root lengths and the inherent growth differences between taproots and lateral roots. However, the interaction between treatment and root type is not statistically significant (\(F\)-value = 2.734, \(p\)-value = 0.0988), indicating a consistent impact of salt stress across both types of roots. It is noteworthy that there are no 0 values for taproot length observations in the control and salt-stressed groups, whereas 10 out of 156 lateral root length observations are 0 in the control group and 31 out of 117 lateral root length observations are 0 in the salt-stressed group (Supplementary Figure S1). To further understand the incidence of zero values, a [PERSON]'s exact test was performed (Supplementary Table S1), which showed a significant difference (Odds Ratio = 5.26, \(p\)-value = 5.24 \(\times\) 10\({}^{-6}\)), highlighting the greater likelihood that lateral roots are not malformed under salt stress. Subsequent Tukey HSD post hoc analyses clarify specific differences between treatments and root types (Supplementary Table S1). The lateral root length is significantly reduced under salt stress compared to the control, with a mean difference of \(-\)12.669 cm (<0.0001 ***), highlighting the inhibitory effect of salt on lateral root development. In addition, the intrinsic growth differences between control taproots and lateral roots are also significant, with the most pronounced difference between salt-stressed taproots and control lateral roots, with a mean difference of \(-\)23.944 cm (<0.0001 ***).