Machine learning identifies the dominant physiological components in intrinsic water-use efficiency and photosynthetic performance index in dryland maize

1. Substantial genotypic variation exists in intrinsic water-use efficiency (WUEi) and photosynthetic performance index (PIabs) in dryland maize. 2. Stomatal conductance ( g s ) and electron transport efficiency beyond Q A ⁻ (ψEo) are identified as the primary physiological determinants of genotypic variation in WUEi and PIabs, respectively. In arid and semi-arid regions, leaf-level intrinsic water use efficiency (WUEi) and photosynthetic performance index (PIabs) are critical traits for climate resilience. WUEi depends on both the photosynthetic rate ( P n ) and stomatal conductance ( g s ), while PIabs integrate antenna size, energy-trapping capacity, and electron transport efficiency. Genotypic variation in WUEi and PIabs has been well studied, however, the contributions of their constituent traits to this variation remain unexplored in dryland maize. To bridge this knowledge gap, we conducted a two-year field study on China's Loess Plateau, aiming to: (1) quantify genotypic variation in WUEi and PIabs across maize hybrids, and (2) identify the dominant physiological components underlying this variation using machine learning approach. Field experiments revealed 1.5- to 1.7-fold genotypic variation in P n and WUEi, respectively. Random forest analysis identified g s as the primary modulator of WUEi differences (%IncMSE: 26.1 in 2023; 38.1 in 2024). PIabs ranged from 0.71 to 1.39 in 2023 and from 1.39 to 2.01 in 2024, with electron transport efficiency beyond Q A - (ψEo) emerging as the main contributor to PIabs variation in both years. Linear regression indicated WUEi was negatively related to PIabs (2023: y =-8.26 x +153.96; 2024: y =-36.36 x +214.98), revealing a potential physiological trade-off between water conservation and photosynthetic performance for dryland maize. These findings clarify the contributions of individual component traits underlying genotypic variation in WUEi and PIabs, offering valuable guidance for the selection and breeding of maize hybrids optimized for dryland environments.