Integrated Control of Biomass Partitioning and Ionic Homeostasis Underpins Salinity Tolerance in Bread Wheat (Triticum aestivum L.)

Salinity severely constrains wheat productivity by disrupting biomass allocation and ionic homeostasis. Although many studies have described individual physiological responses to salinity, fewer have integrated organ-specific ion regulation with whole-plant biomass dynamics to explain why some genotypes tolerate salt better than others do. To address this gap, the present study aimed to identify the physiological mechanisms that underpin salinity tolerance by combining biomass partitioning, organ-resolved ion profiling and multivariate causal modelling. Three wheat genotypes—BAW 1147 (tolerant), BARI Gom 25 (moderately tolerant), and BARI Gom 28 (susceptible)—were evaluated under control, 5 dS m⁻¹ and 10 dS m⁻¹ salinity conditions. The plants were subsequently divided into nine organs to quantify dry weight (DW) and the organ-specific ion status of sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺) and magnesium (Mg²⁺). Salinity reduced the total DW in all the genotypes, but BAW 1147 consistently retained more organ-specific biomass (roots, stems, flag leaf blades) and preserved reproductive allocation. Ion profiles in BAW 1147 revealed increased sequestration of Na⁺ in roots and structural tissues. In contrast, Na⁺ concentrations remained lower in flag leaves and grain. Moreover, K⁺, Ca²⁺ and Mg²⁺ levels were maintained across organs, resulting in superior K⁺:Na⁺ ratios in photosynthetic and reproductive tissues. Correlation analysis indicated stress-dependent strengthening of positive links among biomass traits and beneficial cations, whereas principal component analysis resolved PC₁ as a tolerance axis characterized by increased biomass; higher K⁺, Ca²⁺ and Mg²⁺ concentrations; increased K⁺:Na⁺ ratios; and decreased Na⁺ levels. Structural equation modelling (SEM) revealed K⁺:Na⁺ as the strongest positive causal determinant of shoot biomass and total biomass, with Na⁺ exerting negative effects on photosynthetic and yield tissues. Overall, this integrative approach demonstrates that wheat salinity tolerance arises from coordinated biomass buffering and strategic ion partitioning across organs. These findings provide practical physiological indicators, such as high K⁺:Na⁺, stable Ca²⁺-Mg²⁺ homeostasis and balanced root investment, that can be used to select and develop salt-resilient wheat genotypes for coastal and irrigated saline environments.