Networks of physiological adjustments and defenses, and their synergy with sodium (Na+) homeostasis explain the hidden variation for salinity tolerance across the cultivated Gossypium hirsutum germplasm

<p>The abilities to mobilize and/or sequester excess ions within and outside the plant cell are important components of salt-tolerance mechanisms. Mobilization and sequestration of Na⁺ involves three transport systems facilitated by the plasma membrane H⁺/Na⁺ antiporter (SOS1), vacuolar<i> </i>H⁺/Na⁺ antiporter (NHX1), and Na⁺/K⁺ transporter<i> </i>in vascular tissues (HKT1). Many of these mechanisms are conserved across the plant kingdom. While <i>Gossypium hirsutum</i> (upland cotton) is significantly more salt-tolerant relative to other crops, the critical factors contributing to the phenotypic variation hidden across the germplasm have not been fully unraveled. In this study, the spatio-temporal patterns of Na⁺ accumulation along with other physiological and biochemical interactions were investigated at different severities of salinity across a meaningful genetic diversity panel across cultivated upland <i>Gossypium. </i>The aim was to define the importance of holistic or integrated effects relative to the direct effects of Na⁺ homeostasis mechanisms mediated by <i>GhHKT1, GhSOS1,</i> and <i>GhNHX1. </i>Multi-dimensional physio-morphometric attributes were investigated in a systems-level context using univariate and multivariate statistics, <i>randomForest,</i> and path analysis. Results showed that mobilized or sequestered Na⁺ contributes significantly to the baseline tolerance mechanisms. However, the observed variance in overall tolerance potential across a meaningful diversity panel were more significantly attributed to antioxidant capacity, maintenance of stomatal conductance, chlorophyll content, and divalent cation (Mg²⁺) contents other than Ca²⁺ through a complex interaction with Na⁺ homeostasis. The multi-tier macro-physiological, biochemical and molecular data generated in this study, and the networks of interactions uncovered strongly suggest that a complex physiological and biochemical synergy beyond the first-line-of defense (Na⁺ sequestration and mobilization) accounts for the total phenotypic variance across the primary germplasm of <i>Gossypium hirsutum</i>. These findings are consistent with the recently proposed Omnigenic Theory for quantitative traits and should contribute to a modern look at phenotypic selection for salt tolerance in cotton breeding.</p>