Networks of Physiological Adjustments and Defenses, and Their Synergy With Sodium (Na) Homeostasis Explain the Hidden Variation for Salinity Tolerance Across the Cultivated Germplasm.

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 H/Na antiporter (NHX1), and Na/K transporter in vascular tissues (HKT1). Many of these mechanisms are conserved across the plant kingdom. While (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 The aim was to define the importance of holistic or integrated effects relative to the direct effects of Na homeostasis mechanisms mediated by , and Multi-dimensional physio-morphometric attributes were investigated in a systems-level context using univariate and multivariate statistics, , 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 (Mg) contents other than Ca 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 . 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.