Comparative physiological and ionic mechanisms underlying differential salt tolerance in two kiwifruit cultivars

Soil salinization, exacerbated by climate-induced degradation, poses a major threat to agricultural sustainability, severely limiting kiwifruit ( Actinidia chinensis var. deliciosa ) production in vulnerable regions like the Spanish Mediterranean. However, the specific physiological and biochemical mechanisms that trigger salinity resilience in contrasting commercial kiwifruit genotypes remain poorly understood, creating a critical knowledge gap for stress-tolerance breeding. To address this challenge, this study introduces a novel comparative framework by evaluating the distinct physiological and biochemical responses of the global commercial genotype ’Hayward’, and its primary pollinator in Spain ‘Tomuri’, as contrasting physiological models. Under severe salinity stress, both cultivars suffered biomass and photosynthetic efficiency losses. However, ‘Tomuri’ exhibited superior resilience. This tolerance was driven by an enhanced capacity to maintain cellular ion homeostasis, where optimized potassium transport prevented sodium toxicity by sustaining a favorable leaf Na⁺/K⁺ ratio. Furthermore, 'Tomuri' actively triggered biochemical protection against osmotic stress through targeted proline accumulation, which served as both an osmolyte and a cellular stabilizer under saline pressure. By uncovering these distinct genotype-specific mechanisms, this work provides a foundational breakthrough for kiwifruit breeding programs to engineer resilient rootstocks, while offering scalable physiological insights to secure fruit crop production within the broader framework of sustainable saline agriculture.