Stomata-Photosynthesis Synergy Mediates Combined Heat and Salt Stress Tolerance in Sugarcane Mutant M4209

Sugarcane (Saccharum officinarum L.) is an economically important long-duration crop which is currently facing concurrent heat waves and soil salinity. The present study evaluates an inducible salt-tolerant sugarcane mutant M4209, developed via radiation-induced mutagenesis of elite check variety Co 86032, under heat (42/30°C; day/night), NaCl (200 mM) or heat + NaCl (HS)-stress conditions. Though heat application significantly improved plant growth and biomass in both genotypes, this beneficial impact was partially diminished in Co 86032 under HS-stress conditions, coinciding with higher Na⁺ accumulation and lower triacylglycerol levels. Besides, heat broadly equalised the negative impact on NaCl stress in terms of various physiological and biochemical attributes in both the genotypes, indicating its spaciotemporal advantage. The simultaneous up- and downregulation of antagonistic regulators, epidermal patterning factor (EPF) 9 (SoEPF9) and SoEPF2, respectively attributed to the OSD (Open Small Dense) stomatal phenotype in M4209, which resulted into enhanced conductance, transpirational cooling and gaseous influx. This led to improved photoassimilation, which was supported by higher plastidic:nonplastidic lipid ratio, upregulation of SoRCA (Rubisco activase) and better source strength, resulting in overall plant growth enhancement across all the tested stress scenarios. Taken together, the present study emphasised the knowledge-driven harnessing of stomatal-photosynthetic synergy for ensuring global sugarcane productivity, especially under "salt-heat" coupled stress scenarios.