Carbon Quantum Dots and Ectoine Foliar Application Enhance Heat Tolerance of Gentiana rhodantha

Abstract

Heat stress exerts detrimental effects on the survival, growth, and productivity of Gentiana rhodantha Franch. ex Hemsl., a medicinal herb endemic to Southwestern China. The application of exogenous protective substances has been widely recognized as an effective and economical strategy for alleviating heat damage in crops. However, most previous studies have focused on conventional agents such as phytohormones, osmolytes, or mineral nutrients, while the potential of emerging nanomaterials and microbial-derived compatible solutes remains largely unexplored. In the present study, we investigated the effects of foliar application of two promising exogenous candidates, (i) carbon quantum dots (CQD), a newly emerged carbon-based nanomaterial with unique photochemical and electron-transfer properties, and (ii) ectoine (ECT), a microbial-derived compatible solute renowned for its exceptional protein and membrane-stabilizing capacity under abiotic stress, applied individually or in combination (CQD + ECT) on the agronomic and physiological traits of two G. rhodantha genotypes (Kaili and Dafang) under normal temperature (NT, 25/20 °C) and high temperature (HT, 35/30 °C) conditions, with H2O application as the control. The results showed that HT inhibited plant growth, reduced canopy traits, altered root architecture, and disrupted antioxidant enzyme activities and osmolyte accumulation, with pronounced genotypic differences. The KL genotype exhibited greater inherent heat tolerance than the DF genotype, as reflected by relatively less injury to canopy traits and root architecture, as well as more favorable physiological adjustments under HT. Exogenous application of CQD and ECT, particularly their combination, generally mitigated heat-induced injury. Under HT, the combined CQD + ECT treatment preserved canopy structure and root morphology, enhanced SPAD values and leaf nitrogen concentrations, and modulated antioxidant enzyme activities and osmolyte levels, with the most pronounced effects observed in the KL genotype. The combined application of CQD and ECT consistently exhibited superior protective effects across multiple parameters, suggesting that it represents a novel and promising strategy for alleviating heat injury in G. rhodantha cultivation. These findings provide new insights into the application of nanomaterial and compatible solute-based approaches for improving heat tolerance in medicinal plants.