## 1 Introduction  
Calcium (Ca\({}^{2+}\)) is an essential element in plants, acting as a second messenger in response to extracellular signals. The Ca\({}^{2+}\)/cation antiporter (CaCA) superfamily is critical in regulating and accumulating calcium [1, 2, 3]. This superfamily encompasses distinct clades with varied phylogenetic, structural, and functional characteristics [4]. In plants, CaCA proteins are classified into four families: H\({}^{+}\)/cation exchangers (CAXs); Na\({}^{+}\)/Ca\({}^{2+}\) exchanger-like proteins (NCLs); cation/Ca\({}^{2+}\) exchangers (CCXs); and Mg\({}^{2+}\)/H\({}^{+}\) exchangers (MHXs) [5, 6]. Among these, CAXs and NCLs are implicated in abiotic stress responses [3, 7], and play roles in hormone signaling and flowering [8, 9]. CAXs may form heteromeric transporters influencing guard cell and mesophyll cell functions related to environmental adaptability [10]. Additionally, CaCA proteins are being explored for their potential in biofortification and phytoremediation due to their roles in metal ion uptake [11; 12; 13]. Notably, _CAX1a_ in _Brassica rapa_ and the heterologous expression of _Triticum urartu TucCAX1a_ and _TuCAX1b_ in _Arabidopsis thaliana_, play a pivotal role in calcium homeostasis, nutrient translocation, and metabolic regulation under calcium stress and heavy metal conditions [14; 15]. _CAX1_ also inhibits the formation of reactive oxygen species induced by Cd, a stress tolerance mechanism [16]. These varied reports highlight the multifunctionality of the CaCA gene family in plants. _Rosa roxburghii_, commonly known as 'Cili' in China, is a wild deciduous perennial shrub of the Rosaceae family. It is noted for its golden, spine-covered fruits. The fruits emit a faint aroma and have a slightly tart taste but are primarily valued for their nutritional and medicinal benefits [17]. This shrub thrives in the cool, hilly regions of Southwestern China, particularly in Guizhou Province [18]. Beyond wild varieties, cultivated germplasm resources like 'Guinong 5' (Rr-5) were developed [19]. With growing research interest due to its medicinal and commercial values, _R. roxburghii_ fruit is rich in carbohydrates, amino acids, vitamins, proteins, minerals, and dietary fibers. Other bioactive compounds, such as superoxide dismutase (SOD), organic acids, polysaccharides, flavonoids, polyphenols, triterpen, and glycosides, contribute significantly to its medicinal properties. Studies have demonstrated its antioxidative, anti-tumor, anti-inflammatory, anti-radiation, anti-diabetic, and anti-aging effects [20; 21; 22]. _R. roxburghii_ is also widely used in the food industry, enhancing the flavor of products such as herbal teas, jams, vinegar, yogurts, and mooncakes [23]. Its diverse applications and economic significance establish it as a valuable horticultural crop. Originating from the karst regions of Southwestern China, _R. roxburghii_ is a distinctive economic tree species with broad adaptability to various calcareous habitats [24]. As a calciphilic species, _R. roxburghii_ demonstrates a robust response to the addition of exogenous Ca\({}^{2+}\), marked by the upregulation of the genes involved in the ascorbate (AsA) synthesis pathway, including _L-galactono-1,4-lactone dehydrogenase (GalLDH)_ and _galactose-1-phosphate phosphatase (GPP)_, as well as genes related to AsA regeneration, such as _dehydroascorbate reductase (DHAR)_ and _monodehydroascorbate reductase (MDHAR)_[25; 26]. This leads to enhanced AsA synthesis and accumulation, which plays a pivotal role in improving the antioxidant capacity of _R. roxburghii_ fruits. Exogenous Ca\({}^{2+}\) also significantly promotes the accumulation of bioactive compounds, such as flavonoids and triterpen, which contribute to the fruit nutritional and medicinal properties [27]. Despite multiple studies on the impact of calcium on the quality of _R. roxburghii_, comprehensive research on its adaptation to high-calcium conditions is still lacking.