Species- and Processing-Dependent Variability of Ascorbic Acid in Fruits of 14 <i>Rosa</i> Species and its Redox Behavior toward Iron and Copper Ions

Ascorbic acid (AA) is a key phytochemical marker of rosehip (Rosa spp.) fruits, but its functional behavior in complex plant matrices remains unclear. Here, we analyzed AA variability in fruits of 14 Rosa spp. (0.26–5.57% dry weight) and evaluated the effects of processing and redox activity toward iron and copper. AA content varied markedly among species and was strongly influenced by postharvest conditions, with short-term drying at 60–80 °C preserving AA most effectively, whereas prolonged drying caused degradation and freezing (−18 °C, 1 year) had no significant effect. Despite large differences in AA content, copper reduction was observed for all samples and showed no correlation with AA concentration. In contrast, iron reduction was weaker, restricted to acidic conditions (pH 4.5–5.5), and correlated with AA content at pH 4.5 after exclusion of an outlier species (R2 = 0.72, p = 0.008) but not at pH 5.5. A substantial proportion of commercial samples contained little or no detectable AA, indicating high variability in product quality. These findings demonstrate that the redox behavior of rosehip fruits is governed by phytochemical matrix effects rather than AA concentration alone and highlight the importance of species selection and processing conditions in determining AA content.