Influence of nitrogen starvation on physiology and biochemistry of red and green Beta vulgaris leaves: insights into salt stress

Main conclusion: Nitrogen starvation stimulates betacyanin accumulation and shared stress responses in Beta vulgaris, despite nitrogen costs, whereas salt stress reduces pigments and antioxidants, indicating stress-specific, genotype-dependent regulation and unresolved nitrogen sourcing. Abstract: Betacyanins, unlike nitrogen-free anthocyanins, are betalamic immonium derivatives containing core nitrogen and are red-violet pigments found only in plants of the Caryophyllales order, possibly evolving to replace anthocyanins in this order. Their biosynthesis is influenced by endogenous and environmental factors like drought, salinity, and nutrient deficiency, yet their regulation under combined stresses remains poorly understood. This study investigated the role of betacyanins in Beta vulgaris (green- and red-leafed genotypes) under nitrogen starvation (NSt), salinity stress (SS), and their combination. Plants were assessed for biometric traits, gas exchange, photosynthetic pigments, betacyanin content, oxidative stress markers, antioxidant metabolites, and ion accumulation. NSt promoted betacyanin accumulation in green plants but not in red ones, highlighting a genotype-dependent response. In contrast, salinity reduced pigment levels and impaired photosynthetic traits in both genotypes. The combined stress resulted in intermediate responses, suggesting an interaction between nitrogen availability and salinity effects. Overall, our findings indicate that NSt and SS differentially regulate betacyanin accumulation, with responses strongly influenced by genotype, and point to a complex relationship between nitrogen metabolism and pigment biosynthesis under abiotic stress conditions.