Physiological and Nutritional Mechanisms Underlying the Crimson Coloration of Boxwood on Ultramafic Soils of Sugashima Island, Japan

ABSTRACT The evergreen shrub Buxus microphylla (Buxaceae; Japanese boxwood) forms distinctive ultramafic vegetation in Japan, with populations on Sugashima Island exhibiting unusually vivid crimson leaves in winter. To clarify the physiological and nutritional mechanisms underlying this coloration, we combined field monitoring with laboratory experiments to reproduce leaf reddening under controlled conditions. Field observations revealed that leaves of B. microphylla growing on ultramafic soils turned bright red each winter, whereas those from non‐ultramafic sites remained mostly green, except for slight reddening at the Mie University site. The crimson coloration coincided with a seasonal decline in leaf nitrogen (N) and chlorophyll content, suggesting that nutrient limitation enhances pigment imbalance. In laboratory assays, light exposure and sucrose addition strongly promoted chlorophyll degradation, increasing the carotenoid‐to‐chlorophyll ratio (Car/Chl) by more than 20%. Under dark conditions, both sucrose and N enhanced pigment retention. The Car/Chl ratio consistently ranked as water‐only > N‐only > multi‐nutrient media, indicating that nutrient sufficiency counteracts sugar‐induced photoinhibition and chlorophyll breakdown. These results demonstrate that leaf reddening in boxwood is not due to anthocyanin synthesis but rather to a relative enrichment of carotenoids following chlorophyll loss under photoinhibitory conditions. The combination of high irradiance, low temperature, and N deficiency on ultramafic soils promotes this imbalance, producing the intense “crimson boxwood” phenotype. Our findings highlight how nutrient limitation and photoprotection interact to shape adaptive pigmentation in plants growing on ultramafic soils.