SEASONAL DIFFERENCES IN XANTHOPHYLL CYCLE CHARACTERISTICS AND ANTIOXIDANTS IN MAHONIA REPENS GROWING IN DIFFERENT LIGHT ENVIRONMENTS

We investigated differences between summer and winter in photosynthesi s, xanthophyll cycle-dependent energy dissipation, and antioxidant sys tems in populations of Mahonia repens (Lindley) Don growing in the eas tern foothills of the Colorado Rocky Mountains in deep shade, full exp osure, and under a single-layered canopy of Finns ponderosa (partially shaded). In summer, increasing growth irradiance (from deep shade to partial shade to full exposure) was associated with increased xanthoph yll cycle-dependent energy dissipation in PSII and an increased capaci ty to detoxify reactive reduced oxygen species, as measured by increas es in the activities of ascorbate peroxidase, superoxide scavenging, g lutathione reductase, and monodehydroascorbate reductase, as well as i ncreases in leaf ascorbate and glutathione content. Leaves of exposed and partially shaded plants exhibited decreased capacities for photosy nthetic O-2 evolution in winter compared to summer, while in the deepl y shaded plants this parameter did not differ seasonally. Seasonal dif ferences in the levels of antioxidants generally exhibited an inverse response to photosynthesis, being higher in winter compared to summer in the exposed and partially shaded populations, but remaining unchang ed in the deeply shaded population. In addition, total pool size and c onversion state of the xanthophyll cycle were higher in winter than in summer in all populations. These trends suggest that both xanthophyll cycle-dependent energy dissipation in PSII and the capacity to detoxi fy reactive reduced oxygen species responded to the level of excess li ght absorption.