| The carotenoids of the xanthophyll cycle are involved in an important photoprotective mechanism in plants whereby excessive absorbed excitation energy is dissipated harmlessly in the light-harvesting complexes of photosystem II (PSII). The combined exposure of plants to high light and environmental stress can cause increases in the amount of excess light absorbed by plants, and can also result in persistent low PSII efficiency. The research in this dissertation describes a characterization of the xanthophyll cycle in plants acclimated to environmental stress with two major goals in mind; (1) to assess whether there was an increased utilization of xanthophyll cycle-dependent energy dissipation in plants acclimated to conditions of environmental stress, and (2) to characterize a possible role for sustained xanthophyll cycle-dependent energy dissipation that persists days in plants exhibiting stress-induced low PSII efficiency. Two types of environmental stress were examined, the effects of low nitrogen availability on spinach plants, and the effects of low temperatures during winter on several overwintering species.; While increases in xanthophyll cycle-dependent energy dissipation were found in all cases in response to increased stress, low PSII efficiency that persisted through the night was not always observed. The latter was only found in those species where the response to stress involved a downregulation of photosynthesis while chlorophyll was largely retained. This occurred in both the perennial evergreens Euonymus kinutschovicus and Pinus ponderosa during winter. Leaves of spinach plants grown under conditions of limiting N availability, on the other hand, exhibited proportional decreases in photosynthetic activity and chlorophyll content and very little nocturnally persistent lowering of PSII efficiency. Furthermore the herbaceous plant Malva neglecta exhibited much higher rates of photosynthesis in the winter compared to summer and no lowering of PSII efficiency that persisted upon warming.; Studies monitoring the recovery of winter-stressed plants upon warming in the laboratory provided evidence that dark-sustained xanthophyll cycle-dependent energy dissipation was contributing to the persistent low PSII efficiencies. The data suggest that retained xanthophyll cycle-dependent energy dissipation was facilitated by more than one mechanism. The retention of protons within the thylakoid lumen in darkness likely explained a portion of the persistent xanthophyll cycle-dependent energy dissipation which reversed rapidly upon warming leaves. A second component contributing to the reduced PSII efficiencies, which was observed only in sun leaves and reversed only slowly upon warming, appeared to involve protein rearrangement that maintained xanthophyll cycle-dependent energy dissipation via an unknown mechanism. This component may involve a redistribution of the xanthophyll cycle pigments zeaxanthin and antheraxanthin from the major light-harvesting complexes (LHCII) to the inner light harvesting complexes (CP26 and CP24). |
