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Nothofagus dombeyi  (Mirb.) Blume and  Nothofagus nitida  (Phil.) Krasser, two evergreens in the South Chilean forest, regenerate in open habitats and under the canopy, respectively. Both overtop the forest canopy when they are in the adult stage, suggesting that their photoprotective mechanisms differ in ontogenetic dynamics. We postulated that  N .  nitida , a shade-tolerant species increases its capacity to tolerate photoinhibitory conditions (low temperature and high irradiance) by thermal energy dissipation of excess energy during its transition from the seedling to the adult stage, whereas  N .  dombeyi , a shade-intolerant species, maintains a high capacity for photoprotection by thermal energy dissipation from the seedling to the adult stage. To test this hypothesis, the main photoprotective mechanisms in plants – the fast- and slow-relaxing components of thermal energy dissipation (NPQ, non-photochemical quenching) NPQ F  and NPQ S , respectively, and state transitions – were studied in seedlings and adults of both species grown in their natural habitats and in a common garden. In adults, NPQ F  and NPQ S  did not differ between species and seasons. The greatest differences in these parameters were observed in seedlings. The xanthophyll cycle was more active in  N .  dombeyi  seedlings than in  N .  nitida  seedlings at low temperature and high irradiance, consistent with a higher NPQ F  in  N .  dombeyi . Under all study conditions,  N .  nitida  seedlings had higher NPQ S  than  N .  dombeyi  seedlings. The state transition capability was higher in  N .  nitida  seedlings than in  N .  dombeyi  seedlings. Therefore, although (shade-intolerant)  N .  dombeyi  was able to thermally dissipate the excess absorbed energy, under natural conditions its photochemical energy quenching was efficient in both developmental stages, decreasing its need for thermal dissipation. In contrast, the seedlings of  N .  nitida  were more sensitive to photoinhibition than the adult trees, suggesting a change from shade-grown to sun-exposed phenotype from the seedling to the adult stage. These results help to explain the differences in the regeneration patterns of  N .  nitida  and  N .  dombeyi  and the presence of  N .  nitida  adult stage in the upper canopy.

tree physiology

Thermal energy dissipation and its components in two developmental stages of a shade-tolerant species,  Nothofagus nitida , and a shade-intolerant species,  Nothofagus dombeyi