Canopy processes in a changing climate.

Forest canopies exchange a large part of the mass and energy between the earth and the atmosphere. The processes that regulate these exchanges have been of interest to scientists from a diverse range of disciplines for a long time. The International Union of Forest Research Organizations (IUFRO) Canopy Processes Working Group provides a forum for these scientists to explore canopy processes at scales ranging from the leaf to the ecosystem. Given the changes in climate that are being experienced in response to rising [CO(2)], there is a need to understand how forest canopy processes respond to altered environments. Globally, native and managed forests represent the largest terrestrial biome and, in wood and soils, the largest terrestrial stores of carbon. Changing climates have significant implications for carbon storage in forests, as well as their water use, species diversity and management. In order to address these issues, the Canopy Processes Working Group held a travelling workshop in south-east Australia during October 2010 to examine the impact of changing climates on forest canopies, highlighting knowledge gaps and developing new research directions.

Xanthophyll-cycle dynamics and rapid induction of anthocyanin synthesis in Eucalyptus nitens seedlings transferred to photoinhibitory conditions.

The effects of rapid induction and subsequent relaxation of cold-induced photo-inhibition on pigment chemistry of Eucalyptus nitens (Deane and Maiden) Maiden seedling leaves was assessed. The seedlings were subjected to four treatments in a nursery, fertilised or non-fertilised and shaded or non-shaded, before induction of photoinhibition in a growth chamber. Within 2 days, growth chamber conditions decreased photochemical efficiency (Fv/Fm) and increased xanthophyll-cycle conversion ratio. This was associated with decreased levels of total chlorophyll and increased levels of xanthophyll-cycle pigments. After 8 days, Fv/Fm of all treatments rose. Anthocyanin Levels gradually increased until day eight, except in non-shaded, non-fertilised seedlings, which had high levels before the induction of photoinhibition. Visible spectroscopy indicated increased absorption between 500 and 590nm, indicative of anthocyanin absorption. Electron transport rate after xanthophyll-cycle relaxation was half that measured before induction of photoinhibition. The findings indicate that anthocyanins may provide a photoprotective role in E. nitens seedlings under conditions of photoinhibition.

Alternate energy dissipation? Phenolic metabolites and the xanthophyll cycle.

The dynamics of phenolic galloylglucoses (di-, tri-, tetra- and penta-galloylglucose), flavonoids (quercitin and quercitin glycosides) and sideroxylonal were compared with that of xanthophyll cycle-dependent energy dissipation during rapid induction of chilling-dependent photo-inhibition. Pre-dawn xanthophyll cycle engagement of seedlings of Eucalyptus nitens transferred from mild nursery conditions to a low temperature controlled environment increased logarithmically during eight days of treatment. Photochemical efficiency and flavonoids decreased after four days of treatment and non-photochemical quenching after two days of treatment. Galloylglucoses and sideroxylonal decreased linearly during treatment. These results demonstrate that rapid changes in foliar phenolic levels are associated with abrupt changes in the plant environment. It is argued that under these growth-chamber conditions, the xanthophyll cycle facilitated dissipation of excess light energy, lessening the requirement for the dissipation of energy or antioxidant activity through phenolic metabolites.

Interactive effects of nitrogen and irradiance on sustained xanthophyll cycle engagement in Eucalyptus nitens leaves during winter.

Eucalyptus nitens is a species that is adapted to low temperature. This study examines xanthophyll-cycle engagement in E. nitens seedlings exposed to cold-induced photoinhibitory conditions under different levels of irradiance and nutrient status. Xanthophyll-cycle pool size indicated an increased requirement for light energy dissipation under high irradiance and low nutrient status. Greater sensitivity to photoinhibition of non-shaded seedlings indicated that sustained xanthophyll-cycle engagement may occur in response to damaged chlorophyll. Within irradiance treatments, fertilised seedlings had higher photochemical efficiency and faster recovery from photoinhibition than unfertilised seedlings. These results demonstrate that fertilised compared to unfertilised seedlings can utilise a greater proportion of incident light under cold temperature conditions

Cold hardening reduces photoinhibition of Eucalypts nitens and E. pauciflora at frost temperatures.

Photoinhibition of photosynthesis at low temperatures was investigated in two species of subalpine eucalypt, Eucalypts nitens (Deane and Maiden) Maiden and E. pauciflora Sieb. ex Spreng. Imposition of an artificial cold-hardening treatment increased the frost tolerance of leaf tissue and increased tolerance to excess light. Cold-hardened seedlings of both species had a higher photosynthetic capacity than non-hardened seedlings at 6 and 16°C and lower levels of non-photochemical quenching (NPQ) at 20 and 5°C. Furthermore, hardened seedlings had faster rates of NPQ development at 5 and -3.5°C. An increase in minimal fluorescence, which indicates slowly reversible photoinhibition, was evident in all seedlings at -1.5 and -3.5°C but was less pronounced in hardened seedlings, with a threefold faster rate of development of NPQ, at -3.5°C than non-hardened seedlings. Hardened seedlings also recovered faster from photoinhibition at -3.5°C. Thus cold hardening increased tolerance to high light in these species. Differences between E. nitens and E. pauciflora in their response to excess light were small and significant only at -3.5°C. Faster recovery from photoinhibition of E. pauciflora was consistent with its occurrence in colder habitats than E. nitens.