© 2007 Heron Publishing—Victoria, Canada
Photoinhibition, carotenoid composition and the co-regulation of photochemical and non-photochemical quenching in neotropical
savanna trees
Augusto C. Franco (1,2), Shizue Matsubara (3,4) and Birgit Orthen (5)
1. Universidade de Brasília, Depto. de Botânica, Caixa Postal 04457, 70904-970 Brasília, DF, Brazil / 2. Corresponding author (acfranco@unb.br) / 3. Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia / 4. Present address: Forschungszentrum Jülich GmbH, ICG-III, D-52425 Jülich, Germany / 5. Universität Münster, Institut für Ökologie der Pflanzen, D-48143 Münster, Germany / Received February 27, 2006; accepted July 11, 2006; published online February 1, 2007
Summary
Plants in the neotropical savannas of central Brazil are exposed to high irradiances, high air temperatures and low relative
humidities. These conditions impose a selection pressure on plants for strong stomatal regulation of transpiration to maintain
water balance. Diurnal adjustments of non-photochemical energy dissipation in photosystem II (PSII) provide a dynamic mechanism
to reduce the risk of photoinhibitory damage during the middle of the day when irradiances and leaf temperatures are high
and partial closure of the stomata results in considerable reductions in internal CO2 concentration. At the end of the dry season, we measured diurnal changes in gas exchange, chlorophyll fluorescence parameters
and carotenoid composition in two savanna tree species differing in photosynthetic capacity and in the duration and extent
of the midday depression of photosynthesis. Non-photochemical quenching and its quantum yield were tightly correlated with
zeaxanthin concentrations on a total chlorophyll basis, indicating that the reversible de-epoxidation of violaxanthin to antheraxanthin
and zeaxanthin within the xanthophyll cycle plays a key role in the regulation of thermal energy dissipation. In both cases,
a single linear relationship fitted both species. Although efficient regulation of photochemical and non-photochemical quenching
and adjustments in the partitioning of electron flow between assimilative and non-assimilative processes were operating, these
trees could not fully cope with the rapid increase in irradiance after sunrise, suggesting high vulnerability to photoinhibitory
damage in the morning. However, both species were able to recover quickly. The effects of photoinhibitory quenching were largely
reversed by midday, and zeaxanthin rapidly converted back to violaxanthin as irradiance decreased in late afternoon, resulting
in the maximal quantum yield of PSII of around 0.8 just before sunrise.
Keywords:
Cerrado, chlorophyll fluorescence, fluorescence quenching, quantum yield of non-photochemical quenching, photosynthesis, xanthophyll
cycle.
