Is the bark of shining gum (Eucalyptus nitens) a sun or a shade leaf?

Photoinhibition and pigment composition of green stem tissues of field-grown adult Eucalyptus nitens were measured on clear spring days with low morning temperatures-conditions that cause photoinhibition in leaves of many plant species. The sun-exposed (north-facing) bark contained less chlorophyll a+b (531 vs 748 mu mol m(-2)), neoxanthin (29 vs 41), and beta-carotene (54 vs 73), more xanthophyll cycle pigments per unit surface area and per unit chlorophyll (71 vs 53 mu mol m(-2) and 141 vs 66 mmol mol(-1) chlorophyll), and less lutein per unit chlorophyll (239 vs 190) than the shaded (southern) side. Maximum electron flow rates were 60 mu mol m(-2) s(-1) on the sun-exposed side, and about 10 mu mol m(-2) s(-1) on the shaded side. F-v/F-m was always lower than 0.8 on the sun-exposed side and the de-epoxidation state (DEPS) of the xanthophyll cycle was dominated by zeaxanthin in midday samples. F-v/F-m increased quickly after darkening, but DEPS recovered more slowly to 40% overnight. This suggested that rapidly reversible pH-dependent quenching was responsible for the bulk of changes in PS II efficiency. F-v/F-m remained below 0.8 overnight, which may well be indicative of photo-damage to PSII. In contrast, DEPS of the shaded side was lower, and F-v/F-m was higher, than for the sun-exposed side. We conclude that E. nitens chlorenchyma on the sun-exposed stem side has a photosynthetic pigment composition similar to sun leaves and it experiences significant photoinhibition in the field.