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 μmol m−2), neoxanthin (29 vs 41), and β-carotene (54 vs 73), more xanthophyll cycle pigments per unit surface area and per unit chlorophyll (71 vs 53 μmol 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 μmol m−2 s−1 on the sun-exposed side, and about 10 μmol m−2 s−1 on the shaded side. Fv/Fm 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. Fv/Fm 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. Fv/Fm 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 Fv/Fm 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.