Solar UV-B radiation on growth, photosynthesis and the xanthophyll cycle in tropical acacias and eucalyptus

The effects of solar ultraviolet-B (UV-B) radiation on the growth, photosynthesis and the xanthophyll cycle of four acacia and two eucalyptus species in Darwin, Australia, were examined. Solar UV-B radiation seemed to delay plant growth in all species examined even though it did not affect photosynthetic activity significantly. Under solar UV-B radiation, a reduced specific leaf area (SLA), and an increased leaf thickness and size of epidermis were observed in plants. Chlorophyll content decreased but leaf soluble protein content increased in plants under solar UV-B radiation. Solar UV-B radiation apparently had a strong effect on chlorophyll degradation rather than the size of the xanthophyll cycle pool. It seemed that xanthophyll formation was more strongly affected by photosynthetic photon flux density (PPFD) than by solar UV-B radiation. The contents of sugar, sugar alcohol and total sugar did vary dramatically between treatments in all the species examined. Overall, the effect of solar UV-B on plant growth and form was much less detrimental than reported for indoor experiments where high UV-B dosage (relative to growth irradiance) was used. Solar UV-B radiation did not cause any photodamage in acacia and eucalyptus plants examined. It had a significant effect on growth and metabolism such as plant height, canopy, leaf thickness and the contents of chlorophyll and soluble protein.