Photosynthetic responses to overnight frost in Eucalyptus nitens and E. globulus

Significant expansion in the area of eucalypt plantations in Tasmania has led to their establishment at altitudes that are close to the upper limits of the planting distributions of Eucalyptus nitens and E. globulus, the main species planted. This has implications for plantation productivity. We investigated the processes that limit productivity in these environments through a study of freezing-induced depression of photosynthesis of E. nitens saplings in the field and plantlets of E. nitens and E. globulus clones in a controlled environment cabinet. In the field consecutive frosts of around –4.6°C had a cumulative effect, reducing maximum net photosynthesis ( A max) by 17%, and then a further 9%, respectively, compared with saplings insulated from the frosts. Shading saplings pre-dawn had no effect on Amax measured after 1030 hours indicating that the reduction in Amax at this time was independent of photoinhibition. Recovery of Amax to pre-frost levels required at least two consecutive frost-free nights and was dependent on the severity of frost. Photosynthetic light response curves indicated that reduced Amax was associated also with decreased quantum yield and stomatal conductance. Similar intracellular carbon dioxide concentration between exposed and insulated saplings indicated that low stomatal conductance did not limit photosynthesis through carbon dioxide limitation. The timing of frost events was critical: E. nitens saplings took less time to recover from reduced Amax in the field when they were hardened. Unhardened plantlets of E. nitens and E. globulus clones had greater reduction of Amax and took longer to recover from frost events than hardened plantlets. E. globulus was more susceptible to frost-induced reduction of Amax than E. nitens. This is consistent with its planting range which is restricted to mild sites compared with that of E. nitens.