Eucalyptus Physiology. II. Temperature Responses

Abstract

Seedlings of 13 Eucalyptus species were grown at three temperature regimes. One species was exposed to sublethal high temperatures. Another species was frost-hardened by low temperatures. Maximum growth of species from hot low latitudes of Australia involved high-temperature optima. These were higher than the medium optima of species from the cooler southern latitudes. Such relationships between temperature optimum for growth and temperature of location of seed source were also observed among latitudinal provenances of E. camaldulensis and among altitudinal provenances of E. paucifora and E. viminalis. These examples of adaptive variation involved temperature optima, not maximum growth rate. The absence of high optima in southern species and high-altitude provenances was often associated with severe growth abnormalities and poor survival of plants grown at 33°C and above. Frost resistance of E. viminalis depended on exposure to near freezing, hardening temperatures for about 2 days. Altitudinal ecotypes for level of resistance and for rate of hardening were sometimes detected in such low-temperature responses. These results support the concept that the distribution of Eucalyptus species is related to their tolerance of extreme conditions rather than to their maximum growth rate under less severe conditions. The relative insensitivity of the genus to day length suggests that this concept should emphasize the importance of temperature but exclude day length and the combined effects of both.