Frost hardiness of Eucalyptus delegatensis R.T. Baker

A major factor limiting the growth of E. delegatensis is low temperature. This is important both in the natural distribution of the species and as a plantation species both within Australia and elsewhere. This project deals with aspects of the frost hardiness of E. delegatensis including the seasonal variation, genetic variation and comparison with other species of eucalypts. The diffusate electrical conductivity method for measuring frost hardiness of plant tissue was adapted and developed for use with an air-filled frost chamber. The major adaptation was the addition of an ice nucleation agent, silver iodide, to tissue samples during test freezing to prevent supercooling. A thorough evaluation showed that this method is sensitive enough to detect differences of 0.3°C and would be useful for screening large numbers of plants for breeding. Seven provenances of E. delegatensis planted in two provenance trials (planted in 1979) at Tarraleah and Myrtle Bank, Tasmania, were tested for frost hardiness. Plants at Tarraleah were tested at approximately six week intervals throughout 1984, while at Myrtle Bank they were tested three times during the hardening phase. Seasonal differencesin frost hardiness ranged from 2.4°C for the Bicheno provenance (the least hardy) to 4.6°C for the Ben Lomond provenance (the most hardy) at Tarraleah. The maximum hardiness reached ranged from -6.0°C for the Bicheno provenance to -8.6°C for the Ben Lomond provenance at Tarraleah while at Myrtle Bank the range was from -4.7°C for the Bicheno provenance to -7.7°C for the Ben Lomond provenance. The same ranking of provenances at maximum frost hardiness was obtained at both trials. Laboratory simulation of hardening conditions with night temperatures of 12, 4 and 0°C showed that colder night temperatures resulted in greater development of frost hardiness for all provenances tested. The ranking of provenances for frost hardiness corresponded to the field trial. A field trial in the Esperance Valley, Tasmania, (planted in 1983) had two provenances each of E. delegatensis, E. nitens, E. regnans and E. globulus planted at altitudes of 60, 240, 440 and 650 m. One provenance of E. grandis was planted at the 60 and 240 m sites and one provenance of E. pauciflora at the 440 and 650 m sites. All species were tested forfrost hardiness in March and August of 1985. There was no significant difference between species or provenances in March. In August the only species with a significant difference between provenances was E. delegatensis, Significant differences between species were measured in August, when the species ranked in decreasing order of frost hardiness as follows: E. deIegatensis = E. nitens > E. pauciflora > E. globulus > E. grandis > E. regnans. It was found that the lowest minimum temperatures occurred at the 60 m site followed by the 650 m site then the 440 m site with the 240 m site having the highest minimum temperatures. The frost hardiness of the plants tested also followed this pattern with the greatest development of frost hardiness at the 60 m site. Growth of the plants corresponded to the altitudinal sequence with most height and diameter growth at the 60 and 240 m sites which experienced the warmest maximum temperatures. The development of a reliable method of testing plant tissue for frost hardiness is important for plant breeding, since both seedlings and established plants can be tested. Testing of established E. delegatensis showed that there is significant variation in frost hardiness within the species and that there is no significant interaction between frost hardiness and site within Tasmania. Frost hardiness development in E. delegatensis is a response to low night temperatures and is independent of day temperature and growth rate. E. delegatensis compared favourably with other commercially planted species of eucalypts in terms of frost hardiness. It was shown that it is possible to achieve good growth of eucalypts on a frost-prone site provided suitable provenances are planted.