Frost resistance ln Eucalyptus nitens (deane & maiden). maiden.

Genetic and physiological aspects of variation in frost resistance were investigated in Eucalyptus nitens seedlings. Frost resistance was primarily determined by measuring the relative leakage of electrolytes from leaf discs subjected to artificial frosts, in an air-filled freezing chamber. The method used was a modification of that developed by other researchers with eucalypts. Frost resistance assessments made using leaf discs compared favourably with both artificially frosting whole seedlings and visual assessments of relative frost damage to field plantings subjected to natural frosts. Transmission electron microscope observations indicated that frosted leaf discs with a high relative leakage of cellular electrolytes, had cells that were characterised by rupture of the plasma membrane and tonoplast, and protoplasm coagulation. The effects of photoperiod, temperature regime and differential root and shoot temperatures on frost resistance were examined in both hardening and dehardening E.nitens seedlings. Both hardening and dehardening processes were markedly influenced by temperature regime, but no photoperiodic responses were detected. Exposure to temperatures between 1.5 and 4°C for some part of the day/night cycle resulted in significantly increased frost resistance. Three separate experiments showed maximum hardening rates of c. 0.5°C week-1. Increasing the daily amount of low temperature (2 to 3°C), from 8 to 16 h, resulted in increased levels of frost resistance. In contrast, seedlings exposed to constant 24 h low temperature, were almost 2°C less frost resistant than seedlings grown with 16 h day-1 low temperature after eight weeks, and suffered considerable water stress. Whilst heating seedling roots to between 6 and l8°C throughout the constant low air temperature regime maintained high plant water status, it did not confer any increased frost resistance. Rates of dehardening in well hardened seedlings increased with increasing day and/or night temperature in the range 6 to 20°C, but were significantly reduced if roots were kept at 3°C. The apparent quantitative response of both hardening and dehardening processes to temperature regime is discussed. A comprehensive study of the genetic variation for frost resistance in E.nitens was made by means of artificial frostings, of both whole seedlings and leaf samples, and the assessment of damage from natural frosts to field trials. Two groups of provenances were identified. The Western provenance, from the Central Highlands of Victoria, and the provenance in Northern N.S.W. showed superior hardiness to the Southern N.S.W. and East Gippsland (Errinundra) provenances. This was demonstrated for plants that were unhardened, partially hardened and at near maximum hardiness. The critical frost temperature (50% leaf damage), separating the most and least hardy provenances, increased with the overall level of hardiness from only 0.3 to over 1.0°C, for unhardened and winter hardened plants respectively. At well developed levels of hardiness, individual families hardened to below -10C and differed by as much as 2.3 °C. There were also significant trends of increasing frost resistance with altitude of seed source. The components of variance in frost resistance for \provenances\" were about three times larger than components for \"seedlings-within-provenance\" for seedlings subjected to both artificial and natural frosts. The early growth and frost resistance of E.nitens was compared with that of 12 other Eucalyptus species in trial plantations on two frost prone sites in Tasmania. Differences in levels of frost resistance amongst the species were over 7 and 3°C in winter and summer respectively. Species with the poorest frost resistance in winter such as Efraxinoides E.regnans and E.laevopinea suffered almost complete mortality at the coldest site. Only 16 months after planting there were highly significant differences in height and diameter growth amongst the species with E.nitens by far the most productive. The possible interaction between growth rate and relative frost hardiness is discussed. Artificial frostings of reciprocal grafts between E.nitens and E.gunnii (a species with greater frost resistance) clearly demonstrated that levels of frost resistance in leaf tissue were largely determined by foliage genotype irrespective of root stock genotype. Intraspecific and interspecific controlled pollinations were attempted on a number of E.globulus ssp. globulus maternal parents. On the basis of capsule set data indicate that E.globulus stigma are receptive a few days after anthesis reach maximum receptivity after c. seven days and remain receptive for up to 12 days. Intraspecific and interspecific controlled pollinations were successfully made using a number of E.nitens maternal parents. Generally speaking the maternal influence was stronger than the paternal in terms of both number of seed set and seed weight Controlled breeding methods in E.nitens and E.globulus ssp. globulus and some implications of the reproductive biology of Eucalyptus species for breeding programmes are highlighted. Progeny from crosses onto E.nitens were morphologically intermediate with respect to both parents. The inheritance of morphological characters in eucalypts is discussed. The nature of the frost resistance of interspecific hybrids differed amongst the species combinations sometimes appearing recessive or dominant but more commonly additive. E.nitens X E.gunnii seedlings from six different E.nitens mothers (representing the full range of provenances) all exhibited similar patterns in levels of frost resistance with respect to their parents. After only four weeks hardening E.nitens X E.gunnii seedlings were on average l.3°C more frost resistant than their E.nitens mothers but only 0.5°C less frost resistant than their E.gunnii father. Frost resistance of intraspecific crosses generally displayed similar patterns (with respect to parents) to those of interspecific crosses. Factors to consider when selecting and breeding for frost resistance in eucalypts are discussed."