L. Decourtye, W. Harris
A significant correlation coefficient of -0.64 was shown between cold damage and the latitude of origin of 15 wild populations of Leptospermum scoparium from New-Zealand grown at Angers, France. The most southern population (Lake Te Anau) appeared to be more sensitive to cold and the population from a higher altitude (Cass), less sensitive to cold than expected from the general regression.

The ability of plants to regrow after frost damage was also significantly correlated with the latitude of origin (+ 0.81). This relationship was useful in being able to discriminate between the most cold sensitive populations. Integrated with the cold damage score to provide a scale of cold sensitivity of 1 to 9 this improved appreciably the correlation of cold tolerance with the latitude of origin (- 0.75).

Sixty of the plants from nine of the populations have not suffered cold damage in the period of observation. Having undergone selection for cold resistance their selection for other horticultural characteristics can be pursued. The species can be vegetatively propagated which will allow the eventual distribution of selected clones with greater cold resistance.

For three of the wild populations grown at Angers, plants raised from seed obtained from the same populations grown at Lincoln, New Zealand were compared. In four of the five comparisons the pattern of cold damage in progeny populations was very like that of the parent populations. A small divergence of increased cold sensitivity was shown in the other case possibly arising from insect pollination with foreign pollen. Therefore cold resistance appears to be strongly heritable. It should be possible to realise even further improvement in cold resistance by hybridisation between the better populations from geographically separated localities which have distinct electrophoretic profiles.

Hybridisation of cold resistant clones with cultivars which have desirable horticultural characteristics such as red flowers is to be pursued to provide a cultivar better suited to outdoor cultivation in colder localities.

Decourtye, L. and Harris, W. (1992). SELECTION FOR COLD RESISTANCE IN LEPTOSPERMUM SCOPARIUM (J.R. ET G. FORST.). Acta Hortic. 320, 39-48
DOI: 10.17660/ActaHortic.1992.320.5

Acta Horticulturae