Screening eucalypts for frost tolerance in breeding programs

Improvements in a laboratory technique and statistical methods for screening eucalypts for relative frost tolerance using small samples of leaf tissue are described. As a contribution to more efficient genetic selection in a breeding program, these methods aim to provide reliable estimates of relative frost tolerance for ranking genotypes and to allow partitioning of total variation into components due to provenances, families, and individuals. Examination of sampling strategies indicated that four leaf discs per seedling at three frosting temperatures and 10 seedlings per family provided highly reliable estimates of frost tolerance. Mean relative conductivity values were found to be highly correlated with mean leaf damage sustained by a seedling across the whole range of values. Alternative methods of data analysis are compared. Estimation of LD50 values (the temperature at which 50% of leaf discs would be killed) and calculation of percent survival of leaf discs were found to be less efficient means of data analysis than directly analysing relative conductivity values. The nature of genetic control of frost tolerance and the assumptions underlying such nondestructive screening techniques are examined.