sodininkystė ir daržininkystė 25(4)

extremely high frequency as though all membranes were destroyed; τ is a generalized
time constant of the tissue; ψ is a distribution factor of the time constant
(Repo et al., 1993); ψ is a complex number operator [(-1) 1/2 ] and ω is angular
frequency of the alternate current. The ô and ø parameters characterize the whole
system and describe the distribution of cell sizes. Impedance parameters were
normalized to values for 1 cm 3 of tissue. In this report, all data are normalized and
all resistances are expressed as ohm·cm -1 .
For the seedling experiment, mortality tests were conducted on the plants once
they were removed from cold storage, moved to the greenhouse, defoliated with
scissors, and regrown for four weeks. Plant viability was measured as percent seedling
survival. This was collected as binomial data with each plant given a rating of
either 1 or 0. Only plants that had a ≥ 50% stem death were recorded as 0. Percentages
of plant population mortality were then calculated by dividing the number of
plants given a rating of zero over the total number of seedlings per tray per selection
population.
Statistical analysis. With the first method, the mortality of apple cultivars was
related to a series of freezing temperatures by logistic regression, from which the
temperature that 20% of trees died was determined (i.e., LT 20
). The predicted values
for LT 20
were obtained from a mixed model (REML) analysis, with year and cultivar
as fixed factors. The morbidity of the surviving trees, which were planted in the
field, was assessed by the increases in root volume and trunk diameter and reduction
in shoot growth. The increase in root volume was calculated for each individual tree.
Both root volume and trunk diameter increases were regressed on the log scale for
each group of cultivar and replicate trees; the percentage increase was calculated
from the ratio of the mean final to mean initial measurements. This ratio of the
means is a more precise estimate of the fractional increase than from the mean of the
ratios from individual trees. To calculate the temperature at which 50% regrowth
occurred a “linear divided by linear” functional relationship was used, i.e., Y = a + (b /
(1 + c t)), where t = temperature, and from which the temperature at RT 50
was
calculated using the estimated parameters. A hardiness index was computed by averaging
the results of the four regrowth attributes. Correlations between our results
(root volume RT 50
) and the Champlain Valley field results (% survival) were used to
validate our findings.
With the second method, unit weighting statistical procedures were used for all
impedance spectral analyses. In most analyses, the full range impedance data (100
Hz to 800 KHz) were used. Please refer to Privé and Zhang (1996) for details.
With the last method, the survival of the three genotypes to freezing was analysed
as a General Linear Model (GLM), (SAS Institute Inc., Cary, N. C.) with the
number of plants surviving as a binomial variant with a logistic link function (McCullagh
and Nelder, 1983). The data were back-transformed [y = 100×exp(p)/(1 +
exp(p))] in order to express the data as percentages.
Results. The whole-plant controlled freezing experiments followed by regrowth
measurements provided a good segregation of the cold hardiness of various
apple rootstocks (Table 1). In the subsample of rootstocks presented in this
manuscript, KSC28 had the highest (-14.3°C) while M.4 had the lowest (-8.1°C)
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