sodininkystė ir daržininkystė 25(4)

Valley of British Columbia, low temperatures during late fall and early winter were
the main climatic factors limiting apple production in this area. Similar studies have
been done for Atlantic Canada but low temperatures were not found to be associated
with low production (Privé et al., 2000). However, what seems common to apple
rootstock injury in the Maritimes are the multiple freeze-thaw events which occur
throughout the winter, causing a melting of the insulating snow cover and permitting
freeze-thaw cycling to occur deep below the soil surface. Although soil temperatures
at 10 cm in 1992 dropped to near -9°C, this temperature is not known to harm
most of the rootstocks in our region (Privé and LeBlanc, 1999). However, the midwinter
thaw was very stressful to plant integrity since tree mortality and vigour were
seriously affected in the subsequent growing season.
In cold hardiness studies, there is the challenge of evaluating many plant samples
for their survival at various developmental stages and freezing stresses. It is the
purpose of this manuscript to examine three techniques used to assess apple rootstock
hardiness at Agriculture and Agri-Food Canada in Eastern Canada. These include
1) using whole plant controlled freezing tests and regrowth parameters; 2) cell
integrity of root pieces subjected to freeze-thaw cycling and assessed using electrical
bioimpedance spectroscopy (Z) and 3) a seedling screening protocol used to
facilitate rapid progress in breeding programs.
Materials and methods. Plant material. Uniform specimens of various apple
rootstocks were lifted from the nursery in the autumn, packed in moist sawdust,
and placed in refrigerated storage at 3°C. Following two months in storage, whole
plants were removed for whole plant controlled freezing tests and roots pieces from
these same plants were cut from each of the rootstocks for the Z-analysis.
Freezing experiments. In the whole plant controlled freezing experiment, trees
were removed from cold storage, rinsed with water to remove soil and sawdust.
Roots were then equipped with thermocouples, placed in plastic bags as described
by Privé and Embree (1997), and all groups except the control were placed into a 3
x 3 m walk-in controlled-climate chamber preset to -3°C. The bagged control trees
were returned to cold-storage. A CR-7 datalogger (Campbell Scientific, Calgary,
AB) was programmed to read all thermocouples every minute and to output 15
minute average values, including minimum and maximum values. The six freezethaw
treatments were based on freezing cycles of 16 hours at -12°C and thawing
cycles of 16 hours at +2°C. Once all the treatments were completed, trees were
returned to cold storage and sawdust was added to each of the bags to conserve
moisture until the trees could be potted. All trees were bare-rooted in the bags for the
same period, including the control. No root dehydration or injury due to the freezing
treatments was visible directly after the treatments. Prior to potting the plants, initial
measurements were taken on trunk cross-sectional area (TCA/cm 2 ) and root volume
(cm 3 ). Root volume was calculated using the Archimedes principle: W air
– W water
= B
= Vρ; where W = mass; B = buoyant force; V = volume and ρ = liquid density.
Following these initial measurements, rootstocks were planted in 16-liter pots with a
soilless mixture (Agro-Mix, Fafard, Shippagan, N. B.), placed in trenches in the
field, and grown under normal cultural practices for the area. Pots were drip-irrigated
as required and fertilized weekly until late July with 20N-20P-20K (1 g·liter -1 ).
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