Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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R.G. Trischuk, B.S. Schilling, M. Wisniewski and L.V. Gusta
only tolerate -3 o C (Gusta unpublished results). When acclimation to one stress results
in increased tolerance to other stresses it is referred to as cross adaptation. However,
the plant will not achieve the same level of freezing tolerance as attained by low temperature
exposure. This suggests that some genes are specifically controlled by low
temperature. The site of cold perception is not known, but is speculated to be due to a
slight relaxation in the turgor of the plasma membrane (Sangwan et al., 2002), which
results in changes in the intracellular pool of calcium. Not all tissues in a plant acclimate
at the same rate and not all tissues achieve the same level of freezing tolerance. For
example, the crowns of winter cereals are more freezing tolerant than the leaves while
roots possess only a few degrees of freezing tolerance (Chen et al., 1983). Legg et al.
(1983) observed that all tillers on winter cereals did not have the same LT 50
. The point
we are trying to establish here is although all tissues in a plant are genetically identical,
morphologically and anatomically they are very different. These differences may impact
the freezing tolerance that results from the upregulation of stress-associated genes.
Under natural conditions plants germinate, grow and mature under a constant
state of environmental fluxes. Daily temperature variation can be as much as 10-15 o C,
however, it is very rare that a large temperature change of 20°C to 2°C occurs over a
matter of a few minutes, especially in the rhizosphere. Due to the buffering action of the
soil, changes in soil temperature and water availability are stable. In contrast, the roots
of plants grown in pots are subjected to wide variations in temperature and water
conditions, which do not reflect natural conditions. Depending upon the species and
cultivar, plants grown in pots eventually recover after a period of days. Plants are
exposed to large temperature variations during natural acclimation in the fall. For example,
night temperatures may be lower than 8 o C, which triggers the upregulation of
cold associated genes, but actual leaf temperatures during the day may readily exceed
20 o C, which promotes loss of freezing tolerance. Therefore, in the fall, the plant is
exposed to a series of mixed signals. While the upregulation of cold associated genes
are important for induction and maintenance of cold acclimation, equally important is
the prevention in loss of freezing tolerance at non-acclimating temperatures. For example
in nature, spring cultivars of Brassica napus rarely cold acclimates to -10 o C,
however the same cultivar tolerates -19 o C when cold acclimated in controlled environment
chambers. (Schilling, unpublished). Similarly non-acclimated winter cereals tolerate
lower temperatures when hardened in a controlled environment chamber as compared
to natural conditions (Gusta et al., 2001). Thus, the genes for freezing tolerance
may be present in a genotype, however, the upregulation of the cold associated genes
may not be optimized and therefore the regulation of cold-induced gene expression may
be as important as the presence of any specific genes that directly confer cold tolerance.
Another limitation to artificial acclimation is light intensity and quality. For
example, very cold hardy winter wheat seedlings have a prostrate growth habit in the
field, whereas plants in pots in controlled environment chambers subjected to cold
acclimating temperature tend to have an erect growth habit. Photoinhibition can readily
be induced in plants grown at constant low temperatures under high light conditions.