Agriculture Reference
In-Depth Information
soybean are exceptionally subjected to freezing periods, more regularly they endure
chilling and the most common situation is suboptimal growth temperature. While
freezing kills these plant species, chilling and suboptimal temperature constitute an
important constraint to productivity. In the last two cases, damage levels depend on
the magnitude of temperature diminution and the exposure time. In contrast, wheat,
barley and oat crops are normally grown under freezing, chilling and suboptimal
growth temperatures, being freezing tolerated to some extent by these species.
There are two different strategies to overcome low temperature stress: avoidance
and tolerance. In terms of crop production, avoidance may be determined by the
sowing period, growth cycle and agronomic management, but tolerance is a genetic
feature, peculiar to each cultivar, which constitutes a major tool for crop production
management in areas characterized by low temperatures.
Cultivar response to low temperature stress involves important biochemical and
molecular changes. Essentially, plants increase the production of protective com-
pounds that affect cell lipid composition, thus participating in membrane stabili-
zation and maintaining plasma membrane functionality (Janská et al.
2010
). Bio-
chemical changes also include the synthesis of cryoprotectant molecules as soluble
sugars, (saccharose, raffinose, stachyose, trehalose), sugar alcohols (sorbitol, ribi-
tol, inositol) and low-molecular weight nitrogenous compounds (proline, glycine
betaine). Symplastic and apoplastic soluble sugar directly contribute to membrane
stabilization (Livingston et al.
2006
). Also, compounds such as tripeptidthiol, glu-
tathione, ascorbic acid (vitamin C) and a-tocopherol (vitamin E) are important for
their antioxidant activity (Chen and Li
2002
). PAs are also involved in the stress
response to low temperatures. Changes at the transcriptional and metabolic levels
have been reported, mainly in
A. thaliana.
Currently, attempts are being made to
manipulate PAs metabolism genes in order to obtain plants tolerant to low tem-
perature stress. Table
5.2
shows variations in PAs levels during the response to cold
temperature in several crop species.
In five bean (
Phaseolus sp.
) cultivars differing in chilling response, Guye et al.
(
1986
) found that prior to chilling treatment, PAs levels did not appear to be cor-
related with chill-tolerance, since levels in non-chilled controls were highest in
cultivars of medium chill-sensitivity. These authors also found that the Put levels
were increased in tolerant cultivars, whereas no changes were observed in sensitive
ones. They concluded that it is the change in Put titer rather than its absolute level
what appears to be correlated with chill-tolerance. In two wheat cultivars with slight
difference in response to cold tolerance, Nadeau et al. (
1987
) found a 6-9-fold in-
creased Put level during cold acclimation, whereas a smaller raise was observed in
the Spd content and conversely, Spm level decreased. These authors also found an
augmentation in Put level of alfalfa (
Medicago sativa).
ADC activity level declined
in cold treated plants, related to the untreated control, whereas no major variations
were observed in ODC activity levels, suggesting that ADC is the mainly enzyme
responsible for the incremented Put levels under the cold-hardening condition. In
turn, DAO activity varied in parallel with Put content. In a short term freezing stress
experiment, a marked Put and agmatine accumulation was observed in wheat sub-
jected to −2 °C for six hours (Rácz et al.
1996
). The buildup of agmatine (which is
