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of this parameter was lower in the coleoptile than in the mesocotyl, in both culti-
vars. Stepwise regression analysis of these data showed that chilling injury in roots
was generally correlated with Spd concentration while in the mesocotyl, injury was
mainly correlated with Put and Spd concentrations. In the last cultivars, Zheng et al.
(
2009
) found that Put reduced, but Spd and Spm increased during chilling stress
(5 °C, 48 h). However, Spd and Spm contents were higher in the tolerant than in the
sensitive cultivars. The values of (Spd + Spm)/Put were negatively correlated with
malondialdehyde contents, whereas treatment with methylglyoxal-bis-guanyl-hy-
drazone (MGBG; an inhibitor of SAMDC) resulted in raised malondialdehyde con-
tent and reduction of germination percentage and energy, in both maize cultivars.
Working with two rice cultivars differing in their response to chilling condition
(5 °C), Lee et al. (
1997
) observed that levels of Put, Spd and Spm contents, as well
as ADC and SAMDC activities, increased under the stress in the tolerant cultivar
Tainung 67. However, minor changes occurred in the sensitive cultivar Taichung
Native 1. Furthermore, ABA increased, similarly to Put, whereas treatments with
inhibitors of Put synthesis or pre-treatment with ABA provoked enhanced sensitiv-
ity and improved tolerance to cold stress, respectively. Earlier, a high correlation
had been reported between ABA, Put and tolerance levels in 11 rice cultivars (Lee
et al.
1995
).
On the other hand, Pillai and Akiyama (
2004
) found that the rice OsSAMDC
gene was induced in the tolerant cultivar Yukihikari but not in the sensitive TKM9
one. In agreement with last results, Spd levels increased in shoots of Yukihikari and
it was not altered in TKM9, whereas Put and Spm remained unchanged in both cul-
tivars. OsSAMDC was also induced by Ethephon (liquid ethylene) in both cultivars,
but this gene was not responsive to salt, drought, submergence, mannitol or ABA.
In cucumber plants, chilling induced a marked Spd rise in a tolerant cultivar, but
not in a sensitive cucumber cultivar (Shen et al.
2000
). Also, Put built up during the
rewarming period in the tolerant cultivar, but there was no change in the sensitive
one. In contrast, Zhang et al. (
2009
) reported increased Put, Spd and Spm in tolerant
cucumber plants and a slight Put increase in sensitive ones. Such apparent incongru-
ence might be attributed to the different chilling conditions employed: 3 °C in the
first case versus 15/8 °C in the second. Shen et al. (
2000
) informed that augmenta-
tions in Put and Spd were preceded by enhancements in ADC and SAMDC activi-
ties. Pre-treatment of sensitive plants with Spd prevented chill-induced increments
in leaf H
2
O
2
contents and nicotinamide adenine dinucleotide phosphate (NADPH)-
oxidase activity, alleviating chilling injury. On the other hand, the application of
MGBG to a chilling-treated, tolerant cultivar, prevented Spd rise and enhanced
NADPH oxidase activity and chilling injury. A beneficial effect of PAs addition was
reported by He et al. (
2002
). These authors described an improvement of chilling
tolerance of the photosynthetic apparatus in cucumber leaves pre-treated with Spd.
In tomato, Put and Spd increased under cold treatment in the wild type, whereas
only Put was augmented in the
flacca
mutant, (an ABA-deficient tomato plant). The
exogenous application of DFMO (an inhibitor of Put synthesis) intensified the elec-
trolyte leakage, whereas Put addition reversed this phenomenon (Kim et al.
2002
).
Both ABA and Put had a protective effect against cold stress, but exogenously
