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|Title: ||Responses and tolerance mechanisms of mangrove seedlings to heavy metal stress|
|Other Titles: ||Hong shu you miao dui zhong jin shu de xiang ying ji nai shou ji zhi yan jiu|
|Authors: ||Yan, Zhongzheng ( 閆中正)|
|Department: ||Department of Biology and Chemistry|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2011|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Mangrove plants -- Seedlings -- Effect of heavy metals on.|
|Notes: ||CityU Call Number: QK938.M27 Y36 2011|
xxxiii, 274 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references (leaves 253-273)
|Abstract: ||The strategies of higher plants to cope with heavy metal stresses vary with stress
types and durations, as well as plant species. The present research aims to (1) examine
the effects of different levels of lead (Pb) stress on seedlings of Avicennia marina and
compare the growth and biochemical responses to Pb stress between seedlings with
experimentally removed cotyledons prior to their natural fall off and those with
cotyledons; (2) investigate the temporal responses of oxidative enzymes activities,
compatible osmolytes and polyphenols in seedlings of three common mangroves plants,
Kandelia obovata, Excoecaria agallocha and Acanthus ilicifolius under Pb or
manganese (Mn) stresses and (3) compare the responses of three endogenous
phytohormones, namely jasmonic acid (JA) and salicylic acid (SA), in leaves of K.
obovata, E. agallocha and A. ilicifolius to Pb or Mn stresses.
The effects of Pb (0-1000 mg L-1) stress on the growth and biochemical responses
of seedlings of A. marina, with and without cotyledons, were examined. After 50 days
exposure to Pb, the growth of A. marina was not affected at low Pb stress (0-250 mg
L-1). Roots could tolerate high Pb stress and a significant reduction in biomass was only found at the highest Pb level (1000 mg L-1). Pb was mainly accumulated in roots,
with some uptake in cotyledons but very little in leaves. Pb stress level had significant
positive effects on sugar content, malonaldehyde (MDA) concentration and peroxidase
(POD) activity in both roots and leaves, irrespective to the presence or absence of
cotyledons. The removal of cotyledons only showed significant effects on POD
activity and MDA content in roots. Sugar content in the cotyledon of Pb-treated
seedlings was significantly lower than that in the control, suggesting that more
carbohydrate reserves stored in cotyledons had been mobilized.
The changes of growth, accumulation and translocation of heavy metals (Pb or Mn)
in seedlings of K. obovata, E. agallocha and A. ilicifolius under different levels of Pb
or Mn stresses were followed at different stress time. Pb or Mn stress had no
significant effect on growth of mangrove seedlings at Days 1 and 7 but a significant
decline in growth was observed at Day 49. Both Pb and Mn were accumulated in root
tissues of the seedlings at levels higher than that in sediment. The amount of Mn
accumulated in leaves was significantly higher than that in roots, as Mn is an essential
plant micronutrient which has higher mobility than Pb. Among the three species,
translocation of Pb from root to leaf was faster in E. agallocha and A. ilicifolius than in
K. obovata, and significant accumulation of Pb was found in leaves of the first two
species at Day 1.
The temporal responses of antioxidative enzyme activities, including superoxidase
dismutase (SOD) and POD; concentrations of phenolic compounds, total polyphenols
(TP) and extractable condensed tannin (ECT) and compatible osmolyte, proline and soluble sugars in seedlings of K. obovata, E. agallocha and A. ilicifolius to Pb or Mn
stresses were investigated. SOD and POD activities in all three species exhibited rapid
responses to Pb or Mn stresses, with a decline of POD activity but an increase of SOD
activity observed in both roots and leaves of K. obovata at Day 1. However, both POD
and SOD in E. agallocha and A. ilicifolius increased significantly in roots and leaves at
Day 1. With increases in stress time, at Day 7, POD and SOD in leaves of K. obovata,
E. agallocha and A. ilicifolius significantly augmented at the moderated Pb or Mn
levels. At Day 49, both SOD and POD activities in leaves of K. obovata were
significantly reduced but were higher in roots than at Day 7, irrespective to the stress
As compatible osmolytes, the changes of proline and soluble sugar content due to
Pb or Mn stress were not as fast as that of the antioxidative enzymes. Significant
increases of proline concentrations were detected in leaves of K. obovata, E. agallocha
and A. ilicifolius at Days 7 and 49 but not at Day 1. Soluble sugars seemed less
sensitive than proline in leaves of E. agallocha and A. ilicifolius, which only increased
significantly at Day 49. The accumulation of soluble sugars in roots was more rapid
than that in leaves, as Pb or Mn stresses induced significant increases of soluble sugars
in roots of K. obovata, E. agallocha and A. ilicifolius at Day 7. These results showed
that proline changes in leaves of mangrove seedlings were more rapid than soluble
sugars under heavy metal stress, suggesting that proline was the primary compatible
osmolytes adopted by mangrove seedlings in response to heavy metal stress.
The changes of phenolic compounds under metal stress were also slower than that of the antioxidative enzymes. TP and ECT concentrations in both roots and leaves of K.
obovata, E. agallocha and A. ilicifolius showed no significant changes at Day 1. As
stress time increased, TP and ECT decreased significantly in roots with metal levels at
Days 7 and 49. Mn-treated roots had significantly lower phenolic concentrations than
the Pb-treated roots, suggesting that mangrove seedlings were more sensitive to Mn
than Pb stress. In leaves of K. obovata, E. agallocha and A. ilicifolius, TP and ETC
concentrations increased significantly at low levels of metal stress (100 and 200 mg
Kg-1 Pb or Mn), implying that mangrove seedlings used the phenolic compounds as
the defensive mechanism to metal stress.
The quantitative analysis of the responses of two plant hormones, JA and SA to
different levels of Pb or Mn stress in leaves and roots of K. obovata, E. agallocha and
A. ilicifolius at different stress time were followed using gas chromatography-mass
spectrometry. Mangroves seedlings, similar to other wetland plants, increased their
endogenous SA and JA concentrations which contribute to their tolerance to heavy
metal stress. Increases of SA and JA were mainly observed in leaves of K. obovata and
E. agallocha, especially at Day 1, while such increase was only observed at Days 7
and/or 49 in A. ilicifolius.
The present research revealed that different mangrove species showed different
responses and tolerance mechanisms to heavy metal stresses at different stress time. K.
obovata showed stronger tolerance abilities to Pb and Mn than E. agallocha and A.
ilicifolius. The tolerance ability of mangrove seedlings attributed, in part, to the size of
the propagule, and the energy reserves in the propagule may play important role in the anti-stress processes. When the mangrove seedlings were subject to heavy metal stress,
the quickest responses were observed on the hormonal and antioxidative enzymes
levels. Other mechanisms, such as the synthesis of osmolytes, proline and soluble
sugar, and the synthesis of the polyphenols occurred at a later stage of stress. The
tolerant species, like K. obovata and E. agallocha, tended to acclimate to metal stresses
by increasing and maintaining high levels of SOD and POD activities at later stage of
the treatment, while the non-tolerant species such as A. ilicifolius could not acclimate
to the metal stress and enzyme activity decreased with the time.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b4086534|
|Appears in Collections:||BCH - Doctor of Philosophy |
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