Responses and tolerance mechanisms of mangrove seedlings to heavy metal stress

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 level. 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.