| Anatomical changes of Bruguiera gymnorrhiza (L.) Lamk and Avicennia marina (Forsk.) Vierh. seedlings grown in experimental equipment that simulated semidiurnal tides under greenhouse conditions were studied. The equipment included seven tanks, and the inundation periods in each tanks were 12, 10, 8, 6, 4, 2 and 0 h, respectively. There were two tide cycles each day. The leaf, stem and root anatomical features were obtained in order to study the responses to waterlogging and the functional significance, and the differences existed in the organs of mangroves were compared. Based on previous studies on physiology and results in this thesis, the abilities and mechanisms of tolerance of mangroves to waterlogging were further discussed. The main results were as follows:1. It was predicted that the increase of epidermis to leaf thickness ratio might be responsible to the'physiological drought'that accompanies the prolonged immersion duration. Epidermis to leaf thickness ratio of B. gymnorrhiza increased significantly in 8 h treatment, whereas epidermis to leaf thickness ratio of A. marina increased significantly in 8 h treatment. The leaf anatomical features of these plants that have been waterlogged for a short period enable the utilization of light. Mesophyll tissue thickness of B. gymnorrhiza declined markedly in plants under the 2 h treatments compared with the 0 h treatment; the leaf anatomical features of A. marina that have been waterlogged for a long period still enable photosynthesis. Water conduction and safety in leaf of B. gymnorrhiza might be influenced negatively by the reduction of number of vessels per unit area, vessel diameter and vessel wall thickness in the 2 h treatments. In A. marina, water conduction might be promoted by the increment of vessel diameter in short-duration waterlogging (2-4 h), and the conductive safety could be partly attributed to the increment in vessel wall thickness. When waterlogging time was longer than 4 h, vessel diameter and vessel wall thickness declined, which leaded to the reduction of water-transporting capacity and safety.2. In B. gymnorrhiza, both cortex thickness and pith diameter increment could have contributed to the increase in stem diameter with prolonged waterlogging duration in the 0–6 h treatments. It was suggested that the cortex thickness, pith diameter and stem diameter would be promoted by short-term waterlogging conditions (0–6 h); however, when waterlogging time was longer than 6 h, cortex thickness, pith diameter and stem diameter were influenced significantly, which was associated with the inhibition of stem growth in mangrove seedlings under long-duration waterlogging. Stem diameter of A. marina in the 4–12 h treatments was higher than that of 0–2 h treatments, and cortex thickness to stem diameter ratio increased significantly with prolonged inundation. It was suggested that stem diameter was not inhibited under inundation conditions, and the increment of cortex thickness might be adapted to waterlogging.3. In B. gymnorrhiza, vessel density showed a significant decrease with waterlogging time, whereas tangential vessel diameter of the 0–6 h treatments first decreased with increasing duration of waterlogging, and then tended to increase significantly from 6 h to 12 h treatments. It was hypothesized that an increase in vessel diameter might compensate for the loss of hydraulic conductivity due to the reduction of vessel density when plant seedlings were inundated in diluted natural seawater for a long time. Vessel density of A. marina kept constant. The water conduction could be promoted by the vessel diameter increase in the 0–4 h treatments, and then declined due to the reduction in the 6–12 h treatments.4. Both vessel length and bar number per scalariform perforation plate decreased gradually with prolonged waterlogging time in B. gymnorrhiza. It is predicted that longer vessel elements accompanied by more bars in the scalariform perforation plate were required for the trade-off between conductive efficiency and conductive safety for different periods of immersion. Vessel length of A. marina kept constant, and the simple perforation plate in A. marina increased flow rate. Mechanical strength and conductive safety during long-term inundation could be partly attributed to the increment in vessel wall thickness in B. gymnorrhiza. On the contrary, mechanical strength and conductive safety will be influenced by the reduction of vessel wall thickness in A. marina. 5. In B. gymnorrhiza, fibre diameter remained more or less unchanged despite slight fluctuations, whereas fibre wall thickness showed a dramatic reduction in the 2 h treatment. It seemed that the mechanical support was negatively affected by the reduction of fibre wall thickness when waterlogging duration was longer than 2 h. Although fibre wall thickness decreased, longer fibres produced with prolonged inundation in the 6–12 h treatments were considered to be associated with the need for mechanical strength in long-term inundation. However, fibre length in A. marina kept constant with constant fibre diameter, despite slight reduction in the 12 treatment. Mechanical strength would be influenced by the reduction of fibre wall thickness with prolonged inundation. In B. gymnorrhiza, a significant negative linear correlation was found between gelatinous fibre ratio and waterlogging duration. When waterlogging time was longer than 4 h, no gelatinous fibre appeared in A. marina. But the maximum gelatinous fibre ratio in A. marina was higher than that of B. gymnorrhiza.6. In B. gymnorrhiza, cortex thickness decreased with prolonged inundation, and cortex thickness to stem diameter kept constant, which might result in the low tolerance to waterlogging. In the 0–6 h treatments, the increased water conduction in root depended on more vessels per unit area. It was hypothesized that an increase of vessel density in first-order root might compensate for the loss of hydraulic conductivity due to the reduction of vessel diameter in the 6–12 h treatments. The thicker fibre wall in the pneumatophore (4–6 h) and first-order root (6-12 h) could be responsible for the need of mechanical strength.7. The different anatomical response of vascular systems to waterlogging exists among leaf, stem and root of B. gymnorrhiza. When waterlogging time was longer than 2 h, number of vessels per unit area, vessel diameter and vessel wall thickness in leaf declined markedly. In stem, vessel density showed a significant decrease with waterlogging time, whereas tangential vessel diameter of the 0–6 h treatments first decreased with increasing duration of waterlogging, and then tended to increase significantly from 6 h to 12 h treatments, which might compensate for the loss of hydraulic conductivity. Vessel diameter in root kept constant in the 0–6 h treatments, and then tend to decrease with prolonged inundation. But the increase of number of vessels per unit area could maintain the hydraulic conductivity. Fibre diameter in stem and root kept constant, thicker vessel wall in stem and thicker fibre wall in roots could promote the mechanical strength. As mentioned above, the different organs in B. gymnorrhiza showed different mechanisms to waterlogging. It was concluded that the anatomy characteristics with no waterlogging enabled the maximum efficiency of photosynthesis and water conduction, and significant changes in the leaf anatomical features as a result of periods of immersion would have come at the cost of reduction of photosynthesis and water transport when waterlogging duration was longer than 2 h.8. Number of vessels per unit area in leaf and stem of A. marina showe no difference. The maximum vessel diameter occurred in the 4 h treatment, and then tend to decrease. Significant negative linear correlations were found between vessel wall thickness and waterlogging duration in leaf and stem. Compared with B. gymnorrhiza, the anatomical features in different organs showed similar tendencies. Anatomically, water conduction would be promoted by short-duration waterlogging.
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