There were characteristic differences in salt gland morphology of crested and dwarf mangrove leaves, namely in the cell walls, vacuoles and vesicle formation. Numerous large vacuoles were observed in the secretory cells of glands of dwarf mangrove leaves compared to those of marginal plants. Salt gland frequencies were higher in the top, mid-lamina and base of fringe than dwarf mangrove leaves with and 51 %, respectively.
Phosphorus was an element that appeared deficient in dwarf mangrove leaves, being 50% lower compared to fringed mangrove leaves.
INTRODUCTION
All coastal ecosystems are affected by changes in sea level (http://www.response.restoration.gov.za). Due to excessive sedimentation, mangrove growth and productivity are short-lived (http://www.response.restoration.gov.za). Destruction of mangrove areas has been reported in Thailand (55%) and the Philippines (60%) (http://www.response.restoration.gov.za).
Mangroves are distributed within the tropical and subtropical zones of the world, and occur mainly between 25ºN and 25ºS (Fig. 1.) (http://www.response.restoration.gov.za).
LITERATURE REVIEW
High salt concentrations inhibited enzyme activity in glycophytic and halophytic higher plants (Flowers et al., 1977). In mesophyll tissue, thylakoid membranes of chloroplasts appeared swollen and disintegrated after severe salt stress (Mitsuya et al., 2000). Extensive lipid accumulation is another noticeable effect in some chloroplasts of salt-treated plants (Mozafar & Goodin, 1970; Kelly et al., 1982).
In studies reported by Nir et al. 1970), root cells under water stress showed the accumulation of osmiophilic droplets in the cytoplasm.
MATERIALS AND METHODS
Branches containing fully expanded mature leaves were selected from five individual trees from the two sites, placed in plastic bags moistened with distilled water and kept moist on ice. Blades were mounted on brass stubs with carbon conductive tape, sputter coated with gold and set. The number of salt glands per mm2 were counted on freeze-dried leaf segments at X 80 magnification selected from three regions (ie leaf tip, mid-lamina and leaf base) and viewed in a.
The leaves were rinsed in distilled water to remove surface rust and dried in an oven for 48 h at 70 ºC.
RESULTS
ANATOMY AND MORPHOLOGY
- ULTRASTRUCTURAL STUDIES
Freeze fracture of a marginal mangrove leaf exposed the outer walls of the secretory cells, revealing their structure (Fig. 21). The walls of the hypodermal cells in leaves of fringe mangroves appeared thick and well formed (Fig. 28). However, the inner tangential walls of dwarf mangrove leaves were particularly wavy and thicker in appearance (Figs. 29, 30) than those of the marginal mangrove leaves (Fig. 28).
Fence cells in mangrove leaves with margins appeared to be continuously fused to each other; plasma membranes were closely associated with cell walls (Fig. 32). In fringed mangrove leaves, spongy mesophyll cells were oval to spherical in shape, with chloroplasts evenly distributed along the tangential cell walls (Fig. 37). Abundant mitochondria were distributed near the chloroplasts and along the peripheral region of the cell walls (Fig. 38).
In leaves of fringe mangroves, the cytoplasm formed a thin parietal layer, tightly pressed against the wall by a large central vacuole occupying most of the cell volume (Fig. 31). In dwarf mangrove leaves, where plasmolysis had occurred, the cytoplasm and organelles appeared to be withdrawn from the cell wall (Fig. 40). Vacuoles of marginal mangrove leaves were large and of different sizes, due to fusion (Fig. 37).
The irregularity in the outline of the plasma membrane was more extensive in dwarf mangrove leaves (Figure 33). In the cytoplasm of secretory cells, there were elongated membranes near the nucleus and vacuoles (Fig. 58). In the dwarf mangrove leaves, the vacuoles in the excretory cells of the abaxial glands were numerous and larger than in the marginal mangrove leaves (Fig. 59).
The lower epidermis and outer tangential walls of trichomes of dwarf mangrove leaves were incised (Fig. 60).
PHYSIOLOGY
DISCUSSION
MORPHOLOGY AND ANATOMY
There is a reduction in leaf area to leaf weight ratios when plants grow under water stress conditions, minimizing transpiration, salt loading in leaves and heat loading, but possibly at the expense of productivity (Ball &. Passioura, 1995; Fernandez et al., 2002). It is certain that these areas can play an important role in the removal of salt secretions from the leaves. It appeared that non-glandular trichomes on the abaxial leaf surface probably served to reduce light falling on photosynthetic tissue and to protect protruding salt glands and stomata from exposure to desiccation and evaporation, as has been suggested in other species in the literature (Waisel, 1972; Rozema, 1991; Naidoo et al., 2002; Naidoo & Chirkoot, 2004).
It is believed that the glands probably reduce the apoplastic salt load, thereby reducing transpiration losses in the leaf tissues (Fitzgerald & Allaway, 1991). Heavier deposition of crystalline salts was found to occlude the adaxial glands of dwarf mangrove leaves compared to that of edge mangroves, which could be attributed to a greater salt concentration in the transpiration flux of dwarf mangrove leaves. This was consistent with findings by Munns (2002), who found that salts moving in the transpiration stream were deposited in the leaves as water evaporated, causing the gradual accretion of salt over time.
Dwarf mangrove leaf salt glands appear to be deeply sunken in crypts, probably due to less water availability, greater evaporation, and desiccation in the dwarf zone. It is assumed that salts gradually accumulate in the leaves over time by moving along the transpiration flow in the leaves and water evaporation, as also suggested by Munns (2002). Thus, when the mechanism controlling gland secretion in mature leaves of dwarf mangroves is unable to secrete salts to the surface and ion concentrations in leaf tissues reach toxic levels, the salt glands plasmolize, then probably degenerate and finally senesce (Munns, 2002). ).
Similar findings were reported by Naidoo (2006) who showed that in the marginal zone the soil water potential was −3.07 MPa and the xylem water potential was −4.5 MPa, indicating a positive water balance. At the dwarf site, there was a decrease in soil water potential (–7.44 MPa) and xylem water potential by –6.5 MPa, resulting in a water deficit in plants.
ULTRASTRUCTURAL STUDIES
Accumulation of plastoglobules in the chloroplast stroma may be due to premature senescence or senescence of dwarf leaves that cannot tolerate the accumulation of salts within the leaves. Among the many studies on the effects of salts on chloroplasts were those by Ball & Anderson (1986) who reported that the accumulation of sodium chloride in the chloroplasts of the salt-tolerant A. kripe 1999) also associated changes in chloroplast ultrastructure with a decrease in the rate of net uptake of photosynthetic CO2.
In this study, trees in the edge zone effectively secreted salts and chloroplasts and other plant organelles appeared undamaged. In the present study, a characteristic feature of chloroplasts in mature dwarf mangrove leaves was the accumulation of plastoglobuli, which was likely a response to salt stress, and leaf senescence or senescence (Kutik et al., 1999). In the present study, the cell wall of mangrove leaves remained intact and the plasma membrane remained tightly apposed to the cell wall.
They observed the formation of vesicles in the extracytoplasmic space, which is formed by the withdrawal of the plasma membrane from the cell wall in dwarf mangrove leaves. Multivesicular structures were often characteristic of dwarf mangrove leaves and were present in vacuoles, chloroplasts, mitochondria, and along cell walls and plasma membranes compared to those of marginal mangrove leaves. Plants growing at high salinity showed an increased presence of vesicles and myelin-like structures in their vacuoles (Mitsuya et al., 2000).
In both marginal and dwarf mangroves, the various sized vesicles common in salt glands were probably involved in wall material secretion and wall synthesis, as also suggested in the literature (Rachmilevitz & Fahn, 1973; Shimony et al., 1973 ). It has been proposed that the "band" membranes originate from the Golgi apparatus (Shimony et al., 1973).
PHYSIOLOGY
Dwarf mangrove leaf thickening is associated with a xerophytic trait that dwarf mangroves adapt to store or conserve water due to reduced water availability in the dwarf plant's habitat; and reduced further loss through evapotranspiration (Hiralal et al., 2003). In this study, mature marginal mangrove leaves showed a higher frequency of glands than mature dwarf mangrove leaves. In the present study, the concentration of glycinebetaine was 40% higher in dwarf leaves than in marginal plants.
When plants are exposed to NaCl, the ions appear to decrease the apoplastic water potential and overaccumulate in the cytosol (Binzel et al., 1988). In this study, marginal mangrove leaves contained significantly higher concentrations of K + and lower concentrations of Na + compared to those of dwarf mangrove leaves. It is then hypothesized that high Na+ concentrations in dwarf mangrove leaves impair K+ uptake probably due to the physicochemical similarities between these two elements (Marschner, 1995; Läuchli, 1999b).
It has been shown that K+ concentration in leaves decreases with increasing salinity because high Na+ concentrations can hinder K+ uptake in roots and chloroplasts (Munns et al., 1983; Robinson & Jones suggested that the reduction of photosynthetic activity in leaves of A. High Nitrogen concentrations in dwarf mangrove leaves were the result of differences in tidal contribution and upland influence (Naidoo, 2006). Phosphorus limitation at the dwarf site was the result of infrequent tidal influence and low plant productivity (Naidoo, 2006).
Ca2+ has also been shown to enhance K+ uptake under saline conditions, thereby increasing the tissue K+/Na+ ratio as a mechanism of salt tolerance (Cramer et al., 1990; Zhong & Läuchli, 1994). Ca2+ concentrations were significantly lower in dwarf mangrove leaves by 38%, which is in agreement with other halophytes (Gulzar et al., 2003; . Koyro, 2006).
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