Two mangrove tree species of the Sundarban forest viz. Heritiera fomes Buch-Ham and Excoecaria agallocha L. were selected for the present investigation. Three different areas viz. Chandpai, Jungra and Munshiganj as Oligohaline (4 - 8 dS⁻¹∙m), Mesohaline (8 - 15 dS⁻¹∙m) and Polyhaline saline (>15 dS⁻¹∙m) zones respectively were selected as an experiment site. The Chandpai and Jungra zone are in Chandpai range and Munshiganj zone in Satkhira range of the Sundarban forest.

Flowering and fruiting behavior of Heritiera fomes and Excoecaria agallocha were investigated at the three saline zones viz. Oligohaline, Mesohaline and Polyhaline. Initiation of flowering activities of Heritiera fomes took place during the period of December-January and completed the fruiting stage by March-April. Fruit shedding took place during the period of late May to June at Chandpai and Jungra (Oligohaline and Mesohaline zones) but at the Munshiganj (Polihaline zone) in late July to early August. However, flowering and fruiting behavior of E. agallocha performed similar performance at the three different saline zones. They initiated flowering in April-May but fruiting stage covered the months of June-July. Fruit ripen and shedding took place between late July and August (Table 1 & Photos 1-7). Previous observation was found that behavior of fruits/seeds development and servility critically depended on moisture availability [1]. Although flowering started most of the mangrove species during the dry season but fruits mature or ripening occurred just before or during the early monsoon or in monsoon (rainy season) [12].

At the three saline zones viz. Oligohaline (Chandpai), Mesohaline (Jungra) and Polihaline (Munshiganj) zones of the Sundarban forest seed germination status of H. fomes and E. agallocha were observed. Seed germination of Heritiera fomes

Photo 1. Flowering Branch of Heritiera fomes species.

Photo 2. Green Fruiting Branch of Heritiera fomes species.

Photo 3. Matured Fruit/Seeds of Heritiera fomes species.

Photo 4. Branch of Flowering of Excoecaria agallocha L. species.

Photo 5. Green Fruiting branch of Excoecaria agallocha L. species.

Photo 6. Green Fruits with seed of Excoecaria agallocha L. species.

Buch-Ham is hypogeal type has a single seeded capsule with fibrous epicarp and brown papery mesocarp, after contact with tidal water they break up and initiate germination. Seeds remain attached with the adult seedlings. Seed germination of Excoecaria agallocha L seeds were epigeal type and initiated by explosively released seeds from the capsule, then in both species germination started from 7 days of sowing at all three saline zones and continued up to 28 days after sowing.

The final germination rate was counted at 28 days after sowing for the both species. At 28 days after showing, Heritiera fomes showed the highest percentage of germination (96%) at the Oligohaline (Chandpai) and Mesohaline (Jungra) zones, but the lowest at the Polihaline (Munshiganj) zone (88%). On the other hand, Excoecaria agallocha showed almost similar germination rate at the three saline zones (Table 2) from the beginning (7 days after sowing) of the germination up to 28 days after sowing (89% - 90%). The earlier findings noticed 65 2% -

Photo 7. Mature Fruits with seed of Excoecaria agallocha L. species.

94% germination on majority mangrove species [13]. The growth of radical height and plumule length of H. fomes showed the highest and almost similar at the Chandpai (Oligohaline) and Jungra (mesohaline) zones but lower at the Polihaline (Munshiganj) zone at the 28^th day of germination (Table 3 & Photo 8). However, in E. agallocha species, radicle height and plumul length showed almost similar growth trends from 7 days to 28 days at Mesohaline (Jungra) and Polihaline (Munshiganj), but comparatively lower growth found at the Oligohaline zone (Chandpai). Although the seed stage is most of the time well protected against various stresses, after germination due to imbibition, it becomes stress-sensitive and strongly depends on the surrounding environment [5]. Germination and growth in high salinity conditions are insufficient for H. fomes, which show a lower tolerance of salinity, but not for E. agallocha, shown here as a salt-tolerating species.

Photo 8. (a) 28 days Heritiera fomes seedlings with plumul and radical; (b) 28 days Excoecaria agallocha seedlings with plumul and radical.

In this investigation, an attempt has been made to establish a constant relationship between different salinities and the ages of the mangroves. A sharply decreasing trend in the survival rates of both Heritiera fomes Buch-Ham and Excoecaria agallocha L from the ages of 2 months to 6 months in three distinct saline zones was observed. After that, the declining trend continued and became negligible up to the age of 12 months. Later survival rates were minimized at the age of 30 months (Figure 1(a) and Figure 1(b)). At this ages (30 months) H. fomes showed 25% survival rate at Oligohaline (Chandpai) and Mesohaline (Jungra) zones but 10% at the Polihaline (Munshiganj) zone. On the other hand, survival rate of E. agallocha was 15% at Mesohaline and Polihaline but 13% at Oligohaline. Such declining trend of the natural regeneration of H. fomes and E. agallocha were also observed by many authors [14] [15] [16] and [17]. Generally, during seed germination and early growth of the seedlings, depends on minerals stored in the seeds, and after the depletion of this source, plants become dependent on soil minerals. Mangroves are not salt lovers but salt tolerant [18]. Although they inhabit saline sediments, their growth and structure generally decline due to increasing sediment salinity [19] [20]. Salinity, among other abiotic stresses, reduces plant survival rate through ionic toxicity and osmotic stress, and then through nutritional stress.