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Adaptive Phenotypic Plasticity of Avicennia Officinalis L. Across the Salinity Gradient in the Sundarbans of Bangladesh

Md. Rabiul Alam
Forestry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladesh

Hossain Mahmood
Forestry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladesh

Mst. Lulu Rayhan Khushi
Forestry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladesh

Md. Masudur Rahman
Mangrove Silviculture Division, Bangladesh Forest Research Institute, Chittagong, Bangladesh

Abstract/ Executive Summary:

Adaptive phenotypic plasticity of Avicennia officinalis across the salinity gradient in the Sundarbans of Bangladesh was studied. Propagule morphology was compared through use of a completely randomized design. Propagule growth initiation traits across the salinity gradient (from 0 to 35 ppt at 5 ppt interval) were studied by means of a randomized block design. Propagules showed variability in length, width, and weight across the salinity gradient in the Sundarbans. Propagule growth initiation time, mean growth initiation time, growth initiation index, and propagule growth initiation percentage of A. officinalis varied significantly with the increasing salinity and among low, medium, and high saline zones. However, propagules originating from the high and medium saline zones started their growth initiation more rapidly and vigorously at high salinities compared to those from the low saline zone. Therefore, A. officinalis exhibited adaptive phenotypic plasticity in terms of variability in propagule size and weight as well as physiologically adaptive plastic responses during propagule growth initiation across the salinity gradient in the Sundarbans. A. officinalis in high and medium saline zones of Sundarbans is the most salt-adapted phenotype, and a good knowledge about this will be widely useful for successful regeneration, coastal afforestation, and conservation of this species in increasing saline environments in the future.

Keywords: Mangrove, Morphological, Physiological, Plastic response, Propagule growth initiation, Propagule origin

Location:

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Soil salinity

Objectives:

An attempt was undertaken to explore the adaptive phenotypic plasticity of A. officinalis focusing on the influence of salinity and maternal origins on propagule morphology and propagule growth initiation traits across the salinity gradient in the Sundarbans of Bangladesh.

Methodology:

Propagule collection The dominant patches of A. officinalis across the salinity gradient in the Sundarbans were identified by following the Overseas Development Agency (ODA) vegetation map (ODA, 1985) and the salinity map to collect the propagules of this species from three different maternal origins, viz., LSZ, MSZ, and HSZ in the Sundarbans. A minimum of 50 mother trees from each of the three saline zones were selected. The propagules of this species were single units (Zabala, 1990), and were collected from those selected mother trees of A. officinalis in LSZ, MSZ , and HSZ of the Sundarbans during August, 2015. The collected propagules were then kept zone-wise separately in the laboratory. Propagule morphology and viability tests A completely randomized design was adopted for this experiment. From the collected propagules, 3 samples of 1000 propagules were taken from each of the three saline zones. Length (mm), width (mm), and weight (g) of each propagule were measured. Then, the viability of each propagule was tested by using 0.1% 2, 3, 5-Triphenyl Tetrazolium Chloride solution at 40C for 6 h in the laboratory. Only red colored propagules were considered as viable (Copeland & McDonald, 2005). Propagule growth initiation experiment Since the propagules of A. officinalis are crypto viviparous where embryo grows but does not rupture the pericarp (Naskar & Mandal, 1999; Saenger, 2002), propagule growth initiation is termed instead of germination for this experiment. The experiment was conducted through use of a randomized block design in a glass house in the forest nursery of Khulna University to study propagule growth initiation time (PGIT) which indicates the time required to initiate growth; mean growth initiation time (MGT) which indicates the day at which maximum growth initiation occurred; growth initiation index (GI) which indicates vigorousness and speedy growth initiation of the propagules; and propagule growth initiation percentage (PGIP) which indicates growth initiation success (%) at the end of the experiment. 24 trays (75 cm 9 75 cm 9 6 cm of each) filled with a 3 cm thick layer of coarse sand were prepared for each saline zone. Therefore, 72 trays were prepared for LSZ, MSZ, and HSZ. From the zone-wise previous collection, 100 propagules were sown in each of the 24 trays prepared for each of the three saline zones. Distilled water was used for 0 ppt treatment level. The other seven levels of salt solution (5, 10, 15, 20, 25, 30, and 35 ppt) were prepared by using crude sea salt (unrefined sea salt) containing all the chemical constituents of sea water. Then, the eight levels of treatments with three replications for each treatment level were applied to the propagules sown in 24 trays for each of the three saline zones. Salinity in each tray was checked and corrected daily. Mean temperature and relative humidity during the experimental period were recorded as 35C and 66%, respectively. Initiation of both root and shoot was considered as propagule growth initiation. Number of growth initiated propagules were counted and recorded at 24 h interval for 49 days. The propagule growth initiation traits were calculated as follows: PGIT (day)= Day of first growth initiation -Day of propagule sowing.

Source of Information: Hydrobiologia (2018) 808:163–174

Article Link (Online Journal): https://doi.org/10.1007/s10750-017-3420-z

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Achievement/Findings:

The propagules of A. officinalis originating from LSZ were significantly larger and heavier than those originating from MSZ and HSZ zones in the Sundarbans. Propagule growth initiation traits of A. officinalis in terms of PGIT, MGT, GI, and PGIP varied significantly among different salinity treatments as well as among three different maternal origins. The propagules of this species originating from HSZ and MSZ started their growth initiation more rapidly and spontaneously than those from LSZ. Avicennia officinalis of HSZ and MSZ is the most salt-adapted phenotype and that of LSZ is the least salt-adapted phenotype. Therefore, A. officinalis showed adaptive phenotypic plasticity which enabled this species to cope with different saline environments in the Sundarbans of Bangladesh. The most salt-adapted phenotype of A. officinalis proved to be well adapted to high saline environments during their growth initiation. This salinity-regime-specific knowledge on adaptation of A. officinalis during propagule growth initiation will be largely useful for its successful regeneration and conservation as well as for coastal afforestation in the increasing saline environments in the future.

Knowledge Management: Journal