Leaf morphological plasticity of some selected mangrove tree species in the Sundarbans of Bangladesh along salinity gradients

Leaves are the most important organs of plants. Leaves show considerable plasticity in sizes and shapes in contrasting environments. Particularly leaves of mangroves exhibit a high level of morphological plasticity along the salinity gradients. Salinity is crucial to the limit the growth and development of mangroves. However, no such initiatives were taken to investigate the pattern of variability of leaf morphological plasticity of mangrove tree species in the Sundarbans, Bangladesh. In this study, three main tree species Sundri, Gewa and Goran of the Sundarbans were chosen for leaf morphological analysis. Leaf samples were collected from each species from three different position of each tree like base, middle and top positions were taken. From each zone 10 trees for each species were considered for leaf sample collection. Fresh leaves were then scanned using a digital flatbed scanner. Leaf parameters viz. leaf length (cm), leaf width (cm), leaf middle width (cm), leaf upper quarter width (cm), leaf down quarter width (cm), petiole length (cm), leaf base angle (degree), leaf tip angle (degree), leaf area (cm2), leaf perimeter (cm) were measured and quantified using NIH ImageJ software. Ratio of leaf length to leaf width (leaf index), leaf area to leaf perimeter, leaf length to petiole length, ratio of leaf base angle to tip angle and ratio of leaf upper quarter to down quarter width were calculated from the measured parameters. A sub-set of leaf samples was considered for measuring leaf thickness and specific leaf area (SLA). Leaf thickness was measured using digital caliper. All collected data were analyzed using the freely available programming language R. Univariate and multivariate statistical tools were employed to analyze leaf morphological plasticity. A wide range of phenotypic variation was observed in all leaf parameters studied for each species among the salinity zones of the Sundarbans. One-way ANOVA revealed significant differences (P < 0.001) in all leaf parameters among the salinity zones and confirming that there was a high degree of phenotypic variation among the salinity zones of the Sundarbans. Leaf length (L) was decreased in Sundri and Gewa from fresh water zone (FW) to strong saline zone (SS) but increased in Goran. Leaf index is rather stable for a given species. However, this study showed high level of morphological plasticity in leaf index among the salinity zones of the Sundarbans for each species of Sundri, Gewa and Goran.

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Plasticity index (PI) was developed in this study. PL, LA and LL/PL showed highest plasticity for Gewa in moderate saline zone and PL, LA and LL/PL showed the highest plasticity for Goran in strong saline zone. The results indicated that these leaf parameters are more sensitive and plastic to salinity gradients than other leaf parameters measured. High level of phenotypic plasticity in these leaf traits reflects fitness to different saline environments. Among the three species, Gewa showed the highest SLA in fresh water zone of the Sundarbans. Leaf thickness plasticity was the highest in Goran. SLA plasticity was correlated with salinity gradients. Scatter plot constructed based on the first two principal components (PC1 and PC2) analysis demonstrated the pattern of phenotypic plasticity among the salinity zones of three species. These results clearly separated the species among the salinity zones in a two-dimensional plot. Our results provide clear evidence that all the leaf parameters measured for three tree species viz. Sundri, Gewa and Goran effectively utilizes a plastic strategy along the salinity gradients in the Sundarbans. Morphological trait plasticity could serve as powerful biological indicators of upcoming environmental change events.

• Supervision