Adaptive evolution of leaf anatomical features in mangrove Rhizophoraceae cues differential strategies of salt tolerance

The monophyletic mangrove Rhizophoraceae (tribe Rhizophoreae) group has been studied by leaf anatomical features to understand whether the adaptive strategies for salt tolerance are generalized or specific to species or genera (or clade). Thickness data from leaf and different layers in leaf cross-sectional view are acquired to discuss adaptive evolution in the tribe. An increase in leaf thickness is the most common phenomenon in mangroves. The development of thin to thick but always well-developed hypodermis layers in the leaves of Rhizophoreae is a strategic advantage to store water and prevent high-intensity light of solar irradiation. In contrast, the hypo-dermis layer in non-mangrove Carallia brachiata is rudimentary though thicker than some mangrove species. The existence of hypodermis in non-mangrove relatives of Rhizophoreae might pave their way to saline intertidal habitat. The clade bearing all the species of Bruguiera shows a decreasing trend of allocation to the hypodermis layer whereas an increasing trend was observed in case of the palisade layer. The positioning of chloroplasts deep into the elongated palisade cells in Bruguiera may minimize the photodamage effect. An efficient ultrafiltration mechanism in the Bruguiera clade may result in a slow accumulation of salts in mesophyll tissue and thereby allocation to the hypodermis layer is reduced. A lower palisade layer was developed in the ancestor of Ceriops and Kandelia. This layer is more developed in Kandelia than in Ceriops due to the presence of a thinner upper hypodermis layer in the former genus. The thin upper hypodermis is inefficient in preventing high-intensity light; consequently, the photodamage effect is likely in the upper palisade layer of Kandelia. The lower palisade layer possibly compensates photodamage effect of the upper palisade layer. Maximized allocation to the upper hy-podermal layer in Rhizophora clade may indicate its highest salinity tolerance ability. The compact spongy layer in Rhizophoreae divulges their limited primary productivity in comparison to their terrestrial relative. The contrastive allotment of leaf layers in the species or genera (or clades) of Rhizophoreae cues different strategies of salt tolerance despite the fact that they originated from a common ancestor of saline intertidal habitat.

Automated summary

Leaf anatomical features of the monophyletic mangrove Rhizophoraceae (tribe Rhizophoreae) group were studied to understand the general or specific adaptive strategies for salt tolerance in different species or genera (or clades). An increase in leaf thickness, development of well-developed hypodermis layers, and allocation changes in different leaf layers were observed, indicating different strategies for salt tolerance among the species or genera (or clades) of Rhizophoreae. The presence of hypodermis in non-mangrove relatives of Rhizophoreae suggests a possible adaptation to saline intertidal habitat. The positioning of chloroplasts and the allocation to different layers in the leaves play important roles in minimizing the photodamage effect and enhancing salt tolerance in different Rhizophoreae clades.