Characterization of cork warts and aerenchyma in leaves of Rhizophora mangle and Rhizophora racemosa

Abstract

Mangroves are a diverse group of plants that inhabit tidal zones in the tropics and sub-tropics. Some mangrove species occupy the lower tidal zone in which the substrate is anoxic for long time periods while some mangroves inhabit the upper tidal zone in which the substrate should be less anoxic. Recent research has shown that about 12 species of mangroves from the Americas and from Australia transport air internally from openings in leaves called cork warts, as air enters leaves it collects within leaf aerenchyma (special air spaces within leaves) where the air expands when heated by sunlight. Aerenchyma in leaves is connected to aerenchyma in stems and stilt roots that lead to small roots in the substrate. Eventually, youngest root tissues are aerated while they grow within the anoxic substrate. Data of this study show that cork warts and leaf aerenchyma develop during leaf initiation in both shoot terminals of branches as well as in viviparous seedlings. The number of cork warts per leaf is similar for leaf primordia and for fully enlarged leaves for both Rhizophora species. Tissue sections of leaves of less than 30 mm2 area show an area of more translucent cells. Leaves with a surface area of 200 mm2 show a distinct aerenchyma tissue in which cells have no contents. For such leaves the depth of aerenchyma was 54 µm with a total leaf depth of 230 µm. Fully enlarged leaves had an aerenchyma depth of about 81 µm while the total leaf thickness was about 660 µm. For all leaves above 30 mm2, aerenchyma comprised 25% of total leaf volume. Densities of cork warts were the same for emerging leaf primordia as for fully enlarged leaves. The results of this study show that cork warts and aerenchyma develop in leaf primordia of both terminal buds and viviparous seedlings. Aerenchyma and cork warts become active once they emerge from their stipular coverings.