Formation and function of aerenchyma in baldcypress (Taxodium distichum (L.) Rich.) and Chinese tallow tree (Sapium sebiferum (L.) Roxb.) under flooding

To understand flooding adaptation mechanisms of baldcypress (Taxodium distichum (L.) Rich.) and Chinese tallow tree (Sapium sebiferum (L.) Roxb.), we designed field experiments to examine the changes of morphology, growth, aerenchyma formation, porosity and root O2 consumption of one-year-old seedlings of both species. Both trees were divided into two groups. One group was not flooded (NF, soil water content was 75% of field capacity), while the other group was treated with flooding (FL, water level was 5cm above soil surface) for 95days. We found that flooding inhibited growth of both baldcypress and Chinese tallow tree, and biomass increment of baldcypress and Chinese tallow tree under FL decreased 36.4% and 74.5%, respectively, indicating that baldcypress was more tolerant to flooding. Root/shoot ratio of the two tree species increased significantly under flooding, which was primarily due to the decrease of stem and leaf biomass. Flooding also stimulated aerenchyma formation in the roots (lateral and adventitious), stems and leaves of baldcypress and roots of Chinese tallow tree. Porosity in the roots, stems and leaves of the two tree species increased significantly under flooding. The aerenchyma formation and increased porosity enhanced O2 diffusion to the roots. Our results indicate that baldcypress and Chinese tallow tree exhibit a number of adaptive mechanisms in response to flooding, including formation of adventitious roots and new lateral ones, aerenchyma formation, increased porosity of the roots, stems and leaves, and increased O2 release into the rhizosphere. ► Flooding led to an increase in root/shoot ratio of two tree species. ► Flooding facilitates the aerenchyma formation in the root, stem and leaf. ► Flooding led to an increase in porosity of root, stem and leaf. ► The aerenchyma formation and increased porosity enhanced O2 diffusion to roots.