Cavity Tissue for the Internal Aeration in Plants

Internal aeration is essential for the adaptation to excess water stress. Aquatic- and semi-aquatic plants possessed several traits for internal aeration. Cavity tissues in leaves, stem, and roots are aisle for the gas movement between parts above and below the water level under excess water conditions, which are called “aerenchyma.” Aerenchyma is the internal gas space contributed to internal aeration but is morphologically different in each plant species. Aerenchyma is largely classified as primary and secondary aerenchyma. Primary aerenchyma, especially in lysigenous aerenchyma, has been well studied, and several evidences related to the mechanisms of aerenchyma formation are also recently reported. Several plants form lysigenous aerenchyma even under aerobic conditions and enlarge lysigenous aerenchyma in response to low-oxygen conditions. These aerenchyma are distinguished, and the former is defined as a “constitutive” aerenchyma, and the latter is as “inducible” aerenchyma. Additionally, new insight into aerenchymatous phellem (a type of secondary aerenchyma) is also shown. In this chapter, we summarized the characteristics of each type of aerenchyma and present recent advances in understanding the mechanisms of lysigneous aerenchyma formation and secondary aerenchyma.