Cortical aerenshyma formation in hypocotyl and adventitious roots of Luffa cylindrica Roem. subjected to soil flooding.

When soil is flooded, the rhizosphere quickly reaches oxygen shortage conditions because of oxygen consumption during the early stages of flooding, not only by plant roots but also by soil micro-organisms. Although this environment is generally disadvantageous to plant growth, hygrophytes and some mesophytes can adapt to these conditions. In these plants, aerenchyma (tissue with much enlarged gas space) develops in the petioles, stems and roots, and enhances transport of atmospheric and photosynthetic oxygen to flooded tissues, thereby preventing serious damage to root organs (Armstrong, 1979; Dacey, 1980; Drew et al., 1985; Drew, 1997). On the other hand, many mesophytes have little ability to develop aerenchyma so, if the soil becomes flooded, their roots may become oxygen starved and damaged (Smirnoff and Crawford, 1983; Justin and Armstrong, 1987; Laan et al., 1989). Therefore, aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Two main types of cortical aerenchyma in stems and roots can be distinguished by their process of formation: lysigenous and schizogenous aerenchyma (Jackson and Armstrong, 1999; Evans, 2003; Visser and Voesenek, 2004; Seago et al., 2005). The former is created through cell disintegration (death) in the primary cortex of adventitious roots, as in rice (Justin and Armstrong, 1991), maize (Drew et al., 1979, 1981) and wheat (Huang et al., 1997), whereas the latter is formed by the separation of cells from each other, often accompanied by cell divisions and normal expansion as in Rumex species (Laan et al., 1989; Jackson and Armstrong, 1999; Colmer et al., 2004). The anatomical and morphological formation of both types has been reported in detail. In particular, much research has been done on the regulation of lysigenous aerenchyma formation using the roots of maize (Drew, 1997; Gunawardena et al., 2001) and rice (Kawai et al., 1998). However, another type of cortical aerenchyma was described in roots of Jussiaea (now Ludwigia) species by Martins (1866) and Schenck (1889) .100 years ago. Some Ludwigia species have pneumatophores (respiratory roots) that grow vertically upward and emerge above the water surface as in mangrove plants (Hartsema, 1927; Sculthorpe, 1967). The intercellular gas spaces are produced by radial elongation of living, thin-walled primary cortical cells. These aerenchymatous tissues are composed of a regular meshwork of rectangular spaces, and are produced in concentric layers in transverse sections of the roots (Martins, 1866; Schenck, 1889; Ellmore, 1981). The elongated cells appear T-shaped in longitudinal sections (Haberlandt, 1914). Although the variation is different from that of Ludwigia, recently intercellular gas space shave been shown to be produced by primary cortical cell elongation in adventitious roots of the wetland plant Pontederia cordata (Seago et al., 2000) and is termed ‘expansigenous aerenchyma’ by Seago et al. (2005). Sponge gourd (Luffa cylindrica, Cucurbitaceae) is an annual vegetable upland crop that originates in India and southern Asia, and is distributed mainly in tropical to warm-temperate areas. Compared with bitter melon (Momordica charantia), this species is flood tolerant, but its morphological changes in response to flooding stress have not been examined (Liao and Lin, 1996). In Taiwan, yield of bitter melon is increased by grafting with Luffa spp., which allows bitter melon to survive in flooded soils (Palada and Chang, 2003). In sponge gourd, it was found that the primary cortical tissues of newly formed adventitious roots under flooding created an aerenchyma that was neither schizogenous nor lysigenous, but took the form of elongated cortical aerenchyma. This aerenchyma is composed of cells that appear T-shaped in longitudinal sections. According to Jost (1887), intercellular gas spaces are created by T-shaped cells in roots of Luffa amara (now L. acutangulus). T-shaped cells that are secondary tissues derived from a phellogen are also observed in flooded stems of Ludwigia (Schenck, 1889, see the English text in Arber, 1920), and the lacunate tissues seem to be similar to elongated primary cortical aerenchyma. However, the origin of the lacunate tissues in the stem of Ludwigia is strikingly different from that of aerenchyma observed in sponge gourd. Recently, morphological observations of T-shaped cells derived from a phellogen were reported in the root and stem of Decodon species (Little and Stockey, 2003, 2006) and Lythrum salicaria (Stevens et al., 1997), but there was no detailed observation of T-shaped cells derived from primary cortical cells. Ellmore (1981) described elongated cortical aerenchyma in mature pneumatophores of Ludwigia, but did not report how lacunae were created at an early stage; therefore, an investigation has been carried out to determine the developmental processes of this type of aerenchyma. In addition, it was found that such tissues were composed of radially elongated primary cortical cells in flooded hypocotyl of sponge gourd, but not of T-shaped cells. Here, the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation are described for sponge gourd, and the relative amount of aerenchyma is analysed