AOBPreview originally published online on October 6, 2007
Annals of Botany 2007 100(7):1431-1439; doi:10.1093/aob/mcm239
Cortical Aerenchyma Formation in Hypocotyl and Adventitious Roots of Luffa cylindrica Subjected to Soil Flooding
1 National Institute of Crop Science, 2-1-18 Kannondai, Tsukuba, Ibaraki, 305-8518, Japan
2 Biotron Institute, Kyushu University, 6-10-1 Hakozaki, Fukuoka, 812-8581, Japan
3 Department of Plant Resources, Faculty of Agriculture, Kyushu University, 111 Harumachi, Kasuya, Fukuoka, 811-2307, Japan
* For correspondence. E-mail shimamu{at}affrc.go.jp
Received: 4 June 2007 Returned for revision: 10 July 2007 Accepted: 2 August 2007 Published electronically: 6 October 2007
Background and Aims: Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed.
Methods: Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method.
Key Results: Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11–45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root.
Conclusions: Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.
Key words: Aerenchyma, Luffa cylindrica, primary cortex, flooding, oxygen, adventitious root, hypocotyl, porosity