The two types of aerenchyma are classified according to the formation mechanism, that is, primary aerenchyma formed in fundamental tissues and secondary aerenchyma differentiated from phellogen (secondary meristem). The term “aerenchyma” was originally given to secondary aerenchyma, and its morphogenesis has been researched from the beginning of the 19th century. Secondary aerenchyma is homologous with cork tissue and consists of white spongy tissue structured by living cells with non-suberized walls. It is continuously formed in the stem, hypocotyls, tap root, adventitious roots and nodules in plants with flooding tolerance under flooded conditions. It plays a role in lenticels entering atmospheric O₂ into the plant at the stem base and transports O₂ to underground tissues. In the flooding-tolerant legume plants, O₂ for respiration and N₂ for nitrogen fixation are transported to nodules through aerenchyma at the same time. Although the research of the formation mechanism of secondary aerenchyma and its function in aeration is at the fundamental level, it is expected that further progress in the research will lead to a better understanding of how to improve flooding tolerance of crops.

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The growth and morphology of IL-srt1, a short-root near isogenic line derived from rice cultivar Oochikara, were characterized by paddy field and pot experiments, with particular focus on the interrelationship between shoot and root growth. The shoot dry weight, stem number, root length and root dry weight per hill of IL-srt1 were lower than those of Oochikara, but the number of crown roots per hill was the same for two cultivars (line). The root weight/root length ratio of IL-srt1 was higher than that of Oochikara, which was attributed to the difference of the mean root diameter, not to the specific gravity of roots. Root system surveys by monolith method revealed that IL-srt1 had smaller root length density at everywhere in soil, and narrow rooting zone. The number of stem per hill was responsible for the difference of shoot dry weight per hill, and there was no difference in shoot dry weight per stem. It was assumed that larger number of crown roots and higher percentage of young crown roots of IL-srt1 as compared to Oochikara might compensate for shorter root length per stem, and attained the same water and nutrient supply per stem. It was also assumed that the rooting zone would closely relate to the emergence and survival of tillers through the ability of water and nutrient supply.

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