As one of the trial projects aiming to reconstruct a food production area in the Tohoku region following the Great East Japan Earthquake and Tsunami in 2011, we conducted an empirical study on an integrated mechanical cultivation system for cabbages grown for processing. As a countermeasure against the lodging of cabbage heads, which is recognized as one of the problems associated with the mechanical harvesting of cabbages, we investigated the effect of transplant depth of plug-grown seedlings on the yield, head angle, and root system of cabbages. We compared three transplant depths: shallow (+4.0 mm from the soil surface), medium (-4.4 mm), and deep (-22.6 mm). The fresh weights of cabbage leaves and heads did not differ among the planting depths. The population of cabbages with a small head angle (\<20°) from the vertical position was significantly larger in the deep plantation than in the shallow plantation, indicating that the deep plantation of plug-grown seedlings could reduce the probability of lodging. We discuss the possible reasons for these results, focusing particularly on the form of main stem and root system. The ratio of stem length to stem weight was lower in the deep plantation than in the shallow plantation, suggesting that a thicker and more rigid stem could be developed by deep plantation. Root length density tended to be higher in the deep plantations than in the shallow plantation at depths of 5–10 cm. These properties may contribute to lodging resistance in cabbage.

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Erianthus and Napier grass are large, C4 perennial grasses. Currently, we study on cultivation techniques of the two species in an attempt to use them as raw materials for cellulosic bioethanol. Energy crops should be grown in non-arable lands to avoid disturbance of food production so that their roots should be capable of acquiring water and nutrients from degraded soils. Because the entire aboveground biomass is harvested as raw materials, roots are only organic matters incorporated into soils. In energy crop cultivation, few studies were conducted belowground despite its importance. Thus, roots of the two species are also investigated in our study. Erianthus and Napier grass develop a large amount of roots in shallow soil layers while penetrating deeply. As a result, the amount of standing roots becomes very large compared with common crops, and a substantially large amount of dead roots is released into soils. Nodal roots are thick in diameter, and many starch grains accumulate in pith of their steles in autumn. Exodermis, endodermis and stele are unique in structure among gramineous plants. Soil sheath forms on root surfaces due to dense root hairs, and large air spaces are found in cortex. The two species can grow well in hard, poor soils and/or under flooded conditions. Soil carbon concentrations increase even after the repeated removal of entire aboveground biomass. Therefore, Erianthus and Napier grass should be potential energy crops in terms of their root growth and functions as well as their large aboveground biomass.

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Roots of gramineous plants can be morphologically adapted to waterlogging in soil, where dissolved oxygen concentrations are extremely low. To adapt to such conditions, some gramineous plants develop lysigenous aerenchyma, which is formed by the creation of gas spaces as a result of death and the subsequent lysis of some cells, in the root cortex. Internal transport of oxygen from shoots to roots through aerenchyma is essential for plants to survive under waterlogged conditions. Although it is well known that the gaseous phytohormone ethylene is involved in induction of aerenchyma formation, the detail molecular mechanisms underlying inducible aerenchyma formation remained to be elucidated. Previously, we identified genes associated with aerenchyma formation in maize roots by using a microarray analysis combined with laser microdissection, and found that the expression of genes encoding generation/scavenging of reactive oxygen species is confined to cortical cells. This suggests that reactive oxygen species are involved in inducible aerenchyma formation in gramineous plants. Recently, we showed that adventitious roots of wheat seedlings that emerge under waterlogged conditions have thicker root diameters and larger air spaces (i.e., aerenchyma) than adventitious roots that emerge under aerobic conditions. Therefore, the thicker root diameters would contribute to the transport of oxygen from shoots to roots. In this article, we summarize recent advances in understanding the molecular mechanisms of aerenchyma formation in gramineous plants, and discuss the mechanisms for morphological adaptation of roots of gramineous plants to waterlogging in soil.

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