Deep rooting of wheat has been suggested that it influences the tolerance to various environmental stresses. In this study, the nodes from which the deepest penetrated roots had emerged were examined in winter wheat. The wheat was grown in long tubes with or without mechanical stress and in large root boxes. The length and growth angle of each axile root were examined to analyze the difference in the vertical distribution of the roots between the two wheat cultivars, one with a deep and one with a shallow root system. In Shiroganekomugi, a Japanese winter wheat cultivar with a shallow root system, the rooting depths of the seminal and nodal roots decreased as the rooting nodes advanced acropetally. Six out of nine deepest roots were seminal root in the non-mechanical stress conditions. In Mutsubenkei, a Japanese winter wheat cultivar with a deep root system, grown in root boxes, not only the seminal roots but also the coleoptilar and the first nodal roots penetrated to a depth of more than 1.3 m in the root box, and became the deepest roots. In both cultivars, the seminal roots became the deepest roots under the mechanical stress conditions. There were no clear tendencies in the root growth angles among the rooting nodes in the wheat root system. This indicates that the length of the axile roots can explain the differences in the rooting depths among axile roots in a wheat root system. On the other hand, the axile roots of Mutsubenkei elongated significantly more vertically than those of Shiroganekomugi. This suggests that not only seminal but also nodal roots exhibit strong positive gravitropism and penetrate deeply in a cultivar with a deep root system. In wheat cultivars, it is likely that the extent of its Root Depth Index results partly from the gravitropic responses of both seminal and nodal roots.

Asymmetric distribution of acetylcholinesterase (AChE) activity has previously been demonstrated to occur in the lower side of the gravity-stimulated maize shoot. The localization of immunoreacted IAA-inositol synthase, AChE and safranin was detected in selected organs of gravistimulated dark grown maize seedlings using a light microscope. Immunoreacted IAA-inositol synthase was asymmetrically distributed in the lower side of the stele of coleoptile node and mesocotyl in maize seedlings placed horizontally. The positive AChE spots in the coleoptile node and mesocotyl were apparently localized in the lower half of the gravistimulated seedlings. Safranin was also asymmetrically distributed in the lower half of the endodermis and stele cells of coleoptile node and mesocotyl. Namely, transport of safranin in the upper half of the coleoptile node and mesocotyl was blocked by gravistimulation. Furthermore, the asymmetric distribution of immunoreacted IAA-inositol synthase was inhibited by neostigmine bromide, AChE inhibitor. These results show that an asymmetric environmental stimulus induces changes in AChE activity, affecting IAA-inositol synthase localization and safranin transport.