Effects of root-zone environment, humid atmosphere and nutrient solution with different oxygen levels on external and internal structures of tomato roots were investigated. Young tomato plants were grown in the wet-sheet culture (WSC), in which all roots were developed in a humid atmosphere, or hydroponically in the deep flow technique (DFT) with or without aeration. Dissolved oxygen in the DFT without aeration (DFT) or with aeration (DFT+Air) were 45-62% and 91-96% saturation, respectively. Growth of tomato plants in the DFT+Air and the WSC were more vigorous than those in the DFT. No differences in the total lengths and surface areas were observed among the three plots. The fractal dimension, describing complexity of fractal architectures, of lateral roots in the WSC was higher than those in the DFT+Air. The number and mean length of first order lateral roots were similar among the three plots, whereas the DFT had a large proportion of shorter laterals. Aerenchyma was observed in the stele in the DFT. The WSC had more root hairs, larger cortical cells, metaxylem, stele, and larger deposits of lignin lamellae at exodermis than had the DFT. These changes in external or internal structure of the roots are interpreted to be adaptive responses to the root environment, e.g., anoxia in the DFT and water deficit in the WSC.
