Response of tomato root systems to environmental stress under soilless culture systems

Summary The response of tomato grown in soilless culture to root-zone environment was investigated in terms of structural and functional aspects. We demonstrated that the growing system exerts an adaptive response and would be effective in stable production. DFT (Deep flow technique) and WSC (Wet-sheet culture) , that are different in their root-zone environments, were adopted. This study was focused on the characteristic roots of these systems, one in solution and the other in humid atmosphere. It is known empirically that roots in the humid atmosphere are more highly adaptive to the nutrient concentration, pH and temperature than in the solution. There is no applicable observation for practical use. In the present study, we analyzed structure and function of those two root systems under practical conditions. First, oxygen and temperature stresses, that tend to be problems in year-round production by soilless culture, were verified. Young tomato plants were grown in the wet-sheet culture (WSC) , in which all roots were developed in humid atmosphere, or hydroponically in the deep flow technique (DFT) with or without aeration. Effects of root-zone environment on external and internal structures of tomato roots were observed. 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 the exodermis than the DFT. These changes in external or internal structures 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. Short-term differences in water and nitrate absorption between tomato roots in the WSC and DFT were compared at four-root temperatures; 17, 27, 33 and 45 deg C . Root dry weight in the WSC was larger than in the DFT, whereas whole plant weights in both systems were equal. Water absorption rates on whole root were almost equal in both systems at all temperatures. On the other hand, nitrate absorption was greater in the WSC than the DFT at 45 deg C , but no significant differences were observed at other temperatures. Root respiration in the DFT showed an increase as the root temperature increased from 17 deg C to 33 deg C, but decreased at 45 deg C. We suggested that roots in the wet