Chin J Plant Ecol ›› 2008, Vol. 32 ›› Issue (6): 1258-1267.

• Original article •

### ECOLOGICAL ADAPTATION MECHANISMS OF ROOTS TO FLOODED SOIL AND RESPIRATION CHARACTERISTICS OF KNEE ROOTS OF TAXODIUM ASCENDENS

TANG Luo-Zhong1,*(), HUANG Bao-Long1, HAIBARA Kikuo2, TODA Hiroto2

1. 1College of Forest Resources and Environment, Nanjing Forestry University, Nanjing 210037, China
2Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan
• Received:2008-04-11 Accepted:2008-07-29 Online:2008-04-11 Published:2008-11-30
• Contact: TANG Luo-Zhong

Abstract:

Aims Taxodium ascendens is a flood-tolerant tree species. It is important to understand the mechanisms of flood-tolerance by means of study on the root changes of T. ascendens in flooded conditions.

Methods Based on the investigation of the roots of T. ascendens plantations in 17 year-old in Lixiahe wetland, Jiangsu Province, China, the ecological adaptations of the roots were analyzed in the different water table sites divided into three groups: high water table site (HWS, the site was flooded from June to October every year, and the depth of the mean water table in each year was -5 cm), middle water table site (MWS, the site was flooded from August to September every year, and the depth of the mean water table in each year was -18 cm), and low water table site (LWS, the site was not flooded all the time, and the mean water table in each year was -41 cm).

Important findings In HWS, T. ascendens formed aerating roots which were very long and thin, and attached on the tree stem or in the outer epidermis or in the crack of tree bark; In the MWS, T. ascendens formed knee roots which were (7.9±2.2) cm in diameter, (7.7±2.7) cm in height; AlthoughT. ascendens also formed the knee roots in LWS, they were smaller than in MWS. The belowground biomass and the aboveground biomass of the trees were in the order of HWS<MWS<LWS. However, the ratios of the belowground biomass and the aboveground biomass were in the order of LWS<MWS<HWS. It was suggested that althoughT. ascendens was a flood-tolerant tree species, the biomass growth decreased in flooding conditions, especially the aboveground biomass growth obviously decreased in flooding conditions. The ratios of the diameter at ground and at breast height in HWS, MWS and LWS was 2.66±0.11, 2.08±0.10 and 1.75±0.08, respectively. The volume weight of underground roots decreased if the trees were waterlogged for long time. But the volume weight of the aerating roots and the knee roots were greater than that of the underground roots. The concentrations of iron and manganese in fine roots in HWS and MWS were significantly higher than in LWS. However, the concentrations of iron and manganese in leaves were no difference between HWS, MES and LWS. The mean respiration rate of the each knee root was 2.1~2.5 mgCO2·h-1 in August and September, 0.7~0.9 mgCO2·h-1in June and November, and 0.4 mgCO2·h-1in March, respectively. The moles of oxygen absorption was 4.6 times more than that of carbon dioxide flux by knee roots. It appeared that the oxygen absorbed by knee roots was not only supplied to knee roots respiration, but also supplied to underground roots respiration. It was suggested that T. ascendens with high flood-tolerance was due to forming the aerating roots and knee roots, promoting the diameter growth of stem, and decreasing the volume weight of roots in the flooding sites for improving the ventilation condition of roots.