Physiologia Plantarum 130: 177–184. 2007 Copyright ª Physiologia Plantarum 2007, ISSN 0031-9317 Detoxification of hydrogen peroxide maintains the water transport activity in figleaf gourd (Cucurbita ficifolia) root system exposed to low temperature Ji-Ye Rhee † , Seong-Hee Lee † , Adya Prasad Singh ‡ , Gap-Chae Chung and Sung-Ju Ahn* Agricultural Plant Stress Research Center, Environmental Friendly Agricultural Research Center, Department of Plant Biotechnology, Chonnam National University, Gwangju 500-757, Korea Correspondence *Corresponding author, e-mail: asjsuse@chonnam.ac.kr Received 6 November 2006; revised 21 January 2007 doi: 10.1111/j.1399-3054.2007.00895.x The figleaf gourd (Cucurbita ficifolia Bouche ´ ) root system has the ability to take up water and nutrients at low soil temperatures, and in the present paper, we attempt to reveal some of the molecular mechanisms behind this low- temperature tolerance. Exposure of figleaf gourd root system to low temperature induced accumulation of H 2 O 2 along the plasma membrane but not in the cytoplasm. H 1 -ATPase (EC 3.6.1.35) activity of isolated root plasma membranes and root hydraulic conductivity (Lp r ) were largely insensitive to externally applied H 2 O 2 . However, using bromocresol purple, it was shown that the acidification of the medium surrounding the root was strongly inhibited with low temperature- and H 2 O 2 -treated roots. Addition of catalase (EC 1.11.1.6) to the root medium during low-temperature exposure led to a recovery of H 1 -efflux along the root surface and increased Lp r , demonstrating the importance of an H 2 O 2 detoxification system in the root cells. Additional evidence for an increased Lp r was obtained by the Fenton reaction wherein a warming of the solution increased the activity of the detoxification system. All available evidence suggests that the ability of figleaf gourd root system to maintain a low level of H 2 O 2 in the cytoplasm and to detoxify reactive oxygen species is related to the maintenance of water transport activity at low temperatures. Introduction It has been shown that the figleaf gourd root system has the ability to take up water and nutrients at low soil temperatures (Ahn et al. 1999). Using figleaf gourd as a rootstock, this important agronomic characteristic has been exploited for cucumber production especially during winter, when soil temperatures are low. As a possible candidate to explain such tolerant character- istics, we have shown that there is a large increase in the hydraulic conductivity of the figleaf gourd when the root system is preconditioned at low temperatures (Lee et al. 2005a). Because water is the basis of life, it is not surprising to find that various environmental factors affect the water relation of plants. Notably, osmolarity (Johansson et al. 1996), mechanical stress (Wan et al. 2004) and temper- Abbreviations – Lp, cell hydraulic conductivity; Lp r , root hydraulic conductivity; P , cell turgor pressure; P r , root pressure; ROS, reactive oxygen species; T w 1=2 ; half-times of water exchange; TEM, transmission electron microscope. † These authors contributed equally to this work. ‡ Present address: The joint forces of CSIRO & SCION (formerly Forest Research), Te Papa Tipu Innovation Park, 49 Sala Street, Private Bag 3020, Rotorua, New Zealand Physiol. Plant. 130, 2007 177