Wu et al. J. Anim. Plant Sci. 21(1):2011 20 ROOT MORPHOLOGICAL MODIFICATION OF MYCORRHIZAL CITRUS (CITRUS TANGERINE) SEEDLINGS AFTER APPLICATION WITH EXOGENOUS POLYAMINES Q. S. Wu, Y. N. Zou, Y. H. Peng and C. Y. Liu College of Horticulture and Gardening, Yangtze University, No. 88 Jingmi Road, Jingzhou City, Hubei Province 434025, People’s Republic of China Corresponding author. E-mail: wuqiangsh@163.com ABSTRACT The present work tried to determine the effects of the three polyamine (PA) species (100 mg L-1), putrescine (Put), spermidine (Spd) and spermine (Spm), on root colox`nization, root morphology and phosphorus (P) uptake of three- month-old citrus (Citrus tangerine) seedlings inoculated with arbuscular mycorrhizal fungus (AMF), Glomus mosseae. Exogenous PAs significantly increased root colonization three months after application. Sole AMF inoculation notably improved root morphology (total length, total projected area, total surface area and total volume) and growth performance (plant height, stem diameter and leaf number per plant) in comparison with non-mycorrhizal control, whilst PAs application to mycorrhizal seedlings increased more these root morphological and growth traits. AMF significantly reduced guaiacol peroxidase (G-POD) activity of root, whereas G-POD activities of mycorrhizal leaf and root were elevated by Pas, to a certain extent. Compared with sole AMF treatment, exogenous PAs significantly inhibited acid and neutral phosphatase activities of rhizosphere. Moreover, exogenous PAs notably reduced available P content of rhizosphere but significantly increased root P content. It concluded that exogenous PAs could stimulate both root colonization and morphology, and root morphological improvement of mycorrhizal seedlings by PAs was related to not G-POD activity but to root P level and would result in an increment of growth performance. Key words: Arbuscular mycorrhizal fungi, citrus, putrescine, root morphology, spermidine, spermine INTRODUCTİON Citrus, belonging to the genus Citrus of the family Rutaceae, is the most widely produced fruit tree, and it is grown in more than 80 countries (Ladaniya, 2008). China is the second production country of citrus fruit and leads in tangerine/mandarin production, with more than 8.6 million-ton output. The presence of root hairs has been found in many citrus species, but root hairs of citrus are relatively shorter than those of other tree species (Poerwanto et al, 1989). Therefore, citrus must depend on arbuscular mycorrhiza (AM), a symbiosis between arbuscular mycorrhizal fungi (AMF) and roots of plants, which can improve the supply of water and nutrients to the host plant (Parniske, 2008; Wu et al, 2009). However, in the open field, mycorrhizal colonization of citrus is less than 10% (Wu et al, 2009). Polyamines (PAs), namely putrescine (Put), spermidine (Spd) and spermine (Spm), are low molecular weight, aliphatic polycations found in the cells of all living organisms (Kusano et al, 2008). Plant PAs have involved in a variety of divergent processes, such as gene expression, protein and DNA synthesis, cellular homeostasis, cell division and differentiation, growth and developmental processes such as embryogenesis, organogenesis, senescence, and also responses to abiotic and biotic stresses (Moschou et al, 2008). Generally, PA concentration was equivalent in AM and non-AM plants when growth parameters were similar, irrespective of their different phosphorus (P) contents (Parádi et al, 2002). A recent experiment showed that exogenous PAs (100 mg L -1 ), especially Put, obviously increased AM colonization of Glomus versiforme-colonized Poncirus trifoliata (Wu and Zou, 2009). Optimal concentrations (50-200 mg L -1 ) of exogenous PAs had significantly positive effects on spore germination and hyphal growth of G. mosseae and Gigaspora margarita in vitro culture conditions (Zhang et al, 2003). However, it is unknown whether PAs regulate AM colonization of citrus. Moreover, PAs involve in root development, and PA accumulation has also been correlated to adventitious root formation (Couée et al, 2004). Greater root morphology will benefit plants absorbing more water and nutrients, thus increasing plant production. Root morphology modification depends on PA species. Spd and Spm contents showed positive correlations with primary root growth of Pringlea antiscorbutica, whereas Put level showed neutral or negative effects on the trait (Hummel et al, 2002). To date, information regarding the effect of PAs on root development of citrus is lacking. The aim of the present study was to determine whether PAs modify mycorrhizal colonization, root morphology and P uptake of citrus seedlings inoculated with G. mosseae. Additionally, the study tried to clarify the mechanism of root morphological modification based on the analysis of guaiacol peroxidase (G-POD). The Journal of Animal & Plant Sciences, 21(1): 2011, Page: 20-25 ISSN: 1018-7081