Abscisic acid and indole-3-acetic acid contents in orange trees infected by Xylella fastidiosa and submitted to cycles of water stress M.M.A. Gomes 1, *, A.M.M.A. Lagôa 2 , E.C. Machado 2 and C.L. Medina 3 1 Setor de Fisiologia Vegetal/CCTA/LMGV, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000, Cep: 28015-620 Campos dos Goytacazes, RJ, Brasil; 2 Centro de Ecofisiologia e Biofisica, Instituto Agronômico de Campinas, SP, Brasil; 3 Departamento de Fisiologia Vegetal, Universidade Estadual de Campinas, SP, Brasil; *Author for correspondence (e-mail: maramag@uenf.br; phone: (55) 22 2726-1435) Received 9 April 2002; accepted in revised form 18 September 2002 Key words: Abscisic acid, Citrus sinensis L. Osbeck, Citrus variegated chlorosis, Indole-3-acetic acid, Water stress, Xylella fastidiosa Abstract ‘Pêra’ sweet orange plants (Citrus sinensis L. Osbeck) grafted on ‘Rangpur’ lime rootstock (1 year-old) (Citrus limonia Osbeck) were inoculated with Xylella fastidiosa, a xylem-limited bacterium pathogen, which causes Cit- rus Variegated Chlorosis (CVC). Since it was known that water deficiency in the field enhances CVC-effects on the plant, the trees were submitted to three cycles of water stress during a one year period (March and October, 1998; and April, 1999) and divided in four treatments: healthy plants (HP); water-stressed healthy plants (WSHP); diseased plants (DP) and water-stressed diseased plants (WSDP). Stomatal conductance (g s ) of water-stressed diseased plants decreased in the first and second cycles of water deficiency, as the stress was increasing. The low stomatal conductance verified may be due to the high concentrations of abscisic acid (ABA) found in these plants. In the third cycle, values of g s in diseased plants were, usually, lower than in the healthy ones. In healthy plants, g s was reduced when these plants were submitted to water deficiency, independently of the cycle. The drop in leaf water potential in healthy plants was faster after irrigation was withheld, because healthy plants transpired more and, therefore, the water content of the substrate decreased more quickly. When the irrigation of WSDP was withheld in the third cycle, it was not possible to detect increases in ABA contents, suggesting that other factors could be acting to diminish the stomatal conductance in these plants. The presence of Xylella fastidiosa did not induce an increase in indole-3-acetic acid content in the leaves. After three cycles of water deficiency, the concentrations of indole-3-acetic acid in WSHP and WSDP were lower than those concentrations in the irrigated controls on the day water stress was more severe. Introduction Citrus Variegated Chlorosis (CVC) is a disease caused by Xylella fastidiosa bacteria that develops in the xylem of host plants. Plant leaves with CVC are generally smaller than those in healthy plants, with much lower zinc and potassium contents and present small blisters on the abaxial surface. Most of the symptoms of this disease, such as leaf marginal ne- crosis, delayed growth in the spring and decline of vigor leading to death of the plant, suggest a dysfunc- tion of the water conducting system (Hopkins 1989). It was also verified that bacteria, gums and tyloses block the xylem vessels of plants infected by Xylella fastidiosa (Purcell and Hopkins 1996). Citrus leaves with CVC present symptoms of wa- ter stress, associated with significant decreases in the CO 2 assimilation and transpiration rates (Machado et al. 1994). There is greater disease incidence in São Paulo State, Brazil, a region where water stress is higher. In Parthenocissus quinquefolia infected by Xylella fastidiosa, the disease symptoms increased under water stress conditions (McElrone et al. 2001). 263 Plant Growth Regulation 39: 263–270, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.