DTD 5 Comparative effects of allelochemical and water stress in roots of Lycopersicon esculentum Mill. (Solanaceae) T. Romero-Romero a , S. Sa ´nchez-Nieto b , A. SanJuan-Badillo b , A.L. Anaya a , R. Cruz-Ortega a, * a Laboratorio de Alelopatı ´a, Departamento de Ecologı ´a Funcional, Instituto de Ecologı ´a, Universidad Nacional Auto ´noma de Me ´xico, Circuito Exterior, Ciudad Universitaria, 04510 Me ´xico, DF, Me ´xico b Departamento de Bioquı ´mica, Facultad de Quı ´mica, Universidad Nacional Auto ´noma de Me ´xico, 04510 Me ´xico, DF, Me ´xico Received 20 August 2004; received in revised form 26 November 2004; accepted 1 December 2004 Abstract In the present study we compared the effects of allelochemical and water stress on roots of Lycopersicon esculentum. We evaluated the effects on root growth, morphology, H + -ATPase activity, and oxidative damage, to gain an understanding of the modes of action of allelochemicals from the noxious weed Sicyos deppei. Allelochemical stress was produced by 0.5% aqueous leachate of S. deppei and water stress by 12% PEG. Both stresses inhibited root growth by 75% and 76%. Scanning electron microscopy showed that all stressed roots exhibit a lateral swelling in the meristematic zone, but that only allelochemical stress caused an increase in root hairs development. Plasma membrane H + -ATPase activity was reduced by both stresses, allelochemical (50%) and water stress (56%). Water stress caused a higher reduction (42%) in tonoplast H + -ATPase activity than allelochemical stress (28%). Catalase activity was increased 137% by allelochemical stress, and only 47% by water stress. Levels of free radicals increased 44% (allelochemicals), and 32% (water stress), but levels of H 2 O 2 decreased in both (27% by allelochemical, and 52% by water stress). However, water stress caused the highest lipid peroxidation (105%) compared to allelochemical stress (47%). Allelochemical modes of action are discussed. # 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Allelochemical stress; Lycopersicon esculentum; Catalase; Lipid peroxidation; H + -ATPase; Water stress 1. Introduction Allelopathy involves plant chemical interactions in both natural and agro-ecosystems [1]. These interactions are due to the release of secondary metabolites from producer plants to the environment. These ‘allelochemicals’ can then interfere with the metabolism of other plants. If the effect of these allelochemicals is harmful to plant growth and development, it becomes a biotic stress known as allelochemical stress, which can have an indirect or direct effect on the receiver plant [2–6]. Thus, allelochemical stress can act as a mechanism of interference and can influence the pattern of vegetation, weed growth and crop productivity [7]. An allelochemical stress can have several molecular targets in the receiver plants and consequently can interfere with different cellular processes causing inhibition of growth. Our laboratory has focused on understanding the allelochemical modes of action of the weed Sicyos deppei G Don (Cucurbitaceae) on the growth of crop plants. S. deppei is a weed endemic of the central part of Mexico and climbs aggressively over other plants. We have previously reported the allelopathic potential of this weed [8] as well as some of its effects on the root ultrastructure of beans and bottle gourd [9]. In this latter study, we observed that both the allelochemical stress induced by an aqueous leachate of S. deppei and water stress generated by PEG produced similar cell plasmolysis. With the goal of gaining more insight into the mechanisms of interference of this weed on www.elsevier.com/locate/plantsci Plant Science xxx (2005) xxx–xxx Abbreviations: SEM, scanning electron microscope; PEG, polyethy- lene glycol; AS, allelochemical stress; WS, water stress; CAT, catalase * Corresponding author. Tel.: +52 55 5622 9032; fax: +52 55 5616 1976. E-mail address: rcruz@miranda.ecologia.unam.mx (R. Cruz-Ortega). 0168-9452/$ – see front matter # 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2004.12.002