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. deepei 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 H2O2 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.
