In plants, the accelerated production of reactive oxygen species (ROS) is a common response to different stresses. We developed a computer method of leaf image analysis to automatically quantify the O À 2 and H 2 O 2 staining patterns in leaves. The quantification of histochemically detected ROS is given as a percentage of stained area partitioned according to the staining intensity and hence O À 2 =H 2 O 2 accumulation. Thus, this method expands the routine histochemical detection of ROS in situ. The same procedure can be also used to assess necrotic infection symptoms. In plants infected with necrotizing pathogens leaves can be simultaneously examined for pathogeninduced O À 2 and H 2 O 2 accumulation and infection development. This procedure allows rapid and accurate examination of many leaves and provides an advantage over the routine visual assessment of in situ histochemical detection of ROS and infection symptom development.
Summary
We studied the effects of sequentially applied salt stress and Pseudomonas syringae pv lachrymans (Psl) infection in cucumber (Cucumis sativus L.). Infection development, shoot and root growth potential, the concentrations of chlorophyll and proline as well as electrolyte leakage, lipid peroxidation and H2O2 production were determined. Cucumber plants were first exposed to salt stress and irrigated for seven days with 50 or 100 mM NaCl and thereafter inoculated by Psl. Abiotic stress compromised the defence response to pathogen and disease severity was the highest in 100 mM NaCl-treated plants. The reduced performance of salinized plants under biotic stress could be related to salt stressinduced plant growth inhibition with leaf expansion being the most sensitive to salinity, decreased chlorophyll content, increased electrolyte leakage and prolonged H2O2 accumulation in leaves implying perturbations in redox homeostasis. The response of NaCl-treated and control plants to bacterial infection differed in terms of H2O2 generation and lipid peroxidation. This study confirmed that proline is an important component of local and systemic responses to salt stress and infection. The results contribute to our knowledge of the nature of plant response to a combination of abiotic and biotic stresses
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