Effects Of Stresses On ROS Metabolism In Cellorganelles In Cucumber And Tomato Leaves

There is an increasing demand for vegetables with both high productivity and a superior quality in a rapid developing economy characterized with improved standards of living as in China currently. However, sustainance of high productivity and superior quality vegetables is imposing a big challenge on sound environmental management. Improper practices may lead to environmental deterioration which might induce global climate changes in the seasonal or paroxysmal natural disaster, and pathogen infection, etc. Therefore there is a practical need for individual or global researcher to elucidate clearly the response of vegetables to environmental stresses, attempts to improve their resistance, and to develop stress-tolerant cultivars through a combination of knowledge on vegetable physiology and breeding. In this study, we investigated the effect of abiotic stresses (such as heat, moderate heat, MV or H₂O₂) on APX isoenzymes, SOD isoenzymes, antioxidative systems in cellular or cub-cellular level in cucumber seedlings. Also, experiments were carried out to examine the relationship between photosynthetic, respiratory electron transport and antioxidant system in cell organelles in cucumber seedlings under a long-term cucumber mosaic virus infection (CMV). The main results are as follows:1. The response of moderate-heat stress on SOD isoenzymes and antioxidative system was investigated in two (one tolerant and one sensitive) cucumber genotypes. Differences in changes between these two genotypes of cucumber seedling subjected to moderate-heat were observed. Moderate heat resulted in more intensive changes in antioxidative systems for the sensitive genotype. For sensitive genotype, moderate heat induced the activities of Cu/Zn-SOD and Fe-SOD. But no significant changes were observed in Mn-SOD activity. At the same time, moderate-heat resulted in a general increase of AsA-GSH cycle enzymes. Similarly, Consistent closely with their activities, the expression of cAPX and CAT were up-regulated, indicating that sAPX was regulated at the transcriptional level. These changes play a positive role in effectively weeding out redundant ROS.2. The effects of abiotic stresses (heat, MV and H₂O₂) on the APX isoenzymes and AsA regeneration system (AsA-GSH recyle) were examined in cucumber seedlings. The activities of cytosol APX (cAPX), chloroplastic stroma APX (sAPX), microbody APX(mAPX) increased after a slight decline throughout the experiment. Consistent closely with sAPX activity, the expression of sAPX followed a similar change pattern, indicating that sAPX was regulated at the transcriptional level. In contrast, constitutive expression was observed in thylakoid membrane APX (tAPX) activity and no significant changes in tAPX activity were found throughout the experiment. The increases in monodehydroascorbate reductase (MDAR) and glutathione reductase (GR) were accompanied with enhanced level of AsA/DHA, implying that the AsA regeneration system plays an essential role in compensating AsA degradation.3. The effects of methyl viologen (MV)-induced photo-oxidation on the activities of superoxide dismutase (SOD) and enzymes of the ascorbate-glutathione (AsA-GSH) cycle in chloroplasts, mitochondria and cytosol of cucumber leaves were investigated. Photo-oxidation by MV resulted in significant reductions in net photosynthetic rate (Pn) and increases in the ratio of the quantum efficiency of photosystem II (PSII), Opsn to that of the quantum efficiency of CO2 fixation (Oco2), followed by increased activities of SOD, and a general increase of AsA-GSH cycle enzymes in chloroplasts, mitochondria and cytosol. These increases were however, most significant in chloroplasts. There were also significant increases in dehydroascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) except that the content of ascorbate (AsA) in chloroplasts and cytosol was slightly decreased and little effected, respectively. However, GSSG in mitochondria and GSH in cytosol were little influenced by the MV treatment. The activity of ascorbate oxidase (AO) in these organelles was independent of the MV treatment while the activity of L-galactono-l,4-lactone dehydrogenase (GLDH) in mitochondria was slightly inhibited by MV treatment. These results indicate that disturbance of electron transport in chloroplasts by MV influenced the metabolism of whole cell by a crosstalk signaling system and that the AsA-GSH cycle played a primary role in sustaining the levels of AsA.4. The effects of a long-term CMV infection on photosynthetic, respiratory electron transport and its relative response to ROS, together with the SOD and ascorbate-glutathione (AsA-GSH) cycle system were also investigated. Under a long-term CMV infection, reduced net photosynthetic rate (Pn) was accompanied by a lower quantum efficiency of PSII electron transport (