The Response Of Alternative Respiration To Temperature And Its Relation To Tolerance In Fruit Vegetable Crops

Respiration is one of the main processes in plant metabolism. Although it is well known that respiration is greatly influenced by environment, information about the relationship between respiration and tolerance is scanty. The present study was undertaken to examine the response of cucumber (Cucumis sativus L.) and pepper {Capsicum annuum L.) to changes in temperature in term of growth and respiration. Furthermore, changes in each respiration pathway, AOX regulatory mechanism, the relationship between ROS production and scavenging metabolism in chloroplast and mitochondria, and the relationship between AOX activity and chilling-induced photoinhibition were examined. The main results are as follows:1. Chilling at 8℃for 8d significantly decreased the fresh weight of shoots but had little effects on roots weight in cucumber. Result showed growth of shoots was more affected than roots by chilling. Chilled leaves showed a transit increases in total respiration (Vt), cytochrome (ρ'Vcyt) and alternative (ρValt) respiratory pathways then decreased gradually. At the end of the experiment, Chilled leaves showed lower Vt andρ'Vcyt and unchangedρValt valued as compared to unchilled control. On the other hand, chilling resulted in significant decrease ofρ'Vcyt and increases ofρValt and Vt in roots. Mitochondrial state 3 respiratory parameters showed similar trend to those observed in leaves and roots. Chilling also decreased the respiratory control ratio of leaves and roots mitochondria with the decreases in leaves mitochondria being more pronouncing. All these results suggest that chilling had a more significant impact on leaves mitochondria than roots mitochondria.2. Heat stress at 45/35℃(day/night) for 10d resulted in a growth reduction in pepper of Jizhao×Jilin, but not in Zhengjiao No. 13. However, drought, especially under heat stress, significantly inhibited the growth of peppers. These stresses increased ion leakage from leaves and this was especially apparent in Jizhao×Jilin. Heat induced increases of total, cytochrome and alternative pathway respirations. For Zhengjiao No. 13, drought restricted cytochrome respiration, but increased alternative respiration. At last, a slight increase in total respiration was observed at the initial 5 d of drought stress. In sharp contrast to the decreases of total respiration, alternative respiration increased significantly at the initial 1 d and then decreased to the level of control in Zhengjiao No. 13 under heat and drought. Alternative respiration was increased under drought, but decreased under heat and drought. Drought also inhibited both the total and cytochrome respirations of Jizhao×Jilin, especially under heat conditions. ZhengJiao No. 13 showed higher alternative and total respirations than Jizhao×Jilin under heat or/and drought stresses. These results suggested that higher heat or/and drought tolerance of pepper was associated with the capacity to keep high total and alternative respirations under stresses.3. After chilling at 8°C for 4 days, there were greater increases of H₂O₂ and malonaldehyde (MDA) contents in leaves than in roots. The same trend was also observed for ion leakage from tissues. Compared with control, both total respiration and alternative respiration in roots increased at different measurement temperatures. In contrast, cytochrome respiration remained unchanged. In comparison, chilling significantly decreased the total respiration in leaves and this decrease was mostly due to a decrease in cytochrome respiration. Chilling apparently decreased the sensitivity of alternative respiration to changes of temperature. The reduction levels of ubiquinone pool (UQr/UQt) increased both in chilled leaves and roots whilst pyruvate content increased only in chilled roots. Taken together, the results suggested that the higher chilling tolerance of roots was associated with the higher total respiration and alternative respiration.4.In order to clarify the relationship between chill-induced disturbance in photosynthetic, respiratory electron transport and the metabolism of reactive oxygen species, leaf gas exchange, chlorophyll fluorescence quenching, respiration, activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) were investigated in chloroplasts, mitochondria and cytosol of cucumber (Cucumis sativus) leaves subjected to a chill (8℃) for 4d. Chilling decreased net CO₂ assimilation rate (A) and quantum efficiency of PSII (Φ_PSII), but increased the ratio of O_PSII to the quantum efficiency of CO₂ fixation (ΦCO₂) and nonphotochemical quenching (NPQ) in cucumber leaves. While chilling inhibited the activity of cytochrome respiration pathway, it induced an increase of alternative respiration pathway activity and the reduction level of Q-pool. Meanwhile chilling significantly increased O_<sup>·- producing rate and H₂O₂ content and the SOD and APX activity in chloroplast and mitochondria. Higher increases of SOD and APX activities were observed in chloroplast than in mitochondria, and the membrane-bound Fe-SOD and tAPX in chloroplast increased more than other isoenzymes. It was suggested Fe-SOD and tAPX may play important role in protecting against chilling injury of chloroplast.5. The AOX (alternative oxidase) inhibitor salicylhydroxamic acid (1 mM; SHAM) inhibited AOX activity by 50% in vivo in both control and chilled cucumber leaves, but did not induced damage to photosynthetic apparatus. Chlorophyll a fluorescence parameters were not influenced by 1 mM SHAM treatment at room temperature. SHAM at 1 mM, however, significantly enhanced chill-induced both decreases in photochemical quenching and non- photochemical quenching under high light. Under low light, increased non-photochemical quenching was observed. Compared to the full recovery under low light, uncomplete recovery was found in chilled leaves in term of photoinhibition under high light and this trend was more significant in SHAM-treated leaves. Taken together, these results showed that up-regulation of alternative respiration would avoid chill-induced photoinhition, and AOX may play important roles in protecting against chilling under high light.