Salicylhydroxamic Acid-Resistant and Sensitive Components of Respiration in Chilling-Sensitive Plants Subjected to a Daily Short-Term Temperature Drop

Salicylhydroxamic acid (SHAM) is used in studies of plant respiratory metabolism as a specific inhibitor of alternative cyanide-resistant oxidase (AOX). The sensitivity of respiration in intact plant tissues to SHAM provides indirect evidence for electron transport through the AOX-mediated pathway (AOP) in the respiratory electron transport chain. In this study, chilling-sensitive plants (cucumber (Cucumis sativus L.), tomato (Solanum lycopersicum L.), and sweet pepper (Capsicum annuum L.)) were exposed daily to short-term drops of temperature (to 12, 8, 4, and 1 degrees C) by the end of night periods, and the effects of DROP treatments on SHAM-resistant and SHAM-sensitive respiration of leaves were examined. When leaf respiration in cucumber and tomato plants was measured at optimum temperature (25 degrees C), it showed an increase after the DROP treatments; this phenomenon was caused by the activation of SHAM-sensitive respiration ascribed mainly to AOP activity. At low measurement temperature (15 degrees C), the DROP treatments stimulated total respiration and SHAM-resistant respiration of cucumber leaves, which indicates the activation of electron flow through the cytochrome oxidase pathway (COP). Remarkably, the COP activity increased with the severity of DROP treatment. At high temperature (35 degrees C), the respiration of cucumber leaves became less sensitive to SHAM, whereas SHAM-resistant respiration increased with the severity of DROP treatments. The contribution of the SHAM-sensitive component to the total leaf respiration in cucumber increased with the temperature during measurements. Upon changes in temperature, the SHAM-sensitive respiration in cucumber exhibited larger variations than the SHAM-resistant component, which implies a significant contribution of COP to the respiratory chilling tolerance. Our data suggest that although AOP pathway respiration plays an important role in respiratory activity of chilling-sensitive plants, COP pathway is the main mechanism for maintaining the respiration during low-temperature treatments.