Arrhenius plots of the respiration rates of nmitochondria isolated from chilling sensitive plant tissuies (tonmato and cucumber fruit, and sweet potato roots) showed a linear decrease from 23 C to about 9 to 12 C (with Qio values of 1.3 to 1.6), at which point there was a marked deviation with an increased slope as temperatures were reduced to 1.5 C (Qio of 2.2 to 6.3). The phorylative activities (assayed at 25 C) of mitochondria, isolated from chilling sensitive sweet potato roots, were impaired as a result of chilling treatment. However, many factors, such as the polyphenol content of the parent tissue (4, 11), can markedly influence both oxidative rates and apparent phosphorylative ability of isolated plant mitochondria. Thus, where plant material is given a differential pretreatment, it is most difficult to ascertain whether the observed differences in activity represent a direct effect of the chilling treatment on the mitochondria or an indirect effect due to the release or activation of some components which might be operational during isolation. It has also been noted that the method of tissue disruption (8), the amount of mitochondrial protein in the assay mixture (20), and the time involved between tissue disruption and assay (18) influence the activity of isolated mitochondria but which may be of little significance in vivo.The experiments reported here were carried out with mitochondria from plant tissue not subjected to chilling treatment, with the aim of evaluating the influence of temperature on oxidation and phosphorylation of these isolated mitochondria. The results show that in mitochondria from chilling sensitive tissues there is a marked depression in the respiratory rates below the critical temperature for chilling injury (10 C) which is not observed with mitochondria from resistant tissues. It was also established that temperatures between 1.5 and 25 C do not influence the phosphorylative efficiency of mitochondria from either sensitive or resistant plant tissues.Mitochondria isolated from chilling resistant tissues show the capacity to swell to greater extent than the mitochondria from chilling sensitive tissues (14), indicating that the membranes of mitochondria from chilling sensitive tissues are less flexible than those from chilling resistant tissues. Furthermore, this difference in flexibility was related to the relative proportion of saturated and unsaturated fatty acids of the membrane lipids. A study of the temperature at which mixtures of the predominant saturated and unsaturated fatty acids, found in the membranes of plant mitochondria, solidify showed that a small increase in the proportion of unsaturated fatty acids causes a large decrease in the solidification temperature (13). Thus (Beta vulgaris L.).The mitochondria were prepared and respiratory activity was determined by the methods described in the preceding paper (20). Since it was shown (20) that the respiration of isolated mitochondria was greatly reduced and the mitochondria lost respiratory control when th...

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Factors influencing the rate of superoxide (O 2 (-) ) production by thylakoids were investigated to determine if increased production of the radical was related to injury induced by chilling at a moderate photon flux density (PFD). Plants used were Spinacia oleracea L., Cucumis sativus L. and Nerium oleander L. grown at either 200° C or 45° C. Superoxide production was determined by electron-spin-resonance spectroscopy of the (O 2 (-) )-dependent rate of oxidation of 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazolidine (OXANOH) to the corresponding oxazolidinoxyl radical, OXANO ·. For all plants, the steady-state rate of O 2 (-) production by thylakoids, incubated at 25° C and 350 μmol photon · m(-2) · s(-1) (moderate PFD) with added ferredoxin and NADP, was between 7.5 and 12.5 μmol · (mg chlorophyll)(-1) · h(-1). Incubation at 5° C and a moderate PFD, decreased the rate of O 2 (-) production 40% and 15% by thylakoids from S. oleracea and 20° C-grown N. oleander, chillinginsensitive plants, but increased the rate by 56% and 5% by thylakoids from C. sativus and 45° C-grown N. oleander, chilling-sensitive plants. For all plants, the addition of either ferredoxin or methyl viologen increased the rate of O 2 (-) -production at 25° C by 75-100%. With these electron acceptors, lowering the temperature to 5° C caused only a slight decrease in O 2 (-) production. In the absence of added electron acceptors, thylakoids produced O 2 (-) at a rate which was about 45% greater than that when ferredoxin and NADP were present. The addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea reduced O 2 (-) production under all conditions tested. The results show that the rate of O 2 (-) production increases in thylakoids when the rate of electron transfer to NADP is reduced. This could explain differences in the susceptibility of thylakoids from chilling-sensitive and chilling-insensitive plants to chilling at a moderate PFD, and is consistent with the proposal that O 2 (-) production is involved in the injury leading to the inhibition of photosynthesis induced under these conditions.