The effect of temperature on respiration and kinetics of H2PO4-and K+ uptake in corn roots was determined in the range of 2 to 42 C. The response of uptake to temperature, determined from Qio and activation energy (Ea) data, for the anion and the cation differ significantly, especially in the range of uptake mechanism (Mech.) I. At 2.5 micromolar the Ea for K+ uptake below the 13 C transition is 29.3 kilocalories per mole. As the K+ concentration is increased, Ea (22.7 at 1.0 millimolar to 1.0 at 50 millimolar). There is no definable low temperature transition at these concentrations. Ion uptake is found to be much more sensitive to low temperature than respiration in this chillsensitive species. The data suggest that the low temperature reduction of ion transport is more closely related to restriction of function of active transport systems than to either respiration or membrane permeability.An analysis of the relationship of temperature to the polarization of the cell PD4 and respiration of corn roots revealed that oxidative metabolism is not a limiting factor for ion uptake below the apparent membrane transition of corn roots (2). Low temperature inhibition of the ion uptake capabilities of chill sensitive species is well documented (5, 6) and several hypotheses involving membrane localized effects have been advanced to explain this phenomenon (5). The extreme temperature dependence of the membrane potential below the apparent phase transition and the marked decline above it suggested that a major effect of low ' Partially supported by a grant from the Universidad Central de MATERIALS AND METHODS Seedlings of corn (Zea mays L. cv. Golden Bantam) were grown as described in the accompanying paper. After the 48-h period in lx Higinbotham solution (11) 5-cm apical root sections were excised and rapidly transferred to lx solution. These were recut 2.5 cm behind the tip just prior to use and rinsed in deionized H20. Approximately 0.5 g of root segments were placed in culture tubes which contained 50 ml of vigorously aerated absorption solution.The uptake solution for the Pi uptake experiments, unless otherwise specified, was composed of 0.25 mm KH2PO4 buffer plus 0.20 mm CaCl2 at pH 6.0. For the K+ uptake experiments, the absorption solution unless otherwise specified, was composed of 0.25 mm KCI plus 0.20 mm CaCl2 at £H 5.5. Absorption solutions were labeled with 5 ,uCi/l of 32p or Rb, respectively. At the end of the absorption period, which was I h except for the time courses of Pi uptake, the root segments were rinsed for 30 s with two 10-ml aliquots of unlabeled exchange solution. This solution was composed of 5 mm KH2PO4 plus 0.20 mm CaCl2 at pH 6.0 for Pi exchange, and 1.0 mm KCI plus 0.20 mm CaCl2 at pH 5.5 for K+ exchange. Roots were then transferred to ice-cold unlabeled exchange solution for removal of ions from the free space, as described by Epstein et al. (9). Exchange was 30 min for K+ and 10 min for Pi. Apparent ion uptake was determined by liquid scintillation spectrometry, using a Pack...

The effects of cyanide, anoxia, and temperatures varying from 2 to 42 C on the cell membrane electropotential difference (PD) of washed and freshly excised corn roots have been determined. Respiration rates of freshly excised root segments in response to cyanide and to varying temperatures were also measured. The ceDl membrane PD of roots which had been washed for 12 to 15 hours was almost insensitive to cyanide and anoxia but sensitive to low temperature. In contrast, the cell membrane PD of freshly excised roots was reversibly depolarized by all three treatments, cyanide depolarized from -117 to -86 milivolts and the sequential imposition of anoxia further lowered the PD to -69 milivolts. Anoxia applied first depolarized maximaDy and the PD was not further lowered by sequential cyanide treatment. Arrhenius plot analysis of the temperature response of respiration showed an apparent transition at 13 C with an activation energy of 20.0 kilocalories per mole below and 8.8 kilocalories per mole above the transition temperatures. The energy of activation for repolarization of PD is much higher, 53.4 kilocalories per mole below 7 to 8 C and 25.4 kilocalories per mole above this apparent transition. The energy requirement for polarization of the ceOl membrane PD was calculated based on the temperature responses of the ceOl membrane PD and respiration. It was estimated that 3.5% of the energy output from respiration at 22 C is required for cell polarization. It is unlikely that ion transport is limited by energy availability below the 8C transition in this chili sensitive species.Cells of higher plants typically maintain a cell PD4 of about 100 mv, interior negative. This cell PD has been shown to have two components in corn roots and in other higher plants (1,6,7,9). One of the components is of a diffusive nature and contributes about one-half of the total potential (12). The other component, generated by the activity ofelectrogenic ion pumps, can be blocked by treating the tissue with inhibitors of mitochondrial electron transport such as cyanide (7, 17), carbon monoxide (1), azide (20), uncouplers such as fluorocarbonyl cyanide phenylhydrazone (6) and DNP (7,12). Anoxia (10), and low temperatures (8,20) that the cell PD in washed pea and oat stem tissue is sensitive to CN-. Preliminary studies on the relationship of PD to respiratory activity in corn indicated that PD in this species is not affected by CN-when roots are washed by the protocol used for oats and peas by Higinbotham et al (12). A portion of this study examines the effects of CN-, anoxia, and low temperature on washed and freshly excised corn roots. The restriction of respiration by low temperature in chill sensitive species has been amply documented (2,4,5,19). The reduction of respiratory function has been correlated with reduced ion uptake (5). This study allows an analysis of the relationship between low temperature inhibition of respiratory activity and the cell PD in this chill sensitive species.The results are used to define further the r...