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The effect of temperature of the rate of photosynthetic electron transfer in chloroplasts of chilling-sensitive and chilling-resistant plants
Authors:Amir Shneyour  John K Raison  Robert M Smillie
Institution:Plant Physiology Unit, CSIRO Division of Food Research and School of Biological Sciences, Macquarie University, North Ryde, 2113, Sydney Australia
Abstract:1. Photochemical activities as a function of temperature have been compared in chloroplasts isolated from chilling-sensitive (below approximately 12 °C) and chilling-resistant plants.2. An Arrhenius plot of the photoreduction of NADP+ from water by chloroplasts isolated from tomato (Lycopersicon esculentum var. Gross Lisse), a chilling-sensitive plant, shows a change in slope at about 12 °C. Between 25 and 14 °C the activation energy for this reaction is 8.3 kcal·mole?1. Between 11 and 3 °C the activation energy increases to 22 kcal·mole?1. Photoreduction of NADP+ by chloroplasts from another chilling-sensitive plant, bean (Phaseolus vulgaris var. brown beauty), shows an increase in activation energy from 5.9 to 17.5 kcal·mole?1 below about 12 °C.3. The photoreduction of NADP+ by chloroplasts isolated from two chilling-resistant plants, lettuce (Lactuca sativa var. winter lake) and pea (Pisum sativum var. greenfeast), shows constant activation energies of 5.4 and 8.0 kcal·mole?1, respectively, over the temperature range 3–25 °C.4. The effect of temperature on photosynthetic electron transfer in the chloroplasts of chilling-sensitive plants is localized in Photosystem I region of photosynthesis. Both the photoreduction of NADP+ from reduced 2,6-dichlorophenol-indophenol and the ferredoxin-NADP+ reductase (EC 1.6.99.4) activity of choroplasts of chilling-sensitive plants show increases in activation energies at approximately 12 °C whereas Photosystem II activity of chloroplasts of chilling-sensitive plants shows a constant activation energy over the temperature range 3–25 °C. The photoreduction of Diquat (1,1′-ethylene-2,2′-dipyridylium dibromide) from water by bean chloroplasts, however, does not show a change in activation energy over the same temperature range. The activation energies of each of these reactions in chilling-resistant plants is constant between 3 and 25 °C.5. The effect of temperature on the activation energy of these reactions in chloroplasts from chilling-sensitive plants is reversible.6. In chilling-sensitive plants, the increased activation energies below approximately 12 °C, with consequent decreased rates of reaction for the photoreduction of NADP+, would result in impaired photosynthetic activity at chilling temperatures. This could explain the changes in chloroplast structure and function when chilling-sensitive plants are exposed to chilling temperatures.
Keywords:DCIP  2  6-dichlorophenolindophenol  Diquat  1  1′-ethylene-2  2′-dipyridylium dibromide
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