Transport Properties of the Tomato Fruit Tonoplast 1: III. Temperature Dependence of Calcium Transport

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Calcium transport into tomato (Lycopersicon esculentum Mill, cv Castlemart) fruit tonoplast vesicles was studied. Calcium uptake was stimulated approximately 10-fold by MgATP. Two ATP-dependent Ca2+ transport activities could be resolved on the basis of sensitivity to nitrate and affinity for Ca2+. A low affinity Ca2+ uptake system (Km > 200 micromolar) was inhibited by nitrate and ionophores and is thought to represent a tonoplast localized H+/Ca2+ antiport. A high affinity Ca2+ uptake system (Km = 6 micromolar) was not inhibited by nitrate, had reduced sensitivity to ionophores, and appeared to be associated with a population of low density endoplasmic reticulum vesicles that contaminated the tonoplast-enriched membrane fraction. Arrhenius plots of the temperature dependence of Ca2+ transport in tomato membrane vesicles showed a sharp increase in activation energy at temperatures below 10 to 12°C that was not observed in red beet membrane vesicles. This low temperature effect on tonoplast Ca2+/H+ antiport activity could only by partially ascribed to an effect of low temperature on H+-ATPase activity, ATP-dependent H+ transport, passive H+ fluxes, or passive Ca2+ fluxes. These results suggest that low temperature directly affects Ca2+/H+ exchange across the tomato fruit tonoplast, resulting in an apparent change in activation energy for the transport reaction. This could result from a direct effect of temperature on the Ca2+/H+ exchange protein or by an indirect effect of temperature on lipid interactions with the Ca2+/H+ exchange protein.

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