Characteristics of light-induced H+ transport in spinach chloroplast at lower temperatures II. A possible role of structured water in H+ transport of chloroplast near 0°C

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Abstract
Light-induced H+ transport in spinach chloroplasts at lower temperatures was investigated with a glass electrode. Half-decay time of the H+ transport on turning off the illumination increased with lowering of the temperature. However, near the freezing point of bulk water, the H+ transport showed no dependency on temperature. An Arrhenius plot of the apparent first-order rate constant of H+ transport showed a break at about 8°C. Below that critical temperature, activation energy was about 0 kcal/mole. The break temperature in the Arrhenius plot shifted by 2–3°C to a higher temperature in the presence of 10% glycerol, an effective water structure-enhancing agent. On the other hand, addition of 5 mM NaSCN, known as a potent water structure breaker, abolished the break and a constant activation energy was obtained over the entire low temperature range. A similar effect was observed in the presence of 50 mM urea. On the basis of these results, we propose a mechanism for the H+ transport of chloroplasts at lower temperatures involving structured water, where rapid proton transfer between hydrogen-bonded structured water molecules takes place.