Abstract
Previous studies with isolated chloroplasts have indicated that Cl− is an essential cofactor for photosynthesis. Considerable support for the postulated Cl− requirement in photosynthesis came from the observation that Cl− is essential for growth. Data are presented which show that a 60% reduction in growth which occurred in Cl− -deficient sugar beet (Beta vulgaris L.) was not due to an effect of Cl− on the rate of photosynthesis in vivo (net CO2 uptake per unit area of attached leaves). The principal effect of Cl− deficiency was to lower cell multiplication rates in leaves, thus slowing down their growth and ultimately decreasing their area. The absence of an effect of Cl− on photosynthesis in vivo was unlikely to have been due to Cl− retention by the chloroplasts because their Cl− concentration (measured after nonaqueous isolation) decreased progressively with decrease in leaf Cl−.
An effect of Cl− with isolated chloroplasts in vitro, however, was confirmed. Addition of Cl− to the reaction medium after washing chloroplasts in EDTA increased the rate of ferricyanide photoreduction 10-fold. This effect of Cl− did not appear to be related to the Cl− concentration of the chloroplasts since chloroplast Cl− was not decreased further by washing in EDTA. It is concluded that Cl− has not yet unequivocally been shown to be an essential cofactor for photosynthesis and that the response to Cl−in vitro probably does not have a physiological basis.
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