Photosynthesis, Growth, and the Role of Chloride

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 CO(2) 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.