A shortage in the zinc supply to spinach (Spinacia oleracea L.) drastically reduced carbonic anhydrase levels with little effect on net C02 uptake per unit leaf area, except with the most severe zinc stresses. Under these conditions, carbonic anhydrase was below 10% and photosynthesis 60 to 70% of the control levels. When photosynthesis was measured at a range of CO2 supply levels, zinc-deficient leaves were less efficient at 300 to 350 microliters per liter CO2 and above, but the same as controls at lower CO levels. This suggests that carbonic anhydrase does not affect the diffusion of C02, and that the effect of zinc deficiency was on the photosynthetic process itself. Our evidence does not support the hypothesis that carbonic anhydrase has some role in facilitating the supply of C02 to the sites of carboxylation within the chloroplast.Carbonic anhydrase is widespread in plants, but its exact physiological role is still uncertain. It may be directly involved in photosynthesis, facilitating the diffusion of CO2 through the liquid phase of the cell to the chloroplast (9,13,20). In algae grown at high CO2, carbonic anhydrase is absent. After transfer to low CO2, photosynthesis is at first negligible, but later increases in step with increasing carbonic anhydrase levels (8). In higher plants, acetazolamide, an inhibitor of carbonic anhydrase, has been shown to reduce photosynthesis by intact chloroplasts. The inhibition can be overcome by raising the bicarbonate level (4). Acetazolamide also inhibits the photosynthesis of detached leaves of spinach and maize (5). Wood and Sibly (19) found that zinc-deficient plants contained low levels of carbonic anhydrase. In the experiments reported in this paper, we have studied the relationship between carbonic anhydrase and photosynthesis by manipulating the zinc supply to the plant. By this means, we have obtained large changes in carbonic anhydrase. However, these were accompanied by small effects on photosynthesis. The results reported do not support the hypothesis that carbonic anhydrase is directly involved in photosynthesis. MATERIALS AND METHODS

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Subterranean clover plants were grown in river sand or in culture solutions with and without phosphorus. Some phosphorus-deficient plants were sprayed once every 2–3 days with solutions (pH 2.5) of several phosphorus compounds at various concentrations, with and without wetting agents. Dry weights of tops and roots were significantly increased by most spray treatments compared with control plants grown without phosphorus. Foliar applications of 50 mM phosphoric acid solutions, containing little or no wetting agent, generally gave the greatest response. In the first experiment the plant dry weight in the best spray treatment was twice as high and in the second experiment 3.5 times as high as in the controls without applied phosphorus. In these spray treatments plant dry weight was no more than 40% of that in the control treatment receiving root phosphorus. Two days after application of 32P (solution pH 5.5, 0.01 mM phosphate) to the centre leaflet of either the first or the fourth trifoliate leaf, 70% of the tracer could be removed by washing with 20 ml water. Seven days after application of the tracer the treated leaflet of the first or the fourth trifoliate leaf still contained 77 or 70% respectively of the amount absorbed by the plant. In another experiment, in which 32P was applied in a 30 mM H3PO4solution at pH 2.5 or 5.0, washing of the treated leaflet 2 days after application removed 42 and 60% of the tracer respectively. Seven days after application the treated leaflet contained 28 and 34% respectively of the residue after washing. Relatively slow rates of uptake of phosphorus applied to the leaves were considered to be at least partly responsible for the poor growth responses compared with phosphorus applied to the roots. It was concluded that foliar applications of phosphorus offered little scope as a practical means of hastening the recovery from phosphorus deficiency.

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