Influence of pH and plant nutrient status on ion fluxes between tomato plants and simulated acid mists

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Abstract
Diffusion along concentration gradients, ion exchange, active uptake, and physical damage are possible mechanisms causing fluxes of ions between plant tissues and acidified moisture on plant surfaces. The temporal patterns of ion fluxes during individual wetting events should vary in a predictable manner in relation to moisture pH, plant nutritional status, and the operational mechanism. Tomatoes (Lycopersicon esculentum Mill.) were grown hydroponically at relative nutrient addition rates (Ra) of 7, 12, and 15% per day. At similar growth stages, the plants were exposed to HCl-acidified mists of pH 2.5, 4.0, or 5.6 for 4 h, or left unmisted as controls. Whole-plant throughfall was analyzed for pH and ion concentrations after 15, 30, 60, 120, and 240 min. Surface chemical contamination was minimized by pre-rinsing with deionized water. Except for H+ during the first 15-30 min, all ions showed efflux for the duration of the mistings. Effluxes were low (< 150 .mu.equiv m-2 h-1), time-invariant, and unrelated to Ra at mist pH of 4.0 and 5.6. At pH 2.5, effluxes of K+, Mg2+, NO3-, and PO43- increased over time, while Ca2+ efflux increased during the first hour, then remained constant. Maximum efflux (105 to 730 .mu.equiv m-2 h-1) mostly occurred at pH 2.5 and Ra = 17% d-1, increasing in the order NH4+ = SO42- < NO3- < PO43- < Ca2+ = Mg2+ < K+ < H+. These results suggested that although ion diffusion and ion exchange contributed to observed effluxes, an additional mechanism, temporally accumulating physical damage to cuticles and cell membranes below a threshold pH between 4.0 and 2.5, was involved. Foliar concentrations of K, Ca, and Mg did not differ significantly among treatments, but ion effluxes into pH 2.5 mists removed 1-13% of the whole-plant element contents, with the largest removal for Ca.