Wetting and stomatal penetration of plant leaves by spray solutions can be obtained when the surface tension of the spray solution is sufficiently low. Low surface tension fluorocarbon and silicone-based surfactants were active at lower concentrations and were found to be less harmful to plant cell membranes than a comparable carbohydrate-based surfactant. A silicone-based surfactant greatly increased the efficiency of iron solutions sprayed onto chlorotic citrus trees.

Foliar sprays of ferrous sulphate containing the organosilicone surfactant L77 formed films on bean leaf surfaces and caused only marginal burn damage. Sprays containing carbohydrate-based surfactant formed drops on the leaf surfaces and caused severe burn damage. It was found that ferric citrate and some other chemicals inhibited the spreading ability of the organosilicone surfactant, possibly through the formation of complexes.

Growth of bean (Phaseolus vulgaris cv. 'Brittle Wax') seedlings in iron-free media caused large reductions in root size and dye reduction capacity. Shoot growth was also severely retarded and the classical symptoms of chlorosis were observed. A single prophylactic spray of FeSO4, applied to the primary leaves of 7-day seedlings, enabled subsequent growth to equal that obtained when iron was continuously supplied to the roots over a 12-day period, although chlorophyll levels were lower. By adding a silicone-based surfactant to FeSO4 spray solutions, the burn damage normally caused to seedlings by such sprays was largely eliminated and the irreversible adsorption of iron increased. Foliar spray of an iron chelate (ferric ethylenediaminedi(o-hydroxyphenylacetic acid)) were less effective than those of FeSO4.The Fe reserves in seeds are generally adequate for only the first few days of seedling growth (16). Fe deficiencies in seedlings can endanger survival by restricting root and shoot growth and by reducing the plant's ability to recover from water deficits (1,7,10, 15 for a further 4 days in the light on vermiculite irrigated with 0.1 strength Hoagland solution without micronutrients. The cotyledons were removed from 6-day-old plants, and batches of 10 plants were transferred on day 7 to trays containing 8 liters of aerated, full strength Hoagland solution. The plants were supported by their stems between L-shaped polystyrene floats, which also served to reduce evaporation and light levels in the Hoagland solution.Iron was supplied to control plants by adding Fe EDDHA at 12 ,ug/ml to the full strength Hoagland solution. Fe-deficient plants were not provided with Fe EDDHA in the medium. Care was taken to reduce bacterial infection and effects of pH changes by passing the air supply to the solution through cotton-wool filters and by renewing the culture solution every 4 days.Plants were grown in a room at 25 to 26 C with mixed fluorescent and incandescent illumination (350 ft-c) and a 20-hr photoperiod.Spray Treatments. The effects of two widely used Fe compounds, FeSO4 and Fe EDDHA, were compared. Fe concentrations in the sprays were similar to those used in agricultural practice. Spray treatments were applied to runoff to the newly opened primary leaves of 7-day plants. Contamination of the roots by the spray itself or by runoff from the leaves or stem was avoided. All spray treatments included 0.04% (w/v) of a silicone alkylene oxide copolymer surfactant which was found to be particularly effective in preventing leaf buLrn and in facilitating the penetration of applied salts (12, 13).Measurement of Growth and Chlorophyll Levels. Plants were analyzed at 19 days, after they developed first and second trifoliate leaves. Fresh weights of tops and roots were determined directly. Leaves were then dried and Chl levels determined as previously described (1 1