Foliar fertilization of field crops has received renewed interest as a method of increasing seed yield by supplying additional nutrients to the leaves to delay the normal depletion of nutrients in the leaves during reproductive stages. It has been hypothesized that application of foliar fertilization may delay natural leaf senescence. We tested this by estimating leaf photosynthesis in field‐grown corn (Zea mays L.) plants using a 14CO2 uptake procedure. A small segment of the leaf material enclosed in a chamber is exposed to 14CO2 for a short time period. Leaf samples are then analyzed using standard radiotracer techniques. In 1976 and 1977 14CO2 uptake rates were measured on a number of days between silking and plant maturity in conjunction with a larger experiment relating the response of corn to foliar fertilizer application and soil moisture deficits. The experimental plants were field‐grown in a Nicollet loam soil (an Aquic Hapludoll, fineloamy, mixed, mesic). These measurements indicated that the rate of leaf photosynthesis was depressed by as much as 17% the day following foliar‐fertilizer application, but had nearly recovered by the 2nd day. A similar pattern occurred in leaf conductance. No difference in the seasonal trend of photosynthetic rate was detected between control and foliarfertilizer treatments. Photosynthetic patterns after foliar‐fertilizer application may warrant further investigation, as significant yield reductions were noted in these experiments. Because the technique used in our experiments monitors rate of photosynthesis on small, intact leaf segments, totalcanopy photosynthesis and transpiration measurements could reveal helpful information.

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The weather shelter plot The weather shelter plot during construction, showing size of potometers, and drainage sand Diagram of a potometer with neutron probe access tube Soil-moisture patterns in 1976, Experiment 1. Curves are based on means of neutron probe measurements Soil-moisture patterns in 1976, Experiment 2. Curves are based on means of neutron probe measurements Soil-moisture patterns in 1977. Curves are based on means of neutron probe measurements Grain yield response to moisture stress, 1976 Experi ment 1 Yield component response to moisture stress, 1976 Experiment 1 Grain yield and yield component response to moisture stress, 1976 Experiment 2 Yield component response to moisture stress. Compari son between ears in Variety 2, 1976 Experiment 2 Grain yield and seed number response to foliar fertil izer and moisture stress. Variety 1, 1976 Experiment 2 Percent nitrogen in the grain as influenced by mois ture stress, 1976 Experiment 2 Percent phosphorus and percent potassium in the grain as influenced by moisture stress, 1976 Experiment 2 Moisture-stress periods shown in relation to silking and foliar-fertilizer application dates and to open pan evaporation in 1977 Grain yield response to moisture stress, varieties combined

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