Foliar Applications of Nutrients on Maize. II. Physiological Responses

Detailed analyses of plant and plant parts during the grain development period could establish mechanisms by which foliar fertilization, especially N, affects productivity of maize (Zᵉᵃ ᵐᵃʸˢ L.) hybrids. Field-grown maize hybrids were sprayed at various stages of development with a mixture of nutrients (N-P-K-S) in 1981 or urea in 1982 to determine whether foliar application of N would delay the remobilization of leaf N and leaf senescence thereby increasing productivity. In 1981, N-P-K-S (12 kg N ha⁻¹ per treatment) was applied to plants of five hybrids at 7 days before and 12 days after anthesis. In 1982 urea (two equal applications for a total of 67 kg N ha⁻¹) was applied to three hybrids B73 × Me17, Pioneer brand 3382, and Farm Services brand 854, at 12 and 6 days before anthesis and 9 and 15 and 25 and 32 days after anthesis to give treatments 1, 2, and 3, respectively. Soil type was Drummer silt loam (fine-silty, mixed, mesic Typic Haplaquoll) in 1981 and a Flanigan silt loam (fine, montmorillonitic, mesic Aquic Arqiudoll) in 1982. The overall lack of effect of the fertilizer spray treatments on plant metabolism in 1981 was attributed to the small amount of supplemental nutrients applied. In 1982, changes in reduced N and nitrate of leaves, stalks, and whole plants were measured between anthesis and physiological maturity. These changes showed that the first urea treatment interfered with both nitrate uptake and assimilation for P3382 and assimilation for B73 × Mo17 and FS854. The second and third treatments had less effect on nitrate uptake and assimilation for all hybrids. The larger increases in stalk reduced N for all hybrids in response to the first urea treatment, compared to the second and third treatments, indicated that in the absence of a rapidly developing ear the stalk served as a storage reservoir for additional N. Urea applications caused no consistent increase, or alteration of the normal postanthesis decline, in the reduced-N content of the leaves. The immediate decreases and trends in stalk carbohydrate contents show that the first two urea treatments interfered with the accumulation of carbohydrates, especially for P3382 and B73 × Mo l7. The observations that urea sprays did not increase N content of the leaves, decreased the accumulation of carbohydrates by the stalks, and appeared to interfere with indigenous N metabolism help explain the general ineffectiveness of foliar-N treatments of maize.