Role of Nitrogen in Resistance to Striga Parasitism of Maize

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Abstract
Striga [Striga hermonthica (Del.) Benth.] is a parasitic angiosperm that infects tropical cereals causing severe yield losses. This study was conducted to determine if Striga damage in maize (Zea mays L.) can be mediated by the amount, form, and timing of N availability; and if the efficacy of N is contingent upon its regulation of assimilate partitioning. Two experiments were conducted in Kibos, western Kenya, in 1989 and 1990 on fields that had uniform Striga infestation. One experiment evaluated N rates of 0, 30, 60, and 90 kg N ha−1 supplied as either urea, calcium ammonium nitrate, ammonium sulfate, or ammonium sulfate plus the nitrification inhibitor dicyandiamide, while a second experiment evaluated similar N rates applied at 14, 21, 28, and 35 d after planting. Although Striga infection generally declined with increasing N availability, the impact was partially dependent on the severity of infestation as all N rates decreased infection in 1990, while only 90 kg N ha−1 reduced infection in 1989. Under high parasite densities in 1989, only urea reduced Striga (26%), while in 1990, infection was significantly decreased (an average of 30%) by all sources of N. In both years, N application at 28 d after planting resulted in the least Striga infection. Although assimilate partitioning during vegetative growth was unresponsive to N treatments, N availability during reproductive growth altered dry matter partitioning in favor of the ear over the vegetation. Averaged across N rates, this alteration resulted in increases in grain yield (64%) and harvest index (27%), and a decrease in source‐sink partitioning (41%) and in the concentration of nonstructural carbohydrates in the stalk (16%). Based on these data, N fertility can mediate the impact of Striga infestation on maize by altering assimilate partitioning in favor of the ear.

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