Tryptophan, an essential amino acid that acts as a building block in protein synthesis, is a biochemical precursor for serotonin, niacin, and auxin in most organisms. When soil moisture is limited, applying biosolids boosted with auxin from tryptophan may increase root production and endogenous hormone levels that can result in plant growth regulation. Tryptophan is produced industrially, which results in a significant amount of byproducts. Tryptophan byproduct (TRP-B) is considered a waste product, but its amino acid and nutrient content make it a possible growth promoter for turfgrasses. The objective of this research was to determine whether applications of TRP-B improve ‘Penn A-4’ creeping bentgrass (Agrostis stolonifera L.) performance more than applications of pure tryptophan and/or urea.

Creeping bentgrass plugs taken from sand-based greens at both Virginia Polytechnic Institute and State University (Blacksburg, VA), and Iowa State University (Ames, IA) were transplanted into pots and allowed to re-establish in growth chambers before being treated. Treatments included TRP-B, urea, and pure tryptophan + urea applied every 14 days at three different rates. Application rates were based on the amount of nitrogen (N) applied (2.45, 12.23, and 24.46 kg N ha-1). At trial’s end (42 days), plant parts were harvested and used for analysis. On average, TRP-B treatments increased leaf total free amino acid contents by 6.2%, rooting biomass by 9.3%, leaf indole-acetic-acid (IAA) concentration by 20.2%, and root IAA concentration by 145.2%, compared to urea only treatments. Tryptophan + urea treatments increased leaf total free amino acid contents by 5%, rooting biomass by 8.3%, leaf IAA concentrations by 32.6%, and root IAA concentrations by 213% on average, compared to urea only treatments. At 24.46 kg N ha-1, TRP-B increased root biomass by 18.2% and pure tryptophan + urea produced a 16.3% increase compared to urea only. According to the results, creeping bentgrass treated with TRP-B can result in increased performance, and that response is rate-dependent.