Physiological Responses of Creeping Bentgrass (Agrostis stolonifera L.) to a Tryptophan-containing Organic Byproduct

Amino acid-based products have been used as alternative fertilizer nitrogen (N) sources to improve turfgrass performance, especially where there is a strong reliance on synthetic N sources. However, the physiological mechanisms underlying improvements in turfgrass performance are not well documented. The objective of this research was to determine whether applications of a tryptophan-containing organic byproduct (TRP-B) or tryptophan (TRP) + urea improve creeping bentgrass (Agrostis stolonifera L.) performance compared with standalone applications of urea, a commonly used synthetic N source. At two separate universities, mature ‘A-4’ creeping bentgrass plugs were transplanted into containers and allowed to re-establish in growth chambers before being treated. Treatments included TRP-B, urea, and TRP + urea applied every 14 d at three different N rates: 2.5, 12.25, and 24.5 kg N ha⁻¹. At the trial’s end, TRP-B and TRP + urea increased leaf indole-3-acetic acid (IAA) by 227 and 255%, respectively, relative to urea at the high N rate, as measured at day 42 of the study. Applications of TRP-B and TRP + urea also increased root biomass by 22 and 20%, respectively, when compared with urea only at the high N rate. The TRP-B and TRP + urea treatments did not impact leaf total amino acids or photochemical efficiency when compared with urea only. Overall, results indicate that application of TRP-B or TRP + urea at 24.5 kg N ha⁻¹ every 2 wk may improve leaf and root IAA content, root biomass, and subsequent creeping bentgrass quality relative to applications of urea only.