Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Plant and Environmental Science


Liu, Haibo

Committee Member

Bielenberg , Douglas

Committee Member

Martin , Samuel

Committee Member

Luo , Hong


The most widely used foliar nitrogen (N) source for warm-season turfgrass and agriculture is urea [(NH2)2CO], due to its low cost, high percentage of N (46% by mass), and completely soluble nature. Since urea is a soluble N source, it is commonly utilized as a foliar N source when tank mixed with pesticides in warm-season turfgrass management. The N in urea is not directly available to the plant until it is hydrolyzed into ammonia by the enzyme urease in the cytosol. Urease is a nickel (Ni2+) dependant enzyme that is ubiquitous in plants. Its main biochemical function is the hydrolysis of urea; however other physiological roles have been discovered including enhancement in germination and plant defense mechanisms.
Nickel was first recognized as a required nutrient in plants in the 1970s. Critical Ni2+ concentrations in leaf tissue varying between 25- 100 µg kg -1 depending on N source and species. Nickel is a highly mobile trace element that tends to accumulate in newly formed plant parts, as well as seeds and is an important cofactor of many enzymes. Typically, excessive Ni2+ is a more common problem and has shown to affect physiological and biochemical processes including decreased chlorophyll, lowered photosynthetic and transpiration activities, reduced germination and impaired membrane permeability associated with enhanced extracellular peroxidase activity.
Water use and quality have become important issues in turfgrass management due to water use restrictions and mandates in arid climates. In these areas, effluent water irrigation has become commonplace, leading to potential problems with water quality including transition or heavy metal toxicity. Furthermore, seawater intrusion in coastal areas has also led to a need for salinity tolerant turfgrasses and better knowledge of their management techniques. Although Ni2+ is rarely deficient in plants, the widespread use of urea as a N source in turfgrass management and the importance of urease activity requires further examination of urea N metabolism and Ni2+ nutrition under salinity stress.
Research also needs to examine Ni2+ supplementation of warm-season turfgrass supplied with combinations of NH4+, NO3- and [(NH2)2CO] N sources. Analysis of urea and specific amino acid concentrations in plant tissue needs to be conducted to more fully understand the uptake, assimilation, and translocation of foliar applied urea N under the influence of Ni2+ supplementation. The significance of Ni2+ supply is dependent on N source, and species. Critical Ni2+ concentrations in turfgrass tissues need to be determined in those scenarios. Comprehensive research of Ni2+ nutrition needs to be further conducted to determine the effects of supplemental Ni2+ levels, including Ni2+ toxicity symptoms, and long term ecological impact in turfgrass ecology.
Due to the lack of research examining urea N fertility, Ni2+ nutrition and toxicity, and salinity stress of turfgrasses, three studies were conducted. The first study examined the effect of urea fertilization method (root vs. foliar) under salinity stress of five warm-season turfgrasses. We hypothesized that urea delivery method will influence N uptake under salinity stress and the turfgrasses will perform similarly under salinity stress. Treatments included two fertility delivery methods, two salinity levels, and five warm season turfgrass genotypes. Results revealed no difference between root and foliar applications of urea N under salinity stress. There was variability in the performance of the ultradwarf bermudagrass cultivars, with Champion exhibiting the greatest reduction in turf quality and accumulating the greatest concentration of proline in leaf tissue. Seadwarf, the most salinity tolerant genotype examined, exhibited significant increases in N concentration under foliar urea N applications and slight improvements in TQ under moderate salinity stress. In addition, foliar applications of urea N resulted in elevated Na+ concentration in the leaf tissue of Seadwarf at the midpoint and conclusion of the study, which was the only genotype to display such a response. Findings from this study suggest that foliar applications of urea N provide an alternative to traditional granular fertilization when root zone salinity is elevated.
The second study examined urea N metabolism and the effect of Ni2+ supplementation on foliar uptake of urea. We hypothesized that Ni2+ supplementation will enhance urea N metabolism and foliar uptake by stimulating urease activity and increasing total amino acid pools in turfgrass leaf tissue. Treatments included two salinity levels, two turfgrass species and three Ni2+ levels. Results from this study revealed an apparent stimulation of N metabolism under foliar urea nutrition with Ni2+ supplementation. Although urease activity and amino acid pools were increased under Ni2+ supplementation, an overall decrease in N content in leaf tissue was observed over the course of the nine week study. The reduction observed in total N concentration in leaf tissue could be due to the use of a single N source (urea) causing physiological N deficiency which is a common response. Due to this finding, it is important to use multiple N sources to maintain optimal growth.
The third study further examined Ni2+ toxicity of two common warm-season turfgrasses under urea N fertility. We hypothesized that Ni2+ supplementation will stimulate urease activity and increase amino acid pools as recorded in the previous study. Secondly, as Ni2+ concentration in leaf tissue increases, toxicity will cause decreases in turf quality, growth, and fluctuations in micronutrient concentration. Treatments included two turfgrass species, and four Ni2+ levels. Results revealed a stimulation of urease activity and increases in the total amino acid pool with Ni2+ supplementation. However, visual toxicity symptoms occurred when Ni2+ concentrations increased in leaf tissue. Reductions in turf quality and growth were exhibited under 400, 800, and 1600 µM Ni2+ regimes. Results from this study suggest that the critical Ni2+ toxicity level in Diamond and TifEagle begins at a range >25 mg kg-1. Ni2+ concentrations in leaf tissue greater than 25 mg kg-1 caused reductions in growth and symptoms of toxicity.
An additional fertility delivery method experiment was conducted to examine recovery of 15N following root and foliar applications of urea. We hypothesized that total plant recovery of 15N derived from fertilizer would be different between delivery methods and that overall recovery would be greater in foliar applied treatments. Results revealed that total plant recovery of 15N labeled urea derived from fertilizer was not significantly different in either fertility regime or species tested. Although not statistically different, root applications of urea N resulted in 10% higher total 15N recovery than foliar treatments at 8 hours after application. There was variability in total plant recovery across species although not statistically significant. MiniVerde displayed the lowest total 15N recovery at 8 hours at 35.14%, which was much lower than Diamond and Seadwarf at 44.66% and 47.62% respectively. Recovery of labeled urea in each plant part was significantly influenced by fertility regime, and was anticipated. Foliar applications of urea resulted in higher recovery in leaf tissue while root applications resulted in elevated 15N recoveries in root tissue. In addition to recovery in specific plant tissue, root applications of urea N resulted in significantly higher 15N retention in soil than foliar applications, however overall recovery of 15N derived from fertilizer was higher in root treatments. The 10% overall reduction in 15N recovery for foliar treatments compared to root applications could be due to a number of factors, including volatilization. The disparity, although not statistically different, in total 15N recovery due to fertility regime could be biologically significant and is worth examining more closely. 15N labeled urea retained in the soil (5.55%) 8 hrs after root applications has the ability to be taken up by the plant potentially increasing the overall recovery over time. Foliar treatments resulted in 15N recovery in the soil of <1%. Leaching and volatilization losses were not quantified for this study, and account for the large portion of N lost when sampling took place.
Lastly, a field study was conducted to investigate the effects of N fertility levels and plant growth regulator applications on the performance of Diamond zoysiagrass as a putting green surface in the transition zone. We hypothesized that N fertility level and plant growth regulator applications would significantly influence Diamond zoysiagrass putting green performance. Results of this study revealed that Diamond zoysiagrass has the ability to become another warm-season turfgrass option for putting greens in the southern transition zone. Based on finding of this project, N fertilization of Diamond zoysiagrass in putting green scenarios should begin with 147 kg-1 N ha-1 or less over the growing season. Additional quick release N sources should be used following cultivation events to promote growth and recovery. As total N input surpassed 147 kg-1 N ha-1 putting green performance as indicated by ball roll distance suffered. An obvious increase in thatch depth and accumulation was displayed during the two year study. Cultivation, surface management, PGR use, and fertility regimes need to be determined to optimize putting green performance and overall turfgrass health of Diamond zoysiagrass in putting green scenarios.