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Drought stress inhibits shoot growth of cool-season turfgrass species, and elevated CO₂ concentration may mitigate the adverse effects of drought through alteration of hormone production. The objective of this study was to determine whether elevated CO₂–enhanced drought tolerance in creeping bentgrass (Agrostis stolonifera L.) was associated with the stimulation of tiller and stolon growth and the alteration of stress-regulating and growth-regulating hormone accumulation. Creeping bentgrass (cv. Penncross) plants were established for 24 d at ambient (400 μL L⁻¹) or elevated (800 μL L⁻¹) CO₂ concentration and subsequently exposed to drought stress for 23 d by withholding irrigation. Drought stress caused significant reduction in leaf relative water content and tiller density, whereas both parameters, as well as stolon length, were maintained at significantly higher values in CO-treated plants compared with those at ambient CO₂ under drought stress. The positive effects of elevated CO₂ on the maintenance of leaf hydration and the promotion of tiller density and stolon growth in creeping bentgrass exposed to drought stress could be associated with the suppression of drought-induced accumulation of abscisic acid and the increase in the endogenous content of isopentenyladenosine, jasmonic acid, and the jasmonic acid precursor 12-oxo-phytodienoic acid. These results suggest that the elevated CO₂–enhanced growth of tillers and stolons in creeping bentgrass under drought stress could be regulated in part by the adjustment of endogenous hormone levels.

Proline is an important osmoprotectant in plant in response to osmotic stresses. Delta 1-pyrroline-5-carboxylate synthetase (P5CS) is a key enzyme in proline biosynthesis. In this study, two P5CS genes (PrP5CS1 and PrP5CS2) were isolated for the first time from perennial ryegrass. Expression analysis revealed that the transcript of PrP5CS1 in leaves was significantly up-regulated in two ryegrass cultivars exposed to 255 mm NaCl. The up-regulated level of PrP5CS1 was higher in salt-tolerant ‘Overdrive’ than in sensitive ‘Pizzazz’. PrP5CS2 was significantly induced in ‘Overdrive’ but suppressed in ‘Pizzazz’ by NaCl treatment. In stems, however, there was no significant transcript change for both genes under salt treatment. The proline accumulation was significantly induced in both cultivars after salt treatment, and it was higher in ‘Overdrive’ than in ‘Pizzazz’ after 2 days of salt treatment. The results suggested that both genes are salt inducible and may be associated with salt-stress tolerance in perennial ryegrass.

Bermudagrasses (Cynodon spp.) grown in the U.S. transition zone and other regions with a similar climate undergo cold acclimation in fall and deacclimation in spring. Physiological mechanisms of cold acclimation and deacclimation associated with freezing tolerance and spring green-up are not well documented. This study investigated changes in cytokinin (t-zeatin riboside [t-ZR]) and abscisic acid (ABA) during cold acclimation and deacclimation in five bermudagrass entries in growth chambers. One ecotype (GA-851) and four cultivars (Patriot, Riviera, Tifway, and Princess-77) were subjected to cold acclimation at 8/4°C (day/night) for 28 d and then deacclimation at 10/5, 21/7, 23/8, and 26/14°C for either 3 wk (standard deacclimation) or 1 wk (fast deacclimation treatment). Stolon t-ZR content declined while ABA content increased during cold acclimation. Patriot and Riviera had higher levels of stolon ABA relative to Princess-77 at the end of cold acclimation. During deacclimation, t-ZR increased as the temperature increased to 26/14°C, while ABA declined as the temperature increased from 10/5 to 21/7°C. Patriot, GA-851, and Tifway had higher levels of t-ZR relative to Riviera and Princess-77. Abscisic acid level during cold acclimation may be associated with freezing tolerance, while cytokinin level during deacclimation may be associated with rate of spring green-up of bermudagrass. The results suggest that selection and use of entries with higher levels of ABA during cold acclimation and cytokinin during deacclimation could improve bermudagrass winter survival and spring green-up in transition zone climates.

Bermudagrasses (Cynodon spp.) grown in the U.S. transition zone and other regions with a similar climate undergo cold acclimation in fall and deacclimation in spring. Physiological mechanisms of cold acclimation and deacclimation are not well documented or understood. This study investigated changes in quality, photochemical efficiency, N metabolite (proline, total amino acid [AA], and protein) content, and dehydrin expression during cold acclimation and deacclimation in five bermudagrass entries. One ecotype (GA-851) and four cultivars (Patriot, Riviera, Tifway, and Princess-77) were subjected to cold acclimation at 8/4°C (day/night; 10-h photoperiod) for 28 d and then deacclimation at 10/5, 21/7, 23/8, and 26/14°C sequentially (11-h photoperiod) for either 3 wk (standard deacclimation) or 1 wk (fast deacclimation treatment) at each temperature. In response to cold acclimation, turfgrass quality and photochemical efficiency declined, while proline, AA, protein, and abundance of 25 kDa dehydrin increased. Patriot and Riviera had higher levels of stolon proline, AA, protein, and abundance of 25 kDa dehydrin relative to Princess-77 at the end of cold acclimation. Significant deacclimation occurred as temperature changed from 21/7 to 23/8°C. Patriot, GA-851, and Tifway had greater turfgrass quality ratings and photochemical efficiency and also higher levels of N metabolites relative to Princess-77 and Riviera during deacclimation. A similar response was observed with fast deacclimation treatment. The results suggest that selection and use of entries with higher levels of N metabolites during cold acclimation and deacclimation could improve bermudagrass persistence and spring green-up in transition zone climates.

Seaweed (Ascophyllum nodosum Jol.) extract-based cytokinins (SWEC) have been used to improve turfgrass stress tolerance, but optimum dosages for improving creeping bentgrass (Agrostis stolonifera L.) heat tolerance are lacking. This study was designed to examine effects of SWEC at various dosages on creeping bentgrass heat tolerance and to compare their effects to a trans-zeatin riboside (t-ZR)-standard in a growth chamber. The SWEC and t-ZR were applied to the foliage of 'L-93' creeping bentgrass four times (14-d interval) at 1, 10, and 100 μM. After the second application, the grass was subjected to heat stress for 49 d at 35/25°C (day/ night) in the initial experiment or 38/28°C (day/ night) in the second experiment. The SWEC and t-ZR treatments at 10 and 100 μM alleviated the decline of turfgrass quality and chlorophyll content. They also increased leaf t-ZR content and nitrate reductase (NR) activity, relative to the control, especially under 38/28°C (day/night). These data indicate that improvements under heat stress may be associated with an increase in plant t-ZR levels and NR activity due to SWEC or t-ZR application. Results suggest that repeated foliar application of SWEC at 10 μM may be an effective approach for improving turfgrass performance and heat stress tolerance.

The presence of biologically active substances (BAS) in biosolids may enhance plant stress tolerance and growth, but the underlying mechanisms are not well understood. This greenhouse study investigated the effects of untreated biosolids, containing 2.1 μg indole-3-acetic acid (IAA) g -1 , and tryptophan-treated biosolids, containing 15.4 μg IAA g -1 , on tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] drought resistance. Treatments included a fertilizer control, indole-3-butyric acid (IBA) at 2 μM (hormone control), treated biosolids (2.95 g kg -1 soil), and untreated biosolids (2.97 g kg -1 soil). Nitrogen availability was equalized among treatments, and tall fescue physiological responses were measured under well-watered or moisture stress conditions. Treated biosolids improved turfgrass quality and root mass under both soil moisture regimes and delayed leaf wilting under moisture stress. At the end of the second drought cycle (2 February), treated biosolids improved quality and photochemical efficiency by 18 and 26% relative to the control. Treated biosolids increased leaf IAA by 122 and 52%, respectively, and trans-zeatin riboside by 100 and 189%, respectively, during each of two maximum drought periods. Leaf tissue increases of IAA and cytokinin were associated with greater content of these hormones in the soil. The data suggest that biosolids application may alter growth hormone (IAA and cytokinin) content and improve plant drought resistance.

Heat stress is the primary factor limiting summer performance of creeping bentgrass (Agrostis stolonifera L.) in many temperate to subtropical regions. Seaweed extract (SWE)-based cytokinins have been used to improve stress tolerance, but their specific effects on creeping bentgrass under supraoptimal temperatures are lacking. This study was designed to determine whether SWE-based cytokinins affect creep-ing bentgrass heat tolerance, and to compare effects of SWE-based cytokinins to those of a trans-zeatin riboside (t-ZR)-standard. Concentrations of t-ZR in two SWE sources (referred to as Oce and Aca) were determined. Treatments were applied twice to creeping bentgrass at an equivalent t-ZR concentration of 10 μM. One week after the initial treatment, heat stress was imposed (35/25°C [day/night]) for 42 d. The Oce SWE, Aca SWE, and t-ZR treatments resulted in leaf t-ZR concentrations that were 39, 32, and 28% higher, respectively, relative to the control at 14 d of heat stress. The Oce SWE, Aca SWE, and t-ZR treatments also increased superoxide dismutase activity and alleviated the decline of turfgrass quality, photochemical efficiency, and root viability. Ashed SWE provided results similar to the water control. Beneficial effects of SWE on heat tolerance appear to be associated with their organic, especially cytokinin, components and not the mineral (ashed) fraction. Proper application of SWE-based cytokinins may be an effective approach to improve summer performance of creeping bentgrass.

Trinexapac-ethyl (TE) is a popular plant growth regulator in the turfgrass industry not only for its effectiveness in reducing mowing requirements but for its positive effects on turf density and appearance. We hypothesized that reported side effects of TE such as increases in color, photochemical efficiency (PE) of photosystem II, tillering, and chlorophyll, may be related to changes in leaf cytokinin content. Our objective was to determine if TE influences leaf transzeatin riboside (t-ZR) content of three common turfgrass species. Sods of Kentucky bluegrass (Poa pratensis L.), creeping bentgrass (Agrostis stolonifera L.), and hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) were transplanted and grown in flats under a greenhouse mist system. Label-rate TE treatments were applied every 2 wk for 2 mo. Canopy PE, leaf color, canopy height, and leaf t-ZR contents were measured every 2 wk in 2003, with leaf total nonstructural carbohydrates (TNC) measurements added in 2004. A significant increase in tZR content was measured following sequential TE treatment in all three species in both years. Leaf TNC was consistently increased following the second TE application in creeping bentgrass and hybrid bermudagrass. While our data are nonspecific as to how TE increases leaf t-ZR, it appears that a shift in assimilate partitioning to basal organs could be a contributing factor. Whatever the mechanism, increased leaf t-ZR is likely to confer aesthetic, as well as functional, advantages to treated turfgrasses.

This study was conducted to examine the changes in the levels of carbohydrates and N-rich defense compounds during cold acclimation associated with freezing tolerance. Two bermudagrass [Cynodon dactylon (L.) Pers. var. dactylon] cultivars, Riviera (cold tolerant) and Princess-77 (cold sensitive), were selected and either subjected to cold acclimation at 8/4°C (day/night) with a light intensity of 200 μmol m -2 s -1 over a 10-h photoperiod for 21 d or maintained at 25/23°C (day/ night) with natural sunlight in a glasshouse. Cold acclimation induced accumulation of sugars and proline in both cultivars and also an increase in total nonstructural carbohydrates (TNC) and protein in Riviera, but not in Princess-77. Superoxide dismutase (SOD) increased during the first 7 d and then declined, while catalase (CAT) and ascorbate peroxidase (APX) activity decreased in response to cold acclimation in both cultivars. Electrolyte leakage (EL) was reduced in both cultivars following cold acclimation. The LT50 was reduced by 2.2°C (from -6.1°C to -8.3°C) in Riviera and 1.7°C (from -4.6°C to -6.3°C) for Princess-77 following cold acclimation. Riviera had more carbohydrates and N-rich compounds and less EL than Princess-77 at the end of cold acclimation. Significant correlations of LT50 with sugars, proline, protein, CAT, and APX were obtained in Riviera, but only with proline and the antioxidant enzymes in Princess-77. The results suggest selection and use of cultivars with rapid accumulation of C- and N-rich compounds during cold acclimation could improve bermudagrass persistence in transition zone climates.

Late-season fertilization of bermudagrasses (Cynodon spp. L.C. Rich.) in the transition zone of the United States has traditionally been not recommended. This study was conducted to determine whether late-season fertilization could extend the duration of turfgrass color retention and visual quality without negatively impacting cold tolerance. Field plots of 'Midiron' and 'Tifway' bermudagrasses (C. dactylon x C. transvaalensis Burtt Davy), as well as 'Princess-77' and 'Riviera' bermudagrasses [C. dactylon (L.) Pers. var. dactylon] received applications of seaweed extract (SWE) (0.54 kg ha -1 ), N (49 kg ha -1 ), and Fe (1 kg ha -1 ) every 3 wk during the fall of 2001 and 2002. Visual turfgrass assessment showed that cultivar color ratings decreased as the fall progressed, with Princess-77 having greatest color retention in November of both years. Nitrogen was the only treatment to increase turfgrass color ratings relative to the control at the end of each growing season. Stolon samples removed from acclimated plants were artificially frozen to determine freezing tolerance. Midiron displayed the best freezing tolerance followed by Riviera, Tifway, and Princess-77. Chemical treatments did not have a significant effect on shoot regrowth from stolon nodes after freezing. In both years Midiron and Riviera displayed the quickest and greatest amount of spring greenup followed by Tifway and then Princess-77. Cold tolerance indicators proline and linolenic acid were highest in Midiron, followed by Riviera, Tifway, and Princess-77. Nitrogen, SWE, and Fe did not generally have an effect on linolenic acid and no consistent effects of these chemical treatments were noted on proline concentration. The results of this study indicate that judicious N applications during the fall can promote color retention and do not have a negative effect on bermudagrass cold tolerance.

A primary economic concern of sod producers is loss of sod quality during the transportation and storage phases of a sale. Previous research and field experience indicate that soil and plant respiration rates, and thus the rate of pallet heating, may be reduced by harvesting in the morning, lowering mowing heights and removing clippings, and minimizing tissue nitrogen and soil moisture before harvest. However, even when proper cultural guidelines are followed, excessive sod heating and tissue damage often occurs. Various pre- and post-harvest chemical treatments aimed at protecting leaf tissue integrity during and after supraoptimal heating have shown promise for increasing transplant success. One of these compounds is the natural plant growth regulator salicylic acid (SA). This study was conducted to investigate the influence of pre-harvest foliar application of SA on transplant injury and root strength of tall fescue (TF; Festuca arundinacea Schreb.) and Kentucky bluegrass (KBG; Poa pratensis L.) sod following supraoptimal heating. Salicylic acid was applied at 0.5 kg ha -1 to the turfgrass 10 d before harvest and canopy photochemical efficiency was measured 1 d before harvest. Harvested and rolled sod was subjected to high temperature stress (38-40°C for 72 or 96 h), transplanted into the field, and injury and root strength were determined. Application of SA enhanced the pre-harvest canopy photochemical efficiency of KBG and TF sod in both years. Averaged over years and heat duration, SA increased canopy photochemical efficiency by 12% for KBG and 14% for TF. Salicylic acid reduced visual injury and enhanced post-harvest root strength in both years. Averaged over years and heat duration, SA increased transplant root strength by 26% for KBG and 9% for TF. These data suggest that pre-harvest foliar SA application may improve shelf life and transplant success of supraoptimally heated cool-season sod.

Drought continues to be a major limiting factor for creeping bentgrass (Agrostis palustris Huds. A.) quality and persistence on golf course fairways, greens, and tees. Little breeding specifically aimed at improving bentgrass drought resistance has been completed. However, a number of reports indicate that treatment with natural products such as seaweed extracts and humic acids improve cool-season grass drought resistance possibly by hormonal up-regulation of plant defense systems against oxidative stress. This study was conducted to determine the response of exogenous natural product treatment of three creeping bentgrass cultivars subjected to drought. 'Penn G-2', 'L-93', and 'Penncross' creeping bentgrass were treated with seaweed extract (SWE) at 0.5 kg ha -1 , humic add (HA; 80% a.i.) at 1.5 kg ha -1 , alone or in combination, and maintained in a greenhouse at approximately field capacity (-0.01 MPa) or allowed to dry until near the permanent wilting point (-1.5 MPa). Unashed samples of SWE and HA contained 66 μg g -1 and 57 μg g -1 zeatin riboside (ZR), respectively, while ashed samples contained no detectable cytokinins as determined by enzyme-linked immunosorbent assay (ELISA). There were no significant differences between cultivars in response to drought, except for ZR concentration, which was higher in Penn G-2 than in L-93 or Penncross foliage. Turf quality and photochemical efficiency began to decline 14 d into the dry-down for the control and at 21 d in the natural product-treated bentgrass. The combination of HA + SWE enhanced root mass (21-68%), and foliar α-tocopherol (110%) and ZR (38%) contents. This is the first known report indicating that these natural products contain cytokinins and that their application resulted in increased endogenous cytokinin levels, possibly leading to improved creeping bentgrass drought resistance.

A primary economic concern of sod producers is loss of sod quality during the transportation, storage, and transplant stages of sale. Although a number of cultural methods have been documented for potentially reducing sod heating, many of the biological mechanisms are yet to be investigated. This study investigated the influences of selected plant growth regulators (PGRs) on sod tolerance to stress during storage by examining the relationship between preharvest photochemical efficiency (PE) and transplant rooting of Kentucky bluegrass (Poa pratensis L., KBG) sod. The PGRs, including propiconazole (propiconazole [1-(2-(2,4-dichloropheny)-4-propyl-1,3-dioxolan-2yl) methyl-1-H-1,2,4-triazole]; PPC), and seaweed (Ascophyllum nodosum Jol.) extract (SWE) plus humic acid (93% a.i.; HA), were applied alone, or in combination, to the KBG 2 wk before harvest in 1999 and 2000. Photochemical efficiency was measured immediately before harvest. The harvested sod was subjected to high temperature stress (37 degrees C) for 72 or 96 h. The heated sod was replanted in the field and transplant visual turf injury and rooting were determined. Foliar application of SWE at 0.5 kg ha(-1) plus HA at 1.50 kg ha(-1), PPC at 0.44 kg ha(-1) alone, or a combination of SWE + HA with 0.22 kg ha(-1) PPC, enhanced PE of preharvest sod in both years. Extension of heat duration from 72 to 96 h caused significantly more injury to the sod in 1999. All PGR treatment combinations reduced visual turf injury. On average in 1999 and 2000, SWE + HA, PPC, and SWE + HA + PPC enhanced transplant rooting by 21.8, 34.7, and 44.2%, respectively. Regression analysis indicated that KBG with higher preharvest PE suffered less turf injury and produced greater rooting after transplantation. The data suggest that foliar application of SWE + HA, PPC alone, or in a combination, may improve shelf life and transplant rooting of KBG sod.

This study was conducted to determine whether the plant endogenous antioxidant concentration is responsive to exogenous hormone-containing products (HCPs) in two turfgrass species subjected to drought. Two-week-old seedlings of tall fescue (Festuca arundinacea Schreb.) and creeping bentgrass (Agrostis palustris Huds. A.) were treated with two HCPs , seaweed extract (SWE) at 326 g ha-1 or humic acid (HA, 25% a.i.) at 5 L ha-1, applied alone or in combination and grown under either -0.03 or -0.5 MP a soil moisture for 5 wk. Growth and antioxidant status of leaves were determined subsequently. The HCP treatments significantly improved leaf water status (LWS) and shoot and root growth of the grasses grown under high (-0.03 MPa) and low (-0.5 MPa) soil moisture. alpha-Tocopherol concentration increased significantly and ascorbic acid concentration remained unchanged for drought-stressed compared with nonstressed turfgrass. The HCP treatments significantly increased alpha-tocopherol and ascorbic acid concentration of the grasses grown under high and low soil moisture. Positive correlation between antioxidants and shoot or root growth was found in the two grass species. Improvement of growth and LWS of turfgrass treated with HCPs may be related to its high antioxidant concentration.