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Heat and drought are two major factors limiting growth of cool-season grasses during summer. The objective of this study was to compare the effects of heat stress alone (H) or in combination with drought (H+D) on photosynthesis, water relations, and root growth of tall fescue (Festuca arundinacea L.) vs. perennial ryegrass (Lolium perenne L.). Grasses were exposed to H (35 °C day/30 °C night) or H+D (induced by withholding irrigation) in growth chambers for 35 days. Soil water content declined under H+D for both grasses but to a greater extent for fescue than for ryegrass. Declines in canopy net photosynthetic rate (Pn), leaf photochemical efficiency (Fv/Fm), and leaf relative water content (RWC) and the increase in electrolyte leakage (EL) were much more severe and occurred earlier for ryegrass than fescue subjected to both H and H+D and for both species than under H+D then H. Evapotranspiration (ET) rate increased to above the control level within 3 or 6 days of H and H+D for both species, but fescue had a higher ET rate than ryegrass at 3 and 6 days of H and 6 days of H+D. Root dry weight and viability in all soil layers decreased under H and H+D for both species. However, fescue had higher root dry weight and viability than ryegrass in the 20-40 cm layer under H and in both the 0-20 and 20-40 cm layers under H+D. The results indicated that maintenance of higher Pn, Fv/Fm, ET, RWC, and root growth and lower EL would help cool-season turfgrass survive summer stress, and that their characteristics could be used for selecting stress tolerant species or cultivars.

Recovery from submergence stress is vital for plant regrowth. The objective of this study was to characterize plant growth, carbohydrate, and antioxidant metabolism of creeping bentgrass (Agrostis stolonifera) to foliar application of nitrogen and cytokinin (CK) after de-submergence. Creeping bentgrass (cv. Penncross and 007) were submerged under the water for 14 days and then foliar-sprayed at 1, 2, 3, 7, and 14 days after de-submergence with six types of chemical treatments, respectively: 1) water (W); 2) 10 mm urea (N10); 3) 20 mm urea (N20); 4) 10 µm CK; 5) N10 with CK (N10CK); and 6) N20 with CK (N20CK). Leaves were harvested at 20 days after chemical applications for various measurements. Compared with the nonstressed plants, plant height (HT), chlorophyll index (Chl), leaf dry weight (DW), water-soluble carbohydrate content (WSC), activities of superoxide dismutase (SOD), and ascorbate peroxidase (APX) decreased, but catalase (CAT) and peroxidase (POD) activities, malondialdehyde (MDA), and total soluble protein (TSP) content increased in both cultivars exposed to 14 days of submergence. After de-submergence, plants treated with N alone (N10, N20) or combined with CK (N10CK, N20CK) generally had higher HT, DW, Chl, TSP, and a lower amount of MDA, compared with treatments of W or CK alone, whereas treatment using CK resulted in higher WSC for both cultivars. Foliar applications of N and CK had some effect on SOD, CAT, POD, and APX activities after de-submergence, but the effects were not consistent across chemicals and cultivars. The results indicated that foliar application of N or combined with CK promoted plant growth and reduced lipid peroxidation after de-submergence. The results also suggested a more positive role of foliar N application in comparison with a complex regulation of CK on creeping bentgrass regrowth after de-submergence.

Waterlogging (WL) affects the growth and physiological responses of turfgrass. The objectives of this study were to compare the relative WL tolerance of Kentucky bluegrass (Poa pratensis L.) cultivars and to investigate the physiological responses of shoots and roots to WL. Ten cultivars differing in growth habit were subjected to 30 d of WL. The turf quality (TQ) and soil redox potential (Eh), as well as the chlorophyll concentration (Chl), decreased with increasing periods of WL. Among all cultivars, root dry weight (RDW) decreased 16.7% to 39.9% under 10 d and 30.0% to 60% under 30 d of WL, respectively. Waterlogging increased the root electrolyte leakage (REL) from 0.6% to 53.2% under 10 d and from 29.1% to 98.0% under 30 d of WL for all cultivars, respectively. The best correlations were observed between Eh and TQ (r = 0.74), REL and TQ (r = 0.75), RDW and root water-soluble carbohydrate content (RWSC) (r = 0.74), and root oxidase activity and RWSC (r = 0.63), respectively. ‘Moonlight’, ‘Serene’, and ‘Champagne’ showed better tolerance to short-term WL conditions, whereas ‘Kenblue’ and ‘Eagleton’ were the least tolerant cultivars. ‘Limousine’, ‘Unique’, ‘Awesome’, ‘Julia’, and ‘Midnight II’ ranked in the middle group. Variations in WL tolerance among Kentucky bluegrasses could potentially be used for enhancing turfgrass management.

Canopy reflectance has the potential to determine turfgrass shoot status under drought stress conditions. The objective of this study was to describe the relationship of turf quality and leaf firing versus narrow-band canopy spectral reflectance within 400 to 1100 nm for different turfgrass species and cultivars under drought stress. Sods of four bermudagrasses (Cynodon dactylon L. × C. transvaalensis), three seashore paspalums (Paspalum vaginatum Swartz), zoysiagrass (Zoysia japonica), and st. augustinegrass (Stenotaphrum secundatum), and three seeded tall fescues (Festuca arundinacea) were used. Turf quality decreased 12% to 27% and leaf firing increased 12% to 55% in 12 grasses in response to drought stress imposed over three dry-down cycles. The peak correlations occurred at 673 to 693 nm and 667 to 687 nm for turf quality and leaf firing in bermudagrasses, respectively. All three tall fescues had the strongest correlation at 671 nm for both turf quality and leaf firing. The highest correlations in the near-infrared at 750, 775, or 870 nm were found in three seashore paspalums, while at 687 to 693 nm in Zoysiagrass and st. augustinegrass. Although all grasses exhibited some correlations between canopy reflectance and turf quality or leaf firing, significant correlation coefficients (r) were only observed in five grasses. Multiple linear regression models based on selected wavelengths for turf quality and leaf firing were observed for 7 (turf quality) and 9 (leaf firing) grasses. Wavelengths in the photosynthetic region at 658 to 700 nm or/and near-infrared from 700 to 800 nm predominated in models of most grasses. Turf quality and leaf firing could be well predicted in tall fescue by using models, evidenced by a coefficient of determination (R ²) above 0.50. The results indicated that correlations of canopy reflectance versus turf quality and leaf firing varied with turfgrass species and cultivars, and the photosynthetic regions specifically from 664 to 687 nm were relatively important in determining turf quality and leaf firing in selected bermudagrass, tall fescue, zoysiagrass and st. augustinegrass under drought stress.

Accurate, rapid, and nondestructive estimates of turfgrass leaf water status are important for site-specific irrigation and drought stress management. The objective of this study was to identify changes and correlations among the canopy reflectance, canopy temperature, and leaf relative water content (RWC) of perennial ryegrass (Lolium perenne L.) under water deficit conditions. Six cultivars of perennial ryegrass were subjected to dry-downs in the field from May to Aug. 2007 and from June to Aug. 2008. Turf quality was positively correlated with soil moisture (SM), RWC, and normalized difference vegetation index (NDVI), but negatively correlated with canopy and ambient temperature differentials (ΔT). ΔT was well correlated with RWC (r = –0.77 to –0.78) and SM (r = –0.66 to –0.74), whereas SM was correlated with RWC (r = 0.64 to 0.74) across seasons in both years. When a wide range of stress symptoms occurred in July and Aug., RWCs became highly correlated with ΔT (r = –0.80 to –0.89) and NDVI (r = 0.77 to 0.81), whereas ΔT was correlated with NDVI (r = –0.70 to –0.80) in both years. SM was well correlated with RWC (r = 0.71 to 0.80), NDVI (r = 0.70 to 0.73), and ΔT (r = –0.76 to –0.78) in July and August in both years. These results suggest that changes in ΔT can be used to predict well the leaf water and soil moisture content of perennial ryegrass under water deficit conditions. Combined with NDVI, the correlations can be used for direct mapping of the variability in grass water status, thus improving irrigation management.

Nitrogen greatly impacts plant growth and development. The objective of this study was to characterize growth, nitrogen use, and gene expression of perennial ryegrass (Lolium perenne) in response to increasing nitrogen supplies. Perennial ryegrass (cv. Inspire) was grown in sand culture and irrigated with a half-Hoagland solution amended with 0, 0.5, 1.0, 2.5, 5.0, and 7.5 mm nitrogen. Leaf tissues were harvested at 10 days (first cutting) and 20 days (second cutting) and roots were harvested at 20 days. The relatively higher N supply (2.0–7.5 mm) resulted in a larger amount of leaf fresh and dry weight but lower root fresh and dry weight, especially for the second cutting. Root:leaf ratio was higher under low N, but lower under the high N treatment. Leaf N content was relatively higher under 2.5, 5, and 7.5 mm N than under the other three treatments, while 2.5 mm N exhibited relatively higher leaf carbon content for both cuttings. Leaf C:N ratio and leaf nitrogen use efficiency (LNUE) decreased with increasing N supplies for the first cutting but were higher under low N (0–1.0 mm) for both cuttings. Leaf C:N ratio and LNUE did not differ among low N and LNUE also remained unchanged among high N for the second cutting. Root N content increased, but the root C:N ratio and root N use efficiency (RNUE) decreased with increasing N supplies, especially under 2.5, 5.0, and 7.5 mm N. Low (0.5 mm), moderate (2.5 mm), and high (7.5 mm) N were chosen to examine the expression level of NR encoding nitrate reductase and GS1b encoding glutamine synthetase. Treatment of 0.5 mm N had higher expression levels of leaf NR than other two treatments for both cuttings and a higher level of leaf GS for the second cutting. Expression of NR in the roots did not vary among treatments but the expression of GS increased under 2.5 and 7.5 mm, compared with the 0.5 mm N. Differential leaf and root growth and physiological responses to low N (0 to 1 mm) and to moderate to high N (2.5 to 7.5 mm) could be used for examining the natural variation of N use in diverse perennial ryegrass populations.

Perennial ryegrass (Lolium perenne) is a popular cool-season and forage grass around the world. Salinity stress may cause nutrient disorders that influence the growth and physiology of perennial ryegrass. The objective of this study was to identify the genotypic variations in growth traits and nutrient elements in relation to salinity tolerance in perennial ryegrass. Eight accessions of perennial ryegrass [PI265351 (Chile), PI418707 (Romania), PI303012 (UK), PI303033 (The Netherlands), PI545593 (Turkey), PI577264 (UK), PI610927 (Tunisia), and PI632590 (Morocco)] were subjected to 0 (control, no salinity) and 300 mm NaCl for 10 d in a greenhouse. Across accessions, salinity stress decreased plant height (HT), leaf fresh weight (LFW), leaf dry weight (LDW), leaf water concentration (LWC), and concentration of N, C, Ca²⁺, Cu²⁺, K⁺, Mg²⁺, and K⁺/Na⁺ ratio and increased Na⁺ concentration. Negative correlations were found between C and Na⁺, whereas positive correlations of K⁺/Na⁺ with C and N were found under salinity treatment. The principal component analysis (PCA) showed that the first, second, and third principal components explained 40.2%, 24.9%, and 13.4% variations of all traits, respectively. Based on loading values from PCA analysis, LWC, Na⁺ concentration, and K⁺/Na⁺ ratio were chosen to evaluate salinity tolerance of accessions, and eight accessions were divided into the tolerant, moderate, and sensitive groups. The tolerant group had relatively higher LWC and K⁺/Na⁺ ratio and concentrations of C, P, and Fe²⁺ and lower Na⁺ concentrations than the other two groups, especially the sensitive groups. The result suggested that lower Na⁺ accumulation and higher K⁺/Na⁺ ratio and LWC were crucial strategies for achieving salinity tolerance of perennial ryegrass.

Waterlogging can occur in salt-affected turfgrass sites. The objective of this study was to characterize growth and carbohydrate, lipid peroxidation, and nutrient levels in the leaves and roots of two perennial ryegrass (Lolium perenne) cultivars (Catalina and Inspire) to short-term simultaneous waterlogging and salinity stress. Previous research showed that ‘Catalina’ was relatively more tolerant to salinity but less tolerant to submergence than ‘Inspire’. Both cultivars were subjected to 3 and 7 days of waterlogging (W), salinity [S (300 mm NaCl)], and a combination of the two stresses (WS). Across the two cultivars, W alone had little effect on the plants, while both S and WS alone significantly decreased plant height (HT), leaf fresh weight (LFW), leaf dry weight (LDW), root fresh weight (RFW), root dry weight (RDW), leaf nitrogen (LN) and carbon (LC), and leaf and root K⁺ (RK⁺), and increased leaf water-soluble carbohydrate (LWSC) and root water-soluble carbohydrate (RWSC), malondialdehyde (MDA), and Na⁺ content, compared with the control. A decline in chlorophyll content (Chl) was found only at 7 days of WS. Leaf phosphorus (LP) content either decreased or remained unchanged but root phosphorus content increased under S and WS. Reductions in LFW and LDW were found at 3 days of S and WS, whereas RFW and RDW were unaffected until 7 days of S or WS. Both cultivars responded similarly to W, S, and WS with a few exceptions on RDW, LWSC, leaf MDA (LMDA), and root MDA (RMDA). Although WS caused declines in Chl and resulted in higher leaf Na⁺ (LNa⁺) and root Na⁺ (RNa⁺) than S at 7 days of treatment, S and WS had similar effects on growth, carbohydrate, MDA, N, C, and phosphorus, and K⁺ content across the two cultivars. The results suggested that S alone largely accounted for the negative effects of WS on plant growth and physiology including alteration of carbohydrate and nutrient content as well as induction of lipid peroxidation.

Perennial ryegrass (Lolium perenne L.) is a widely used cool-season turfgrass species. The exact ploidy levels of the worldwide perennial ryegrass accessions in the USDA National Plant Germplasm System (NPGS) are unknown, which could complicate future use and breeding efforts. The objective of this study was to determine the ploidy level and DNA content of the 194 USDA NPGS perennial ryegrass accessions and six commercial cultivars (Brightstar SLT, Catalina II, Divine, Inspire, Manhattan 4, Silver Dollar) using flow cytometry. Among the 200 accessions, 194 diploids and six tetraploids were identified. Three tetraploids originated from Canada with the remaining from Ireland, Japan, and The Netherlands. The average DNA content was 5.60 pg/2C for the diploid and 11.45 pg/2C for the tetraploid. The 2C DNA content was positively correlated (r = 0.23, P < 0.01) with seedling plant height but not seedling leaf width. This ploidy data provide important information for future marker trait analysis and cultivar improvement.

Turfgrasses are often exposed to different shade environments in conjunction with traffic stresses (wear and/or compaction) in athletic fields within stadiums. The objective of this study was to assess the effects of morning shade (AMS) and afternoon shade (PMS) alone and in combination with wear and wear plus soil compaction on `Sea Isle 1 seashore paspalum (Paspalum vaginatum Swartz). The study was conducted using two consecutive field trials under sports field conditions from 9 July to 10 Sept. 2001 at the Univ. of Georgia Experiment Station at Griffin. “T” shaped structures constructed of plywood on the sports field were used to provide §90% morning and afternoon shade, respectively, and were in place for 1 year prior to data accumulation. A wear device and a studded roller device simulated turfgrass wear (WD) and wear plus soil compaction (WSC), respectively, to the shaded plots. Only minor differences in turf color, density, or canopy spectral reflectance were found between AMS and PMS under no-traffic treatments in both trials. Grasses under WD generally recovered faster than those exposed to WSC across all light levels, including full sunlight (FL), AMS, and PMS. AMS combined with WD treatment had an average 9% higher rating of color, 11% higher density, and 28% less tissue injury than that of PMS with WD at 7 days after traffic treatment (DAT). Compared to PMS with WSC treatment at 7 DAT, AMS with WSC had 12% higher rating of color, 9% higher density, and 4% less tissue injury. AMS with WD treatment exhibited 11% higher normalized difference vegetation index (NDVI), 4% higher canopy water band index (CWBI), and 13% lower stress index than that of PMS with WD at 7 DAT. AMS with WSC, relative to PMS with WSC, demonstrated 8% higher NDVI, 3% higher CWBI, and 8% lower stress index at 7 DAT. Re sults indicated that AMS (i.e., afternoon sunlight) had less detrimental influences than PMS (i.e., morning sunlight) on turfgrass performance after it was subjected to wear stress or wear plus soil compaction.