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Yiwei Jiang and Bingru Huang

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.

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Kehua Wang and Yiwei Jiang

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.

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Qiang Liu and Yiwei Jiang

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.

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Yiwei Jiang and Robert N. Carrow

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.

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Xin Song, Suo-min Wang, and Yiwei Jiang

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.

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Ying Wang, Cale A. Bigelow, and Yiwei Jiang

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.

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Yiwei Jiang, Huifen Liu, and Van Cline

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.

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Zhongjie Ji, James J. Camberato, Cankui Zhang, and Yiwei Jiang

Plant growth regulators (PGRs) can mediate plant response to salinity stress. Perennial ryegrass (Lolium perenne) cultivars of BrightStar SLT, Catalina, Inspire, and SR4660ST were exposed to 0, 100, or 200 mm NaCl for 14 d. 6-benzyladenine (6-BA, 10 µm), γ-aminobutyric acid (GABA, 500 µm), nitric oxide (NO, 200 µm), and H2O were applied to the foliage every day for 3 days before stress and then every 2 days during salinity stress. Averaged across the four cultivars, a foliar spray of NO increased leaf fresh weight (FW) and dry weight (DW) at 0 mm NaCl, whereas application of 6-BA increased DW and GABA reduced Na+ concentration at 100 mm NaCl, compared with H2O application. Plants treated with 6-BA, GABA, and NO had less chlorotic and necrotic leaf tissue than plants treated with H2O at 200 mm NaCl. Spray of 6-BA and NO increased FW and DW, but application of all three PGRs maintained higher leaf photochemical efficiency and lower leaf Na+ concentration compared with H2O treatment at 200 mm NaCl. Across salinity and PGR treatments, ‘Catalina’ exhibited higher plant height than the ‘Inspire’ and SR4660ST, and SR4660ST had relatively higher Na+ concentration than ‘Catalina’ but not ‘BrightStar SLT’ and ‘Inspire’. The results demonstrate that 6-BA, GABA, and NO ameliorated salinity tolerance of perennial ryegrass by improving growth and photochemical efficiency or reducing Na+ accumulation.

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Yiwei Jiang, Yu Cui, Zhongyong Pei, Huifen Liu, and Shoujun Sun

Perennial ryegrass (Lolium perenne) is an important cool-season forage and turfgrass species. Growth and physiological responses of perennial ryegrasses to chronic deficit irrigation (DI) and recovery following a water deficit event are not well understood. The objective of this study was to characterize plant growth, water status, and gene expression in response to DI and recovery of perennial ryegrass. Two accessions, PI598453 (drought tolerant) and PI403847 (drought susceptible), were subjected to irrigation treatments with 100% evapotranspiration (ET) replacement every other day as the control (100% ET) and 70% ET replacement as DI treatment for 21 days in a greenhouse. After the treatment period, the DI-treated plants were shifted back to 100% ET for 7 days for recovery. The grasses were cut every 7 days, for a total of three times. Leaf relative water content (LRWC) significantly decreased at 21 days of 70% ET for both accessions, compared with the control; but to a greater extent in the more susceptible PI403847. Water-use efficiency (WUE) significantly increased 1.6-fold for PI598453 and 1.3-fold for PI403847 under 70% ET, whereas 33% reduction of leaf dry weight (LDW) was found only in PI403847. Plant height (HT) and leaf length (LL) were unaffected by 70% ET after the first two cuttings, but decreased after the third cut and did not recover to the control level for both accessions. Reductions in leaf width (LW) under 70% ET were found at 8 and 6 days after cutting for PI598453 and for PI403847, respectively. The transcript levels of heat shock protein (HSC70), iron superoxide dismutase (FeSOD), and plasma membrane intrinsic protein type 1 (PIP1) in both leaves and stems were generally downregulated during 70% ET treatment with a few exceptions but fully recovered to 100% ET after rewatering. The expression levels of cytosolic copper/zinc superoxide dismutase (cyto Cu/Zn SOD) and light-harvesting Chl a/b-binding proteins (LHCB) did not alter under 70% ET for both accessions. Differential growth and physiological responses of perennial ryegrass accessions to DI could be used for further studying of molecular mechanisms of drought tolerance in perennial ryegrass.

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Yiwei Jiang, Eric Watkins, Shuwei Liu, Xiaoqing Yu, and Na Luo

Prairie junegrass (Koeleria macrantha) is a native cool-season C3 grass that has shown potential as a low-input turfgrass. An increased understanding of the physiological and molecular responses of prairie junegrass to water-deficit conditions is important for developing cultivars with enhanced drought tolerance. The objective of this study was to characterize the antioxidative responses and candidate gene expression in prairie junegrass subjected to drought stress. Two drought-tolerant (TOL-1 and TOL-2) and two drought-susceptible (SUS-1 and SUS-2) genotypes of prairie junegrass were subjected to 7 days of drought stress. Leaf relative water content (RWC) of SUS-1 and SUS-2 was 72.1% and 73.8% and RWC of TOL-1 and TOL-2 was 90.1% and 85.4% in drought-stressed plants, respectively. Drought stress did not affect chlorophyll fluorescence, lipid peroxidation, and antioxidative enzyme activities of superoxide dismutase (SOD), catalase (CAT), peroxidase, ascorbate peroxidase (APX), or glutathione reductase for tolerant or susceptible genotypes. The TOL-2 and SUS-2 genotypes were further examined for candidate gene expression. Drought stress did not alter expression levels of CAT and chloroplastic copper/zinc SOD (Cu/ZnSOD), but increased levels of APX in either genotype, compared with their relative controls. Expression of P5CS encoding Δ1-pyrroline-5-carboxylate synthetase and P5CR encoding Δ1-pyrroline-5-carboxylate reductase for proline biosynthesis were up-regulated under drought stress for both genotypes; however, expression of P5CR was more strongly induced under drought stress for TOL-2, compared with its control. The expression of 1-FFT encoding fructan:fructan 1-fructosyltransferase, which is involved in fructan biosynthesis, was strongly induced under drought stress for TOL-2 but not detected under either control or drought stress conditions for SUS-2. These results indicate that the genes involved in proline and fructan biosynthesis may play an important role in drought tolerance in prairie junegrass.