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Emily B. Merewitz, Thomas Gianfagna and Bingru Huang

Drought stress is a widespread abiotic stress that causes a decline in plant growth. Drought injury symptoms have been associated with an inhibition in cytokinin (CK) synthesis. The objectives of this study were to investigate whether expression of a gene (ipt) encoding the enzyme adenine isopentenyl phosphotransferase for CK synthesis ligated to a senescence-activated promoter (SAG12) or a heat shock promoter (HSP18.2) would improve drought tolerance in creeping bentgrass (Agrostis stolonifera) and to examine shoot and root growth responses to drought stress associated with changes in endogenous production of CK, and the proportional change in CK and abscisic acid (ABA) due to ipt transformation. Most SAG12-ipt and HSP18.2-ipt transgenic lines exhibited significantly higher turf quality, photochemical efficiency, chlorophyll content, leaf relative water content, and root:shoot ratio under drought stress than the null transformant or the wild-type ‘Penncross’ plants. Transgenic lines that had better growth and turf performance generally had higher CK content and a higher CK-to-ABA ratio, although the direct correlation of CK and ABA content with individual physiological parameters in individual lines was not clear. Our results demonstrated that expressing ipt resulted in the improvement of turf performance under drought stress in creeping bentgrass in some of the transgenic plants with SAG12-ipt or HSP18.2-ipt, which could be associated with the suppression of leaf senescence and promoting root growth relative to shoot growth due to the maintenance of higher CK level and a higher ratio of CK to ABA.

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Xiuju Bian, Emily Merewitz and Bingru Huang

Understanding factors influencing drought resistance traits is important for improving turfgrass growth in water-limited environments. The objectives of this study were to examine effects of a plant growth regulator, trinexapac-ethyl (TE), on turf growth and water use for creeping bentgrass (Agrostis stolonifera L.) exposed to drought stress, and to determine changes in the accumulation of solutes involved in osmotic adjustment associated with TE application. Plant foliage of cultivar L-93 was sprayed with 1.95 mL·L−1 of TE at 0.113% a.i. 14 days before and at the initiation of drought stress. TE-treated and untreated plants were exposed to well-watered or drought stress conditions for 28 days in a growth chamber. TE-treated plants exhibited a reduced rate of water depletion from the soil as demonstrated by higher soil water content, lower evapotranspiration rates, and higher leaf relative water content during 28 days of drought stress compared with non-TE-treated plants. During the later phase of drought stress, TE-treated plants had a greater reduction in leaf ψS at full turgor or greater osmotic adjustment, which was associated with increased accumulation of soluble sugars and inorganic ions (Ca and K) in leaves of TE-treated plants. Proline content increased in response to drought stress, but was unaffected by TE application, suggesting that it may not contribute to the effects of TE on osmotic adjustment. TE-treated plants maintained significantly higher turf quality and leaf photochemical efficiency under drought stress. The results suggest that the promotive effects of TE application on turf growth during drought stress were associated with the reduction in water depletion or lower water use and increases in osmotic adjustment due to the accumulation of inorganic solutes and soluble sugars.

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Hongmei Du, Zhaolong Wang and Bingru Huang

Heat stress may limit the growth of turfgrasses through the induction of oxidative stress, causing cellular and physiological damage. The objective of the study was to examine the association of heat and oxidative stresses between warm-season (C4) and cool-season (C3) turfgrasses. Plants of zoysiagrass (Zoysia matrella L. Merr. cv. Manila) (C4) and tall fescue (Festuca arundinacea Shreber cv. Barlexus) (C3) were exposed to optimal temperature conditions (24 °C for tall fescue and 34 °C for zoysiagrass) or heat stress (10 °C above the respective optimal temperature for each species) in growth chambers. Zoysiagrass exhibited less severe decline in turf quality and photochemical efficiency and less severe oxidative damage in cellular membranes as demonstrated by lower membrane electrolyte leakage and lipid peroxidation compared with tall fescue when both were exposed to heat stress. The activities of superoxide dismutase (SOD) and peroxidase (POD) declined with heat stress for both species, but to a lesser extent in zoysiagrass than in tall fescue, whereas catalase activity did not change significantly under heat stress and did not exhibit species variation. Our results demonstrate that the superior heat tolerance in zoysiagrass in comparison with tall fescue was associated with greater oxidative scavenging capacity as a result of the maintenance of higher SOD and POD activities.

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Nanqing Liu, Yixin Shen and Bingru Huang

Compatible solute accumulation regulating osmotic adjustment (OA) is associated with drought tolerance. The objectives of this study were to examine genetic variations in OA among a diverse group of bentgrass (Agrostis sp.) genotypes or lines with differential drought tolerance, and determine major types of organic osmoregulants contributing to OA and accounting for the genetic variations in drought tolerance. A wild type cultivar of creeping bentgrass [Agrostis stolonifera (Penncross)], a transgenic line of creeping bentgrass (SAGIPT41), and four hybrid bentgrass lines [Agrostis capillaris × Agrostis stolonifera (ColxCr14, ColxCr190, ColxCr481, and ColxCr679)] were exposed to drought stress by withholding irrigation for 17 days in growth chambers. Among genotypes, ColxCr14, ColxCr190, and SAGIPT41 showed superior drought tolerance, as manifested by higher turf quality (TQ) and leaf relative water content (RWC), as well as OA than ‘Penncross’, ColxCr679, and ColxCr481 under drought stress. SAGIPT41 leaves accumulated greater content of soluble sugars (glucose, sucrose, and fructose), proline, glycine betaine (GB), and spermine; ColxCr190 had higher content of soluble sugars and spermidine; and ColxCr14 accumulated more soluble sugars and GB, compared with the three drought-sensitive genotypes. Soluble sugars were predominant contributors to OA, followed by GB and proline, with all three forms of polyamine (PA) as minor contributors in bentgrass genotypes. The osmolytes highly correlated to OA and superior drought tolerance could be used as biomarkers to select for drought-tolerant germplasm of bentgrass and other cool-season turfgrass species.

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Yajun Chen, Jingjin Yu and Bingru Huang

Water availability for plant growth is becoming increasingly limited, whereas rising atmospheric carbon dioxide concentration may have interactive effects with drought stress. The objectives of this study were to determine whether elevated CO2 would mitigate drought-induced water deficit and photosynthesis inhibition and enhance recovery from drought damages on rewatering and to determine whether the mitigating effects during drought stress and the recovery in photosynthesis during rewatering by elevated CO2 were the result of the regulation of stomatal movement or carboxylation activities in tall fescue (Festuca arundinacea Schreb. cv. Rembrandt). Plants were grown in controlled-environment chambers with ambient CO2 concentration (400 μmol·mol−1) or elevated CO2 concentration (800 μmol·mol−1) and maintained well watered (control) or subjected to drought stress and subsequently rewatered. Elevated CO2 reduced stomatal conductance (g S) and transpiration rate of leaves during both drought stress and rewatering. Osmotic adjustment and soluble sugar content were enhanced by elevated CO2. Elevated CO2 enhanced net photosynthetic rate with lower g S but higher Rubisco and Rubisco activase activities during both drought and rewatering. The results demonstrated that elevated CO2 could improve leaf hydration status and photosynthesis during both drought stress and rewatering, and the recovery in photosynthesis from drought damages on rewatering was mainly the result of the elimination of metabolic limitation from drought damages associated with carboxylation enzyme activities.

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Zhou Li, Yan Peng and Bingru Huang

Gama-aminobutyric acid (GABA) is a nonprotein amino acid in plant cells, which responds to changes in environmental factors. The objectives of this study were to evaluate the effects of foliar spray of GABA on drought and heat tolerance in creeping bentgrass (Agrostis stolonifera), and to investigate physiological factors altered by GABA application that contribute to improved drought tolerance and heat tolerance. GABA-treated plants (cv. Penncross) or non-GABA-treated control plants were then subjected to the following three treatments in growth chambers: 1) nonstress control [plants irrigated every 2 days to maintain soil water content at the pot capacity and maintained at 21/19 °C (day/night) for 35 days], 2) heat stress [plants exposed to 35/30 °C (day/night) and well-watered conditions for 35 days], and 3) drought stress [plants unirrigated for 9 days and maintained at 21/19 °C (day/night), and then rewatered for 2 days]. As compared with untreated plants, GABA-treated plants showed 22% to 39% and 8% to 21% significantly lower leaf electrolyte leakage (EL) and 35% to 143% and 21% to 24% significantly higher turf quality (TQ), 8% to 17% and 17% to 24% relative water content (RWC), 22% to 39% and 25% to 27% chlorophyll content, 7% to 11% and 6% to 17% photochemical efficiency, and an 84% to 683% and 57% to 76% osmotic adjustment (OA) exposed to heat or drought stress across days of treatment, respectively. GABA-treated plants accumulated 7% to 10% more water-soluble carbohydrates (WSC) and 11% to 43% more free proline than nontreated plants under heat stress, and 12% to 30% higher accumulation of WSC under drought stress. After 2 days of rewatering, a significantly better recovery also was observed in GABA-treated plants than that in nontreated plants previously exposed to drought stress. The results suggest that foliar application of GABA significantly improved heat and drought tolerance of creeping bentgrass, which was associated with maintenance of cell membrane stability, delaying in leaf senescence, and enhancing OA. The effectiveness of exogenous GABA application was more pronounced under heat stress than under drought stress.

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Liang Cheng, Ning Zhang and Bingru Huang

The accumulation of 1-aminocyclopropane-1-carboxylate (ACC), which is a precursor for ethylene production, in plant roots exposed to salinity stress can be detrimental to plant growth. The objectives of this study were to determine whether inoculating roots with bacteria containing deaminase enzymes that break down ACC (ACC-deaminase) could improve plant tolerance to salinity in perennial ryegrass (Lolium perenne) and to examine growth and physiological factors, as well as nutrition status of plants affected by the ACC-deaminase bacteria inoculation under salinity stress. Plants of perennial ryegrass (cv. Pangea) were inoculated with either Burkholderia phytofirmans PsJN or Burkholderia gladioli RU1 and irrigated with either fresh water (control) or a 250 mm NaCl solution to induce salinity stress. The bacterium-inoculated plants had less ACC content in shoots and roots under both nonstressed and salinity conditions. Salinity stress inhibited root and shoot growth, but the bacterium-inoculated plants exhibited higher visual turf quality (TQ), tiller number, root biomass, shoot biomass, leaf water content, and photochemical efficiency, as well as lower cellular electrolyte leakage (EL) under salinity stress. Plants inoculated with bacteria had lower sodium content and higher potassium to sodium ratios in shoots under salinity stress. Shoot and root nitrogen content and shoot potassium content increased, whereas shoot and root calcium, magnesium, iron, and aluminum content all decreased due to bacterial inoculation under salinity treatment. ACC-deaminase bacteria inoculation of roots was effective in improving salinity tolerance of perennial ryegrass and could be incorporated into turfgrass maintenance programs in salt-affected soils.

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Nanqing Liu, Shaoyan Lin and Bingru Huang

Glycine betaine (GB) and spermidine (Spd) are known to play roles in plant adaptation to stresses that induce dehydration, including drought stress. The objectives of this study were to examine whether improved drought tolerance by exogenous application of GB or Spd is associated with the increases in the endogenous accumulation of those solutes under drought stress in cool-season turfgrass species, and to determine the relative effects of those solutes on osmotic adjustment (OA) and antioxidant metabolism for improving drought tolerance. Creeping bentgrass (Agrostis stolonifera cv. Penncross) plants were treated with 200 mm GB or 0.1 mm Spd for 3 weeks by weekly foliar application before the exposure to drought stress; those plants were subsequently subjected to drought stress by withholding irrigation. The endogenous content of GB and Spd increased significantly through the exogenous application of either compound, to a greater magnitude for GB. The comparison of GB- or Spd-treated plants to untreated plants exposed to drought stress for growth [turf quality (TQ)] and physiological responses (water relations, membrane stability, and antioxidant metabolism) demonstrated that both compounds led to significant improvement in drought tolerance in creeping bentgrass. The improved drought tolerance by exogenous GB could be due to its contribution to OA by significant accumulation of endogenous GB, and activation of antioxidant enzymes with the greatest effects on ascorbate peroxidase (APX). Exogenous Spd did not cause increases in leaf OA despite of the increased endogenous accumulation, but significantly enhanced antioxidant enzyme activities, with the most pronounced effects on catalase (CAT). This study demonstrated that GB and Spd had different effects on OA and activated different antioxidant defense pathways, protecting plants from drought damages in creeping bentgrass.

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Qingzhang Xu, Bingru Huang and Zhaolong Wang

High air and soil temperatures are major factors limiting growth of cool-season grasses. A previous study by the authors reported that a soil temperature reduction of only 3 °C when air temperature was maintained at 35 °C significantly improved shoot and root growth of creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw. (syn. A. palustris Huds.)]. This study was designed to investigate the responses of photosynthetic activities of creeping bentgrass to lowered root-zone temperatures from the supraoptimal level when shoots were exposed to high air temperature. Two cultivars of creeping bentgrass, `L-93' and `Penncross', were exposed to the following air/root-zone temperature regimes in growth chambers and water baths: 1) optimal air and soil temperatures (20/20 °C, control); 2) lowering soil temperature by 3, 6, and 11 °C from 35 °C at high air temperatures (35/32, 35/29, and 35/24 °C); and 3) high air and soil temperatures (35/35 °C). Soil temperature was reduced from 35 °C by circulating cool water (18 °C) in water baths at variable flow rates. Both cultivars had similar responses to high or low root-zone temperatures with high air temperature. High air and root-zone temperatures caused significant reductions in canopy photosynthetic rate (Pcanopy), single-leaf photosynthetic rate (Pleaf), leaf chlorophyll content, photochemical efficiency (Fv/Fm), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, beginning on day 1 of high air and soil temperature stress for Pcanopy and Pleaf, and day 7 for chlorophyll content, Fv/Fm, and Rubisco activity. The 3 °C reduction in root-zone temperature at high air temperature had no effect on those photosynthetic parameters, except chlorophyll content. Reducing root-zone temperature by 6 °C or 11 °C while maintaining air temperature at 35 °C significantly improved Pcanopy, Poleaf, leaf chlorophyll content, Fv/Fm, and Rubisco activity. Single leaf photosynthetic rate at 35/24 °C was not different from the control level, but Pcanopy at 35/24 °C was lower than the control level. A reduction in root-zone temperature enhanced canopy and single-leaf photosynthetic capacity even though shoots were exposed to supraoptimal air temperature, which could contribute to improved turfgrass growth.

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Zhaolong Wang, Bingru Huang and Qingzhang Xu

Abscisic acid (ABA) is an important hormone regulating plant response to drought stress. The objective of this study was to investigate effects of exogenous ABA application on turf performance and physiological activities of kentucky bluegrass (Poa pratensis L.) in response to drought stress. Plants of two kentucky bluegrass cultivars, `Brilliant' (drought susceptible) and `Midnight' (drought tolerant), were treated with ABA (100 μm) or water by foliar application and then grown under drought stress (no irrigation) or well-watered (irrigation on alternate days) conditions in a growth chamber. The two cultivars responded similarly to ABA application under both watering regimes. Foliar application of ABA had no effects on turf quality or physiological parameters under well-watered conditions. ABA application, however, helped maintain higher turf quality and delayed the quality decline during drought stress, compared to the untreated control. ABA-treated plants exposed to drought stress had higher cell membrane stability, as indicated by less electrolyte leakage of leaves, and higher photochemical efficiency, expressed as Fv/Fm, compared to untreated plants. Leaf water potential was not significantly affected, whereas leaf turgor pressure increased with ABA application after 9 and 12 d of drought. Osmotic adjustment increased with ABA application, and was sustained for a longer period of drought in `Midnight' than in `Brilliant'. The results suggested that exogenous ABA application improved turf performance during drought in both drought-sensitive and tolerant cultivars of kentucky bluegrass. This positive effect of ABA could be related to increased osmotic adjustment, cell turgor maintenance, and reduced damage to cell membranes and the photosynthetic system.