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Zhaolong Wang, John Pote and Bingru Huang

This study was designed to examine whether shoot injury induced by high root-zone temperature is associated with changes in shoot detoxifying metabolism and to determine the level and duration of high root-zone temperatures that would induce physiological changes in two cultivars of creeping bentgrass (Agrostis stolonifera var. palustris Huds) differing in heat tolerance. Plants of `Penn A-4' (heat tolerant) and `Putter' (heat susceptible) were grown in sand and exposed to root-zone temperatures of 20 (control), 21, 22, 23, 25, 27, 31, and 35 °C in water baths while air temperature was maintained at 20 °C in a growth chamber. Turf quality, leaf cytokinin content, and antioxidant enzyme activities declined at increased soil temperatures and the duration of treatment for both cultivars. A decline in turf quality occurred following 40 days of exposure to 35 °C for `Penn A-4' and 26 days of exposure to 31 °C for `Putter'. The root-zone temperature causing the decline of isopentenyl adenosine and zeatin cytokinins was 25 °C at 37 d for `Putter' and 27 °C at 47 days for `Penn A-4'. The temperature causing the decline of superoxide dismutase and catalase activities was 25 °C and 27 °C at 33 days for `Putter' and 27 °C and 31 °C at 43 days for Penn A-4, respectively. Malondialdehyde content increased at 27 °C for `Putter' and 31 °C for `Penn A-4' at 43 days of treatment. The decline in cytokinin content and antioxidant enzyme activity occurred at a lower soil temperature and earlier during the treatment than the decline in turf quality, possibly contributing to turf quality decline. The root-zone temperatures causing the decline in turf quality, cytokinin content, and oxidative damage were higher in the heat-tolerant cultivar than heat-susceptible cultivar.

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Qingzhang Xu, Bingru Huang and Zhaolong Wang

Heat injury in creeping bentgrass (Agrostis stolonifera var. palustris Huds) has been associated with decreases in carbohydrate availability. Extending light duration may increase carbohydrate availability and thus improve growth of creeping bentgrass under heat stress. The objective of this study was to investigate whether turf performance and carbohydrate status could be improved by extending daily light duration for creeping bentgrass exposed to supraoptimal temperature conditions. `Penncross' plants were initially grown in growth chambers set at a day/night temperature of 20/15 °C and 14-hour photoperiod and then exposed to a day/night temperature of 33/28 °C (heat stress) and three different light durations: 14 (control), 18, and 22 hours (extended light duration) for 30 days. Turf quality and tiller density decreased with the duration of heat stress, as compared to the initial level at 20 °C, regardless of the light duration. However, both parameters increased with extended light duration from 14 to 18 or 22 hours. Extended light duration, particularly to 22 hours, also improved canopy net photosynthetic rate from -1.26 to 0.39 μmol·m-2·s-1 and daily total amount of carbon assimilation from -6.4 to 31.0 mmol·m-2·d-1, but reduced daily total amount of carbon loss or consumption to 50% through dark respiration compared to 14 hours treatment by the end of experiment. In addition, extending light duration from 14 to 22 hours increased water-soluble carbohydrate content in leaves both at the end of light duration and the dark period. These results demonstrated that extending light duration improved turf performance of creeping bentgrass under heat stress, as manifested by the increased tiller density and turf quality. This could be related to the increased carbohydrate production and accumulation. Supplemental lighting could be used to improve performance if creeping bentgrass is suffering from heat stress.

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Yiming Liu, Hongmei Du, Kai Wang, Bingru Huang and Zhaolong Wang

Salinity is a detrimental abiotic stress for plant growth in salt-affected soils. The objective of this study was to examine photosynthetic responses to salinity stress in two warm-season turfgrasses differing in salinity tolerance. Salt-tolerant species seashore paspalum (Paspalum vaginatum) and salt-sensitive species centipedegrass (Eremochloa ophiuroides) were exposed to salinity at three NaCl concentrations (0, 300, and 500 mm) in a growth chamber. Turf quality, relative water content (RWC), and leaf photochemical efficiency (Fv/Fm) declined, whereas electrolyte leakage (EL) increased under the two NaCl regimes for both grass species, and the changes were more dramatic in centipedegrass than that in seashore paspalum as well as in the higher salinity concentration. Two grass species showed different phytosynthetic responses to salinity stress. The earlier inhibition of photosynthesis in seashore paspalum was mainly associated with stomatal closure. As salinity increased and salinity stress prolonged, the inhibition of photosynthesis in seashore paspalum was mainly associated with non-stomatal factors. The inhibition of photosynthesis in centipedegrass was associated with both stomatal closure and non-stomatal factors at both salinity levels. The sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis demonstrated the Rubisco large subunit had no obvious decrease during the whole stress period under the 300-mm and 500-mm treatments in seashore paspalum, whereas it significantly decreased in centipedegrass under both the 300-mm and 500-mm treatments. The results indicated that the superior salinity tolerance in seashore paspalum, compared with centipedegrass, could be attributed to its maintenance of Rubisco stability, chlorophyll content, photochemical efficiency as well as photosynthetic rate (Pn) capacity under salinity stress.

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June Liu, Zhimin Yang, Weiling Li, Jingjin Yu and Bingru Huang

Cold stress is a major factor limiting the growth of warm-season turfgrass species. Cold tolerance in warm-season turfgrass species could be improved through in vitro selection for somaclonal variations. The objectives of this study were to establish an effective in vitro culture protocol for generating plants from calli using mature seeds of seashore paspalum (Paspalum vaginatum) and to determine whether in vitro cold selection of somaclonal variations would lead to improved cold tolerance in seashore paspalum. The optimal concentrations of supplemental compounds in the culture medium for callus induction, embryogenic callus formation, and plant regeneration were determined. The supplemental compounds included 2,4–dichlorophenoxy acetic acid (2,4-D), 6-benzylaminopurine (6-BA), kinetin (KT), naphthalene-1-acetic acid (NAA), CuSO4, and acidic hydrolysis casein (AHC). The highest rates of callus induction (97.50%), embryogenic callus formation (66.88%), and regeneration (55.94%) were obtained with the supplemental compounds of 3.0 mg·L−1 2,4-D and 10.0 mg·L−1 CuSO4 for callus induction; with 3.0 mg·L−1 2,4-D, 15 mg·L−1 CuSO4, and 1.0 g·L−1 AHC for embryogenic callus formation; and with 8.0 mg·L−1 6-BA, 0.2 mg·L−1 KT, 0.5 mg·L−1 NAA, and 10 mg·L−1 CuSO4 for plant regeneration. Embryogenic calli were subjected to 2 or 6 °C treatment for 90 days for in vitro cold selection of somaclonal variation. Plants regenerated from calli surviving cold treatment (cold-selected) for 45 or 60 days were then exposed to low temperatures [15/10 or 5/3 °C (day/night)]. Plant variants derived from cold-selected calli exhibited significant improvement in their tolerance to low temperature of either 15/10 or 5/3 °C (day/night), as manifested by higher turf quality, leaf chlorophyll content, and membrane stability as well as lower levels of lipid peroxidation compared with the control plants. This study demonstrated the feasibility of in vitro selection for cold tolerance in seashore paspalum. The cold-tolerant variants could be useful germplasm for breeding programs and further molecular characterization of cold tolerance mechanisms.

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Shaoyun Lu, Zhongcheng Wang, Yuejing Niu, Zhenfei Guo and Bingru Huang

Improving the drought tolerance of widely used bermudagrass [Cynodon dactylon (L.) Pers. var. dactylon] is important for water conservation and producing quality turf with limited irrigation. Mutants of bermudagrass were generated using gamma-ray irradiation with an aim toward developing dwarf and drought-resistant bermudagrass. The objectives of this study were to compare morphological characteristics between radiation-induced mutants and the wild-type of bermudagrass and to determine antioxidant responses associated with changes in drought resistance in the bermudagrass mutants. Three mutant lines (7-9, 10-5, and 10-12) that exhibit slow growth and good turf quality were chosen for this study. Plants were exposed to drought stress by withholding irrigation in a greenhouse. Mutant lines had lower canopy height, shorter internodes, and shorter leaves than the wild type under well-watered conditions. Under drought stress, all three dwarf mutant lines maintained higher relative water content and lower ion leakage and malondialdehyde content than the wild type. Antioxidant enzyme activities decreased in response to the drought stress in the mutant lines and the wild type, whereas nonenzymatic antioxidants increased under drought stress. Compared with the wild type, higher enzyme activities and antioxidant contents were maintained in mutant lines under drought stress. Our results indicated that bermudagrass mutants induced by gamma radiation exhibited dwarf characteristics and improved drought resistance, which was associated with maintenance of higher levels of antioxidant enzyme activities and nonenzymatic antioxidant contents.

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Diheng Zhong, Hongmei Du, Zhaolong Wang and Bingru Huang

Fatty acid metabolism may be involved in plant adaptation to drought stress. The objective of this study was to identify saturated and unsaturated fatty acids associated with leaf dehydration tolerance by comparing fatty acid composition and unsaturation levels at equivalent leaf water status of two bermudagrass genotypes contrasting in drought resistance. A drought-resistant hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) genotype (‘Tifway’) and a drought-sensitive bermudagrass (C. dactylon) genotype (‘C299’) were maintained under well-watered (control) or water-withheld (drought) conditions. Drought treatment was imposed until soil water content decreased to 5% or leaf relative water content (RWC) dropped to 28% to 29%. ‘Tifway’ maintained higher RWC and lower electrolyte leakage (EL) at 5 and 10 days of drought stress. Leaves of ‘Tifway’ maintained lower EL when RWC of both genotypes declined to the same level of water deficit (28% to 29%) by the end of drought periods. The degree of fatty acid unsaturation, expressed as the double bond index, decreased in both genotypes during drought stress, which was mainly associated with the decline in linoleic (C18:2) and linolenic acids (C18:3) and an increase in palmitic (C16:0) and stearic acids (C18:0). A lipid composition characterized by a greater amount of unsaturated fatty acids was detected in ‘Tifway’ relative to ‘C299’ exposed to the same level of water deficit, mainly as a result of a greater content of C18:2 and a lower content of C16:0 and C18:0. Our results suggest that the ability to maintain a greater composition of unsaturated fatty acids in membrane lipids may contribute to superior leaf dehydration tolerance in bermudagrass.

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Yali Song, Patrick Burgess, Hairong Han and Bingru Huang

Turfgrass growth and physiological activities are sensitive to temperatures and are affected by mowing height. Increasing temperatures associated with global climate change may limit photosynthetic capacity of established turfgrass stands. The objective of this study was to determine the effects of mowing height on carbon exchange of a turfgrass system and consequential effects on turfgrass growth in response to temperature variations across the growing season in kentucky bluegrass (Poa pratensis cv. Baron) stands. Mature (8 years old) turfgrass was mowed at 7.6 cm [high mowing height (HM)] or 3.8 cm [low mowing height (LM)] during 2012 and 2013. Both LM and HM plots displayed significant decline in turf quality (TQ), shoot biomass, and canopy photosynthetic rate (Pn) with increasing air temperature above 23–24 °C in both years and the decline was more pronounced for LM plots. Turf plots were carbon emitters when total respiration rate of shoots, roots, and soil (Rtotal) exceeded canopy Pn under high temperatures during July–September but maintained net carbon gain during cooler seasons (May and June) due to greater Pn to Rtotal ratio (Pn:Rtotal). Lowering mowing height accelerated carbon loss by reducing canopy Pn, particularly under high temperatures. Our results suggested that whether mature turfgrass stands fix or emit carbon is heavily dependent on interaction between seasonal temperatures and mowing height gauging whole-stand photosynthetic capacity. Furthermore, increasing mowing height during summer months may offset the deleterious effects of high temperature by maintaining positive carbon balance within the turfgrass system.

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Zhaolong Wang, Bingru Huang, Stacy A. Bonos and William A. Meyer

Drought is a major factor limiting plant growth, which has been associated with the accumulation of absicsic acid (ABA) in various species. The objective of the study was to determine the relationship between ABA accumulation and drought tolerance for kentucky bluegrass (Poa pratensis L.) during short-term drought stress. Eight kentucky bluegrass cultivars (`Midnight', `A82-204', `RSP', `Alpine', `Moonlight', `Brilliant', `Washington', and `Baruzo') were subjected to drought stress in a growth chamber. Water relations, gas exchange rate, and ABA content of leaves were determined at various times during drought stress. Turf quality decreased with drought duration for all eight cultivars. Leaf ABA content increased linearly with drought stress within 11 days of treatment; the rate of the increase was negatively related to the rate of turf quality decline. The rate of ABA accumulation during drought stress was positively correlated with the rates of decrease in turf quality (r 2 = 0.6346), increase in electrolyte leakage (r 2 = 0.7128), and decrease in relative water content (r 2 = 0.5913). There were highly significant negative correlations between ABA content and leaf water potential (r 2 = 0.9074), stomatal conductance (r 2 = 0.6088), transpiration rate (r 2 = 0.6581), net photosynthesis rate (r 2 = 0.6956), and a positive correlation between ABA content and electrolyte leakage (r 2 = 0.7287). The results indicate that drought tolerance is negatively related to ABA accumulation during shortterm drought stress. ABA accumulation in response to drought stress could be used as a metabolic factor to select for drought tolerance in kentucky bluegrass.

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Hua Shen, Hongmei Du, Zhaolong Wang and Bingru Huang

The objective of this study was to compare differential nutrient responses to heat stress in relation to heat tolerance for warm-season (C4) common bermudagrass [Cynodon dactylon (L.) Pers.] and cool-season (C3) kentucky bluegrass (Poa pratensis L.). Both species were exposed to two temperature regimes in growth chambers: optimal day/night temperature conditions (24/20 °C for kentucky bluegrass and 34/30 °C for bermudagrass) or heat stress (10 °C above the respective optimal temperature for each species). Heat injury in leaves was evaluated and the concentrations of several major macronutrients [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)] in both grass species were measured at 0, 7, 14, 21, and 28 days of treatment. Heat stress reduced leaf photochemical efficiency and cellular membrane stability in both species, but bermudagrass leaves exhibited less damage in these parameters than kentucky bluegrass. Heat stress caused a significant decline in N, P, and K concentration, beginning at 7 days in kentucky bluegrass, but had no significant effects on N, P, and K concentration in bermudagrass during the 28-day treatment period. The concentration of Ca and Mg increased under heat stress in both kentucky bluegrass and bermudagrass, but there were no significant differences between the species under optimal or high-temperature conditions, suggesting they were not involved in heat responses in either species. The differential responses of N, P, and K to heat stress could at least partially account for the differences in heat tolerance between the two species and demonstrate the importance of sufficient N, P, and K in turfgrass adaptation to heat stress.

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Lili Zhuang, Mengxian Liu, Xiuyun Yuan, Zhimin Yang and Bingru Huang

Aquaporin (AQP) proteins serve important roles in regulating water movement across cellular membranes and affect plant responses to drought stress. The objective of this study was to characterize and examine functions of an AQP gene FaPIP2;1, isolated from a drought-tolerant perennial grass species tall fescue (Festuca arundinacea), for involvement in leaf dehydration status during water stress by overexpressing the gene in arabidopsis (Arabidopsis thaliana). FaPIP2;1 had characteristic transmembrane domains and Asn–Pro–Ala motifs and was similar to PIP2;1 in rice (Oryza sativa) and maize (Zea mays). Quantitative real-time reverse transcriptase polymerase chain reaction analysis showed that FaPIP2;1 was upregulated during moderate water stress (hydroponic culture, osmotic potential (ΨS) at −0.47 and −0.78 MPa) and the transcript level decreased as ΨS further decreased. Transgenic arabidopsis plants overexpressing FaPIP2;1 showed greater number of leaves per plant and improved survival rate compared with the wild type (WT) during drought stress. Transgenic plants also maintained higher leaf relative water content (RWC), chlorophyll content (Chl), net photosynthetic rate (Pn), and lower leaf electrolyte leakage (EL) than the WT. However, there was no difference in root length between the transgenic and WT plants following drought stress. The results demonstrated that overexpressing FaPIP2;1 could improve plant tolerance to drought stress by enhancing leaf water status, Chl, and photosynthetic rate, as well as maintaining improved cellular membrane stability relative to the WT plants. FaPIP2;1 may be used as a candidate gene for genetic modification of perennial grasses to develop new drought-tolerant germplasm and cultivars.