Effects of Arbuscular Mycorrhiza on Primary Metabolites in Phloem Exudates of Plantago major and Poa annua and on a Generalist Aphid.

Arbuscular mycorrhiza (AM), i.e., the interaction of plants with arbuscular mycorrhizal fungi (AMF), often influences plant growth, physiology, and metabolism. Effects of AM on the metabolic composition of plant phloem sap may affect aphids. We investigated the impacts of AM on primary metabolites in phloem exudates of the plant species Plantago major and Poa annua and on the aphid Myzus persicae. Plants were grown without or with a generalist AMF species, leaf phloem exudates were collected, and primary metabolites were measured. Additionally, the performance of M. persicae on control and mycorrhizal plants of both species was assessed. While the plant species differed largely in the relative proportions of primary metabolites in their phloem exudates, metabolic effects of AM were less pronounced. Slightly higher proportions of sucrose and shifts in proportions of some amino acids in mycorrhizal plants indicated changes in phloem upload and resource allocation patterns within the plants. Aphids showed a higher performance on P. annua than on P. major. AM negatively affected the survival of aphids on P. major, whereas positive effects of AM were found on P. annua in a subsequent generation. Next to other factors, the metabolic composition of the phloem exudates may partly explain these findings.

Choline-Mediated Lipid Reprogramming as a Dominant Salt Tolerance Mechanism in Grass Species Lacking Glycine Betaine.

Choline, as a precursor of glycine betaine (GB) and phospholipids, is known to play roles in plant tolerance to salt stress, but the downstream metabolic pathways regulated by choline conferring salt tolerance are still unclear for non-GB-accumulating species. The objectives were to examine how choline affects salt tolerance in a non-GB-accumulating grass species and to determine major metabolic pathways of choline regulating salt tolerance involving GB or lipid metabolism. Kentucky bluegrass (Poa pratensis) plants were subjected to salt stress (100 mM NaCl) with or without foliar application of choline chloride (1 mM) in a growth chamber. Choline or GB alone and the combined application increased leaf photochemical efficiency, relative water content and osmotic adjustment and reduced leaf electrolyte leakage. Choline application had no effects on the endogenous GB content and GB synthesis genes did not show responses to choline under nonstress and salt stress conditions. GB was not detected in Kentucky bluegrass leaves. Lipidomic analysis revealed an increase in the content of monogalactosyl diacylglycerol, phosphatidylcholine and phosphatidylethanolamine and a decrease in the phosphatidic acid content by choline application in plants exposed to salt stress. Choline-mediated lipid reprogramming could function as a dominant salt tolerance mechanism in non-GB-accumulating grass species.

The Role of APOSTART in Switching between Sexuality and Apomixis in Poa pratensis.

The production of seeds without sex is considered the holy grail of plant biology. The transfer of apomixis to various crop species has the potential to transform plant breeding, since it will allow new varieties to retain valuable traits thorough asexual reproduction. Therefore, a greater molecular understanding of apomixis is fundamental. In a previous work we identified a gene, namely APOSTART, that seemed to be involved in this asexual mode of reproduction, which is very common in Poa pratensis L., and here we present a detailed work aimed at clarifying its role in apomixis. In situ hybridization showed that PpAPOSTART is expressed in reproductive tissues from pre-meiosis to embryo development. Interestingly, it is expressed early in few nucellar cells of apomictic individuals possibly switching from a somatic to a reproductive cell as in aposporic apomixis. Moreover, out of 13 APOSTART members, we identified one, APOSTART_6, as specifically expressed in flower tissue. APOSTART_6 also exhibited delayed expression in apomictic genotypes when compared with sexual types. Most importantly, the SCAR (Sequence Characterized Amplified Region) derived from the APOSTART_6 sequence completely co-segregated with apomixis.

Physiological and transcriptome analysis of Poa pratensis var. anceps cv. Qinghai in response to cold stress.

BACKGROUND: Low temperature limits the growth and development and geographical distribution of plants. Poa pratensis is a cool-season turfgrass mainly grown in urban areas. However, low winter temperature or cold events in spring and autumn may cause P.pratensis mortality, affecting the appearance of lawns. P.pratensis var. anceps cv. Qinghai (PQ) is widely distributed in the Qinghai-Tibet Plateau above 3000 m. PQ has greater cold tolerance than the commercially cultivated P.pratensis varieties. However, existing studies on the response mechanism of PQ to low temperatures have mainly focused on physiological and biochemical perspectives, while changes in the PQ transcriptome during the response to cold stress have not been reported. RESULTS: To investigate the molecular mechanism of the PQ cold response and identify genes to improve the low-temperature tolerance of P.pratensis, we analyzed and compared the transcriptomes of PQ and the cold-sensitive P.pratensis cv. 'Baron' (PB) under cold stress using RNA sequencing. We identified 5996 and 3285 differentially expressed genes (DEGs) between the treatment vs control comparison of PQ and PB, respectively, with 5612 DEGs specific to PQ. Based on the DEGs, important Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, such as "starch and sucrose metabolism", "protein processing in endoplasmic reticulum", "phenylalanine metabolism" and "glycolysis/gluconeogenesis" were significantly enriched in PQ, and "starch and sucrose metabolism", "phenylpropanoid biosynthesis", "galactose metabolism" and "glutathione metabolism" were significantly enriched in PB. In addition, the "glycolysis" and "citrate cycle (TCA cycle)" pathways were identified as involved in cold tolerance of P.pratensis. CONCLUSIONS: As we know, this is the first study to explore the transcriptome of P.pratensis var. anceps cv. Qinghai. Our study not noly provides important insights into the molecular mechanisms of P.pratensis var. anceps cv. Qinghai responds to cold stress, but also systematically reveals the changes of key genes and products of glycolysis and TCA cycle in response to cold stress, which is conductive to the breeding of cold-tolerance P.pratensis genotype.

Determining nitrogen isotopes discrimination under drought stress on enzymatic activities, nitrogen isotope abundance and water contents of Kentucky bluegrass.

Drought stress is the most pervasive threat to plant growth, which predominantly encumbers turf grass growth by causing alterations in plant functions. This study appraised the role of nitrogen isotopes in providing a theoretical basis for developing and improving Kentucky bluegrass cultivar performance under drought stress. Nitrogen isotopes labelled 15NH4Cl and K15NO3 were prepared to replace KNO3 in Hoagland's solution at concentrations of 15NH4+ and 15NO3 at 1.5, 15, and 30 mM; the solutions were imposed on stressed plants under glasshouse conditions. Nitrogenous nutrition reduced oxidative stress by elevating the enzymatic activities and proline contents of all three clonal ramet leaves, particularly under stress conditions. Apart from nitrogen content, nitrogen isotope abundance, relative water content and water potential within controls were enhanced in treated with 15NH4+ than in with 15NO3 in both the roots and leaves of Kentucky bluegrass. Nevertheless, an application of 15NH4Cl and K15NO3 at 30 mM had a positive influence to some extent on these attributes under drought stress. Overall, our results suggested that nitrogen isotopes contributed to drought tolerance in all three clonal ramets of Kentucky bluegrass by maintaining a better osmoprotectant and antioxidant defence system, which helped the plants eliminate reactive oxygen species.

Photosynthetic performance of five cool-season turfgrasses under UV-B exposure.

Turfgrasses are monocotyledonous plants from the family Poaceae. They are widely used in green spaces and are considered one of the most economically important horticultural crops in the world. Turfgrass quality is affected by several environmental factors including light, which is involved in the quality decline of transplanted sod. Ultraviolet-B (UV-B) is an important regulator of plant growth and development. Plants growing and/or stored in protected systems, such as in sod production, may be more vulnerable to UV-B damage than those growing in the field due to acclimation. Few studies on the effects of UV-B on turfgrass physiology have been published. Therefore, the aim of this study was to evaluate the influence of UV-B irradiation on the photosynthetic performance of five cool-season turfgrasses, namely Agrostis stolonifera L., Festuca arundinacea Schreb., Poa supina Schrad., Poa pratensis L. and Lolium perenne L. Turfgrasses were exposed to 18.25 kJ m-2 d-1 biologically effective UV-B in growth chambers under controlled conditions. Measurements included photosynthetic pigments, chlorophyll fluorescence and gas exchanges monitored for 16 d-UV-B treatment and after recovery. Content of pigments decreased with UV-B exposure with significant differences among the species. UV-B also affected the photosystem II (PSII) efficiency depending on the exposure period and species. Similarly, gas exchange parameters showed different effects among species after UV-B exposure compromising the assimilation of CO2. Multivariate analysis highlighted three main clusters of species confirming their different UV-B tolerance and ability to restore PSII photochemistry after recovery, from which Festuca arundinacea resulted to be the most tolerant.

Transcriptional regulation and expression network responding to cadmium stress in a Cd-tolerant perennial grass Poa Pratensis.

Kentucky bluegrass has good capability to absorb and accumulate cadmium (Cd) through developed root system, thus having potential phytoremediation function in Cd contaminated soils. Understanding the molecular mechanisms of Cd tolerance and accumulation in this species will be crucial to generating novel Cd-tolerance cultivars through genetic improvement, while it has not well documented yet. In the present study, comparative transcriptome analysis was performed for the seedlings of high Cd-tolerant genotype (M) and low Cd-tolerant genotype (R) under Cd stress. A total of 7022 up-regulated and 1033 down-regulated transcripts were identified in M genotype, whereas, only 850 up-regulated and 846 down-regulated transcripts were detected in R. Further transcriptional regulation analysis in M genotype showed that Dof, MADS25, BBR-BPC, B3, bZIP23 and MYB30 might be the hub transcription factors in response to Cd stress due to the orchestrated multiple functional genes associated with carbohydrate, lipid and secondary metabolism, as well as signal transduction. Differential expressed genes involved in auxin, ethylene, brassinosteroid and ABA signalling formed signal transduction cascades, which interacted with hub transcription factors, thereby finally orchestrated the expression of multiple genes associated with cell wall and membrane stability, cell elongation and Cd tolerance, including IAAs, ARFs, SnRK2, PP2C, PIFs, BES1/BZR1, CCR, CAD, FATB, fabF and HACD. Additionally, post-transcriptional modification of CIPKs, MAPKs, WAXs, UBCs, and E3 ubiquitin ligases were identified and also involved in plant signalling pathways and abiotic resistance. The study could contribute to our understanding the transcriptional regulation and complex internal network associated with Cd tolerance in Kentucky bluegrass.

Drought-induced injury is associated with hormonal alteration in Kentucky bluegrass.

Plant hormones play an important role in plant adaptation to abiotic stress, but hormonal responses of cool-season turfgrass species to drought stress are not well documented. This study was to investigate responses of hormones and photosynthesis to drought stress and examine if drought stress - induced hormone alteration is associated with stress tolerance in kentucky bluegrass (KBG, Poa pratensis L.). The grass was grown in a growth chamber for 6 weeks and then subjected to drought stress [40%-50% evapotranspiration (ET) replacement)] for 28 d. Drought stress caused cell membrane damage, resulting in decline in photosynthetic rate (Pn), chlorophyll content, and visual quality in KBG. Drought stressed grass had higher leaf abscisic acid (ABA), lower leaf trans-zeatin riboside (ZR), isopentenyl adenosine (iPA), and indole-3-acetic acid (IAA), but similar level of leaf gibberellin A4 (GA4) when compared to the control (well-watered). On average, drought stress treatment reduced leaf ZR by 59.1%, iPA by 50.4%, IAA by 26.7%, while increased ABA by 108.5% when compared to the control at the end of drought stress (28 d). The turf quality and photosynthetic rate was positively correlated with cytokinins and IAA, but negatively correlated with ABA and ABA/cytokinins (CK) ratio under drought stress. The results of this study suggest drought stress-induced injury to Kentucky bluegrass may be associated with hormonal alteration, and the plants with higher cytokinins and IAA and less ABA under drought stress may have better photosynthetic function and performance.

How Kentucky bluegrass tolerate stress caused by sodium chloride used for road de-icing?

Salts used in road de-icing during winter season inhibit the growth and development of lawn grass species. The mechanism of plant tolerance/sensitivity to such treatments is still not clear. Moreover, there is a lack of fast and non-invasive tool to detect the effect of these salts on plants growth. This study was designed to understand the tolerance mechanism of Kentucky bluegrass plants on salinity, based on some biometric and physiological parameters. In this experiment, we simulated the urban conditions where salts are used intensively for roads de-icing. Germination capacity was evaluated at five salt solutions of NaCl (0, 50, 100, 150 and 200 mM), and physiological parameters were measured during the tillering phase at salinity levels of 0, 150 and 300 mM of NaCl. Seeds of Kentucky bluegrass did not germinate under salinity. During tillering phase, salinity affected length, area and dry mass of roots as well as the relative water content of plants, negatively. Moreover, it influenced the maximum chlorophyll fluorescence yield, quantum yield of photosystem II and electron transport rate at early period of stress. This allows us to recommend these parameters for early detection of soil salinity effects on Kentucky bluegrass plants. It seems to be that the tolerance of this plant towards salinity is based on retaining water content in leaves that allow more efficient functioning of photosynthetic apparatus.

Water transport properties of root cells contribute to salt tolerance in halophytic grasses Poa juncifolia and Puccinellia nuttalliana.

Plant water uptake and aquaporin-mediated root water transport are among the most salt-sensitive processes in most plants, but even relatively high salt concentrations do not appear to impair water transport processes in halophytes. To develop better understanding of these processes in halophytic plants, we compared the responses to NaCl of the two halophytic grasses varying in salt tolerance, Puccinellia nuttalliana and Poa juncifolia, with the glycophytic grass Poa pratensis. The plants were hydroponically grown and subjected to different NaCl concentrations for up to 10 days. At the lower NaCl concentrations, shoot and root dry weights were drastically reduced in Poa pratensis, but increased in Puccinellia nuttalliana and Poa juncifolia. The examined treatment concentrations of up to 300 mM NaCl had either no effect (Puccinellia nuttalliana) or little effect (Poa juncifolia) on the net photosynthesis and transpiration rates in plants, but severely decreased the gas exchange parameters in Poa pratensis. Similarly, to growth and gas exchange, leaf water content in Puccinellia nuttalliana was not affected even by the highest, 300 mM NaCl concentration, while Poa pratensis showed decreased shoot water content in all examined NaCl treatments and Poa juncifolia in 150 and 300 mM NaCl. Cell hydraulic conductivity in roots of Poa pratensis also showed high sensitivity to NaCl and was drastically reduced in all examined NaCl concentrations. Cell hydraulic conductivity in Poa juncifolia roots was less affected by NaCl compared with Poa pratensis and in Puccinellia nuttalliana, cell hydraulic conductivity increased in response to NaCl treatments. Both Puccinellia nuttalliana and Poa juncifolia accumulated less Na in their shoot tissues compared with Poa pratensis. The concentrations of K in the roots of Poa pratensis sharply decreased with increasing NaCl treatment concentrations while in Puccinellia nuttalliana, K root concentrations remained high in all NaCl treatments and in Poa juncifoila, root K decreased only in the 300 mM NaCl treatment. Since K efflux from the cytoplasm can contribute to the acidification of the cytoplasm, this process could potentially lead to the inhibition of aquaporin function and reduction of root hydraulic conductivity. The, significance of stable K root concentrations in the roots of halophytes should be further investigated as a possible salt tolerance mechanism that could contribute to the maintenance of aquaporin function and root water transport under salt stress conditions.

Soil microarthropods alter the outcome of plant-soil feedback experiments.

Plant-soil feedback (PSF) effects are studied as plant growth responses to soil previously conditioned by another plant. These studies usually exclude effects of soil fauna, such as nematodes, soil arthropods, and earthworms, although these organisms are known to influence plant performance. Here, we aimed to explore effects of a model microarthropod community on PSFs. We performed a PSF experiment in microcosms with two plant species, Phleum pratense and Poa pratensis. We added a model microarthropod community consisting of three fungivorous springtail species (Proisotoma minuta, Folsomia candida, and Sinella curviseta) and a predatory mite (Hypoaspis aculeifer) to half of the microcosms. We measured seedling establishment and plant biomass, nematode and microbial community composition, microbial biomass, and mycorrhizal colonization of roots. Microarthropods caused changes in the composition of nematode and microbial communities. Their effect was particularly strong in Phleum plants where they altered the composition of bacterial communities. Microarthropods also generally influenced plant performance, and their effects depended on previous soil conditioning and the identity of plant species. Microarthropods did not affect soil microbial biomass and mycorrhizal colonization of roots. We conclude that the role of soil microarthropods should be considered in future PSF experiments, especially as their effects are plant species-specific.

Females engage in stronger relationships: positive and negative effects of shrubs are more intense for Poa ligularis females than for males.

Background and Aims: Dioecious plants are of particular concern in view of global environmental changes because reproductive females are more sensitive to abiotic stresses, thus compromising population viability. Positive interactions with other plants may counteract the direct effects of any abiotic environmental stress, allowing them to thrive and maintain a viable population in suboptimal habitats, although this process has not been tested for dioecious species. Furthermore, almost no data are available on the outcome of such species interactions and their link with local spatial patterns and sex ratios. Methods: We set up a field experiment with Poa ligularis, a dioecious native grass from the arid grasslands of South America. We studied the interaction of male and female plants with cushion shrubs of contrasting ecological strategies. We experimentally limited direct shrub-grass competition for soil moisture and transplanted plants to evaluate the amelioration of abiotic stress by shrub canopies (i.e. sun and wind) on grasses. We also studied the distribution of naturally established female and male plants to infer process-pattern relationships. Key Results: Positive canopy effects as well as negative below-ground effects were more intense for females than for males. Deep-rooted Mulinum spinosum shrubs strongly facilitated survival, growth and reproduction of P. ligularis females. Naturally established female plants tended to distribute more closely to Mulinum than co-occurring males. Female growth suffered intensive negative root competition from the shallow-rooted Senecio filaginoides shrub. Conclusions: Interactions with other plants may reduce or enhance the effect of abiotic stresses on the seemingly maladapted sex to arid environments. We found that these biased interactions are evident in the current organization of sexes in the field, confirming our experimental findings. Therefore, indirect effects of climate change on population sex ratios may be expected if benefactor species abundances are differentially affected.

Contrasting survival and physiological responses of sub-Arctic plant types to extreme winter warming and nitrogen.

MAIN CONCLUSION: Evergreen plants are more vulnerable than grasses and birch to snow and temperature variability in the sub-Arctic. Most Arctic climate impact studies focus on single factors, such as summer warming, while ecosystems are exposed to changes in all seasons. Through a combination of field and laboratory manipulations, we compared physiological and growth responses of dominant sub-Arctic plant types to midwinter warming events (6 °C for 7 days) in combination with freezing, simulated snow thaw and nitrogen additions. We aimed to identify if different plant types showed consistent physiological, cellular, growth and mortality responses to these abiotic stressors. Evergreen dwarf shrubs and tree seedlings showed higher mortality (40-100%) following extreme winter warming events than Betula pubescens tree seedlings and grasses (0-27%). All species had growth reductions following exposure to - 20 °C, but not all species suffered from - 10 °C irrespective of other treatments. Winter warming followed by - 20 °C resulted in the greatest mortality and was strongest among evergreen plants. Snow removal reduced the biomass for most species and this was exacerbated by subsequent freezing. Nitrogen increased the growth of B. pubescens and grasses, but not the evergreens, and interaction effects with the warming, freezing and snow treatments were minor and few. Physiological activity during the winter warming and freezing treatments was inconsistent with growth and mortality rates across the plants types. However, changes in the membrane fatty acids were associated with reduced mortality of grasses. Sub-Arctic plant communities may become dominated by grasses and deciduous plants if winter snowpack diminishes and plants are exposed to greater temperature variability in the near future.

Does cross-acclimation between drought and freezing stress persist over ecologically relevant time spans? A test using the grass Poa pratensis.

Despite evidence that prior exposure to drought can increase subsequent plant freezing tolerance, few studies have explored such interactions over ecologically relevant time spans. We examined the combined effects of drought and subsequent freezing on tiller growth and leaf sugar concentrations in the grass, Poa pratensis. We exposed tillers to no drought (-0.04 MPa), moderate drought (-0.19 MPa) or severe drought (-0.42 MPa) for 3 weeks in summer. Tillers were then frozen in autumn or spring at -5 °C (frost damage) or at 0 °C (control) for 3 days and harvested after a re-growth period. For shoot growth, there was a significant interaction between drought and autumn freezing, whereby the relative effect of freezing on growth was least for the plants previously exposed to severe drought; however, there was no significant interaction between drought and spring freezing. For root growth, there were no significant interactions between drought and freezing in either season. Leaf sugar concentrations increased significantly with drought intensity, but these effects dissipated within a month, prior to the onset of the autumn freezing treatment. Overall, our results suggest that interactions between prior drought and subsequent freezing in P. pratensis may be most relevant in the context of autumn freezing, and despite the important role of soluble sugars in increasing both drought and freezing tolerance in this species, the retention of these compounds after drought stress does not appear to explain the occurrence of drought-frost interactions at ecologically relevant time scales.

Can't live with them, can't live without them? Balancing mating and competition in two-sex populations.

Two-sex populations are usually studied through frequency-dependent models that describe how sex ratio affects mating, recruitment and population growth. However, in two-sex populations, mating and recruitment should also be affected by density and by its interactions with the sex ratio. Density may have positive effects on mating (Allee effects) but negative effects on other demographic processes. In this study, we quantified how positive and negative inter-sexual interactions balance in two-sex populations. Using a dioecious grass (Poa arachnifera), we established experimental field populations that varied in density and sex ratio. We then quantified mating success (seed fertilization) and non-mating demographic performance, and integrated these responses to project population-level recruitment. Female mating success was positively density-dependent, especially at female-biased sex ratios. Other demographic processes were negatively density-dependent and, in some cases, frequency-dependent. Integrating our experimental results showed that mate-finding Allee effects dominated other types of density-dependence, giving rise to recruitment that increased with increasing density and peaked at intermediate sex ratios, reflecting tension between seed initiation (greater with more females) and seed viability (greater with more males). Our results reveal, for the first time, the balance of positive and negative inter-sexual interactions in sex-structured populations. Models that account for both density- and sex ratio dependence, particularly in mating, may be necessary for understanding and predicting two-sex population dynamics.

Comparison of growth and physiological characteristics between roughstalk bluegrass and tall fescue in response to simulated waterlogging.

Roughstalk bluegrass (Poa trivialis) is a weed in cool season grass seed production fields in Oregon. Populations of this weed are often greater in fields prone to waterlogging. A greenhouse study was conducted to investigate the morphological and physiological differences between recently established roughstalk bluegrass and tall fescue (Lolium arundinaceum) plants in response to simulated waterlogging. Differences in root morphological development and root respiration were found between waterlogged tall fescue and roughstalk bluegrass. Plants after 4 weeks of waterlogging, leaf number, plant height, and root biomass were reduced more in tall fescue than in roughstalk bluegrass plants. The root length increased 6% in waterlogged tall fescue plants, and decreased 42% in waterlogged roughstalk bluegrass plants, which lead to a shallower root system in roughstalk bluegrass. Root aerenchyma area increased more in waterlogged roughstalk bluegrass than in tall fescue. Alcohol dehydrogenase and lactate dehydrogenase activities increased in the roots of both species, but not in the leaves. The increases were greater in tall fescue than in roughstalk bluegrass. Turf quality, aboveground biomass, photosynthetic capacity, and water-soluble carbohydrate concentrations were reduced by waterlogging, but there were no differences over time or species. Thus, the shallower root system, larger aerenchyma, and reduced fermentation rates were the characteristics most likely to contribute to better waterlogging tolerance in roughstalk bluegrass compared to tall fescue and invasion of roughstalk bluegrass in waterlogged cool season grass seed fields.

Poa secunda local collections and commercial releases: A genotypic evaluation.

The genetics of native plants influence the success of ecological restoration, yet genetic variability of local seed collections and commercial seed releases remains unclear for most taxa. Poa secunda, a common native grass species in Intermountain West grasslands and a frequent component of restoration seed mixes, is one such species. Here, we evaluate the genetic variation of local Poa secunda collections in the context of wild populations and commercial seed releases. We evaluated AFLP markers for seven Poa secunda collections made over a 4000-hectare area and four commercial releases (High Plains, MT-1, Opportunity, and Sherman). We compare the genetic distance and distribution of genetic variation within and between local collections and commercial releases. The extent and patterns of genetic variation in our local collections indicate subtle site differences with most variation occurring within rather than between collections. Identical genetic matches were usually, but not always, found within 5 m2 collection sites. Our results suggest that the genetic variation in two Poa secunda releases (High Plains and MT-1) is similar to our local collections. Our results affirm that guidelines for Poa secunda seed collection should follow recommendations for selfing species, by collecting from many sites over large individual sites.

Prior exposure to freezing stress enhances the survival and recovery of Poa pratensis exposed to severe drought.

PREMISE OF THE STUDY: Both freezing and drought cause cellular dehydration, and they elicit similar increases in protective compounds, which suggests that these stresses could potentially interact. We examined whether the physiological changes that occur in response to freezing in the fall and spring could affect subsequent survival and growth after summer drought. METHODS: We froze Poa pratensis tillers in the late fall, early spring, or late spring at 0, -5, or -10°C for 3 d and then subjected them to no drought (-0.025 MPa), moderate drought (-0.140 MPa), or severe drought (-0.250 MPa) for 3 wk in the summer. We quantified survival and total biomass after a 3-wk recovery period, and we determined leaf soluble sugar concentrations before and then 0, 30, and 55 d after freezing. KEY RESULTS: For survival and biomass, there were significant interactions between freezing and drought. Spring frozen tillers had the highest biomass and survival following severe drought, whereas fall freezing did not significantly increase the biomass of tillers following the severe drought. Increased drought tolerance after spring freezing did not appear to be associated with increased soluble sugar content, given that the freezing effects on leaf glucose, fructose, and sucrose content were absent 55 d post freezing. CONCLUSIONS: Our results demonstrate that multiple stresses that occur over different seasons can interact; this interaction is highly relevant to herbaceous species in northern temperate regions that are experiencing more intense and frequent stress as a result of changes in snow cover and extreme climatic events.

Responses of Poa annua and three bentgrass species (Agrostis spp.) to adult and larval feeding of annual bluegrass weevil, Listronotus maculicollis (Coleoptera: Curculionidae).

The annual bluegrass weevil (ABW), Listronotus maculicollis Kirby, is an economically important pest of short-cut turfgrass in Eastern North America. Wide spread insecticide resistance warrants the development of alternative management strategies for this pest. ABW damage typically occurs in areas with a high percentage of annual bluegrass, Poa annua L., the preferred ABW host. Damage to bentgrasses, Agrostis spp., is much rarer and usually less severe. To aid the implementation of host plant resistance as an alternative ABW management strategy we investigated the tolerance of three bentgrass species to ABW feeding. Responses of P. annua, creeping bentgrass, Agrostis stolonifera L., colonial bentgrass, Agrostis capillaris L., and velvet bentgrass, Agrostis canina L., to adult and larval feeding were compared in greenhouse experiments. Grass responses were measured as visual damage, dry weight of the grass stems and leaves, color, density and overall grass quality. To determine possible mechanisms of grass tolerance constitutive fiber and silicon content were also determined. The three bentgrass species tolerated 2-3 times higher numbers of ABW adults and larvae than P. annua before displaying any significant quality decrease. Creeping bentgrass had the lowest damage ratings. ABW infestation caused higher plant yield reduction in P. annua (up to 42%) than in bentgrasses. Observed differences among the grass species in fiber and silicon content in the plant tissue are unlikely to play a role in the resistance of bentgrasses to ABW. Our findings clearly show that A. stolonifera is the best grass species for the implementation of host plant resistance in ABW management.

Transcriptome profiling of Kentucky bluegrass (Poa pratensis L.) accessions in response to salt stress.

BACKGROUND: Kentucky bluegrass (Poa pratensis L.) is a prominent turfgrass in the cool-season regions, but it is sensitive to salt stress. Previously, a relatively salt tolerant Kentucky bluegrass accession was identified that maintained green colour under consistent salt applications. In this study, a transcriptome study between the tolerant (PI 372742) accession and a salt susceptible (PI 368233) accession was conducted, under control and salt treatments, and in shoot and root tissues. RESULTS: Sample replicates grouped tightly by tissue and treatment, and fewer differentially expressed transcripts were detected in the tolerant PI 372742 samples compared to the susceptible PI 368233 samples, and in root tissues compared to shoot tissues. A de novo assembly resulted in 388,764 transcripts, with 36,587 detected as differentially expressed. Approximately 75 % of transcripts had homology based annotations, with several differences in GO terms enriched between the PI 368233 and PI 372742 samples. Gene expression profiling identified salt-responsive gene families that were consistently down-regulated in PI 372742 and unlikely to contribute to salt tolerance in Kentucky bluegrass. Gene expression profiling also identified sets of transcripts relating to transcription factors, ion and water transport genes, and oxidation-reduction process genes with likely roles in salt tolerance. CONCLUSIONS: The transcript assembly represents the first such assembly in the highly polyploidy, facultative apomictic Kentucky bluegrass. The transcripts identified provide genetic information on how this plant responds to and tolerates salt stress in both shoot and root tissues, and can be used for further genetic testing and introgression.