C4 Perennial Grass Research Articles

Cattle adapted to tropical and subtropical environments: social, nutritional, and carcass quality considerations.

Beef production needs to increase from 60 million to 130 million tons by 2050 to feed a growing world population, and 70% of this production increase is expected from beef industries located in subtropical and tropical regions of the world. Bos indicus-influenced cattle predominate in these regions but are often managed using practices developed for Bos taurus breeds reared in temperate climates. Hence, a fundamental step to meet the increasing global demand for beef is to develop specific management for B. indicus-influenced cattle in tropical or subtropical environments. Bos taurus and B. indicus are different subspecies, and diverge in social and biological functions due to selection pressure caused by complex evolutionary and domestication processes. Bos indicus cattle display different social responses compared with B. taurus counterparts, which must be taken into account by management planning as these traits directly impact cattle performance and welfare. In tropical and subtropical regions, warm-season perennial C4 grasses are the dominant forages, and their availability has a significant influence on the productivity of beef cattle systems. The resilience of C4 grasses under adverse conditions is one of their most important characteristics, even though these forages have reduced nutritive value compared with forages from temperate climates. Accordingly, nutritional planning in tropical and subtropical conditions must include management to optimize the quantity and quality of C4 forages. Nutritional requirements of cattle raised within these conditions also require special attention, including inherent metabolic compromises to cope with environmental constraints and altered energy requirements due to body composition and heat tolerance. Nutritional interventions to enhance beef production need to be specifically tailored and validated in B. indicus-influenced cattle. As an example, supplementation programs during gestation or early life to elicit fetal programming or metabolic imprinting effects, respectively, yield discrepant outcomes between subspecies. Bos indicus-influenced cattle produce carcasses with less marbling than B. taurus cattle, despite recent genetic and management advances. This outcome is mostly related to reduced intramuscular adipocyte volume in B. indicus breeds, suggesting a lesser need for energy stored intramuscularly as a mechanism to improve thermotolerance in tropical and subtropical climates.

SMRT sequencing of a full-length transcriptome reveals transcript variants involved in C18 unsaturated fatty acid biosynthesis and metabolism pathways at chilling temperature in Pennisetum giganteum

BackgroundPennisetum giganteum, an abundant, fast-growing perennial C4 grass that belongs to the genus Pennisetum, family Poaceae, has been developed as a source of biomass for mushroom cultivation and production, as a source of forage for cattle and sheep, and as a tool to remedy soil erosion. However, having a chilling-sensitive nature, P. giganteum seedlings need to be protected while overwintering in most temperate climate regions.ResultsTo elucidate the cold stress responses of P. giganteum, we carried out comprehensive full-length transcriptomes from leaf and root tissues under room temperature (RT) and chilling temperature (CT) using PacBio Iso-Seq long reads. We identified 196,124 and 140,766 full-length consensus transcripts in the RT and CT samples, respectively. We then systematically performed functional annotation, transcription factor identification, long non-coding RNAs (lncRNAs) prediction, and simple sequence repeat (SSR) analysis of those full-length transcriptomes. Isoform analysis revealed that alternative splicing events may be induced by cold stress in P. giganteum, and transcript variants may be involved in C18 unsaturated fatty acid biosynthesis and metabolism pathways at chilling temperature in P. giganteum. Furthermore, the fatty acid composition determination and gene expression level analysis supported that C18 unsaturated fatty acid biosynthesis and metabolism pathways may play roles during cold stress in P. giganteum.ConclusionsWe provide the first comprehensive full-length transcriptomic resource for the abundant and fast-growing perennial grass Pennisetum giganteum. Our results provide a useful transcriptomic resource for exploring the biological pathways involved in the cold stress responses of P. giganteum.

Canopy structure of mixed kikuyugrass–tall fescue pastures in response to grazing management

ABSTRACTThe effects of different management strategies on improving growth and modifying the botanical composition of C3 and C4 perennial grasses grown in a mixed canopy are underexplored. We aimed to evaluate the botanical composition and vegetation dynamics of a mixed canopy containing kikuyugrass (Pennisetum clandestinum Hochst. Ex Chiov) and tall fescue (Festuca arundinacea Schreber cv. Rizomat) subjected to four grazing management strategies over 22 mo. The experimental design was a randomized complete block in a factorial arrangement (two maintenance canopy heights: grazed to 7 cm in the autumn, or uninterrupted and maintained at 12 or 17 cm) with three replicates. The plots were sampled every month throughout the experimental period; the forage mass data were used to determine the botanical composition and leaf area index of the canopy. Although autumn grazing to 7‐cm height promoted an increase in pasture leaf proportion, it did not represent an effective augment in pasture leaf mass in the following winter–spring season. Canopies heights managed at 17 cm presented a greater proportion of tall fescue in herbage mass (81%) compared with those maintained at 12 cm (73%). Severe defoliation in the autumn delayed the regrowth to 12‐ or 17‐cm canopy heights by approximately 90–120 d in both evaluation years. From a farm‐level perspective, this result would restrict the recommendation of this grazing management strategy because this time lapse would demand that the pastures not be grazed for the 25–33% period of the year compared with the pastures that were not grazed in the autumn.

Fall Water Effects on Growing Season Soil Water Content and Plant Productivity

Understanding fall precipitation effects on rangelands could improve forage production forecasting and inform predictions of potential climate change effects. We used a rainout shelter and water addition to test effects of seasonal precipitation on soil water and annual net primary production of C3 perennial grass, C4 perennial grass, annual grasses, forbs, and all plants combined. Treatments were 1) drought during September−October and April−May (DD); 2) drought plus irrigation during September−October and drought during April−May (WD); 3) year-long ambient conditions (WW); and 4) ambient plus irrigation during September−October (W + W). Treatments created conditions ranking among the driest and wettest September−October periods since 1937. Fall water effects on soil water were not detectable by May at 15 cm and 30 cm. Effects persisted into July at 60 cm and 90 cm, depths below the primary root zone. With spring drought, annual net primary production was 344 kg ha−1 greater when the previous fall was wet rather than dry. No differences were detected between fall water treatments when spring was wet and fall was about 184% (1 938 ± 117 kg ha−1) or 391% of the median (1 903 ± 117 kg ha−1). Fall water increased C3 perennial grass when spring was also wet and had no effect under spring drought, when forage production concerns are greatest. Fall water did not affect C4 perennial grass, and extremely wet fall conditions reduced forb production about 50%. The greatest effect of fall water was increased annual grass production. Even record high levels of fall water had minor effects on biomass, functional group composition, and soil water that were short-lived and overwhelmed by the influence of spring precipitation. Movement of fall water to deep soil by the growing season suggests plants that would most benefit from fall precipitation are those that could use it during fall (winter annuals), or deep-rooted species (shrubs).

Conjunction of Vetiveria zizanioides L. and oil-degrading bacteria as a promising technique for remediation of crude oil-contaminated soils

Abstract Oil pollution is a great threat to all forms of aquatic and terrestrial life. Vetiver (Vetiveria zizanioides L.) is a C4 perennial grass which can grow on diverse environments and may have the potential to be used for remediation of contaminated areas. Therefore, a controlled greenhouse experiment was conducted to study growth performance, petroleum tolerance and total petroleum hydrocarbons (TPHs) removal potency of vetiver and hydrocarbon-degrading bacteria over a period of 120 days in contaminated soils with various oil concentrations (0, 2, 4, 6, 8, 10, and 12% w/w). The results showed that chlorophyll (Chl) a and Chl b content decreased in contaminated soils, although carotenoid content increased. Vetiver grass showed no sign of toxicity and thrived well in contaminated soils by applying survival approaches. Total antioxidant activity, malondialdehyde, and proline contents in root and shoot of vetiver increased in the presence of crude oil. The results revealed that decreasing of TPHs was in the range of 47–77% by vetiver, 53.3–87.4% by bacteria, and 57.5–84.6% by plant-bacteria treatments. Therefore, this plant could be used effectively for cleansing crude oil-contaminated soil, particularly in the presence of degrading bacteria. However, it needs more studies in field conditions where the physicochemical and biological characteristics of natural polluted soils may affect plant and bacteria remediation efficiency.

Gross and net nitrogen export from leaves of a vegetative C4 grass

Nitrogen (N) mobilization from mature leaves plays a key role in supplying amino acids to vegetative and reproductive sinks. However, it is unknown if the mobilized N is predominantly sourced by net N-export (a senescence-related process) or other source of N-export from leaves. We used a new approach to partition gross and net N-export from leaf blades at different developmental stages in Cleistogenes squarrosa (a perennial C4 grass). Net N-export was determined as net loss of leaf N with age, while gross N-export was quantified from isotopic mass balances obtained following 24 h-long 15N-labeling with nitrate on 10–12 developmentally distinct (mature and senescing) leaves of individual major tillers. Net N-export was apparent only in older leaves (leaf no. > 7, with leaves numbered basipetally from the tip of the tiller and leaf no. 2 the youngest fully-expanded leaf), while gross N-export was largely independent of leaf age category and was ∼8.4 times greater than the net N-export of a tiller. At whole-tiller level, N import compensated 88 ± 14 (SE) % of gross N-export of all mature blades leading to a net N-export of 0.51 ± 0.07 (SE) μg h−1 tiller−1. N-import was equivalent to 0.09 ± 0.01 (SE) d−1 of total leaf N, similar to reported rates of leaf protein turnover. Gross N-export from all mature blades of a tiller was ∼1.9-times the total demand of the immature tissues of the same (vegetative) tiller. Significant N-export is evident in all mature blades, and is not limited to senescence conditions, implying a much shorter mean residence time of leaf N than that calculated from net N-export. Gross N-export contributes not only to the N demand of the immature tissues of the same tiller but also to N supply of other sinks, such as newly formed tillers. N dynamics at tiller level is integrated with that of the remainder of the shoot, thus highlights the importance of integration of leaf-, tiller-, and plant-scale N dynamics.

Can Mowing Substitute for Fire in Semiarid Grassland?

Accumulating data indicate the importance of fire in rangeland systems. Mowing is a common management technique sometimes considered a surrogate for fire. However, direct comparisons of fire and mowing effects are limited. Our objective was to determine whether mowing can substitute for fire in rangeland by comparing effects on plant biomass, composition, cover, soil nutrients, and forage quality. Three disturbance treatments (nontreated control, spring mowing with clipping removal, and spring fire) were randomly assigned to 21 plots (5 × 5 m) each on silty and claypan ecological sites in a completely randomized design, with seven replications per site. Current-yr biomass was similar among control, mowed, and burned treatments (1 003, 974, 1 022 ± 64 kg ● ha−1). Mowing shifted functional group composition by reducing C3 perennial grass 12% and increasing forbs 8%. Non-native species were a larger component of mowed (12%) than control (6%) or burned plots (4%). Fire increased bare ground 35%, reduced litter 32%, and eliminated previous yrs’ growth the first growing season. Plant-available soil N and S more than doubled with fire, and there was a trend for more P in burned plots. Mowing effects were limited to a trend for less soil Fe. Mowing affected 42% of the forage quality variables with a 2% average improvement across all variables. Fire affected 84% of the variables, with a 12% average improvement. Mowing increased forage P and K, whereas fire increased forage concentrations of N, K, P, S, Mg, Fe, Mn, and Cu. Total digestible nutrients increased 1.1% with mowing and 2.1% with fire. In vitro dry matter disappearance increased 2.2% with mowing and 6.7% with fire. Burned plots had greater in vitro fermentation than controls or mowed plots. Although mowing can be a useful management tool, it is not a substitute for the ecological effects of rangeland fire.

Invasion promotes invasion: Facilitation of C3 perennial grass dominance in mixed C3/C4 grassland by an invasive C3 woody sprouter (Prosopis glandulosa).

In the Southern Great Plains (SGP) of the United States, encroachment of the native invasive woody legume, honey mesquite (Prosopis glandulosa Torr.), has caused a decline in C4 mid‐grass abundance. Prosopis glandulosa invasion has also facilitated growth of the C3 mid‐grass species, Texas wintergrass (Nassella leucotricha [Trin & Rupr.] Pohl) initially beneath its canopy but extending to interspaces between P. glandulosa as stand density increases. Little is known about the stability of the Prosopis/Nassella association or C4 grass recovery following P. glandulosa disturbance.We quantified C3 and C4 grass production in interspaces, and basal cover in interspaces and P. glandulosa subcanopy microsites for 9 years following P. glandulosa suppression (top‐kill) and compared this to untreated P. glandulosa woodland (woodland).The Prosopis/Nassella association limited the window of C4 mid‐grass recovery to only a few years. Nassella leucotricha dominated grass production during the first 3 years after top‐kill. C4 mid‐grass recovery began in year 4, but was interrupted by severe drought in years 5 through 7. Recovery resumed in year 8, due to above‐average summer rainfall, but P. glandulosa regrowth was large enough by this time to limit C4 mid‐grass production to a third of its potential. Nassella leucotricha basal cover remained dominant and stable in woodland subcanopy microsites, even during drought, and only briefly declined in top‐kill subcanopy microsites before returning to pretreatment levels by year 8 as P. glandulosa regrowth increased and provided shade. Synthesis and applications. A single suppression event had little impact on disrupting the Prosopis/Nassella association and allowing C4 mid‐grass recovery. The coupling of a deciduous, N‐fixing C3 woody species with this C3 perennial grass may be a vegetative “state” that is resistant to multiple woody suppression disturbances and permanently limits the transition back to C4 grassland.

Stem elongation in Pennisetum purpureum results from a fixed pattern of vegetative development potentially enhanced by the initiation of flowering

AbstractThe characterization of stem elongation is of fundamental importance in C4 tropical grasses as it affects forage quality and determines optimal management practices. The objectives of this study were to analyse the determinants of stem elongation and leaf area production in shoots of Pennisetum purpureum cv. Napier (elephant grass) using unstressed isolated plants. Three experiments were conducted in Brazil during the spring, summer and autumn seasons. Regular measurements of leaf and pseudostem length were performed on the main and primary axes. Ten destructive measurements were also performed during each experiment to monitor apical meristem height, internode length and the number of initiated leaves. The onset of stem elongation occurred at the same vegetative stage (i.e., appearance of leaf 13) irrespective of the seasons and experiments. The first internode to elongate belonged to phytomer 8, and a constant lag of five phyllochrons was systematically observed between internode production and its rapid elongation period. Higher stem and internode elongation rates were observed during the reproductive phase (autumn) versus the vegetative phase (summer and spring group). Maximal internode length reached 8–10 cm in summer and spring and 20 cm in autumn, at approximately phytomers 12–13. A similar pattern was reported for all primary axes irrespective of the experiments, the position of the first internode to elongate descending regularly down the main axis. These results provide key elements to predict the onset of stem elongation in the field from simple measurements. They could contribute to improving crop models for perennial tropical C4 grasses.

Field Performance of Saccharum × Miscanthus Intergeneric Hybrids (Miscanes) Under Cool Climatic Conditions of Northern Japan

Physiological susceptibility to early- and late-season chilling limits commercial production of sugarcane (Saccharum spp. hybrid), a major crop for lignocellulosic biomass, refined sugar, and bioethanol, to tropical and the warmest subtropical regions. Interspecific and intergeneric hybridization have been used to broaden the genetic base of sugarcane and improve its adaptation to temperate climates. Chilling tolerance can be introgressed in sugarcane through intergeneric hybridization with Miscanthus, a cold-tolerant C4 perennial grass, which is genetically homologous to sugarcane. This study evaluated intergeneric F1 hybrids of Saccharum × Miscanthus, miscanes, which included two genotypes of sugarcane × Miscanthus sinensis and sixteen genotypes of sugarcane × Miscanthus sacchariflorus, for their seasonal variation in photosynthesis and biomass production under field conditions in Hokkaido, Japan, to identify promising genotypes and traits, which can be selected to further improve sugarcane. Results showed several of the miscane genotypes had high early- and late-season photosynthesis coupled with high biomass production, which likely indicates chilling tolerance. High broad-sense heritabilities for traits, including stem diameter, tiller number, leaf width, leaf and stem dry weight, and high correlations between these traits and dry matter yield indicate selections can be made efficiently to improve sugarcane. Although none of the miscanes overwintered at the experimental location, we identified miscane “JM 14-09” as a superior genotype for introgression breeding programs and as a potential energycane cultivar for its high biomass-production capacity.

Breeding Strategies to Improve Miscanthus as a Sustainable Source of Biomass for Bioenergy and Biorenewable Products

Miscanthus, a C4 perennial grass native to Eastern Asia, is being bred to provide biomass for bioenergy and biorenewable products. Commercial expansion with the clonal hybrid M. × giganteus is limited by low multiplication rates, high establishment costs and drought sensitivity. These limitations can be overcome by breeding more resilient Miscanthus hybrids propagated by seed. Naturally occurring fast growing indigenous Miscanthus species are found in diverse environments across Eastern Asia. The natural diversity provides for plant breeders, the genetic resources to improve yield, quality, and resilience for a wide range of climates and adverse abiotic stresses. The challenge for Miscanthus breeding is to harness the diversity through selections of outstanding wild types, parents, and progenies over a short time frame to deploy hybrids that make a significant contribution to a world less dependent on fossil resources. Here are described the strategies taken by the Miscanthus breeding programme at Aberystwyth, UK and its partners. The programme built up one of the largest Miscanthus germplasm collections outside Asia. We describe the initial strategies to exploit the available genetic diversity to develop varieties. We illustrate the success of combining diverse Miscanthus germplasm and the selection criteria applied across different environments to identify promising hybrids and to develop these into commercial varieties. We discuss the potential for molecular selections to streamline the breeding process.

Effects of fertilization, plant species, and intra-specific diversity on soil carbon and nitrogen in biofuel cropping systems after five growing seasons

Land-use change for bioenergy production can release greenhouse gases (GHG) through disturbance of soil carbon (C) pools, but use of native species with extensive root systems as bioenergy crops may help mitigate GHG emissions by enhancing soil C sequestration. Here, we investigated how (1) fertilization, (2) plant species and cultivars, and (3) inter- and intra-specific diversity affect soil C and N accumulation five growing seasons after conversion of an old-field dominated by C3 grasses to a grassland dominated by C4 perennial grasses managed for biofuel production. We manipulated diversity at both the species- and cultivar level, and applied nitrogen (N) at two levels (0 and 67 kg ha−1). Establishment of C4 grass treatments on soils that supported C3 pasture grasses for 36 years enabled us to use the natural abundance C isotope ratio technique to estimate the contribution of new C4 plant-derived C to soil organic matter pools. Our study yielded three main results: 1) annual fertilization did not significantly affect soil C and N concentrations after five growing seasons; 2) increasing inter- and intra-specific diversity did not significantly increase soil C and N concentrations; 3) cultivar- and species identity influenced C4-derived C and total soil C concentrations: big bluestem dominated stands exhibited greater soil C accrual relative to stands dominated by switchgrass and mixed-species treatments. Future research is needed to further assess how big bluestem can aid in the sustainable provisioning of second generation biofuel feedstocks.

Selection for Flowering Time as a Mechanism to Increase Biomass Yield of Upland Switchgrass

Switchgrass is a perennial C4 grass that is undergoing development as a dedicated biomass feedstock for conversion to bioenergy. Increasing biomass yield is the principal goal of most breeding programs. Delayed flowering time is one of the principal approaches toward achieving increased biomass yield. The objective of this study was to conduct and evaluate up to three cycles of phenotypic selection for altered flowering time in two populations of upland switchgrass, one tetraploid, and one octoploid. Selection was conducted in southern Wisconsin and evaluations were conducted at four locations ranging from 42 to 46° N latitude (USDA hardiness zones 4a to 5b). Realized heritability for anthesis date was high, approximately 0.7 in both populations, with selection responses averaging 5 days cycle−1. Biomass yield was strongly influenced by delayed flowering, averaging + 0.03 to 0.13 Mg ha−1 for each day delay in anthesis date. However, the biomass yield responses to delayed flowering were strongly associated with latitude of the evaluation location, decreasing by 75% as latitude increased from 42 to 46° N. There is likely a threshold latitude, above which the growing season is too short for later flowering to have a significant impact on increasing biomass yield of upland populations. Future increases in biomass yield for USDA hardiness zone 3 may be extremely difficult or impossible, or at least require different strategies.

Influence of double-cut harvest system on biomass yield, quality and biogas production from C4 perennial grasses

This study's aim was to determine the impact of two harvests per year on the yield, biomass quality, and suitability for ensiling and biogas production of cordgrass (Spartina pectinata Bosc ex Link), switchgrass (Panicum virgatum, var. Dacotah) and big bluestem (Andropogon gerardii, var. Bison). Grasses were harvested at the end of June (I harvest) and at the beginning of October (II harvest). Single harvest at the beginning of October was also conducted. Biomass from I and II harvest was ensiled and then subjected to anaerobic digestion. There were no differences between dry matter yield of investigated grasses from I and a single harvest, which indicated the validity of two harvests per year in order to increase biomass yield during a growing season. Double-cut harvest system had an effect on C/N ratio and lignin content decreasing in the biomass compared to a single harvest. Biomass from I and II harvest was suitable for ensiling. Double-cut harvest system did not influence on biogas yield from switchgrass and big bluestem. Cordgrass and big bluestem have been found to be better feedstock than switchgrass for biogas production in Polish climate conditions because they offered higher methane yield per hectare.

Biosynthesis and Emission of Stress-Induced Volatile Terpenes in Roots and Leaves of Switchgrass (Panicum virgatum L.).

Switchgrass (Panicum virgatum L.), a perennial C4 grass, represents an important species in natural and anthropogenic grasslands of North America. Its resilience to abiotic and biotic stress has made switchgrass a preferred bioenergy crop. However, little is known about the mechanisms of resistance of switchgrass against pathogens and herbivores. Volatile compounds such as terpenes have important activities in plant direct and indirect defense. Here, we show that switchgrass leaves emit blends of monoterpenes and sesquiterpenes upon feeding by the generalist insect herbivore Spodoptera frugiperda (fall armyworm) and in a systemic response to the treatment of roots with defense hormones. Belowground application of methyl jasmonate also induced the release of volatile terpenes from roots. To correlate the emission of terpenes with the expression and activity of their corresponding biosynthetic genes, we identified a gene family of 44 monoterpene and sesquiterpene synthases (mono- and sesqui-TPSs) of the type-a, type-b, type-g, and type-e subfamilies, of which 32 TPSs were found to be functionally active in vitro. The TPS genes are distributed over the K and N subgenomes with clusters occurring on several chromosomes. Synteny analysis revealed syntenic networks for approximately 30–40% of the switchgrass TPS genes in the genomes of Panicum hallii, Setaria italica, and Sorghum bicolor, suggesting shared TPS ancestry in the common progenitor of these grass lineages. Eighteen switchgrass TPS genes were substantially induced upon insect and hormone treatment and the enzymatic products of nine of these genes correlated with compounds of the induced volatile blends. In accordance with the emission of volatiles, TPS gene expression was induced systemically in response to belowground treatment, whereas this response was not observed upon aboveground feeding of S. frugiperda. Our results demonstrate complex above and belowground responses of induced volatile terpene metabolism in switchgrass and provide a framework for more detailed investigations of the function of terpenes in stress resistance in this monocot crop.

Glycosyl Hydrolases Producing Bacterial Endophytes from Perennial Grass Species (Neyraudia reynaudiana L.) for biomass deconstruction

Endophytes associated with C4 perennial grasses are a treasure trove of biocatalysts, and have emerged as a viable option for biofuel production. Studies were conducted to investigate the production of extracellular cellulases and xylanases from newly isolated Bacillus tequilensis BT5 and Alcaligenes faecalis B12 from perennial grass (Neyraudia reynaudiana L.). The total cellulase activity (FPase) for B. tequilensis BT5 and A. faecalis B12 were recorded as 0.61 and 0.34 FPU.ml-1 at 28 h and 32 h, respectively. Similarly, β-glucosidase, carboxymethyl cellulase (CMCase) and exoglucanase activities of BT5 and B12 were 1.12 and 0.94 U.ml-1; 0.92 and 0.86 U.ml-1 ; 1.98 and 1.58 U.ml-1, respectively. Xylanase activities of 1.58 U.ml-1and 1.22 U.ml-1 at 28 h and 36 h for B. tequilensis BT5 and A. faecalis B12, respectively were also recorded. Furthermore, the presence of cellulases and xylanases genes was confirmed by PCR screening using the gene-specific primers. This study revealed that the cellulases and xylanases from endophytic bacteria from perennial grasses could serve as a natural reservoir for biomass deconstruction and production of cellulose derived commodities.

Nitrogen Addition Increases the Sensitivity of Photosynthesis to Drought and Re-watering Differentially in C3 Versus C4 Grass Species.

Global change factors, such as variation in precipitation regimes and nitrogen (N) deposition, are likely to occur simultaneously and may have profound impacts on the relative abundance of grasses differing in functional traits, such as C3 and C4 species. We conducted an extreme drought and re-watering experiment to understand differences in the resistance and recovery abilities of C3 and C4 grasses under different N deposition scenarios. A C3 perennial grass (Leymus chinensis) and two C4 grasses (annual species Chloris virgata and perennial species Hemarthria altissima) that co-occur in Northeast China were selected as experimental plants. For both C3 and C4 grasses, N addition caused a strong increase in biomass and resulted in more severe drought stress, leading to a change in the dominant photosynthetic limitation during the drought periods. Although N addition increased antioxidant enzyme activities and protective solute concentrations, the carbon fixing capacity did not fully recover to pre-drought levels by the end of the re-watering period. N addition resulted in lower resilience under the drought conditions and lower resistance at the end of the re-watering. However, N addition led to faster recovery of photosynthesis, especially in the C3 grass, which indicate that the effect of N addition on photosynthesis during drought was asymmetric, especially in the plants with different photosynthetic nitrogen use efficiency (PNUE). These findings demonstrated that nitrogen deposition may significant alter the susceptibility of C3 and C4 grass species to drought stress and re-watering, highlighting the asymmetry between resistance and resilience and to improve our understanding about plant responses to climate change.

Root-zone hypoxia reduces growth of the tropical forage grass Urochloa humidicola in high-nutrient but not low-nutrient conditions.

The perennial C4 grass Urochloa humidicola is widely planted on infertile acidic and waterlogging-prone soils of tropical America. Waterlogging results in soil anoxia, and O2 deficiency can reduce nutrient uptake by roots. Interestingly, both nutrient deficiencies and soil waterlogging can enhance root cortical cell senescence, and the increased gas-filled porosity facilitates internal aeration of roots. We tested the influence of nutrient supply and root-zone O2 on root traits, leaf nutrient concentrations and growth of U. humidicola. Plants were grown in pots in a completely randomized design under aerated or stagnant deoxygenated hydroponic conditions and six nutrient regimes, with low to high concentrations of all essential elements, for 28 d in a controlled-temperature greenhouse. The standard acid solution (SAS) used was previously designed based on infertile acidic soils of the tropical America savannas, and step increases in the concentration of SAS were used in aerated or deoxygenated 0.1 % agar solution, which mimics changes in gas composition in waterlogged soils. Measurements included shoot and root growth, root porosity, root anatomy, radial O2 loss, and leaf tissue nutrient concentrations. Shoot dry mass was reduced for plants in stagnant compared with aerated conditions at high, but not at low, levels of mineral nutrition. In low-nutrition stagnant solution, roots were shorter, of greater porosity and had smaller radial thickness of the stele. Suberized lamellae and lignified sclerenchyma, as well as a strong barrier to radial O2 loss, were documented for roots from all treatments. Leaf nutrient concentrations of K, Mg and Ca (but not N, P and S) were higher in aerated than in stagnant conditions. Under low-nutrient conditions, plant growth in stagnant solution was equal to that in aerated solution, whereas under higher-nutrient regimes growth increased but dry mass in stagnant solution was less than in aerated solution. Slow growth in low-nutrient conditions limited any further response to the low O2 treatment, and greater porosity and smaller stele size in roots would enhance internal O2 movement within roots in the nutrient-limited stagnant conditions. A constitutive barrier to radial O2 loss and aerenchyma facilitates O2 movement to the tips of roots, which presumably contributes to maintaining nutrient uptake and the tolerance of U. humidicola to low O2 in the root-zone.

Linkage between leaf development and photosynthetic response at hyperosmotic salinity in the C-4 grass Panicum antidotale

This study aimed at elucidating the impact of salinity on the differential responses of the photosynthetic apparatus at different stages of leaf development in the C4 perennial grass Panicum antidotale. Plants were grown for a period of 15 days at hyper-osmotic salinity (up to 400 mM NaCl) or at control conditions (no addition of NaCl to the culture medium). Each day plants were harvested and salt stress effect on biomass production (fresh and dry weight) was recorded. In parallel leaves were divided from leaf tip to leaf base in five segments for analysis of stress effects on the following parameters: chlorophyll fluorescence parameters (Fv/Fm, ΦPSII, ETR), MDA (an indicator of peroxidation of membrane lipids), and ion leakage.A direct correlation between membrane lipid peroxidation and (i) membrane integrity as well as (ii) inhibition of PSII function was found under salinity. In monocots the leaf meristem is located at the base and mature parenchymatic cells preferentially are found at the leaf tip. This may explain our finding that the degree of salt mediated inhibition of photosynthetic activity (ETRcontrol/ETRstressed) was 20-fold higher at leaf tip than at leaf base. Thus, we discuss observed variance in apparent salt resistance in terms of heterogeneity of the photosynthetic apparatus (due to difference in spatial and temporal development) along the leaf lamina.

Cultivation of C4 perennial energy grasses on heavy metal contaminated arable land: Impact on soil, biomass, and photosynthetic traits

The objective of this study was to evaluate the potential of three C4 perennial grasses (Miscanthus x giganteus, Panicum virgatum and Spartina pectinata) for biomass production on arable land unsuitable for food crop cultivation due to Pb, Cd and Zn contamination. We assessed soil properties, biomass yield, metal concentrations, and the photosynthetic performance of each species. Physico-chemical and elemental analyses were performed on soil samples before plantation establishment (2014) and after three years of cultivation (2016), when leaf area index, plant height, yield and heavy metal content of biomass were also determined. Physiological measurements (gas exchange, pigment content, chlorophyll a fluorescence) were recorded monthly between June and September on mature plants in 2016. Cultivation of investigated plants resulted in increased pH, nitrogen, and organic matter (OM) content in soil, although OM increase (13%) was significant only for S. pectinata plots. During the most productive months, maximal quantum yield values of primary photochemistry (Fv/Fm) and gas exchange parameter values reflected literature data of those plants grown on uncontaminated sites. Biomass yields of M. x giganteus (15.0 ± 0.4 t d.m. ha−1) and S. pectinata (12.6 ± 1.2 t d.m. ha−1) were also equivalent to data published from uncontaminated land. P. virgatum performed poorly (4.1 ± 0.4 t d.m. ha−1), probably due to unfavourable climatic conditions, although metal uptake in this species was the highest (3.6 times that of M. x giganteus for Pb). Yield and physiological measurements indicated that M. x giganteus and S. pectinata were unaffected by the levels of contamination and therefore offer alternatives for areas where food production is prohibited. The broad cultivatable latitudinal range of these species suggests these results are widely relevant for development of the bioeconomy. We recommend multi-location trials under diverse contaminant and environmental regimes to determine the full potential of these species.