C4 Tropical Grass Research Articles

Using Multi‐Homolog Plant‐Wax Carbon Isotope Compositions to Reconstruct Tropical Vegetation Types

AbstractThe stable carbon isotope composition (δ13C) of plant components such as plant wax biomarkers is an important tool for reconstructing past vegetation. Plant wax δ13C is mainly controlled by photosynthetic pathways, allowing for the differentiation of C4 tropical grasses and C3 forests. Proxy interpretations are however complicated by additional factors such as aridity, vegetation density, elevation, and the considerable δ13C variability found among C3 plant species. Moreover, studies on plant wax δ13C in tropical soils and plants have focused on Africa, while structurally different South American savannas, shrublands and rainforests remain understudied. Here, we analyze the δ13C composition of long‐chain n‐alkanes and fatty acids from tropical South American soils and leaf litter to assess the isotopic variability in each vegetation type and to investigate the influence of climatic features on δ13C. Rainforests and open vegetation types show distinct values, with rainforests having a narrow range of low δ13C values (n‐C29 alkane: ‰ ; Suess‐effect corrected). This allows for the detection of even minor incursions of savanna (δ13C n‐C29 alkane ‰) into rainforests. While Cerrado savannas and semi‐arid Caatinga shrublands grow under distinctly different climates, they can yield indistinct δ13C values for most compounds. Cerrado soils and litter show pronounced isotopic spreads between the n‐C33 and n‐C29 alkanes, while Caatinga shrublands show consistent values across the two homologs, thereby enabling the differentiation of these vegetation types. The same multi‐homolog isotope analysis can be extended to differentiate African shrublands from savannas.

Heteropogon‐Themeda grasses evolve to occupy either tropical grassland or wetland biomes

AbstractSpecies of the Heteropogon‐Themeda clade are ecologically important grasses distributed across the tropics, including widespread species, such as the pantropical Heteropogon contortus and Themeda triandra, and range‐restricted species such as Heteropogon ritchiei and Themeda anathera. Here, we examine habitat preferences of the grassland/savanna and wetland species by describing bioclimatic niche characteristics, characterizing functional traits, and investigating the evolution of functional traits of 31 species in the Heteropogon‐Themeda clade in relation to precipitation and temperature. The climatic limits of the clade are linked to mean annual precipitation and seasonality that also distinguish seven wetland species from 24 grassland/savanna species. Tests of niche equivalency highlighted the unique bioclimatic niche of the wetland species. However, climatic factors do not fully explain species geographic range, and other factors are likely to contribute to their distribution ranges. Trait analyses demonstrated that the wetland and grassland/savanna species were separated by culm height, leaf length, leaf area, awn length, and awn types. Phylogenetic analyses showed that the wetland species had tall stature with long and large leaves and lack of hygroscopic awns, which suggest selective pressures in the shift between savanna/grassland and wetland. The two most widespread species, H. contortus and T. triandra, have significantly different bioclimatic niches, but we also found that climatic niche alone does not explain the current geographic distributions of H. contortus and T. triandra. Our study provides a new understanding of the biogeography and evolutionary history of an ecologically important clade of C4 tropical grasses.

Citrus Pulp Replacing Corn in the Supplement Decreased Fibre Digestibility with No Impacts on Performance of Cattle Grazing Marandu Palisade Grass in the Wet-Dry Transition Period.

Simple SummaryDeterioration of forage quality as the dry season approaches has detrimental effects on the performance of cattle grazing tropical grasses unless supplementation strategies are implemented. In this study, corn was replaced by citrus pulp as an energy source to evaluate the effects of supplementation on liveweight performance and metabolic parameters of Bos indicus cattle grazing Urochloa brizantha cv. Marandu in the wet-dry transition period.The wet-dry transition period brings unique challenges to cattle producers in the tropics as the forage quality starts to decrease and animal performance is negatively affected unless supplementation strategies are applied. Two experiments were conducted concomitantly to evaluate the performance and metabolic parameters of cattle supplemented with two different energy sources under a C4 tropical grass continuous grazing system in the wet-dry transition period. In experiment (exp) 1, the liveweight (LW) gain of 42 growing Bos indicus Nellore bulls allocated to 12 paddocks of Uruchloa brizantha cv. Marandu, in a completely randomized block design, was evaluated to compare corn and citrus pulp-based supplements offered at 0.5% LW. Metabolism was evaluated in exp 2 with eight rumen-cannulated Nellore steers in an incomplete replicated Latin square design (RLSD) 4 × 2 (steers × treatment) fed the same two treatments as in exp 1. No differences in animal performance were observed between corn or citrus pulp-based supplements. Rumen pH, ammonia nitrogen, and blood urea nitrogen were lower in the rumen of animals supplemented with citrus pulp. Despite this, there were no differences in total dry matter intake amongst the two energy sources. The neutral detergent fibre digestibility of the diet containing corn decreased, but it neither affected performance nor liveweight gain per area. The findings suggest that corn can be replaced by citrus pulp with neither detrimental effects on animal performance nor indirect effects on pastures productivity.

Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights.

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane δ13CCH4 isotopic signatures were in the range −55 to −49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely −60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ13CCH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around −59‰) and the overall δ13CCH4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was −59 ± 2‰. These results were more negative than expected. For African cattle, δ13CCH4 values were around −60 to −50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, δ13CCH4 values were around −28‰. By contrast, African C4 tropical grass fire δ13CCH4 values were −16 to −12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ13CCH4 around −37 to −36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ13CCH4 values predicted by global atmospheric models are highly sensitive to the δ13CCH4 isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa.This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Is a C4 tropical grass still an option in the revegetation of iron ore tailings in face of climate change?

Is a C4 tropical grass still an option in the revegetation of iron ore tailings in face of climate change?

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.

Nutritive Value and Enteric Methane Production of Brachiaria spp. Under Elevated [CO2

Despite tropical grasses being the main feed source for major part of ruminant livestock, information about nutritional quality changes under climate changes scenario are still scarce. The novelty of this work was to determine the effects of season and elevated CO2 under field conditions upon biomass production and nutritional value of Brachiaria decumbens during 2 years. The Free-Air Carbon Dioxide Enrichment was established in twelve rings: six of them being the control (current atmosphere) and others six fumigated with pure CO2 to achieve a higher concentration (e[CO2]). In each ring, two 0.25 m2 plots have been established with B. decumbens and after standardization cut, green forage yield was evaluated frequently. Plant samples were collected at 20 cm height and evaluated for determining the biomass production, nutritive value and in vitro enteric methane (CH4) production. Season and year had significant effect (P 0.05). Overall, e[CO2] and rainfall seasons significantly increased forage production, contributing to sequester carbon in plant biomass, but this C4 tropical grass must be grazed at its best protein and fermentable carbohydrate ratio for advantages in the rumen microbial synthesis and lowering CH4 production intensity of the production system.

Warming and water deficit impact leaf photosynthesis and decrease forage quality and digestibility of a C4 tropical grass.

Global warming is predicted to cause more intense extreme events such as heat waves, flooding and severe droughts, producing significant effects on agriculture. In tropics, climate change will severely impact livestock production affecting water availability, forage quality and food for cattle. We investigated the isolated and combined effects of soil water deficit (wS) and + 2°C increase in canopy temperature (eT) on leaf gas exchange, chlorophyll fluorescence, carbohydrate content, forage quality and in vitro dry matter digestibility (IVDMD) of a field-grown C4 tropical forage grass Panicum maximum Jacq. using a temperature-free air-controlled enhancement (T-FACE) system. The wS and eT treatments showed no effects on photosystem II photochemistry. However, wS under ambient temperature decreased net photosynthesis rate (A), stomatal conductance (gs ) and maximum rate of carboxylation of Rubisco (Vcmax ), leading to a reduced starch content in leaves. A 16% reduction in leaf dry mass (LDM) and reduction in forage quality by increasing fibers, reducing crude protein (CP) and decreasing the IVDMD was also observed by effect of wS. Warming under adequate soil moisture (eT) significantly increased LDM by 25% but reduced the forage quality, increasing the lignin content and reducing starch, CP and digestibility. The combined wSeT treatment reduced A, gs , Vcmax and the forage quality. When compared to control, the lignin content in leaves increased by 43, 28 and 17% in wS, eT and wSeT, respectively, causing a significant reduction in IVDMD. We concluded that despite physiological mechanisms to acclimate to warming, both warming and water deficit will impair the quality and digestibility of C4 tropical pastures.

Aerial Imagery Analysis - Quantifying Appearance and Number of Sorghum Heads for Applications in Breeding and Agronomy.

Sorghum (Sorghum bicolor L. Moench) is a C4 tropical grass that plays an essential role in providing nutrition to humans and livestock, particularly in marginal rainfall environments. The timing of head development and the number of heads per unit area are key adaptation traits to consider in agronomy and breeding but are time consuming and labor intensive to measure. We propose a two-step machine-based image processing method to detect and count the number of heads from high-resolution images captured by unmanned aerial vehicles (UAVs) in a breeding trial. To demonstrate the performance of the proposed method, 52 images were manually labeled; the precision and recall of head detection were 0.87 and 0.98, respectively, and the coefficient of determination (R2) between the manual and new methods of counting was 0.84. To verify the utility of the method in breeding programs, a geolocation-based plot segmentation method was applied to pre-processed ortho-mosaic images to extract >1000 plots from original RGB images. Forty of these plots were randomly selected and labeled manually; the precision and recall of detection were 0.82 and 0.98, respectively, and the coefficient of determination between manual and algorithm counting was 0.56, with the major source of error being related to the morphology of plants resulting in heads being displayed both within and outside the plot in which the plants were sown, i.e., being allocated to a neighboring plot. Finally, the potential applications in yield estimation from UAV-based imagery from agronomy experiments and scouting of production fields are also discussed.

Effect of fibrolytic enzymes added to a Andropogon gayanus grass silage-concentrate diet on rumen fermentation in batch cultures and the artificial rumen (Rusitec)

In vitro batch cultures were used to screen four fibrolytic enzyme mixtures at two dosages added to a 60 : 40 silage : concentrate diet containing the C4 tropical grass Andropogon gayanus grass ensiled at two maturities – vegetative stage (VS) and flowering stage (FS). Based on these studies, one enzyme mixture was selected to treat the same diets and evaluate its impact on fermentation using an artificial rumen (Rusitec). In vitro batch cultures were conducted as a completely randomized design with two runs, four replicates per run and 12 treatments in a factorial arrangement (four enzyme mixtures×three doses). Enzyme additives (E1, E2, E3 and E4) were commercial products and contained a range of endoglucanase, exoglucanase and xylanase activities. Enzymes were added to the complete diet 2 h before incubation at 0, 2 and 4 μl/g of dry matter (DM). Gas production (GP) was measured after 3, 6, 12, 24 and 48 h of incubation. Disappearance of DM (DMD), NDF (NDFD) and ADF (ADFD) were determined after 24 and 48 h. For all four enzyme mixtures, a dosage effect (P<0.05) was observed for NDFD and ADFD after 24 h and for DMD, NDFD and ADFD after 48 h of incubation of the VS diet. For the FS diet, a dosage effect was observed for GP and NDFD after 24 h and for GP, DMD, NDFD and ADFD after 48 h of incubation. There was no difference among enzyme mixtures nor was there an enzyme×dose interaction for the studied parameters. Because of the greatest numerical effect on NDF disappearance and the least cost price, enzyme mixture E2 at 4 µl/g of diet DM was selected for the Rusitec experiment. The enzyme did not impact (P>0.05) DM, N, NDF or ADF disappearance after 48 h of incubation nor daily ammonia-N, volatile fatty acids or CH4 production. However, enzyme application increased (P<0.05) microbial N production in feed particle-associated (loosely-associated) and silage feed particle-bound (firmly associated) fractions. With A. gayanus silage diets, degradation may not be limited by microbial colonization, but rather by the ability of fibrolytic enzymes to degrade plant cell walls within this recalcitrant forage.

13Carbon Isotope Discrimination in Roots and Shoots of Major Weed Species of Southern U.S. Rice Fields and Its Potential Use for Analysis of Rice–Weed Root Interactions

Assessing belowground plant interference in rice has been difficult in the past because intertwined weed and crop roots cannot be readily separated. A 13C discrimination method has been developed to assess distribution of intermixed roots of barnyardgrass and rice in field soils, but the suitability of this approach for other rice weeds is not known. 13C depletion levels in roots and leaves of rice were compared with those of 10 troublesome weed species grown in monoculture in the greenhouse or field. Included were C4 tropical grasses: barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleaf signalgrass, fall panicum, and large crabgrass; C4 sedge, yellow nutsedge; and C3 species: red rice, gooseweed, and redstem. Rice root δ13C levels averaged ∼ −28‰, indicating that these roots are highly 13C-depleted. Root δ13C levels ranged from −12‰ to −17‰ among the tropical grasses, and were −10‰ in yellow nutsedge, indicating that these species were less 13C depleted than rice, and were C4 plants suitable for 13C discrimination studies with rice. Among the C4 species, bearded sprangletop and yellow nutsedge were most and least 13C depleted, respectively. δ13C levels in shoot and root tissue of pot-grown plants averaged 6% greater for C4 plants and 9% greater for rice in the field than in the greenhouse. In pots, shoots of rice typically were slightly more 13C depleted than roots. A reverse trend was seen in most C4 species, particularly for broadleaf signalgrass and plants sampled from field plots. Corrections derived from inputs including the total mass, carbon mass, carbon fraction, and δ13C levels of roots and soil increased greatly the accuracy of root mass estimates and increased slightly the accuracy of root δ13C estimates (∼ 0.6 to 0.9%) in samples containing soil. Similar corrective equations were derived for mixtures of rice and C4 weed roots and soil, and are proposed as a labor-saving option in 13C discrimination root studies.

Differences between the in vitro digestibility of extrusa collected from oesophageal fistulated steers and the forage consumed

In a study that included C4 tropical grasses, C3 temperate grasses and C3 pasture legumes, in vitro dry matter digestibility of extrusa, measured as in vitro dry matter loss (IVDML) during incubation, compared with that of the forage consumed, was greater for grass extrusa but not for legume extrusa. The increase in digestibility was not caused by mastication or by the freezing of extrusa samples during storage but by the action of saliva. Comparable increases in IVDML were achieved merely by mixing bovine saliva with ground forage samples. Differences were greater than could be explained by increases due to completely digestible salivary DM. There was no significant difference between animals in relation to the saliva effect on IVDML and, except for some minor differences, similar saliva effects on IVDML were measured using either the pepsin–cellulase or rumen fluid–pepsin in vitro techniques. For both C4 and C3 grasses the magnitude of the differences were inversely related to IVDML of the feed and there was little or no difference between extrusa and feed at high digestibilities (&gt;70%) whereas differences of more than 10 percentage units were measured on low quality grass forages. The data did not suggest that the extrusa or saliva effect on digestibility was different for C3 grasses than for C4 grasses but data on C3 grasses were limited to few species and to high digestibility samples. For legume forages there was no saliva effect when the pepsin–cellulase method was used but there was a small but significant positive effect using the rumen fluid–pepsin method. It was concluded that when samples of extrusa are analysed using in vitro techniques, predicted in vivo digestibility of the feed consumed will often be overestimated, especially for low quality grass diets. The implications of overestimating in vivo digestibility and suggestions for overcoming such errors are discussed.

The sensitivity of photosynthesis to phosphorus deficiency differs between C3 and C4 tropical grasses.

Phosphorus (P) is an important determinant of plant productivity, particularly in the tropical grasslands of Australia, which contain both C3 and C4 species. Few studies have compared the responses of such species to P deficiency. Previous work led us to hypothesise that C3 photosynthesis and the three subtypes of C4 photosynthesis have different sensitivities to P deficiency. To examine their dynamic response to P deficiency in more detail, four taxonomically related tropical grasses (Panicum laxum (C3) and Panicum coloratum, Cenchrus ciliaris and Panicum maximum belonging to the C4 subtypes NAD-ME, NADP-ME and PCK, respectively) were grown under contrasting P supplies, including P withdrawal from the growing medium. Changes in photosynthesis and growth were compared with leaf carbohydrate contents and metabolic fingerprints obtained using high-resolution proton nuclear magnetic resonance (1H-NMR). The response of CO2 assimilation rates to leaf contents of inorganic phosphate ([Pi]) was linear in the C3 grass, but asymptotic for the three C4 grasses. Relative growth rate was affected most by low P in the C3 species and was correlated with the leaf content of glucose 6-phosphate more than with carbohydrates. Principal component analysis of the 1H-NMR spectra revealed distinctive profiles of carbohydrates and amino acids for the four species. Overall, the data showed that photosynthesis of the three C4 subtypes behaved similarly. Compared with the C3 counterpart, photosynthesis of the three C4 grasses had a higher P use efficiency and lower Pi requirement, and responded to a narrower range of [Pi]. Although each of the four grass species showed distinctive 1H-NMR fingerprints, there were no differences in response that could be attributed to the C4 subtypes.

Ruminal protein degradability of a range of tropical pastures

The rumen degradability parameters of the diet selected by two to four oesophageal-fistulated Brahman steers grazing a range of tropical pastures were determined by incubation of extrusa in nylon bags suspended in the rumen of rumen-fistulated (RF) Brahman steers. The effective protein degradability (Edg) was determined by measuring the rate of disappearance of neutral detergent insoluble nitrogen (NDIN) less acid detergent insoluble nitrogen (ADIN) in the incubated extrusa. Six to eight RF steers also grazed each of the pastures along with the oesophageal-fistulated steers, to allow determination of key rumen parameters and rumen particulate matter fractional outflow rates (FOR). The seven pastures studied included: native tropical grass (C4) pasture (major species Heteropogon contortus and Bothriochloa bladhii), studied in the early wet (NPEW), the wet/dry transition (NPT) and the dry (NPD) seasons; introduced tropical grass (C4) pasture (Bothriochloa insculpta), studied in the mid wet season (BB); the introduced tropical legumes (C3), Lablab purpureus (LL) and Clitoria ternatea (BP); and the temperate grass (C3) pasture, ryegrass (Lolium multiflorum, RG). Using the measured particle FOR values in calculations, the Edg estimates were very high for both C4 and C3 species: 0.82–0.91 and 0.95–0.98 g/g crude protein (CP), respectively. Substitution of an assumed FOR (kp = 0.02/h) for the measured values for each pasture type did not markedly affect estimates of Edg. However, C4 tropical grasses had much lower effective rumen degradable protein (ERDP) fractions (23–66 g/kg DM) than the C3 pasture species RG and LL (356 and 243 g/kg DM, respectively). This was associated with a lower potential degradability and degradation rate of organic matter (OM) in sacco, lower in vitro organic matter digestibility (IVOMD) and CP concentrations in the extrusa, and lower ammonia-N and branched-chain fatty acid concentrations in rumen fluid for the tropical grasses. As tropical grass pastures senesced, there was a decline in Edg, the ERDP and rumen undegradable protein (UDP) fractions, the potential degradability and degradation rate of OM and the IVOMD. These results provide useful data for estimating protein supply to cattle grazing tropical pastures.

Histochemical Localization of Citral Accumulation in Lemongrass Leaves (Cymbopogon citratus(DC.) Stapf., Poaceae)

Lemongrasses (Cymbopogonspp., Poaceae) are a group of commercially important C4tropical grasses. Their leaves contain up to 1.5% (d.wt) essential oils with a typical lemon-like aroma, consisting mainly of citral (a mixture of the isomeric acyclic monoterpene aldehydes geranial and neral). To specifically locate the sites of citral accumulation in lemongrass we employed Schiff's reagent, which reacts with aldehydes and gives a purple-red coloration with citral. Using this technique, single oil-accumulating cells were detected in the adaxial side of leaf mesophyll, commonly adjacent to non-photosynthetic tissue, and between vascular bundles. Cell walls of these oil cells are lignified. Our results suggest that citral accumulation takes place in individual oil cells within the leaf tissues.

Effect of shade on the growth and mineral nutrition of a C4 perennial grass under field conditions

The effect of shading by a shrub legume on the growth and nutrient uptake of a C4 tropical grass was studied during four regrowth cycles. Regrowth periods were characterised by contrasting soil water availability. Dichanthium aristatum (Poir.) C. E. Hubbard swards were grown in full sun and under Gliricidia sepium (Jacq.) Walp. and Leucaena leucocephala (Lam.) de Wit with a light transmission level ranging from 80 to 30% of the incoming photosynthetically active radiation (PAR), depending on shrub regrowth. A treatment with high N and water supply was included in one of the cycles to quantify the effect of shade alone on potential growth.