Belowground carbon allocation, root trait plasticity, and productivity during drought and warming in a pasture grass

Sustaining grassland production in a changing climate requires an understanding of plant adaptation strategies, including trait plasticity under warmer and drier conditions. However, our knowledge to date disproportionately relies on aboveground responses, despite the importance of belowground traits in maintaining aboveground growth, especially in grazed systems. We subjected a perennial pasture grass, Festuca arundinacea, to year-round warming (+3 °C) and cool-season drought (60% rainfall reduction) in a factorial field experiment to test the hypotheses that: (i) drought and warming increase carbon allocation belowground and shift root traits towards greater resource acquisition and (ii) increased belowground carbon reserves support post-drought aboveground recovery. Drought and warming reduced plant production and biomass allocation belowground. Drought increased specific root length and reduced root diameter in warmed plots but increased root starch concentrations under ambient temperature. Higher diameter and soluble sugar concentrations of roots and starch storage in crowns explained aboveground production under climate extremes. However, the lack of association between post-drought aboveground biomass and belowground carbon and nitrogen reserves contrasted with our predictions. These findings demonstrate that root trait plasticity and belowground carbon reserves play a key role in aboveground production during climate stress, helping predict pasture responses and inform management decisions under future climates.

Drought and warming alter gross primary production allocation and reduce productivity in a widespread pasture grass

Carbon allocation determines plant growth, fitness and reproductive success. However, climate warming and drought impacts on carbon allocation patterns in grasses are not well known, particularly following grazing or clipping. A widespread C₃ pasture grass, Festuca arundinacea, was grown at 26 and 30°C in controlled environment chambers and subjected to drought (65% reduction relative to well-watered controls). Leaf, root and whole-plant carbon fluxes were measured and linked to growth before and after clipping. Both drought and warming reduced gross primary production and plant biomass. Drought reduced net leaf photosynthesis but increased the leaf respiratory fraction of assimilated carbon. Warming increased root respiration but did not affect either net leaf photosynthesis or leaf respiration. There was no evidence of thermal acclimation. Moreover, root respiratory carbon loss was amplified in the combined drought and warming treatment and, in addition to a negative carbon balance aboveground, explained an enhanced reduction in plant biomass. Plant regrowth following clipping was strongly suppressed by drought, reflecting increased tiller mortality and exacerbated respiratory carbon loss. These findings emphasize the importance of considering carbon allocation patterns in response to grazing or clipping and interactions with climatic factors for sustainable pasture production in a future climate.

Unexpected Intraspecific Variability of Perennial Ryegrass (Lolium perenne L.) in Response to Constant Temperature During Germination and Initial Heterotrophic Growth

Lolium perenne (L.) is one of the most important species in temperate grasslands. Temperature is a major factor controlling plant development. Breeding L. perenne cultivars adapted to new ranges of temperature could be necessary under most climate change scenarios. However, before any breeding effort in such direction, knowing the intraspecific variability of responses to temperature is essential. Our objective was to analyze a sample of accessions of L. perenne for their response to constant temperature during germination and initial heterotrophic growth. Eight accessions were taken out from a genebank containing 160 accessions. Six accessions were wild populations collected in different places in France and the other two were populations from a selection program. For germination, four replicates of 100 seeds per accession were tested in Petri dishes in the dark at eight constant temperatures, from 5 to 40°C with increases of 5°C. Germination countings were carried out at variable time intervals and durations that depended on treatments. For initial heterotrophic growth analysis, seeds were germinated at 25°C. Sets of 30 seedlings per accession were placed in polypropylene boxes on blotter blue paper and transferred to each one of the eight treatments in the dark. They were pictured at variable time intervals for root and shoot growth measurement by image analysis. Neither seed germination nor heterotrophic growth was observed at 40°C, for any accession. Temperature and time course response surfaces were markedly different between accessions. Equally, maximum germinability and the shape of the response curves to temperature were significantly different between accessions. This means that limited similarities between responses were observed. Furthermore, germination rates followed the non-linear beta function with significant differences between some accessions. These also showed significant differences in their root and shoot growth rate in response to temperature. In general, the relative growth rates of roots and shoots were slow at 5°C, peaked between 25 and 30°C, and showed a sharp reduction afterward. These results reveal, for the first time, high genetic variability within L. perenne germplasm for the response to temperature in the initial life phases. This discovered variability should serve breeders to create perennial ryegrass varieties for the future.

Vacuolar Sequestration of Paraquat Is Involved in the Resistance Mechanism in Lolium perenne L. spp. multiflorum

Lolium perenne L. spp. multiflorum (Lam.) Husnot (LOLMU) is a winter annual weed, common to row crops, orchards and roadsides. Glyphosate-resistant populations of LOLMU are widespread in California. In many situations, growers have switched to paraquat or other postemergence herbicides to manage glyphosate-resistant LOLMU populations. Recently, poor control of LOLMU with paraquat was reported in a prune orchard in California where paraquat has been used several times. We hypothesize that the low efficacy observed is due to the selection of a paraquat-resistant biotype of LOLMU. Greenhouse dose-response experiments conducted with a susceptible (S) and the putative paraquat-resistant biotype (PRHC) confirmed paraquat resistance in PRHC. Herbicide absorption studies indicated that paraquat is absorbed faster in S than PRHC, although the maximum absorption estimates were similar for the two biotypes. Conversely, translocation of 14C-paraquat under light-manipulated conditions was restricted to the treated leaf of PRHC, whereas herbicide translocation out of the treated leaf was nearly 20 times greater in S. To determine whether paraquat was active within the plant cells, the photosynthetic performance was assessed after paraquat application using the parameter maximum quantum yield of photosystem II (Fv/Fm). Paraquat reaches the chloroplasts of PRHC, since there was a transitory inhibition of photosynthetic activity in PRHC leaves. However, PRHC Fv/Fm recovered to initial levels by 48 h after paraquat treatment. No paraquat metabolites were found, indicating that resistance is not due to paraquat degradation. LOLMU leaf segments were exposed to paraquat following pretreatments with inhibitors of plasma membrane- and tonoplast-localized transporter systems to selectively block paraquat intracellular movement. Subsequent evaluation of membrane integrity indicated that pre-exposure to putrescine resulted in the resistant biotype responding to paraquat similarly to S. These results strongly indicate that vacuolar sequestration is involved in the resistance to paraquat in this population of LOLMU.

Atmospheric vapor pressure deficit is critical in predicting growth response of "cool-season" grass Festuca arundinacea to temperature change

There is a lack of information on plant response to multifactor environmental variability including the interactive response to temperature and atmospheric humidity. These two factors are almost always confounded because saturated vapor pressure increases exponentially with temperature, and vapor pressure deficit (VPD) could have a large impact on plant growth. In this study using climate controlled mini-greenhouses, we examined the interacting influence of temperature and VPD on long-term growth of tall fescue (Festuca arundinacea Schreb), a cool season grass. From past studies it was expected that growth of tall fescue would decline with warmer temperatures over the range of 18.5-27 degrees C, but growth actually increased markedly with increasing temperature when VPD was held constant. In contrast, growth declined in experiments where tall fescue was exposed to increasing VPD and temperature was held constant at 21 degrees C. The inhibited growth appears to be in response to a maximum transpiration rate that can be supported by the tall fescue plants. The sensitivity to VPD indicates that if VPD remains stable in future climates as it has in the past, growth of tall fescue could well be stimulated rather than decreased by global warming in temperate climate zones.

Responses to the renovation of an irrigated perennial pasture in northern Victoria. 1. Pasture consumption and nutritive characteristics

A field experiment was established in northern Victoria in the autumn of 1999 to quantify the effects of renovating a 15-year-old irrigated perennial pasture which had a high paspalum content [>40% dry matter (DM)] in summer. The treatments were: (i) control, the existing pasture; (ii) oversown, in which the existing pasture was grazed, topped and direct drilled; and (iii) resown, in which the existing pasture was sprayed, cultivated and sown with a new pasture. The grass species used in both renovation treatments were perennial ryegrass, Italian ryegrass and tall fescue. The treatments were grazed by dairy cows when the perennial ryegrass had reached the 2.5–3 leaf stage. Grazing of the resown tall fescue coincided with the resown ryegrass in years 1 and 2, but in subsequent years, resown tall fescue was grazed at a rising plate meter height of 80 mm. All treatments were grazed to a residual pasture height of 40–45 mm, as measured with a rising plate meter. Pasture consumption (measured as DM removed by dairy cows), in vitro DM digestibility (in vitro DMD) and crude protein (CP) contents were measured. Oversowing increased pasture consumption over the 4-year period, compared with the control, by an average of 1.1 t DM/ha.year when oversown once with perennial ryegrass and by 1.6 t DM/ha.year when oversown annually with Italian ryegrass. This increase occurred principally during the winter–spring period for pastures oversown with both perennial (0.7 t DM/ha) and Italian (1.6 t DM/ha) ryegrass. Oversowing with perennial or Italian ryegrass did not affect the in vitro DMD or CP content of the pasture on offer. These results show that oversowing with either perennial or Italian ryegrass is a viable means of increasing pasture availability over winter and spring from perennial pastures consisting of a mixture of perennial ryegrass, white clover and paspalum. Pasture consumption in the first 12 months after resowing was 3.5–4.1 t DM/ha lower from the resown than from the control pasture. This was because of two fewer grazings in autumn–winter and to reductions in pasture consumption of 20% in spring and of 40% in summer. These reductions would add considerably to the cost of resowing through increasing the need for supplementary feeding. Pasture consumption from the resown perennial ryegrass pasture in years 2–4 was, on average, the same as the control, although it was higher during winter and spring and lower during summer. Pasture consumption from the resown tall fescue pasture in years 2–4 was, on average, 2.5 t DM/ha.year higher than that of the resown perennial ryegrass pasture, with most of this increase occurring in summer and autumn. The resown pastures had higher in vitro DMD and CP contents than the control with little difference between the resown perennial ryegrass and tall fescue pastures. These findings show that tall fescue is a viable alternative to perennial ryegrass when resowing pastures. The use of nitrogen fertiliser did not affect the in vitro DMD or CP contents of the pasture on offer but allowed an increase in DM consumption, with this increase being greater for the control and oversown pastures than for the resown pasture.