The response of white clover (Trifolium repens L.) seedlings to spring root temperatures: The relative roles of the plant and the Rhizobium bacteria

Three experiments are reported which examine the relative roles of host and Rhizobium genotypes as factors limiting clover (Trifolium repens L.) growth at low soil temperatures.In the first experiment un-nodulated clover and perennial ryegrass (Lolium perenne L.) were grown with non-limiting nitrate at root temperatures of 8, 10 and 12°C. The ryegrass had substantially better relative growth rates (RGR) than the clover with the biggest difference occurring at 8°C. Alterations in growth rate with temperature were more marked in clover than in ryegrass but the latter still produced several times more dry matter than clover at each temperature.In the subsequent experiments clover nodulated with different strains of rhizobia was grown with and without non-limiting additions of nitrate at root temperatures of 9, 12 and 15°C. Plants receiving nitrate generally produced more dry matter than those dependent upon Rhizobium for nitrogen but differences in yield between these treatments did not alter with temperature. This suggests that limitations imposed by nitrogen fixation are similar at both high and low temperatures. Indeed, there was some evidence that nitrogen limitations were rather more pronounced at the highest temperature. The first experiment clearly demonstrated that the clover genotype makes particularly poor use of nitrate at low root temperatures when compared to its common companion perennial ryegrass.It can be concluded that improvements in spring growth of clover will rest largely with alterations to the plant genotype and its ability to use combined nitrogen for growth at lower temperatures rather than with changes in rhizobia or any symbiotic characters.     

Vegetation control effects on untreated wood,crude cellulose and holocellulose δ<Superscript>13</Superscript>C of early and latewood in 3- to 5-year-old rings of Douglas-fir

The stable carbon (C) composition of tree rings expressed as δ¹³C, is a measure of intrinsic water-use efficiency and can indicate the occurrence of past water shortages for tree growth. We examined δ¹³C in 3- to 5-year-old rings of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) trees to elucidate if decreased water supply or uptake was a critical factor in the observed growth reduction of trees competing with understory herb and shrub vegetation compared to those growing without competition. We hypothesized that there would be no differences in δ¹³C of earlywood in trees growing in plots with competing vegetation and those in plots receiving complete vegetation control during 5 years because earlywood formed early in the growing season when soil water was ample. We also hypothesized that δ¹³C in latewood which was formed during the later half of the growing season when precipitation was low, would be greater (less negative) in trees in plots without vegetation control. We then separated early and latewood from rings for three consecutive years and analyzed their δ¹³C composition. No significant differences in earlywood δ¹³C in years 3–5 were observed for trees in the two vegetation control treatments. δ¹³C of untreated latewood separated from wood cores was greater in 4- and 5-year-old rings of trees growing with competing vegetation compared to trees growing without vegetation competition (i.e., −25.5 vs. −26.3‰ for year 4, and −26.1 vs. −26.8‰ for year 5). Results suggest that water shortages occurred in Douglas-fir trees on this coastal Washington site in the latewood-forming portion of the growing season of years 4 and 5 in the no-vegetation control treatment. We also compared δ¹³C from untreated wood, crude cellulose extracted with the Diglyme–HCl method, and holocellulose extracted with toluene–ethanol to see if the extraction method would increase the sensitivity of the analysis. δ¹³C values from the two extraction methods were highly correlated with those from untreated samples (r ² = 0.97, 0.98, respectively). Therefore, using untreated wood would be as effective as using crude cellulose or holocellulose to investigate δ¹³C patterns in young Douglas-fir.     

Influence of arbuscular mycorrhiza on clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons

 The effect of arbuscular mycorrhiza (AM) on white clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons (PAH) was assessed in a pot experiment. The soil was spiked with 500 mg kg^–1 anthracene, 500 mg kg^–1 chrysene and 50 mg kg^–1 dibenz(a,h)anthracene, representing common PAH compounds with three, four and five aromatic rings, respectively. Three treatments and two harvest times (8 and 16 weeks) were imposed on plants grown in spiked soil: no mycorrhizal inoculation, mycorrhizal inoculation (Glomus mosseae P2, BEG 69) and mycorrhizal inoculation and surfactant addition (Triton X-100). Pots without PAH were also included as a control of plant growth and mycorrhizal colonization as affected by PAH additions. The competitive ability of clover vis-à-vis ryegrass regarding shoot and root growth was enhanced by AM, but reduced by PAH and the added surfactant. This was reflected by mycorrhizal root colonization which was moderate for clover (20–40% of total root length) and very low for ryegrass (0.5–5% of total root length). Colonization of either plant was similar in spiked soil with and without the added surfactant, but the PAH reduced colonization of clover to half that in non-spiked soil. P uptake was maintained in mycorrhizal clover when PAH were added, but was reduced in non-mycorrhizal clover and in mycorrhizal clover that received surfactant. Similar effects were not observed on ryegrass. These results are discussed in the context of the natural attenuation of organic pollutants in soils. Accepted: 12 June 2000     

Root turnover in pasture species: chicory,lucerne, perennial ryegrass and white clover

For pastures, root turnover can have an important influence on nutrient and carbon cycling, and plant performance. Turnover was calculated from mini‐rhizotron observations for chicory (Cichorium intybus), lucerne (Medicago sativa), perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) grown in the Manawatu, New Zealand. The species were combined factorially with four earthworm species treatments and a no‐earthworm control. Split plots compared the effects of not cutting and cutting the shoots at intervals. Observations were made c. 18 days apart for 2.5 years. This article concentrates on differences between plant species in root turnover in the whole soil profile to 40 cm depth. At this scale, earthworm effects were generally small and short lived. For ryegrass and white clover, root length and mass were linearly related (R² = 0.82–0.99). For chicory and lucerne, the relationships were poorer (R² = 0.38–0.77), so for those species length turnover may be a poor indicator of mass turnover. Standing root length, total growth and death generally decreased in the sequence ryegrass > lucerne > chicory = white clover. In length terms, scaled turnover (growth divided by average standing root length) generally followed the sequence lucerne > white clover > perennial ryegrass = chicory. Across species the scaled turnover rate averaged 3.4 per year or 0.9% per day. Cutting shoots reduced standing root length, growth and death, but increased scaled turnover. These results indicate fast and prolonged root turnover. For ryegrass and white clover, at least there is need to reappraise how to measure and model shoot : root ratios, dry matter production and carbon cycling.     

Association between tree-ring and needle δ<Superscript>13</Superscript>C and leaf gas exchange in <Emphasis Type="Italic">Pinus halepensis</Emphasis> under semi-arid conditions

Associations between δ¹³C values and leaf gas exchanges and tree-ring or needle growth, used in ecophysiological compositions, can be complex depending on the relative timing of CO₂ uptake and subsequent redistribution and allocation of carbon to needle and stem components. For palaeoenvironmental and dendroecological studies it is often interpreted in terms of a simple model of δ¹³C fractionation in C₃ plants. However, in spite of potential complicating factors, few studies have actually examined these relationships in mature trees over inter- and intra-annual time-scales. Here, we present results from a 4 years study that investigated the links between variations in leaf gas-exchange properties, growth, and dated δ¹³C values along the needles and across tree rings of Aleppo pine trees growing in a semi-arid region under natural conditions or with supplemental summer irrigation. Sub-sections of tissue across annual rings and along needles, for which time of formation was resolved from growth rate analyses, showed rapid growth and δ¹³C responses to changing environmental conditions. Seasonal cycles of growth and δ¹³C (up to ~4‰) significantly correlated (P<0.01) with photosynthetically active radiation, vapour pressure deficit, air temperature, and soil water content. The irrigation significantly increased leaf net assimilation, stomatal conductance and needle and tree-ring growth rate, and markedly decreased needle and tree-ring δ¹³C values and its sensitivity to environmental parameters. The δ¹³C estimates derived from gas-exchange parameters, and weighted by assimilation, compared closely with seasonal and inter-annual δ¹³C values of needle- and tree-ring tissue. Higher stomatal conductances of the irrigated trees (0.22 vs. 0.08 mol m⁻² s⁻¹ on average) corresponded with ~2.0‰ lower average δ¹³C values, both measured and derived. Derived and measured δ¹³C values also indicated that needle growth, which occurs throughout the stressful summer was supported by carbon from concurrent, low rate assimilation. For Aleppo pine under semi-arid and irrigated conditions, the δ¹³C of tree-ring and needle material proved, in general, to be a reasonable indicator of integrated leaf gas-exchange properties.     

Relationships of stable carbon isotopes, plant water potential and growth: an approach to asses water use efficiency and growth strategies of dry land agroforestry species

The relationships between annual wood stable carbon isotope composition (δ¹³C), dry season midday plant water potential, and annual growth rate were investigated to asses the ability of agroforestry species to adapt to climate changes. 6–8 stem disks from four co-occurring species (Acacia senegal, A. seyal, A. tortilis and Balanites aegyptiaca) were collected for radial growth measurements using tree-ring analysis spanning 1930–2003. Annual δ¹³C was measured on three tree disks per species for the period 1970–2002. Midday plant water potential was measured during the dry season. Annual radial growth and midday plant water potential ranged from 0.27 to 9.12 mm and −1.0 to −5.0 MPa, respectively, with statistically significant differences. After correcting annual wood δ¹³C for atmospheric changes in δ¹³C, carbon isotopic composition ranged from −22.22 to −26.58‰. Relationships between δ¹³C, radial growth and plant water potentials revealed the interaction of water availability, stomatal conductance, δ¹³C values and growth. Two contrasting water use strategies and competitive advantages can be distinguished. Species with lower mean δ¹³C values (A. senegal and A. seyal) show high plant water potential and, hence, better growth during moist years. Thus, they indicate low water use efficiency (WUE) and opportunistic water use strategy. On the other hand, species with lower water potentials (A. tortilis and B. aegyptiaca) showed relative better growth performance and less increase in δ¹³C in drought years, reflecting their high WUE and conservative water use strategy. These results suggest that δ¹³C in tree rings can be useful in estimating historic changes in plant WUE and hence in screening drought tolerant species in the face of expected climate changes, as well as for assessing the functional diversity and risk reduction in mixed vegetation.     

The nitrogen handling characteristics of white clover (Trifolium repens L.) cultivars and a perennial ryegrass (Lolium perenne L.) cultivar

Ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) have contrasting responses to soil mineral N availability and clover has the ability to fix atmospheric N(2) symbiotically. It has been hypothesized that these differences are the key to understanding grass-clover coexistence and vegetative dynamics in pastures. However, the whole plant response of clover and ryegrass to mineral N availability has not been fully characterized and inter-cultivar variability in the N-handling dynamics of clover has not been assessed. A detailed experimental study to address these issues was undertaken. For all clover cultivars and ryegrass, mass specific mineral N uptake rates (of whole plants) were similar saturating functions of mineral N availability. For all clover cultivars total N assimilation rates, whole plant C : N ratios and root : shoot ratios were independent of mineral N availability. Clover growth rates were also independent of mineral N availability except for a slight (<10%) reduction at very low N availability levels. Specific N(2) fixation rate (whole plant) was precisely controlled to ensure fixation balanced the deficit between mineral N uptake and the total N assimilation required to maintain constant whole plant C : N ratio. There was always a deficit between N uptake and the total N assimilation required to maintain C : N ratio. Consequently, some N(2) fixation remained engaged even at high mineral N availability levels. All inter-cultivar variation in N(2) fixation dynamics could be attributed to variations in growth rate. Clover mass specific growth rate declined as plant size increased. Ryegrass specific growth rate, whole plant C : N ratio and root : shoot ratio were dependent on N availability. Increased N availability led to increased growth rate and decreased C : N and root : shoot ratios. Specific growth rate was also dependent on plant size, growth rate declining as plant size increased. It is concluded that clover inter-cultivar variation in field performance is unlikely to be a consequence of variation in N-handling characteristics. Inter-cultivar differences in growth rate are likely to be a much more important source of variation.     

Phenanthrene toxicity and dissipation in rhizosphere of grassland plants (Lolium perenne L. and Trifolium pratense L.) in three spiked soils

Rhizodegradation is a technique involving plants that offers interesting potential to enhance biodegradation of persistent organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). Nevertheless, the behaviour of PAHs in plant rhizosphere, including micro-organisms and the physico-chemical soil properties, still needs to be clarified. The present work proposes to study the toxicity and the dissipation of phenanthrene in three artificially contaminated soils (1 g kg^-1 DW). Experiments were carried out after 2 months of soil aging. They consisted in using different systems with two plant species (Ryegrass—Lolium perenne L. var. Prana and red clover—Trifolium pratense L. var. fourragère Caillard), three kinds of soils (a silty-clay-loam soil “La Bouzule”, a coarse sandy-loam soil “Chenevières” and a fine sandy-loam soil “Maconcourt”). Phenanthrene was quantified by HPLC in the beginning (T ₀) and the end of the experiments (30 days). Plant biomass, microbial communities including mycorrhizal fungi, Rhizobium and PAH degraders were also recorded. Generally phenanthrene contamination did not affect plant biomass. Only the red clover biomass was enhanced in Chenevières and La Bouzule polluted soils. A stimulation of Rhizobium red clover colonisation was quantified in spiked soils whereas a drastic negative phenanthrene effect on the mycorrhization of ryegrass and red clover was recorded. The number of PAH degraders was stimulated by the presence of phenanthrene in all tested soils. Both in ryegrass and red clover planted soils, the highest phenanthrene dissipation due to the rhizosphere was measured in La Bouzule soils. On the contrary, in non-planted soils, La Bouzule soils had also the lowest pollutant dissipation. Thus, in rhizospheric and non-rhizospheric soils the phenanthrene dissipation was found to depend on soil clay content.     

Photosynthetic and growth responses of <Emphasis Type="Italic">Camelina sativa</Emphasis> (L.) Crantz to varying nitrogen and soil water status

Water and nitrogen (N) deficiency are two major constraints limiting the yield and quality of many oilseed crops worldwide. This study was designed to assess the response of Camelina sativa (L.) Crantz to the availability of N and water resources on photosynthesis and yield parameters. All the measured variables, which included plant height, root and shoot dry matter, root:shoot ratio, xylem pressure potential (XPP), yield components, photosynthetic parameters, and instantaneous water-use efficiency (WUE) were remarkably influenced by water and nitrogen supply. Net photosynthetic rate (P _N) and yield components were significantly decreased more by water deficit than by N deficiency. XPP, stomatal conductance (g _s), and intercellular CO₂ concentration (C _i) decreased substantially as the water deficit increased irrespective of the level of N application. WUE at the high N supply [100 and 150 kg(N) ha⁻¹] dropped in a large degree as the increased water deficit due to a larger decrease in P _N than transpiration rate (E). The results of this study suggest that the regulative capacity of N supply on photosynthetic and plant growth response is significantly affected by soil water status and C. sativa is more sensitive to water deficit than N supply.     

Water supply regulates structure, productivity, and water use efficiency of Acacia koa forest in Hawaii

We studied changes in stand structure, productivity, canopy development, growth efficiency, and intrinsic water use efficiency (WUE=photosynthesis/stomatal conductance) of the native tree koa (Acacia koa) across a gradient of decreasing rainfall (2600–700 mm) with increasing elevation (700–2000 m) on the island of Hawaii. The stands were located on organic soils on either smooth (pahoehoe) or rough (aa) lava flows. In the greenhouse, we also examined growth and WUE responses to different water regimes of koa seedlings grown from seeds collected in the study area. We tested the hypotheses that (1) stand basal area, aboveground net primary productivity (ANPP), leaf area index (LAI), and growth per unit leaf area decreased with decreasing rainfall, (2) WUE increased with decreasing rainfall or water supply, and (3) WUE responses were caused by stomatal limitation rather than by nutrient limitations to photosynthesis. The carbon isotope composition of phyllode tissues (δ¹³C) was examined as an integrated measure of WUE. Basal area and LAI of koa stands on both pahoehoe and aa lava flows, and ANPP on aa lava flows decreased with elevation. Basal area, LAI, and ANPP of koa in mixed stands with the exotic tropical ash (Fraxinus udhei) were lower compared to single-species koa stands at similar elevations. Along the gradient, phyllode δ¹³C (and therefore WUE) increased with elevation from –30.2 to –26.8‰. Koa in mixed stands exhibited higher (less negative) δ¹³C than in single-species stands suggesting that koa and tropical ash competed for water. In the greenhouse, we observed the same trend observed in the field, as phyllode δ¹³C increased from –27.7 to –24‰ as water supply decreased. Instantaneous gas exchange measurements in the greenhouse showed an inverse correlation of both maximum (morning) photosynthesis (A) and conductance (g) with δ¹³C values and, also, a good agreement between instantaneous (A/g) and integrated measures of WUE. Phyllode δ¹³C was not correlated with foliar concentrations of N or other nutrients in either the field or the greenhouse, indicating that differences in δ¹³C were caused by stomatal limitation rather than by nutrient-related changes in photosynthetic capacity. This study provided evidence that long-term structural and growth adjustments as well as changes in WUE are important mechanisms of koa response to water limitation.     

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C₃–C₄ vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C₄ plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ¹³C and δ¹⁵N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ¹³C and δ¹⁵N values of the residual SOM remained unchanged. The δ¹³C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ¹³C = −11.8‰) compared to the δ¹³C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ¹³C, the δ¹⁵N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ¹³C.     

Indications of Hydraulic Lift by Pinus halepensis and Its Effects on the Water Relations of Neighbour Shrubs

We measured the stable deuterium isotopic composition of xylem sap, the shoot predawn and midday water potentials, and the leaf δ¹³C of Mediterranean shrubs Pistacia lentiscus, Globularia alypum and Rosmarinus officinalis in a south-oriented transect from a large (12 m tall) Aleppo pine tree, Pinus halepensis. We aimed to study the possibility of hydraulic lift from the deep roots of this pine tree to the shallow soil layers and its influence on these neighbour shrubs. These same traits were also studied in several individuals of the shrub Pistacia lentiscus growing with different types of neighbours: just shrubs, a small (3 – 4 m) pine tree, and the above mentioned large pine tree. The greater the distance from P. halepensis the plants grew, the higher xylem water δD, the lower the soil water content, and, the lower the predawn and midday water potentials were found. These results suggest the existence of an hydraulic lift from deep roots to shallow soil in this big tree. Further indication of this existence is provided by the improved water status of P. lentiscus (higher water potentials and δD, and lower δ¹³C and, therefore, lower water use efficiencies) when growing close to the big pine in comparison with the same shrub species growing close to small pines or just surrounded by other shrubs. Moreover, all these trends occurred in the dry summer season, but disappeared in the wet spring season. This revised version was published online in July 2006 with corrections to the Cover Date.     

Coinoculation of Bacillus thuringeinsis-KR1 with Rhizobium leguminosarum enhances plant growth and nodulation of pea (Pisum sativum L.) and lentil (Lens culinaris L.)

Nodulation and the subsequent nitrogen fixation are important factors that determine the productivity of legumes. The beneficial effects of nodulation can be enhanced when rhizobial inoculation is combined with plant-growth-promoting bacteria (PGPB). The PGPB strain Bacillus thuringiensis-KR1, originally isolated from the nodules of Kudzu vine (Pueraria thunbergiana), was found to promote plant growth of field pea (Pisum sativum L.) and lentil (Lens culinaris L.) under Jensen’s tube, growth pouch and non-sterile soil, respectively, when co-inoculated with Rhizobium leguminosarum-PR1. Coinoculation with B. thuringiensis-KR1 (at a cell density of 10⁶ c.f.u. ml⁻¹) provided the highest and most consistent increase in nodule number, shoot weight, root weight, and total biomass, over rhizobial inoculation alone. The enhancement in nodulation due to coinoculation was 84.6 and 73.3% in pea and lentil respectively compared to R. leguminosarum-PR1 treatment alone. The shoot dry-weight gains on coinoculation with variable cell populations of B. thuringiensis-KR1 varied from 1.04 to 1.15 times and 1.03 to 1.06 times in pea and lentil respectively, while root dry weight ratios of coinoculated treatments varied from 0.98 to 1.14 times and 1.08 to 1.33 times in pea and lentil respectively, those of R. leguminosarum-PR1 inoculated treatment at 42 days of plant growth. While cell densities higher than 10⁶ c.f.u. ml⁻¹ had an inhibitory effect on nodulation and plant growth, lower inoculum levels resulted in decreased cell recovery and plant growth performance. The results of this study indicate the potential of harnessing endophytic bacteria of wild legumes for improving the nodulation and growth of cultivated legumes.     

Deficit and excess of soil water impact on plant growth of Lotus tenuis by affecting nutrient uptake and arbuscular mycorrhizal symbiosis

The impact of deficit and excess of soil water on plant growth, morphological plant features, N and P plant nutrition, soil properties, Rhizobium nodulation and the symbiosis between arbuscular mycorrhizal (AM) fungi and Lotus tenuis Waldst. & Kit. were studied in a saline-sodic soil. Water excess treatment decreased root growth by 36% and increased shoot growth by 13% whereas water deficit treatment decreased both root and shoot growth (26 and 32%, respectively). Differences between stress conditions on shoot growth were due to the ability of L. tenuis to tolerate low oxygen concentration in the soil and the sufficiency of nutrients in soil to sustain shoot growth demands. Water excess treatment decreased pH, and increased available P and labile C in soil. Water deficit treatment decreased available P and also increased labile C. In general, N and P acquisition were affected more by water excess than water deficit. The number of nodules per gram of fresh roots only increased in water excess roots (97%). Under both stress conditions there was a significant proportion of roots colonized by AM fungi. Compared to control treatment, arbuscule formation decreased by 55 and 14% under water excess and water deficit, respectively. Vesicle formation increased 256% in water excess treatment and did not change under water deficit treatment. L. tenuis plants subjected to water deficit or excess treatments could grow, nodulated and maintained a symbiotic association with AM fungi by different strategies. Under water excess, L. tenuis plants decreased root growth and increased shoot growth to facilitate water elimination by transpiration. Under water deficit, L. tenuis plants decreased root growth but also shoot growth which in turn significant decreased the shoot/root ratio. In the present study, under water excess conditions AM fungi reduced nutrient transfer structures (arbuscules), the number of entry points and spore, and hyphal densities in soil, but increased resistance structures (vesicles). At water deficit, however, AM fungi reduced external hyphae and arbuscules to some extent, investing more in maintaining a similar proportion of vesicles in roots and spores in soil compared to control treatment.     

Photosynthesis,water-use efficiency and δ<Superscript>13</Superscript>C of five cowpea genotypes grown in mixed culture and at different densities with sorghum

A field experiment involving two planting densities (83,333 and 166,666 plants per ha), two cropping systems (monoculture and mixed culture) and five cowpea [Vigna unguiculata L. (Walp.)] genotypes was conducted at Nietvoorbij (33°54S, 18°14E), Stellenbosch, South Africa, to select cowpea material with superior growth and water-use efficiency (WUE). The results showed significantly higher photosynthetic rates, stomatal conductance and transpiration in leaves of plants at low density and in monoculture due to greater chlorophyll (Chl) levels relative to those at high density and in mixed culture. As a result, C concentration in leaves and the amount of C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B accumulated in shoots at low density and under monoculture were also much higher. Even though no marked differences in photosynthetic rates were found between and among the five cowpea genotypes, leaf C concentration and shoot C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B contents differed considerably, with Sanzie exhibiting the highest C concentration and C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B contents in shoots, followed by Bensogla and Omondaw, while ITH98-46 and TVu1509 had the lowest shoot concentration and contents of C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B. WUE (calculated as photosynthate produced per unit water molecule transpired) was significantly greater in plants at low density and monoculture relative to those at high density and in mixed culture. Isotopic analysis revealed significant differences in δ¹³C values of sorghum [Sorghum bicolor L. (Moench.)] and cowpea, with higher δ¹³C values being obtained for plants at low density and in monoculture relative to those at high density or in mixed culture. The five cowpea genotypes also showed significant differences in δ¹³C values, with Sanzie exhibiting the most negative value (i.e. low WUE) and ITH98-46, the least negative δ¹³C value (i.e. high WUE). Whether measured isotopically or from gas-exchange studies, sorghum (a C₄ species) exhibited much higher WUE relative to cowpea (a C₃ species). Both correlation and regression analyses revealed a positive relationship between WUE from gas-exchange studies and δ¹³C values from isotopic analysis of cowpea and sorghum shoots.     

Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in arid regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 ± 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0–15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0–120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that δ ¹⁸O values of water in exotic annuals (e.g., −11.8 ± 0.4 ‰ for Bromus tectorum L.) were similar to δ ¹⁸O values of surface soil water (e.g., −13.3 ± 1.4 ‰ at −15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that δ ¹⁸O values of water in taprooted exotics (e.g., −17.4 ± 0.3 ‰ for Centaurea diffusa Lam.) were similar to δ ¹⁸O values of deep soil water (e.g., −18.4 ± 0.1 ‰ at −120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.     

Quantitative trait loci controlling water use efficiency and related traits in Quercus robur L.

Genetic variation for intrinsic water use efficiency (W _i) and related traits was estimated in a full-sib family of Quercus robur L. over 3 years. The genetic linkage map available for this F1 family was used to locate quantitative trait loci (QTL) for W _i, as estimated by leaf carbon stable isotope composition (δ ¹³C) or the ratio of net CO₂ assimilation rate (A) to stomatal conductance to water vapour (g _w) and related leaf traits. Gas exchange measurements were used to standardize estimates of A and g _w and to model the sensitivity of g _w to leaf-to-air vapour pressure deficit (sg^VPD). δ ¹³C varied by more than 3‰ among the siblings, which is equivalent to 40% variation of W _i. Most of the studied traits exhibited high clonal mean repeatabilities (>50%; proportion of clonal mean variability in global variance). Repeatabilities for δ ¹³C, leaf mass per area (LMA) and leaf nitrogen content were higher than 70%. For δ ¹³C, ten QTLs were detected, one of which was detected repeatedly for all 3 years and consistently explained more than 20% of measured variance. Four genomic regions were found in which co-localizing traits linked variation in W _i to variations in leaf chlorophyll and nitrogen content, LMA and sg^VPD. A positive correlation using clonal means between δ ¹³C and A/g _w, as well as a co-localisation of QTL detected for both traits, can be seen as validation of the theoretical model linking the genetic architecture of these two traits.     

Interspecific variability of δ13C among trees in rainforests of French Guiana: functional groups and canopy integration

The interspecific variability of sunlit leaf carbon isotope composition (δ¹³C), an indicator of leaf intrinsic water-use efficiency (WUE, CO₂ assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ¹³C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ¹³C values, suggesting a close genetic control of species δ¹³C. Marked differences in species δ¹³C values were found with respect to: (1) the leaf phenology pattern (average δ¹³C=–29.7‰ and –31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative δ¹³C values (average δ¹³C=–30.5‰) (i.e. lower WUE) than hemitolerant species (–29.3‰). However, tolerant species (–31.4‰) displayed even more negative δ¹³C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ¹³C and midday leaf water potential (Ψ_wm) suggests that low δ¹³C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (Δ _A ) calculated from the basal area-weighed integral of the species δ¹³C values was similar in the three stands (average Δ _A =23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands. Received: 30 November 1999 / Accepted: 23 March 2000     

Seasonal, daily and diurnal variations in the stable carbon isotope composition of carbon dioxide respired by tree trunks in a deciduous oak forest

The stable C isotope composition (δ¹³C) of CO₂ respired by trunks was examined in a mature temperate deciduous oak forest (Quercus petraea). Month-to-month, day-to-day and diurnal, measurements were made to determine the range of variations at different temporal scales. Trunk growth and respiration rates were assessed. Phloem tissue was sampled and was analysed for total organic matter and soluble sugar ¹³C composition. The CO₂ respired by trunk was always enriched in ¹³C relative to the total organic matter, sometimes by as much as 5‰. The δ¹³C of respired CO₂ exhibited a large seasonal variation (3.3‰), with a relative maximum at the beginning of the growth period. The lowest values occurred in summer when the respiration rates were maximal. After the cessation of radial trunk growth, the respired CO₂ δ¹³C values showed a progressive increase, which was linked to a parallel increase in soluble sugar content in the phloem tissue (R = 0.95; P < 0.01). At the same time, the respiration rates declined. This limited use of the substrate pool might allow the discrimination during respiration to be more strongly expressed. The late-season increase in CO₂ δ¹³C might also be linked to a shift from recently assimilated C to reserves. At the seasonal scale, CO₂ δ¹³C was negatively correlated with air temperature (R = −0.80; P < 0.01). The diurnal variation sometimes reached 3‰, but the range and the pattern depended on the period within the growing season. Contrary to expectations, diurnal variations were maximal in winter and spring when the leaves were missing or not totally functional. By contrast to the seasonal scale, these diurnal variations were not related to air temperature or sugar content. Our study shows that seasonal and diurnal variations of respired ¹³C exhibited a similar large range but were probably explained by different mechanisms.     

Symbiotically fixed nitrogen from field- grown white and red clover mixed with ryegrasses at low levels of15N-fertilization

A field study was carried out near Zürich (Switzerland) to determine the yield of symbiotically fixed nitrogen (¹⁵N dilution) from white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L) and from red clover (Trifolium pratense L.) grown with Italian ryegrass (Lolium multiflorum Lam.). A zero N fertilizer treatment was compared to a 30 kg N/ha per cut regime (90 to 150 kg ha⁻¹ annually). The annual yield of clover N derived from symbiosis averaged 131 kg ha⁻¹ (49 to 227 kg) without N fertilization and 83 kg ha⁻¹ (21 to 173 kg) with 30 kg of fertilizer N ha⁻¹ per cut in the seeding year. Values for the first production year were 308 kg ha⁻¹ (268 to 373 kg) without N fertilization and 232 kg ha⁻¹ (165 to 305 kg) with 30 kg fertilizer N ha⁻¹ per cut. The variation between years was associated mainly with the proportion of clover in the mixtures. Apparent clover-to-grass transfer of fixed N contributed up to 52 kg N ha⁻¹ per year (17 kg N ha⁻¹ on average) to the N yield of the mixtures. Percentage N derived from symbiosis averaged 75% for white and 86% for red clover. These percentages were affected only slightly by supplemental nitrogen, but declined markedly during late summer for white clover. It is concluded that the annual yield of symbiotically fixed N from clover/grass mixtures can be very high, provided that the proportion of clover in the mixtures exceeds 50% of total dry mass yield.