Most natural and semi-natural communities are limited by major nutrients such as nitrogen (N) or phosphorus (P), but most experiments on the effects of ozone on wild plants have used nutrient-rich composts or soil. In order to investigate interactions between ozone and low nutrient supply, two artificially selected lines (ozone-resistant and ozone-sensitive) of two populations Plantago major ssp. major L. were grown on a sandy loam, with (HN) and without (LN) addition of fertilizer. The soil was from a semi-natural grassland that has never been fertilized. Plants were exposed to either charcoal/Purafil®-filtered air (CF=<5 nl O3 l−1) or 70 nl O3 l−1 7 h d−1 from the seedling stage to seed production.

Poor growth (c. 25% of that in HN) of the low-nutrient plants, and leaf concentrations of N and P showed that the LN plants were severely nutrient-limited. In addition to affecting the total dry mass of the plants, the nutrient supply altered seed production, reproductive effort (number of seeds per total mass) and root-to-shoot allocation. Exposure to ozone had significant effects on physiology, growth, and seed production that varied with population, selection line, time, and plant development. There also were significant interactions between ozone effects and nutrient regime. In the Lullington Heath population, ozone reduced plant dry weight at 4 wk only in the LN treatment, and in the sensitive line of the Bush population, seed production was reduced by ozone only in LN. Therefore, contrary to what was expected, in the present experiment, plants given the LN treatment were often more sensitive to ozone than those grown under the high-nutrient regime. This increase in sensitivity was despite the fact that the LN treatment reduced stomatal conductance and ozone flux. It is concluded that there are potentially important interactions between ozone and low nutrient supply that need further investigation, particularly under field conditions.

We tested how nitrogen availability affects suitability of ectomycorrhizal (ECM) and non-mycorrhizal Pinus sylvestris (L.) for a polyphagous insect herbivore. Seedlings grown on sterile agar at two N levels, were infected or not with the ECM fungus Cenococcum geophilum (Sow.) Ferd. & Winge. Ultrastructural observations showed that C. geophilum formed well-developed ectomycorrhizas in pine roots. After mycorrhizal establishment, oviposition preference of the polyphagous tarnished plant bug (Lygus rugulipennis Popp.) was determined in three different experiments. The duration of these experiments and the amount of irradiance were different. Nitrogen level or mycorrhizal treatment did not have significant effects on oviposition preference. However, mycorrhizal seedlings were favoured as an oviposition site at the lower N level, but only in expt 1. Total or individual terpene, total resin acid and total or individual sugar concentrations in pine shoots were not altered by treatment. Total phenolics concentration in pine shoots and roots was also the same in different treatments. Of individual resin acids, the concentration of dehydroabietic acid was increased after mycorrhizal treatment. Total resin acid, abietic acid and sandaraco pimaric acid concentrations correlated negatively with the number of Lygus eggs, but the correlations were only marginally significant. Of individual terpenes, β-pinene and bornylacetate concentrations had a significant positive correlation with the number of Lygus eggs. Greater amounts of N predictably increased total N concentration in pine seedlings in expts 1 and 2, but mycorrhizal infection increased total nitrogen concentration only in expt 3. Total N concentration did not correlate with the number of Lygus eggs in expts 1 and 2. An early stage of mycorrhizal development did not provide mycorrhizal benefits to the host plant as a higher nutrient concentration or as increased growth. These results suggest that host–plant quality might be involved in selection of oviposition site by L. rugulipennis, but ectomycorrhizal infection at this level does not seem to be an important mediator.

Swards of cocksfoot (cvs KM2, Lutetia) and perennial ryegrass (cvs Aurora, Vigor) were grown under full irrigation or severe (80 d) drought in a field experiment in the South of France. Responses of the bases of immature leaves plus enclosed tissues were made during the drought period and after rewatering.

By the end of the drought, water content had fallen from 3·0 to 0·8 gwater g−1dm, and osmotic potential from −1·0 to −4·5 MPa in all cvs. Measured minerals and water-soluble carbohydrates contributed, respectively, c 19 and 44% to osmotic potential in droughted leaf bases. The drought-sensitive cocksfoot cv. Lutetia was characterized by a large proportion of fructans having a low degree of polymerization (DP=3, 4). As drought progressed, accumulation of dehydrin transcripts and ABA were higher in leaf bases of the sensitive cv. Lutetia than in the resistant cv. KM2.

After rewatering, the water status of immature leaf bases returned to control levels in 1–2 d, and then increased further as leaves began to grow and new tissue was produced. High-DP-fructans remained unchanged in leaf bases of ‘Lutetia’ but were depleted by over 55%, and therefore remobilized, in leaf bases of other cvs after 8 d.

It is concluded that enclosed immature leaf bases survive drought by tolerating a low water status and that changes conventionally associated with desiccation tolerance are expressed most strongly in susceptible plants least able to maintain their water supply.

The response of photosynthetic CO2 exchange to tissue water content in five spp. of Sphagnum from contrasting microhabitats (S. fuscum (Schimp.) Klinggr., S. papillosum H. Lindb., S. magellanicum Brid., S. balticum (Russ.) C. Jens. and S. cuspidatum Ehrh. ex Hoffm.) was measured in the laboratory using an infrared gas analyser technique. Experiments were designed to test recovery of net photosynthesis after periodic and long lasting desiccation.

The contact between capitula and basal parts of the mosses seems to be important for survival. Isolated capitula cut off from any contact with the water table were not able to recover after complete desiccation (at 15°C for 2–4 d). When contact with the water table is lost, e.g. during long periods of desiccation, recovery of net photosynthesis can take place but only if the water content of the capitula does not fall too far below c. 10–20% of the water content at compensation point.

There was no relationship between the ability of net photosynthesis to recover from desiccation and the wetness of the natural microhabitat. Sphagna survive dry periods by avoidance of drying out by high capillarity or dense growth form (as in S. fuscum).

We tested whether plants allocate proportionately less biomass to roots in response to above-ground competition as predicted by optimal partitioning theory. Two population densities of Abutilon theophrasti were achieved by planting one individual per pot and varying spacing among pots so that plants in the two densities experienced the same soil volume but different degrees of canopy overlap. Density did not affect root[ratio ]shoot ratio, the partitioning of biomass between fine roots and storage roots, fine root length, or root specific length. Plants growing in high density exhibited typical above-ground responses to neighbours, having higher ratios of stem to leaf biomass and greater leaf specific area than those growing in low density. Total root biomass and shoot biomass were highly correlated. However, storage root biomass was more strongly correlated with shoot biomass than was fine-root biomass. Fine-root length was correlated with above-ground biomass only for the small subcanopy plants in crowded populations. Because leaf surface area increased with biomass, the ratio between absorptive root surface area and transpirational leaf surface area declined with plant size, a relationship that could make larger plants more susceptible to drought. We conclude that A. theophrasti does not reallocate biomass from roots to shoots in response to above-ground competition even though much root biomass is apparently involved in storage and not in resource acquisition.

From laboratory experiments with seedlings and young trees of Norway spruce (Picea abies L. Karst.), a cycling pool of soluble non-protein N compounds is thought to be indicative of the N-nutritional status of trees. In order to test whether this assumption can be transferred to mature trees grown in the field, xylem sap and phloem exudate were collected from spruce trees in two remote forest stands: (1) a N-limited stand (Villingen site), and (2) a stand where trees are sufficiently supplied with N from the soil (Schluchsee site). Trees at these sites were c. 80–100 (Villingen site) and c. 40–60 (Schluchsee site) yr old. In addition to untreated control areas, one entire watershed area at both sites was subjected to (NH2)2SO4 fertilization to the soil.

In the xylem sap of the spruce roots at both sites Gln, Asp and Arg were the dominant total soluble non-protein nitrogen (TSNN) compounds. In the xylem sap of the trunk and the twigs Arg was virtually absent and Gln plus Asp dominated TSNN. On average, TSNN in the xylem sap of trees at the Schluchsee site was 1·5–2-fold higher than those at Villengen. Highest TSNN contents in the xylem sap were found during growth and development of current year tissues, while the lowest TSNN contents were found in summer. At the Villingen site (NH4)2SO4 fertilization caused an increase in the Gln content in the xylem sap of all tree sections analysed as well as an increase in the Arg content in the xylem sap of the roots. At the Schluchsee site only a small increase in TSNN contents of the xylem was observed, mostly in the xylem sap of the roots. In phloem exudates TSNN contents were much higher in trees on the Schluchsee than on the Villingen site. The seasonal pattern of TSNN in phloem exudates was similar to the seasonal pattern found in the xylem sap. During spring and early summer Gln was the predominant TSNN compound in phloem exudates, but during late summer and autumn Arg became predominant. At the Villingen site (NH4)2SO4 fertilization caused a significant increase in TSNN contents in phloem exudates of twigs and roots, but at Schluchsee an increase in TSNN was found only in phloem exudates of the roots. At both field sites Arg that was not transported to the shoot by xylem transport, was allocated from the leaves to the roots by phloem transport and was cycled within the root system by both xylem and phloem transport. From these results it is calculated that shoot-to-root signalling by long-distance transport of amino compounds can also contribute to the regulation of N-nutrition of mature spruce trees. Apparently, the internal cycling of individual N compounds within spruce trees differs considerably.

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO2 concentrations (350 and 550 μmol mol−1). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root[ratio ]shoot ratios) and within tissues[ratio ]infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO2. Elevated CO2 stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO2 on growth, with infected plants showing a greater response, such that elevated CO2 partly alleviated the effects of the parasite on host growth. Elevated CO2 did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO2 but was unaffected by O. minor. There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO2 in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO2 on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO2.

A range of marine photosynthetic picoeukaryote phytoplankton species grown in culture were screened for the presence of extracellular carbonic anhydrase (CAext), a key enzyme in inorganic carbon acquisition under carbon- limiting conditions in some larger marine phytoplankton species. Of the species tested, extracellular carbonic anhydrase was detected only in Micromonas pusilla Butcher. The rapid, light-dependent development of CAext when cells were transferred from carbon-replete to carbon-limiting conditions was regulated by the available free- CO2 concentration and not by total dissolved inorganic carbon. Kinetic studies provided support for a CO2- concentrating mechanism in that the K0.5[CO2] (i.e. the CO2 concentration required for the half-maximal rate of photosynthesis) was substantially lower than the Km [CO2] of Rubisco from related taxa, whilst the intracellular carbon pool was at least seven fold greater than the extracellular DIC concentration, for extracellular DIC values [les ]1.0 mm.

It is proposed that when the flux of CO2 into the cell is insufficient to support the photosynthetic rate at an optimum photon irradiance, the development of CAext increases the availability of CO2 at the plasma membrane. This ensures rapid acclimation to environmental change and provides an explanation for the central role of M. pusilla as a carbon sink in oligotrophic environments.

Field-grown trees of Alnus incana (L.) Moench, Alnus glutinosa (L.) Geartner and Betula pendula Roth displayed pronounced differences in responses of light-saturated net photosynthesis (Asat) to herbivory by the alder beetle (Agelastica alni L., Galerucinae), a specialized insect which primarily defoliates alders. We found that photosynthetic rates of grazed leaves increased following herbivory in Alnus but not in Betula. Area- and mass-based Asat of grazed leaves declined linearly with increasing amount of leaf perforation in B. pendula, by as much as 57%. By contrast Alnus glutinosa and Alnus incana increased area-based rates of Asat by 10–50% at all levels of leaf grazing. Given increased Asat in the remaining portion of grazed leaves, a net reduction in photosynthesis per leaf occurred only when the proportion of leaf area grazed exceeded 40% for Alnus incana and 23% for Alnus glutinosa. Since vein perforation by Agelastica alni was observed much more frequently in leaves of Betula than in Alnus, we hypothesized that declining Asat in herbivorized Betula was related to this disruption of water transport. A field experiment with artificial leaf perforation demonstrated a greater decline in Asat in vein-perforated Betula leaves than perforated leaves with midrib veins intact. However, regardless of leaf perforation regime, birch never showed post-perforation increases in Asat. In all species, rates of transpiration of grazed leaves linearly increased and water-use efficiency decreased with increased amount of leaf perforation. In grazed Alnus incana leaves, increasing leaf area consumption by Agelastica alni resulted in an increase of total phenols, a reduction in starch content and no changes in nitrogen concentration in the remaining portion. The increase in photosynthesis in Alnus incana might be related to declining leaf starch concentration or increasing stomatal conductance, but was unrelated to leaf nitrogen concentration. These gas exchange and leaf chemistry measurements suggest that in contrast to B. pendula, Alnus incana and Alnus glutinosa, which are the major host species for Agelastica alni, possess leaf-level physiological adaptations and defence mechanisms which can attenuate negative effects of herbivory by the alder leaf-beetle.

Differences between individuals collected from sun and shade populations could result from either somatic or genetic differences, particularly in populations of perennial plants. Our objective in this study was to separate somatic from genetic differences. We collected Anthoxanthum odoratum and Plantago lanceolata from sun and shade populations and made measurements on both vegetatively propagated clones and seed progeny from each clone. The parental populations differed in a wide range of physiological and morphological traits. However, only photosynthetic capacity was significantly different between both the original sun and shade populations and their seed progeny. In both species, plants from the sun population had higher photosynthetic capacities than those from the shade population when grown in a common environment. This demonstrates that there was genetic differentiation between the sun and shade populations. Photosynthetic capacity of parents and offspring also differed, suggesting a somatic effect. Since many of the original clones were virus-infected, but all but one of the offspring were virus-free, this might have been a result of virus infection. However, in spite of the fact that the parents and offspring clones were propagated vegetatively so that the plants were at the same developmental stage at the time of measurement, we cannot rule out the possibility that differences in age of cell lines could also have been a factor.

The interactive influences of elevated carbon dioxide, water stress, and ontogeny on carbon assimilation and biomass production were investigated in northern red oak, a species having episodic shoot growth characteristics. Seedlings were grown from acorns through three shoot-growth flushes (8–11 wk) in controlled-environment chambers at 400, 530 or 700 μmol mol−1 CO2 and under well watered or water-stressed soil-moisture regimes. Increasing CO2 growth concentration from 400 to 700 μmol mol−1 resulted in a 34% increase in net assimilation rate (A), a 31% decrease in stomatal conductance to water vapour (gs) and a 141% increase in water use efficiency (WUE) in well watered seedlings. In contrast, water-stressed seedlings grown at 700 μmol mol−1 CO2 demonstrated a 69% increase in A, a 23% decrease in gs, and a 104% increase in WUE. However, physiological responses to increased CO2 and water stress were strongly modified by ontogeny. During active third-flush shoot growth, A in first-flush and second-flush foliage of water-stressed seedlings increased relative to the quiescent phase following cessation of second-flush growth by an average of 115%; gs increased by an average of 74%. In contrast, neither A nor gs in comparable foliage of well watered seedlings changed in response to active third-flush growth. Whereas seedling growth was continuous through three flushes in well watered seedlings, growth of water-stressed seedlings was minimal following the leaf-expansion stage of the third flush. Through three growth flushes total seedling biomass and biomass allocation to root, shoot and foliage components were very similar in water-stressed seedlings grown at 700 μmol mol CO2 and well watered seedlings grown at 400 μmol mol−1 CO2. Enhancement effects of elevated CO2 on seedling carbon (C) assimilation and biomass production may offset the negative impact of moderate water stress and are likely to be determined by ontogeny and stress impacts on carbon sink demand.

The aim of this work was to assess whether or not oxidative stress had developed in a dwarf shrub bilberry (Vaccinium myrtillus L.) under long-term exposure to enhanced levels of ultraviolet-B (u.v.-B) radiation. The bilberry plants were exposed to increased u.v.-B representing a 15% stratospheric ozone depletion for seven full growing seasons (1991–1997) at Abisko, Swedish Lapland (68°N). The oxidative stress was assessed on leaves and stems by analysing ascorbate and glutathione concentrations, and activities of the closely related enzymes ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2). The affects of autumnal leaf senescence and stem cold hardening on these variables were also considered. The results showed that the treatment caused scarcely any response in the studied variables, indicating that u.v.-B flux representing a 15% ozone depletion under clear sky conditions is not sufficient to cause oxidative stress in the bilberry. It is suggested that no strain was evoked since adaptation was possible under such u.v.-B increases. The studied variables did, however, respond significantly to leaf senescence and especially to stem cold hardening.

Northern red oak in the western Lake States area of the USA exists on the most xeric edge of its distribution range. Future climate-change scenarios for this area predict decreased water availability along with increased atmospheric CO2. We examined recent photosynthate distribution and growth in seedlings as a function of CO2 mole fraction (400, 530 and 700 μmol mol−1 CO2), water regime (well watered and water-stressed), and ontogenic stage. Water stress effects on growth were largely offset by elevated CO2.

Water stress increased root mass ratio without concurrently increasing allocation of recent photosynthate to the roots. However, apparent sink strength of water-stressed seedlings at the completion of the third growth stage tended to be greater than that of well watered seedlings, as shown by continued high export, which may contribute carbon reserves to support preferential root growth under water-stressed conditions.

Elevated CO2 decreased apparent shoot sink strength associated with the rapid expansion of the third flush. Carbon resources for the observed enhanced growth under elevated CO2 could be provided by enhanced photosynthetic rate over an increased leaf area (Anderson & Tomlinson, 1998, this volume).

Increased sink strength of LG seedlings under water-stressed conditions, together with decreased apparent shoot sink strength associated with growth in elevated CO2 provide mechanisms for offsetting water stress effects by growth in elevated CO2.

Careful control of ontogeny was necessary to discern these changes and provides further evidence of the need for such careful control in mechanistic studies.

We grew cuttings of two early (mid Oct.) and two late (early Nov.) leaf-fall Populus tremuloides Michx. genotypes (referred to as genotype pairs) for c. 150 d in open-top chambers to understand how twice-ambient (elevated) CO2 and soil N availability would affect growth and C allocation. For the study, we selected genotypes differing in leaf area duration to find out if late-season photosynthesis influenced C allocation to roots. Both elevated CO2 and high soil N availability significantly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO2 stimulated leaf-level photosynthesis rates, whereas high soil N availability resulted in greater total plant leaf area. The early leaf-fall genotype pair had significantly higher photosynthesis rates per unit leaf area than the late leaf-fall genotype pair and elevated CO2 enhanced this difference. The early leaf-fall genotype pair had less leaf area than the late leaf-fall genotype pair, and their rate of leaf area development decreased earlier in the season. Across both genotype pairs, high soil N availability significantly increased fine root length production and mortality by increasing both the amount of root length present, and by decreasing the life span of individual roots. Elevated CO2 resulted in significantly increased fine root production and mortality in high N but not low N soil and did not affect fine root life span. The early leaf-fall genotype pair had significantly greater fine root length production than the late leaf-fall genotype pair across all CO2 and N treatments. These differences in belowground C allocations are consistent with the hypothesis that belowground C and N cycling is strongly influenced by soil N availability and will increase under elevated atmospheric CO2. In addition, this study reinforces the need for better understanding of the variation in tree responses to elevated CO2, within and among species.

Barley yellow dwarf virus (BYDV) causes significant losses in yield and in overwintering ability of winter cereals. Mechanisms by which the physiology of plants is affected by the virus are not clear. To see how carbohydrates in the crown of winter cereals were affected by BYDV, fructan isomers of degree of polymerization (DP) 3–5, fructan DP>6 and the simple sugars, glucose, fructose and sucrose, were measured before and during cold hardening in three oat (Avena sativa L.) cultivars, ‘Wintok’, ‘Coast Black’ and ‘Fulghum’. On a fresh weight basis fructan DP>6 decreased by 50% in infected ‘Wintok’ and ‘Coast Black’ and by 25% in ‘Fulghum’. Two DP3, one DP4 and one DP5 isomer were significantly higher than non-infected controls. The percentages of simple sugars in infected crowns were significantly higher than controls in all three cultivars in every week except the first week of hardening. Crude enzyme extracts from BYDV infected plants incubated with sucrose suggested higher invertase and lower sucrose-sucrosyl transferase activity. When incubated with 1-kestose and neokestin, no significant difference was found in fructose fructosyl transferase or in hydrolase activity. The activity of unidentified enzymes catalysing the synthesis of larger (DP>5) fructan was altered by BYDV. The decrease of carbohydrates in the crown induced indirectly by BYDV may alter the plant's capacity to regenerate tillers in the spring. The ability of plants to prevent or tolerate carbohydrate fluctuations induced by BYDV infection may be an important genetically regulated characteristic for developing virus-resistant cultivars.

Impacts of defoliation on the growth and physiology of sugar maple (Acer saccharum Marsh.) and trembling aspen (Populus tremuloides Michx.) were examined in ambient and CO2-enriched atmospheres. Saplings were grown for 70 d in controlled environments, wherein CO2 mole fractions averaged either 356 μmol mol−1 or 645 μmol mol−1, under a PPF of 500 μmol m−2 s−1. On day 49 of the study, 50% of the leaf area was removed from a subset of each species in both CO2 environments. Relative growth rate (rgr) and its physiological and morphological determinants were monitored before and after defoliation. For non-defoliated saplings of both species, a slight stimulation of rgr (c. 5%) in elevated CO2 led to a modest increase (9–11%) in final sapling weight. In the case of maple, the minimal growth response corresponded with minor CO2 effects on specific leaf area (sla) and leaf weight ratio (lwr), and an apparent CO2-induced down-regulation of photosynthetic metabolism. For aspen, the CO2 stimulation of photosynthesis was largely offset by a decrease in sla. Responses to defoliation differed markedly between species and CO2 environments. Defoliation decreased maple rgr in ambient CO2, whereas the opposite occurred in elevated CO2. The latter led to complete recovery of plant weight (compensation), and was attributed to a defoliation-induced increase in carbon allocation to new leaves, along with a reversal of photosynthetic CO2 acclimation in that foliage. In both environments, aspen rgr increased after defoliation, facilitating almost full compensation. Defoliation increased light penetration into the aspen canopy, and it was estimated that the resultant stimulation of photosynthesis in lower leaves would have more than offset the concomitant decrease in lwr. CO2 enrichment might substantially enhance the ability of certain tree species to recover from herbivory. Moreover, responses to elevated CO2 might be largest in the presence of stresses, such as herbivory, that decrease plant source[ratio ]sink ratios.

The transport of sugars and amino acids into the mycelium of Erysiphe pisi DC. was investigated using two different systems, intact leaf discs and mycelial suspensions. Of the sugars tested, glucose was preferentially taken up by both uninfected and mildew-infected leaf discs, whereas glutamine was taken up by both tissues at a higher rate than lysine or aspartic acid. Leaf discs from infected tissue had a greater uptake capacity than those from healthy tissue for both sugars and amino acids. The uptake of glucose was inhibited more markedly than that of sucrose and fructose by 10 μm carbonyl cyanide m-chlorophenylhydrazone (CCCP), 1 mmN-ethylmaleimide (NEM), 1 mm diethyl pyrocarbonate (DEPC) and 1 mm phenylglyoxal, whereas 1 mm PCMBS (p-chloro-mercuribenzenesulphonic acid) inhibited sucrose uptake to the greatest extent. Uptake of glutamine, lysine and aspartic acid was inhibited similarly by CCCP (80%), NEM (20%), DEPC (70%) and PCMBS (60%). Additionally, leaf discs were used to determine which solutes could be taken up from leaf tissue by the fungus. The uptake of sugars into the mycelium was greater than that of amino acids.

Suspensions of powdery mildew mycelium accumulated glucose at about three times the rate of sucrose or fructose, and the amino acid glutamine was taken up at three times the rate of lysine or aspartic acid. Spores separated from the suspension had a low uptake capacity.

When the reducing sugar concentration of leaf apoplastic fluid was estimated, leaves infected by powdery mildew had much higher amounts in the apoplast, whereas the activity of acid invertase also appeared to be higher in apoplastic fluids from infected leaves. When apoplastic fluid samples were run on SDS gels, an invertase antibody detected two bands in samples from infected tissues that were not found in the uninfected samples.

A detailed study was carried out to obtain information on the relative rates of NH4+ and NO3−-N uptake and assimilation by perennial ryegrass (Lolium perenne L.), when grown in soil and supplied with different ratios of NH4+ and NO3−-N. Following two preliminary defoliations, plants were supplied with 15N-(differentially) labelled NH4+ and NO3−-N in three different ratios (20[ratio ]80, 50[ratio ]50 and 80[ratio ]20), followed by sequential destructive harvests of shoots and roots taken at four points during a 35-d regrowth period. When supplied with equimolar concentrations of NH4+ and NO3−-N, perennial ryegrass absorbed both forms of nitrogen at almost identical rates; and even when the two N forms were supplied in widely disproportionate concentrations, both forms appear to have been absorbed at equal rates, at least until the supply of the minor nitrogen component in each treatment was almost exhausted, i.e. by day 3 of regrowth. It is suggested that this matching of NH4+ and NO3−-N absorption rates by plants maximizes the advantages and minimizes the disadvantages associated with the exclusive use of either form of nitrogen, and thereby optimizes the potential for vegetative and reproductive regeneration. Fifteen days after the application of 15N-labelled fertilizer, shoot and root yields differed little between treatments. However, plants grown with NH4+ and NO3−-N at a ratio of 50[ratio ]50 had accumulated significantly more root (soluble) carbohydrate than those in the other two treatments. Consequently, when subjected to defoliation at day 15, these former plants were clearly in the best position to re-mobilize root carbohydrate and initiate shoot regeneration. However, further work is needed in order to determine whether or not this finding holds true for plants subjected to a multiple (bi- to tri-weekly) defoliation regime, e.g. in a grazing situation.

Phytosiderophore (PS) release occurs under both iron and zinc deficiencies in representative Poaceae and has been speculated to be a general adaptive response to enhance the acquisition of micronutrient metals. We tested this hypothesis within an on-going study of the role of micronutrient metal nutrition for patterns of spontaneous vegetation in relation to soil pH and carbonate content. Hordelymus europaeus (L.) Harz, a negative grass species commonly found on soils rich in CaCO3 in Western Central Europe, was subjected to deficiencies of Fe, Zn, Mn and Cu using chelator-buffered nutrient solutions. PS release rates were determined at 3–5 d intervals during onset and development of deficiency symptoms. Plant dry matter yields and nutrient concentrations, measured at three time points were used to construct growth curves for calculation of PS release per unit root mass. In comparison with trace metal-sufficient control plants, dry matter production was markedly reduced in the Fe, Zn, Mn and Cu-deficiency treatments, with final relative yields of 6, 11, 15 and 31%, respectively. The phytosiderophore produced under Fe- and Cu-deficiency treatments was identified, using HPLC, as desoxymugineic acid. The highest rate of PS release (18 μmol g−1 root d. wt in 2 h) was measured in the Fe-deficiency treatment, and there was substantial release in the Cu-deficiency treatment (7·25 μmol g−1 root d. wt in 2 h). No PS release above control levels (2·6 μmol g−1 root d. wt in 2 h) was observed in the Zn- or Mn-deficiency treatments (1·5 and 2·6 μmol g−1 root d. wt in 2 h, respectively). It remains to be clarified whether PS release in response to Cu deficiency is a primary reaction to the deficiency, or is caused by a chain of events similar to that observed in Zn-deficient wheat, which involves internal Fe deficiency. Our results suggest that PS release in the native plant species H. europaeus is a specific response to Fe and Cu deficiency and is not significantly induced in response to deficiencies of Zn and Mn. Induction of the PS mechanism in different plant species might be more diverse than previously thought.

This is the first extensive investigation that quantifies natural mycoparasitic relationships in the phyllosphere. The presence of Ampelomyces spp. was quantified in naturally occurring powdery mildew fungi collected in Hungary and Romania between 1992 and 1995. A total of 570 samples was studied representing 27 species (nine genera) of the Erysiphaceae infecting 41 host plant genera. The incidence of Ampelomyces spp., determined as the proportion of samples in which intracellular pycnidia were present, varied between 4.3 and 68.8% in the host fungal genera studied. The intensity of mycoparasitism, defined as a percentage of the powdery mildew mycelia parasitized by Ampelomyces, ranged from 0.15 to 65%. Both the incidence and the intensity of mycoparasitism showed the lowest values in Blumeria graminis (DC.) Speer collected from wild and cultivated monocotyledons, while the highest values were found in Arthrocladiella mougeotii (Lév.) Vassilkov infecting Lycium halimifolium Mill. plants. The paper reports for the first time the natural occurrence of Ampelomyces in Sawadaea bicornis (Wallr.[ratio ]Fr.) Homma on maple.