Despite the usually high abundance of iron (Fe) in soils, the low solubility of Fe-bearing minerals restricts the available Fe pools in most aerobic soils to levels that are far below those required for microbial or plant growth. To acquire the necessary amounts of Fe from the environment, organisms have evolved mechanisms that enhance the solubility and dissolution rate of Fe(iii) oxyhydroxides prevailing in aerobic soils. Chemically, these mechanisms are based on weakening of the Fe–O bond by reduction, chelation and protonation. Physiologically, two distinct and in all known cases mutually exclusive strategies can be distinguished: the excretion of siderophores capable of solubilizing external ferric Fe and subsequent uptake of the ferric siderophore complex; and reduction of Fe(iii) prior to uptake of the more soluble Fe2+ ion. With the exception of graminaceous species, in which Fe uptake is based on the former mechanism, the latter strategy is found in all cormophytes and certain algae, yeast and bacteria. In higher plants, the increase in their capacity to convert extracellular ferric to ferrous Fe is part of a series of physiological and morphological events that act in concert to achieve appropriate internal levels of Fe. It is this amalgam of features that determines the Fe efficiency of a species or cultivar that in turn affects the yield of economically important plants and the natural distribution of species. Adaptive changes to limited Fe availability have been studied at the molecular, physiological and whole-plant level. This review summarises current knowledge of the components of reduction-based Fe uptake in plants and presents an integrated view of the present understanding of mechanisms that control the rate and extent of Fe absorption by roots.

Solution culture studies have demonstrated that apoplasmic iron (Fe) deposited in the roots of dicotyledonous and graminaceous plants can be mobilized to improve plant iron status in conditions of limited Fe supply. The present study investigated the formation of the apoplasmic Fe pool in dicotyledonous (soybean and cucumber) and graminaceous (wheat) plants in a pot experiment. The pots had three compartments such that plants could take up Fe and other nutrients from two calcareous soils treated with different Fe forms without their roots touching with soil directly. In this way overestimating Fe accumulation in root apoplasm was avoided. The results showed that while the root d.wt of wheat did not vary when soils were supplied with different Fe resources, the root d. wt of soybean and cucumber supplied with FeEDTA decreased compared with the control (without Fe treatment). Supplying FeEDTA in the side compartment increased shoot d. wt and Fe concentration in shoots of all species. However, supplying Fe(OH)3 had no effect on shoot d. wt or Fe concentration in the shoots of any species. Soybean and cucumber accumulated little or no Fe in the root apoplasm in controls or in Fe(OH)3 treatments. By contrast, a large amount of Fe was deposited in the root apoplasm of wheat grown in similar conditions. Remarkably, when FeEDTA was supplied in the soils, large apoplasmic iron pools were formed in the roots of all three species. Therefore, in dicotyledonous plants grown on calcareous soils, little or no apoplasmic iron pool forms, because there is not enough available Fe in the soil solution and the plants have little ability to mobilize Fe3+ in the soil. By contrast, a larger apoplasmic iron pool could form in graminaceous plants at lower concentrations of available soil-Fe possibly by enhancing the release of phytosiderophores which could mobilize Fe3+ in the soil and then transfer the Fe3+-complexes to the root apoplasm.

When freshly cut segments of naturally decorticated steles of horizontal roots of the Australian grass tree (Xanthorrhea preissii) are subjected to gentle suction, bubbles of gas sometimes appear, as well as liquid, in capillaries attached to the aspirated ends of the steles. We tested the hypothesis that this gas comes from vessels embolized in the intact xylem stream, and that the gas volume extruded is therefore an effective measure of the extent of this embolism. To do this, twin samples were taken from individual roots of X. preissii in the field, one was fast-frozen intact for subsequent estimation of vessel embolisms in the cryo-scanning electron microscope (CSEM), the other 15-cm segment rapidly assessed for volume of gas aspirated into a standard micropipette tube. The two measures mutually confirmed one another by showing a strong positive correlation between numbers of embolized vessels and extracted gas volume. Similar gas volumes were obtained from replicate root segments excised directly from a root when the ends of the segment were frozen before excision, and aspiration conducted after subsequent thawing of the ends under water. The pattern of changes in embolisms during unstressed conditions in early summer, shown by both CSEM and aspiration, indicated almost no embolisms before dawn, followed by a rapid rise to a peak in mid morning, than a progressive loss of embolisms in late afternoon. It was also shown that the amount of embolism did not change with time after excision of the roots up to at least 30 min. A comparison of changes in leaf transpiration with gas volumes in steles during a 24-h cycle at peak transpiration stress in mid summer showed rapid rates of transpiration in early morning and late afternoon, with an intervening period of low water loss during the rest of the day. Numbers of embolisms rose to an early morning peak, followed by apparent repair of these before noon. There was a second spate of embolisms in late afternoon, followed by complete refilling of all xylem with liquid by an hour or so after dusk. All vessels then remained fully recharged until the following dawn. We believe that aspiration is a direct and reliable technique, which offers a simple, inexpensive means of assessing the relative extent of embolism of vessels in xylem, and a means to test earlier findings by the other direct method of the CSEM. In a broad context, the technique should provide new opportunities for evaluating water relations of the xylem of whole plants.

This study aims to characterize the translocation of photosynthates within and from developing tall fescue (Festuca arundinacea) leaves at the time of transition from sink to source. The developing leaf contains a source, the exposed tip, and a sink, the growing basal portion. When the exposed tip of the developing blade is labelled with 14CO2, it exports photosynthates exclusively to sinks within the developing blade until the blade reaches 80% of its final length, when photosynthates begin to be exported from the blade and pass through the collar to reach the growing sheath and the next expanding leaf. Concurrently, the previous mature leaves reduce their level of photosynthate export to the developing blade; export stops as soon as the sheath of the developing leaf elongates beyond 10 mm. Export from the mature leaves to the growing sheath and to the next expanding leaf blade increases rapidly. Thus, in a developing tall fescue leaf blade photosynthate importation and exportation are exclusive events: the expanding blade imports photosynthate from mature leaves until it reaches 80% of its final length, then exportation begins and importation ceases.

Elimination of calcium ions from the medium of undifferentiated cell cultures of Digitalis thapsi increased cardenolide production and induced extracellular H2O2 accumulation, as measured by the quenching of pyranine fluorescence. The addition of catalase reduced the response and the inclusion of superoxide dismutase enhanced the loss of fluorescence. This suggested that, besides H2O2, the superoxide anion was also formed before dismutating to H2O2. Additionally, exogenous H2O2 or superoxide dismutase stimulated cardenolide production whereas the addition of catalase markedly reduced it. These results point to a connection between H2O2 and cardenolide formation. The absence of calcium did not alter the levels of lipid peroxidation products; however, changes in the antioxidant system of D. thapsi cells were observed. Catalase activity was extremely low in control cultures and remained unaltered upon calcium elimination. Ascorbate peroxidase activity was not modified in calcium-free cultures. By contrast, calcium deprivation stimulated superoxide dismutase activity and strongly inhibited glutathione reductase activity. Also, a significant decrease in reduced glutathione was observed. These responses were emulated by treatment of the cultures with the glutathione biosynthesis inhibitor buthionine sulfoximine and by ethyleneglycol-bis-β-aminoethyl ether and LaCl3. All these results indicate that the depletion of extracellular calcium induces changes in the redox state of cells and suggest that this alteration stimulates cardenolide formation in D. thapsi cultures.

Addition of 2 mM nitrite or ammonium to aerobically incubated cultures of Gloeothece rapidly inhibited N2 fixation (measured as acetylene reduction). In contrast, 2 mM nitrate inhibited N2 fixation less rapidly and less extensively, and often temporarily stimulated nitrogenase activity. The inhibitory effects of both nitrate and ammonium could be prevented by addition of 3 mM L-methionine-DL-sulphoximine, suggesting that the true inhibitor of N2 fixation was an assimilatory product of ammonium rather than either ammonium or nitrate itself. The inhibition of N2 fixation by nitrite could not, however, be prevented by addition of L-methionine-DL- sulphoximine. On the other hand, nitrite (unlike nitrate and ammonium) did not inhibit N2 fixation in cultures incubated under a gas phase lacking oxygen. These findings suggest that the mechanism whereby nitrite inhibits N2 fixation in Gloeothece differs from that of either nitrate or ammonium. The inhibitory effect of nitrite on N2 fixation did not involve reduction of nitrite to nitric oxide, though nitric oxide was a potent inhibitor of nitrogenase activity in Gloeothece. Nitrate and nitrite inhibited the synthesis of nitrogenase in Gloeothece, while ammonium not only inhibited nitrogenase synthesis but also stimulated degradation of the enzyme. In addition, all three compounds favoured the appearance of the Fe-protein of nitrogenase in its larger, presumed inactive, form.

The physiological potential for acquisition of atmospheric ammonia (NH3) was investigated in three European meadow grasses (Arrhenatherum elatius, Bromus erectus and Brachypodium pinnatum) competing in chalk grasslands. Experiments were carried out with plants cultivated for about three months on a soil–sand mixture at high root nitrogen supply or in nutrient solutions at low root nitrogen supply. Two different root nitrogen regimes were applied to the solution-grown plants: 130 μmol NO3 plant−1 wk−1 (approx. 50 kg nitrogen ha−1 in three months); or 130 μmol NO3− plus 130 μmol NH4+ plant−1 wk−1. Each regime was combined with two levels of NH3 fumigation (0 and 70 nmol mol−1 air for 24 d). Uptake of gaseous NH3 in the shoots was investigated under controlled environmental conditions including NH3 concentrations ranging from 0 to 30 nmol mol−1 air. Concurrently, photosynthesis, glutamine synthetase activity, nitrogen allocation, biomass allocation and apoplastic cation composition were measured. For A. elatius, the influence of photorespiration on NH3 acquisition was also assessed. Independently of plant nitrogen status, ammonia compensation points in A. elatius and B. erectus plants were <0.5 nmol mol−1. The total leaf conductance to NH3 absorption remained constant at increasing NH3 concentrations, showing that the capacity for assimilation was unaltered. Whereas internal factors in the leaves did not cause differences in the potential for NH3 acquisition between the species, other factors of NH3 acquisition were quite different: B. erectus had higher stomatal conductance and, thus, higher NH3 uptake rates per unit leaf area compared to A. elatius and B. pinnatum; higher stomatal conductances of B. erectus were to a large extent offset by a lower leaf area per plant, resulting from a lower growth rate and thicker leaves than in the two other species. The rate of photorespiration in Arrhenatherum constituted at least 15% of the net photosynthetic rate. Surprisingly, suppression of photorespiration indicated that NH3 uptake was supported by photorespiration. Bromus responded to fumigation with 70 nmol NH3 mol−1 air for 24 d by lowering the root[ratio ]shoot ratio and increasing the nitrogen concentration in the stem dry matter. The total leaf conductance to NH3 uptake decreased in all three species upon exposure to NH3, while the stomatal conductance was unaffected. The NH3 exposure caused lower apoplastic concentrations of H+, Mg2+ and Ca2+ in A. elatius and B. erectus.

The development of fruits of blackcurrant (Ribes nigrum) cv. Ben Alder from flower to maturity was studied non-invasively by nuclear magnetic resonance (NMR) microscopy, using attached and detached fruits, and the images were compared with those from low temperature scanning electron microscopy (LTSEM) and conventional resin histology. The NMR images derived from 2-D and 3-D datasets showed the previously unreported growth of arillar tissues to the extent that they almost completely occlude the locular cavity, but LTSEM and resin histology revealed that no fusion occurs between the arillar tissues and the gelatinous sheath surrounding each seed, or between the arillar tissues and the endocarp. The discontinuities between these tissues cause magnetic inhomogenities which result in these structures being clearly resolved by gradient echo imaging sequences. During seed maturation the endosperm changed from high (bright) to low (dark) signal intensity as lipid reserves formed and solidified, whereas the gelatinous sheath had high signal intensity throughout maturation. The high lipid concentration in the seed was manifested by chemical shift effects in the images and the increasing viscosity of the endosperm was reflected in the decrease in spin–lattice (T1) relaxation times. The funiculi, throughout development of seeds, appeared in NMR images with low signal intensity and 3-D surface-rendered reconstructions illustrated the complexity of the spatial array of seeds and funiculi arising from parietal placentas within the loculus. All other vascular tissues in the pericarp and placentas were resolved as a small bright core surrounded by a dark region, within a matrix of moderate signal intensity. Conventional microscopical studies then showed that the bright core discernible by NMR imaging encompassed an entire vascular bundle whereas the darker surrounding region represented small parenchyma cells with pronounced intercellular gas spaces. Other regions of the pericarp which included extremely large parenchyma cells, however, had few intercellular spaces and consequently gave rise to brighter regions of the image.

Floerkea proserpinacoides (Limnanthaceae) is a spring ephemeral annual species that grows in deciduous forests throughout eastern North America. Seeds germinate from late November to December, although the first leaf emerges only from late March to early April. Growth begins in early April at the onset of favourable temperatures, following snowmelt, and continues through mid-June. Senescence coincides with increasing air temperature and decreasing light level as a result of canopy closure. In this paper, we present the results of a growth chamber study designed to determine the effect of light level on growth, biomass allocation and reproduction of F. proserpinacoides. The study consists of two parts: in a first experiment, plants were grown at five constant photosynthetic photon fluence rates (PPFR: 90, 180, 360, 540 or 900 μmol m−2 s−1), and in a second experiment, PPFR was reduced from 900 μmol m−2 s−1 to 180 μmol m−2 s−1 after 0, 14, 21, 28 or 35 d of growth. Relative humidity, temperature, nutrient and water supply were kept constant in a hydroponic sand culture experiment. Total biomass, leaf mass and leaf area increased with increasing PPFR up to 540 μmol m−2 s−1. Plants grown at the highest (900 μmol m−2 s−1) and the lowest (90 μmol m−2 s−1) PPFR had a substantially lower biomass by the end of the 35-d growth period than plants grown at intermediate PPFRs (360 or 540 μmol m−2 s−1). Despite differences in total biomass, there were no significant differences in seed production among treatments. The mean relative growth rate (RGR) increased with increasing light levels between 90–540 μmol m−2 s−1, and it was reduced at 900 μmol m−2 s−1. However, differences in RGR were not significant among treatments. Specific leaf area did not vary consistently as a function of light level, whereas leaf area ratio and leaf mass ratio tended to increase with increasing PPFR, reaching maximum values at 360–540 μmol m−2 s−1. However, none of these growth variables differed significantly across the range of PPFR levels. The transfer of plants to lower PPFR had no significant effect on any of the growth components. Biomass production for the species appeared to be optimized at PPFR of 360–540 μmol m−2 s−1. Growth might be restricted by an insufficient supply of photosynthates at low PPFR and by photoinhibitory processes at higher PPFRs.

Previous work on Lolium perenne showed that sucrose[ratio ]sucrose fructosyltransferase (SST) activity did not increase concomitantly with fructan synthesis in regrowing leaves or in mature leaf blades of plants that have been subjected to carbohydrate-accumulating conditions. This was contrary to the pattern of SST activity in roots and stubble. To obtain further insight into the fructan synthesizing activities and to explain this discrepancy, total fructosyltransferase activity (FT) was assayed by increasing the sucrose and the soluble enzyme concentrations and was compared to sedimentable phlein sucrase activity (PS) throughout the regrowth period following defoliation in leaves, stubble and roots. Before analysis on 2-month-old plants, PS activity was characterized in dark-grown coleoptiles, using [U-14C]sucrose. PS activity had a pH optimum of 6.0 and produced 1-kestose in addition to high molecular weight fructans with a mean DP of 9. In 2-month-old plants, sedimentable PS and FT soluble reactions contained an initial sucrose concentration of 160 mM and 400 mM and proteins equivalent to 1.4 and 2.1 g f. wt of tissue, respectively. In stubble and roots, the FT preparation catalysed the synthesis of large fructans, and the overall pattern resembled the native fructans when separated by anion exchange HPLC. In regrowing leaves, the FT preparation produced low-DP fructans relatively more than in vivo but synthesized the high-DP fructan characteristic of the tissue. Moreover, FT activity did not remain at a low level like SST activity but increased from day 5 after defoliation when fructans began to accumulate. PS activity formed very few low- DP fructans and 1-kestose was the main product. Trisaccharides generated by PS activity represented 2–5% of the total trisaccharide synthesis. High-DP fructans were detectable only when the products of the reaction were concentrated 100 times. Results are discussed with respect to the relative contribution of FT and PS activities for the synthesis of 1-kestose and fructans in roots, stubble and leaves of Lolium perenne.

Natural flooding is one of the major factors affecting vegetation dynamics in many regions of the world. The Flooding Pampa Grasslands (Argentina) are frequently exposed to flooding events of diverse intensity and duration, some of which Leontodon taraxacoides, an exotic dicot. frequent in these grasslands, seems to survive. Its responses to four different water depths (0, 1, 7 and 13 cm) were studied. The results indicate that plants in conditions of total submergence (depth of 13 cm) did not survive. In less severe flood conditions, increases in the leaf insertion angle resulted in the maintenance of a large proportion of the total leaf area above the water. Differences in leaf length and a decrease in the width and the proportion of lobes per leaf were also found under partial submergence conditions (depth of 7 cm). Root and leaf aerenchyma, present in unflooded plants, showed a significant increase in flood conditions. In spite of the anatomical and morphological responses, total biomass and leaf area were severely affected by water depth. Control plants allocated more biomass to reproductive organs, while partly submerged plants allocated more to leaves and less to reproductive organs. Mature L. taraxacoides plants presented a wide range of plastic adjustment as a survival strategy in soil anaerobiosis, and appear to be able to survive short spring floods in a vegetative state; in contrast, they might not tolerate total submergence conditions imposed by more intense and long-lasting floods.

Seeds of cherry (Prunus avium) were germinated and grown for two growing seasons in ambient (∼350 μmol mol−1) or elevated (ambient+∼350 μmol mol−1) CO2 mole fractions in six open-top chambers. The seedlings were fertilized once a week, following Ingestad principles in order to supply mineral nutrients at free-access rates. In the first growing season gradual drought was imposed on rapidly growing cherry seedlings by withholding water for a 6-wk drying cycle. In the second growing season, the rapid onset of drought was imposed at the height of the growing season on the seedlings which had already experienced drought in the first growing season. Elevated [CO2] significantly increased total dry-mass production in both water regimes, but did not ameliorate the growth response to drought of the cherry seedlings subjected to two sequential drying cycles. Water loss did not differ in either well watered or droughted seedlings between elevated and ambient [CO2]; consequently whole-plant water- use efficiency (the ratio of total dry mass produced to total water consumption) was significantly increased. Similar patterns of carbon allocation between shoot and root were found in elevated and ambient [CO2] when the seedlings were the same size. Thus, elevated [CO2] did not improve drought tolerance, but it accelerated ontogenetic development irrespective of water status.

Cherry seedlings (Prunus avium) were grown from seed for two growing seasons in three ambient [CO2] (∼350 μmol mol−1) and three elevated [CO2] (ambient+∼350 μmol mol−1) open-top chambers, and in three outside blocks. A drying cycle was imposed in both the growing seasons to half the seedlings: days 69–115 in the first growing season, and in the second growing season days 212–251 on the same seedlings which had already experienced drought. Stomatal conductance was significantly reduced in elevated [CO2]-grown, unstressed seedlings in both the first and second growing seasons, but was not caused by a decrease in stomatal density. Droughted seedlings showed little or no reduction in stomatal conductance in response to elevated [CO2]. However, stomatal conductance was highly correlated with soil water status. Photosynthetic rate increased significantly in response to elevated [CO2] in both water regimes, leading to improvement in instantaneous transpiration efficiency over the whole duration of the experiment, but there was no relationship between instantaneous transpiration efficiency and long-term water use efficiency. The Amax was strongly reduced in the second growing season, but unaffected by [CO2] treatment. Although photosynthetic rate was not down-regulated, Rubisco activity was decreased by elevated [CO2], possibly because of the increased leaf carbon: nitrogen ratio which had occurred by the ends of the two growing seasons. Elevated [CO2] did not improve plant water relations (for example, bulk leaf – water potential, osmotic potentials at full and zero turgor, relative water content at zero turgor, bulk modulus of elasticity of the cell) and thus did not increase water-stress tolerance of cherry seedlings.

The association between willow (Salix) and rust (Melampsora) is highly specific. Willows named Salix burjatica, S. dasyclados (S.×dasyclados) and S.×calodendron are important in renewable-energy plantations in the UK and western Europe. There has been much controversy over their origin, species status and nomenclature. It has been suggested that they have originated from hybridization between. S. caprea, S. viminalis and S. cinerea. In the present work, 59 willow clones were investigated through morphological examination and detached leaf inoculation using willow differentials, for their association, in southwest England, with M. capraearum and three pathotypes of Melampsora epitea (Me-A, B and C). M. capraearum was found on all clones of S. caprea and its hybrids with S. aurita; Me-A on all S. viminalis clones; Me-B on wild S. cinerea, S.×calodendron, S.×dasyclados ‘De Biardii 445’ and S. ‘Spaethii’; Me-C on all S. burjatica clones and most S.×dasyclados clones. Both M. caprearum and Me-A infected all S.×sericans (S. caprea×viminalis) clones and S.×dasyclados ‘LA041/03’. We suggest that S.×dasyclados ‘LA041/03’ should be treated as S.×sericans (S. caprea×S. viminalis); S. burjatica, S. dasyclados and S.×dasyclados as synonyms; S.×dasyclados ‘De Biardii 445’ as S.×calodendron ‘De Biardii 445’; and S. ‘Spaethii’ as S.×calodendron ‘Spaethii’.

Arbuscular mycorrhizas are a widespread symbiosis between soil fungi and plant roots. Flow cytometry, after DNase I partial digestion and DAPI staining, and light and electron microscopy were used to analyse chromatin condensation and nuclear conditions in mycorrhizal and control roots of Allium porrum. The 2C peak, detected by flow cytometry, split into two peaks representing two populations of nuclei, one more resistant and one more susceptible to the enzyme action. The microscopic analyses showed the presence of pyknotic and chromatolytic nuclei, two typical features of senescence. In order to quantify the senescing process, a terminal deoxynucleotidyl transferase assay was performed on extracted nuclei, later analysed by flow cytometry. The numbers of senescing nuclei and their DNA cleavage were higher in control plants. Our results show the existence of senescing nuclei in cortical cells of the bulbous monocotyledon A. porrum and the delaying effect of arbuscular mycorrhizas on senescence.