Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity (LP) decreased to one half during both drying treatments; LP was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.

The developmental regulation and organ-specific expression of shikimate dehydrogenase was studied in seedlings of Capsicum annuum L. during their early development. The results obtained suggest that there is both a developmental regulation and an organ-specific expression of shikimate dehydrogenase in seedlings of C. annuum, seen mainly in the cotyledons. For example, shikimate dehydrogenase isoenzyme 3 was differentially expressed in cotyledons, but not at all in roots or hypocotyls. When the amounts of shikimate dehydrogenase were compared with the end-products of the shikimate pathway, chlorogenic acid and lignins, it was observed that the fall in shikimate dehydrogenase activity in cotyledons coincided with a decrease in the concentrations of phenolics during primary leaf development. However, this did not seem to be the case with the hypocotyls, where the fall in phenolics was not related to the amount of shikimate dehydrogenase but rather to an increase in lignin deposition, supporting the view that chlorogenic acid might act as a precursor of the aromatic moiety of cinnamoyl alcohols during cell wall lignification.

Modern cone scales and seeds of Pinus strobus and Sequoia sempervirens, and their fossil (Upper Miocene, c. 6 Mar) counterparts Pinus leitzii and Sequoia langsdorfii have been studied using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), electron-microprobe and scanning electron microscopy. Microscopic observations revealed only minor microbial activity and high-quality structural preservation of the fossil material. The pyrolysates of both modern genera showed the presence of ligno-cellulose characteristic of conifers. However, the abundance of (alkylated)phenols and 1,2-benzenediols in modern S. sempervirens suggests the presence of non-hydrolysable tannins or abundant polyphenolic moieties not previously reported in modern conifers. The marked differences between the pyrolysis products of both modern genera are suggested to be of chemosystematic significance. The fossil samples also contained ligno-cellulose which exhibited only partial degradation, primarily of the carbohydrate constituents. Comparison between the fossil cone scale and seed pyrolysates indicated that the ligno-cellulose complex present in the seeds is chemically more resistant than that in the cone scales. Principal component analysis (PCA) of the pyrolysis data allowed for the determination of the discriminant functions used to assess the extent of degradation and the chemosystematic differences between both genera and between cone scales and seeds. Elemental composition (C, O, S), obtained using electron-microprobe, corroborated the pyrolysis results. Overall, the combination of chemical, microscopic and statistical methods allowed for a detailed characterization and chemosystematic interpretations of modern and fossil conifer cone scales and seeds.

Two Holocene lake sequences from Lac de Creno, Corsica were analysed on the basis of 119 pollen spectra and with the support of 13 14C-calibrated dates. The lower part of one of these sequences, corresponding to the late- glacial period, has been published previously. The first third of the Post-glacial is characterized by very particular forest dynamics, namely the absence of a role for deciduous Quercus and Corylus, the presence of mesophilous vegetation types dominated by Taxus, and the major forest role of Erica arborea at lower and mean altitude. At about 7440 cal BP, the occurrence of a major anthropogenic action brought about significant changes in the vegetation, notably an increase of deciduous Quercus and the expansion of Quercus ilex. Later, three major human-induced events are identified: the first, at about 2290 cal BP, is the cause of a short local expansion of Abies; the second, at about 1150 cal BP, is the degradation of deciduous forests to the benefit of Fagus; the third, at about 310 cal BP, corresponds to the disappearance of Fagus and its replacement by Pinus. Pollen data indicate that Q. ilex, Abies and Fagus are not indigenous in Corsica but spread there during the Postglacial ; this probably took place at about 6980 cal BP for Q. ilex.

Several different fructan and sucrose hydrolysing enzyme activities were induced in roots and stubble (mainly leaf sheaths) of Lolium perenne L. plants after defoliation. Among those activities, a fructan-β-fructosidase (EC 3.2.1.80) that hydrolyses predominantly β-(2-6)-fructosyl-fructose linkages (6-FEH) was purified from the stubble. The use of the substrate 6,6-kestotetraose and high-performance anion-exchange chromatography with pulsed amperometric detection allowed linkage-specific screening and sensitive analysis of enzyme activity. A 6-FEH was extensively purified to yield one protein band as revealed by one-dimensional sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE). The 6-FEH was separated from the contaminating β-(2-1)-linkage-specific fructan-β-fructosidase and invertase activities (EC 3.1.2.26) by ammonium sulphate precipitation, lectin-affinity, anion-exchange and size-exclusion chromatography. The purified 6-FEH was a glycoprotein with an apparent molecular mass of 65000, as determined by size-exclusion chromatography, and of 69000 by SDS–PAGE. The 6-FEH had an activity optimum in the range of pH 5·1 to 5·6. Temperatures above 30°C affected the stability of the enzyme activity; however, its temperature stability was increased in the presence of 6,6-kestotetraose. The purified 6-FEH activity hydrolysed the β-(2-6)-linkages in 6,6-kestotetraose and (1&6)-kestotetraose at rates five times faster than the β-(2-1)-linkages in 1,1-kestotetraose and (1&6)-kestotetraose. Fructose up to 50 mM did not affect 6-FEH activity; conversely, sucrose substantially inhibited the enzyme activity. Other disaccharides did not affect 6-FEH. It is suggested that sucrose might modulate 6-FEH activity in vivo.

Androdioecy (the coexistence of two genders, cosexuals and males, in a single population) is a rare breeding system. In terms of functional gamete production, androdioecy has been reported in a small number of wind-pollinated and insect-pollinated species. In this study we examine the floral biology, stability of gender, and fruit and seed production after self-pollination and outcrossing pollination in a potentially androdioecious tree, Fraxinus ornus, in southern France. Individual trees have either hermaphrodite flowers or male flowers, the latter lacking a well-developed gynoecium. The two genders produce morphologically similar pollen and were present in 1[ratio ]1 or slightly male-biased ratios. Only hermaphrodites set fruit during 2–4 yr of observation at four different sites in southern France. Experimental pollinations of eight trees in two different sites showed that hermaphrodites produce viable pollen in dehiscent anthers and viable seeds. Thus, in terms of functional gamete production, F. ornus is a new case of functional androdioecy. Seven hermaphrodites were self-compatible, and such self-compatibility of hermaphrodites might improve the colonizing ability of F. ornus in a region where this species is actively expanding its range.

The frequent co-occurrence of ectomycorrhizas and arbuscular mycorrhizas in the same ecosystem raises the question about how two functionally different mycorrhizal associations exploit a spatially heterogeneous habitat. Horizontal (in plots along a transect of 3750 m crossing forest types of low and high abundance of ectomycorrhizal (ECM) host species) and vertical (across profiles of litter and 0–5 cm, 10–15 cm, 25–35 cm soil layers) distribution of ectomycorrhizas and arbuscular mycorrhizas in relation to edaphic parameters (pH, moisture, total carbon, total nitrogen and extractable-phosphorus concentration) was compared in Korup National Park rain forest, to test the hypothesis that the two types of mycorrhiza exploit different niches. No pattern was observed in either ECM or arbuscular mycorrhizal (AM) fractional colonization with depth in the profile even though most of the variation in soil parameters was vertical. Only local (between monoliths within plots) horizontal differences were found in soil parameters. By contrast, horizontal differences in fractional colonization were found between forest types, and between monoliths, for ectomycorrhizas but only between monoliths for arbuscular mycorrhizas. The lack of a clear relationship between both horizontal and vertical distribution of ECM and AM fractional colonization and soil parameters suggests that both fungal groups are equally able to colonize the same niche, at the scale of our experiment. By contrast, a negative relationship was found between AM and ECM fractional colonization in the litter and 0–5 cm and 10–15 cm soil layers. This, in addition to the more heterogeneous horizontal distribution of arbuscular mycorrhizas in forest areas dominated by ectomycorrhizas and a patchy vertical distribution of the former as opposed to a more coherent distribution of the latter, suggests that both types of mycorrhizal fungi compete for the same niche. The co-occurrence of ectomycorrhizas and arbuscular mycorrhizas might be explained by differences between ECM and AM hosts and fungi in their colonizing abilities.

Different aspects of salt excretion from leaves of the xero-halophyte Reaumuria hirtella Jaub. et Sp. (Tamaricaceae) were investigated under the extremely arid conditions of the desert. The diurnal excretion pattern, which gradually decreased toward midday, showed a negative correlation with the daily transpiration pattern. The relative excretion, which is the ratio between the absolute excretion of Na+ and the change in its internal content, was maximal at sites with low salinity, and decreased when the concentration of NaCl in the root environment increased. By contrast, the absolute excretion increased from 253 to 323 mmol Na+ g−1 f. wt per 12 h with increase in the total soil salinity from 0·24 to 1·23%, respectively. Water stress conditions, caused either by increasing soil salinity or by dryness of the atmosphere, greatly influenced the efficiency of excretion process.

Six ions: Na+, K+, Ca2+, Mg2+, Cl− and SO42− constituted 96·8% of the d. wt of the field-collected secreted salts. Na+ and Cl−, which were the predominating ions in the soil solution of the root environment, were also the predominantly secreted, constituting c. 89% of salts secreted. The proportion of Na+ which was prevented from entry into the roots increased with increasing Na+ concentration in the soil solution. More than 67% of the absorbed NaCl was secreted by leaves during the day. However, the accumulated salts contributed to the osmotic potential (ψs) and improved the plant water status. The osmotic potential of the plant sap at the end of the day or during periods of low excretion rates was shown to be decreased −7·3 to −13·7 bar from that in the morning. The fraction of osmotic potential decreased during the day (Δψs) was positively correlated with the soil salinity. These retained salts are expelled from the leaves by increased efficiency of extraction during the night and early morning, so that the plant begins the day with low internal salt content, and the leaf reservoirs become suitable for a new supply. The rejection mechanism at the plant roots and the secretion mechanism at shoots allow the plant to maintain its internal salt content at an approximately constant level despite the great variation in soil salinity.

Three ectomycorrhizal legume trees, Microberlinia bisulcata, Tetraberlinia bifoliolata and T. moreliana, form discrete groves in the southern part of Korup National Park, in southwest Cameroon and contribute c. 45–70% of stand basal area locally in a matrix of otherwise species-rich arbuscular mycorrhizal forest. A transplant experiment was performed to assess the importance of ectomycorrhizal infection associated with proximity to parents in seedling establishment of the grove-forming species. Nonectomycorrhizal seedlings of the three species were transplanted into plots of two forest types, one of high (HEM, within-grove) and one of very low (LEM, outside the grove) abundance of all three species as adult trees. For two species (T. moreliana and M. bisulcata) there was no difference in survival over 16 months, but for the third (T. bifoliolata) survival was best in HEM forest, and correlated with the basal area of adult trees of ectomycorrhizal species. Only one species (T. moreliana) increased in biomass over the experimental period; the others declined. There was no effect of forest type on overall growth of any species, but the survivors of two (T. moreliana and M. bisulcata) had heavier stems in the HEM forest. Differences in survival and growth of transplants between the three species were in accord with the ecology of the species as inferred from the frequency distributions of adult tree size in the forest. Seedlings became infected with ectomycorrhizas in both forest types; where there was a difference in extent of infection (T. moreliana) this was not related to survival or growth; and where there was a difference in survival (T. bifoliolata) this was not related to extent of infection. These results confirm that mycorrhizal inoculum associated with conspecific adults is neither a prerequisite nor a guarantee of seedling establishment, but indicates that in some circumstances there might be benefits of being close to parents. Further research is required to unravel the complexities of ectomycorrhizal community structure in this spatially and temporally heterogeneous forest, and to clarify the extent to which the various hosts share ectomycorrhizal partners.

Three MADS-box cDNA clones and two corresponding genomic sequences (gDNAs) have been isolated from the bryophyte Physcomitrella patens and sequenced. Our findings indicate that the genes may be expressed in a tissue- or age-specific manner, and that expression of one of them is regulated by an alternative splicing mechanism. Conceptual translation of the clones reveals that the encoded MADS-domain proteins have the typical plant-domain pattern (MIKC). Additionally, there is a high degree of conservation of intron number and positions between angiosperm MADS-box genes and the moss loci. These observations confirm the homology of moss and higher plant MADS-box genes. We conclude that the MIKC pattern evolved in MADS-box genes after the separation of the plant lineage from that of fungi and animals, and that it must have been present in the common ancestor of mosses, ferns and seed plants. Therefore it evolved at least 400 million yr ago. Phylogenetic analysis of a large subset of the sequenced plant MADS-box genes, incorporating those from P. patens, indicates that the bryophyte genes are not orthologues of spermatophyte genes belonging to any of the well recognized higher plant gene subfamilies. This conclusion accords well with reports that the known fern MADS-box genes also comprise subfamilies distinct from those of higher plants. Therefore we tentatively propose that the gene duplication and diversification events that created the MADS-box gene subfamilies, discernible in extant angiosperm and other spermatophyte groups, occurred after separation of the moss and fern lineages from the lineage which produced the higher plants.

Spring wheat (Triticum aestivum cv. Nandu) cultivated under glasshouse conditions was exposed to ozone in large fumigation chambers for 2 wk. Different exposure regimes were applied as constant concentrations as well as with ozone peaks, partly under equal dose-conditions, in times of high solar radiation during different stages of development (seedling, late tillering, anthesis). Chlorophyll fluorescence was monitored and amounts of carbohydrates (hexoses, sucrose, starch) and chlorophyll were measured in young leaves (seedling) and flag leaves (late tillering, anthesis) during and after ozone exposure. Although seedlings showed no significant response in photosynthesis, strong effects on photosynthesis and carbohydrate accumulation were measured when plants were fumigated during anthesis, especially after a heat stress period preceding ozone treatments. Under equal dose conditions chlorophyll fluorescence parameters (Fv[ratio ]Fm) and electron transport rate decreased and sucrose content of flag leaves increased significantly if ozone at a concentration of 220 μg m−3 was supplied for 4 h, indicating that peak concentrations show stronger effects than constant concentrations. The reaction of wheat plants is dependent on environmental conditions such as preceding heat stress and on the developmental stage during exposure. The results favour the hypothesis that photoinhibition and disturbance of photosynthesis are only secondary effects as a consequence of retarded sucrose export from the leaf, because of damage at the plasma membrane.

The aim of this study was to investigate whether genetically different pollen donors (Betula pendula clones) differed in pollen-tube growth rate across 11 maternal plants and in vitro, and whether the differences between the donors were consistent across the recipients. To compare the seed-siring success of competing pollen donors, a two-donor hand-pollination experiment with six donors and six recipients was conducted. The experiments were performed at a plastic-house seed orchard. The donors showed significant variation in pollen-tube growth rate on all the 11 recipients. The rankings of the pollen donors were statistically consistent across different maternal plants. A significant positive correlation between pollen tube growth in vivo and in vitro was found. The seed-siring success of two competing pollen donors was unequal in 20 of 29 cases and there was a significant positive correlation between seed-siring success and pollen-tube growth rate in vivo and in vitro. The results show that fertilizations are not random and pollen competition operates in a B. pendula seed orchard population.

Two mycotrophic species (Lactuca sativa and Abutilon theophrasti) and one nonmycotrophic species (Beta vulgaris) were grown in a P-deficient soil, and the effects of mycorrhizal inoculation on three variables that determine growth rate were assessed for each. The phosphorus-use efficiency (PUE, dW/dP) is the ratio of d. wt increase to P content increase. Plant P is the amount of P (the limiting resource) controlled by the plant, which can be allocated to various purposes. The phosphorus efficiency index (PEI, dP/Pdt) is the efficiency with which plant P is used to acquire P from the soil. Inoculated and control plants of a given species initially contained the same amount of P because all plants were grown from seed. Mycorrhizal colonization significantly increased the PEI of Lactuca and Abutilon (by 23 and 32%, respectively). As expected, mycorrhizal inoculation did not significantly increase the PEI of Beta. As a result, mycorrhizal inoculation significantly increased the P content of Lactuca and Abutilon, but not Beta. Mycorrhizal colonization decreased the PUE of lettuce, but did not significantly affect that of Abutilon or Beta. Mycorrhizal inoculation therefore slightly increased the growth rate of Lactuca, greatly increased the growth rate of Abutilon, and ultimately had no significant effect on the growth rate of Beta.

Three successive emergent macrophytes (Typha latifolia L., Phragmites australis (Cav.) Steudel and Glyceria maxima (Hartman) Holmbly) were each grown in substrates collected from three different zones of shoreline vegetation development (non-vegetated sediment, the interface between T. latifolia and P. australis, and degenerating P. australis). The aim of the study was to assess whether accumulation of litter changes growth conditions of P. australis, and to determine its effects on pre- and post successional plant species. The study was carried out by means of pot experiments in a glasshouse. Seedlings of the three species were cultured in fertilized and unfertilized substrates under both waterlogged and drained conditions.

In its own litter, growth of P. australis was strongly reduced, compared with the productivity of plants in substrates from preceding successional stages, and could not be compensated for by fertilization or soil drainage. The redox potential of the substrate was not strongly reduced and the sediment density was well above the critical level. Soil sterilization by gamma-irradiation did not improve growth substantially, although there was some positive effect in unfertilized substrate. Phytotoxic compounds might have caused poor growth of P. australis in its own litter. T. latifolia and G. maxima were relatively less affected by the P. australis litter. The possible importance of litter accumulation on species replacement in shoreline vegetation is discussed. It is concluded that the accumulation of organic matter should be considered as a factor affecting spatio-temporal processes in littoral vegetation owing to its specific impact on the functioning of individual dominant plant species.

Soil-borne spores and hyphae of arbuscular mycorrhizal fungi are important propagules in cracking clay soils of northern NSW, Australia. In these soils, senescent roots were uncommon. Although c. 4–200 spores g−1 soil were found, less than 6% established arbuscular mycorrhizas in trap plants, and this percentage declined over 24 months. Using tetrazolium red as a vital stain, 16–21 % of spores from field soils were found to be viable in fresh soil and 6–7% after 24 months of storage. Using fluorescein diacetate, the length of stained hyphae of c. 0·5 m g−1 soil was shown to be halved over 32 wk. The density of viable propagules of arbuscular mycorrhizal fungi in soil declined over time and was reduced by severe disturbance. The fungi that survived to 12 months included a species thought to form dormant spores, while those initiating infection after 24 months, did not.

The responses to CO2 of perennial grasses (Danthonia richardsonii and Phalaris aquatica) and legumes (Lotus pedunculatus and Trifolium repens) were compared under controlled conditions for isolated plants, monoculture stands and mixed-species stands along a N gradient to test whether: plant–plant interactions between species in mixed stands changed with concentration of CO2; responses to CO2 of species in mixtures could be related to their responses as single stands; responses of mixtures to CO2 could be related to the responses of individual species to CO2 and to competition. Plants were grown for 60 d in sand, using nutrient solutions (six nitrate concentrations from 0.25 to 16 mM NO3), at ambient (c. 357 μl l−1) or elevated CO2 (c. 712 μl l−1). Species dominance in the mixtures depended more on the range of N than of CO2 concentration provided: T. repens and L. pedunculatus dominated at low concentrations of N; L. pedunculatus and P. aquatica performed better at high concentrations. Responses of species in mixtures to CO2 were related to their responses in monocultures but not to those of isolated plants. Species biomass proportions in mixtures under ambient CO2 determined the outcome of mixture responses to CO2 more than of individual species responses to CO2. These results emphasize the influence of plant–plant interactions on community responses to CO2, since mixture behaviour under elevated CO2 could not be scaled-up from responses by isolated plants in this experiment.

A centrifugal method for extracting apoplastic sap from roots of lupin (Lupinus angustifolius) and pea (Pisum sativum) plants, and a method to analyse malic dehydrogenase in the sap using capillary electrophoresis, are described. Osmolality of apoplastic sap was relatively constant at relative centrifugal forces (RCFs) between 600 and 3000 g for lupin, and between 600 and 4000 g for pea. Electropherograms of a marker enzyme (malic dehydrogenase) and other components in apoplastic and symplastic saps revealed that contamination occurred at 7000 g. It is suggested that apoplastic sap expelled from plant roots at RCF between 600 and 3000 g is free from symplastic contamination, and is regarded as being of apoplastic origin. The proposed method was used to measure apoplastic pH changes in the plant roots in response to external pH, ammonium, nitrate and vanadate.

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol−1 CO2) and double ambient (720 μmol mol−1 CO2) carbon dioxide (CO2) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO2 concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO2 concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO2 concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation (Vcmax) and the light saturated rate of electron transport (Jmax) between the control and elevated CO2 treatment.

Five clonal plants of three genotypes of Lotus corniculatus were grown in each of eight controlled environments under combinations of two temperature regimes (18/10°C and 25/15°C), two CO2 concentrations (ambient and 700 ppmv) and two water applications (ad libitum or 60% droughted). Plants were harvested at full flower and measurements made of plant growth and development. Of the three environmental variables studied, higher growth temperatures resulted in the largest number of significant changes to the measured variables. Reproductive capacity, growth rate, shoot biomass, water use efficiency and chlorophyll content were all enhanced by raising the growth temperature from 18 to 25°C. Doubling the CO2 concentration enhanced the growth rate, shoot biomass and water use efficiency and ameliorated some of the effects of drought, including reproductive capacity, and biomass production, but reduced flowering time, specific leaf area, and chlorophyll content of both droughted and undroughted plants. Drought alone reduced reproductive capacity, growth rate and above ground biomass but significantly increased root biomass in all environments. The agronomic effects resulting from a combined increase in growth temperature, doubled CO2 concentration and mild drought in this experiment were a shorter vegetative period and an increase in biomass, but a fall in reproductive capacity.

The apoplastic pH of intact green leaves of Bromus erectus was measured non-invasively by inserting blunt microelectrodes through stomatal openings. After making electrical contact, the recorded signal was stable for hours, yielding a pH of 4.67±0.10. The leaves responded to ‘light-off’ with an initial transient acidification and subsequent sustained alkalinization of 0.2–0.3 pH; ‘light-on’ caused the opposite response. Flushing the leaves with 280 nmol NH3 mol−1 air within 18±6 s alkalinized the apoplast by 0.22±0.07 pH, followed by a slower pH increase to reach a steady-state alkalinization of 0.53±0.14 after 19±7 min. This pH shift was persistent as long as the NH3 was flushed, and readily returned to its initial value after replacing the NH3 with clean air. The resultant [NH4+] increase within the apoplast was measured with a NH4+-selective microelectrode. In the presence of 280 nmol NH3 mol−1 air, apoplastic NH4+ initially increased within 15±10 s to 1.53±0.41 mM, to reach a steady state of 1.62±0.16 mM after 27±7 min. An apoplastic buffer capacity of 6 mM pH−1 unit was calculated from the initial changes of pH and [NH4+ ], whereas the steady-state values yielded 2.7 mM pH−1. Infiltrated leaves responded to NH4+ with concentration-dependent depolarizations, the maxima of which yielded saturation kinetics indicating carrier-mediated NH4+ uptake into adjacent cells, as well as a linear component indicating non-specific transport. We infer that the initial alkalinization is due to rapid conversion of NH3 to NH4+, whereas the slower pH increase would be caused by regulatory processes involving both membrane transport, and (mainly) NH4+ assimilation. Possible consequences of the NH3-induced pH shift for the development of plants growing in polluted areas are discussed.