The aim of this paper is to review the present knowledge of the main aspects of differentiation of Acetabularia, a unicellular, eukaryotic organism, and to underline the multiple control pathways modulated by circadian rhythmicity. Growth and morphogenesis are sequentially programmed. Timing of cap differentiation is highly dependent on external conditions. The importance of the sequence of processes is shown by experimental disregulation.

The alga is a highly polarized cell, both in morphology and in the relative concentrations of a number of the molecules it contains. Apical cap differentiation is regulated at the post-transcriptional level and could also depend in part on polyamines and on proteolytic activity.

To investigate the extent and size of root–soil air gaps that develop during soil drying, resin casts of roots of the desert succulent Agave deserti Engelm. were made in situ for container-grown plants and in the field. Plants that were droughted in containers for 7 and 14 d had 24 and 34% root shrinkage, respectively, leading to root–soil air gaps that would reduce the hydraulic conductivity at the root–soil interface by a factor of about 5. When containers were vibrated during drought, root–soil air gaps were greatly diminished, and the predicted conductivity at the interface was similar to that of the control (moist soil). For plants in the field (4 and 6 wk after the last rainfall), root shrinkage was greater than for container-grown plants, but root–soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root–soil interface. To test the hypothesis that root–soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root, and shoot of plants from vibrated containers (with gaps eliminated or reduced) and non-vibrated containers were compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting more rapid depletion of soil water due to better root–soil contact, and the root water potential was lower as well, suggesting greater water loss by roots in the absence of root–soil air gaps. Thus, air gaps could benefit A. deserti by helping to maintain a higher root water potential in the early stages of drought and later by limiting root water loss at the root–soil interface when the water potential exceeds that of the soil.

Net CO2 assimilation rates and chlorophyll a fluorescence were measured on leaves of an ABA-unresponsive poplar (Populus trichocarpa × koreana cv. Peace) during a period of drought. Stomata of fully expanded leaves of ‘Peace’ partly closed during drought, while those of Populus euramericana cv. Robusta closed almost completely. The measured data were used to calculate total light-driven electron flows and to derive an estimate of a CO2 mole fraction in the chloroplasts. Two major results were obtained. (i) In well watered plants, photosynthesis operated at lower CO2 mole fractions in the chloroplasts of cv. Peace than in those of cv. Robusta, whereas CO2 mole fractions in the substomatal spaces were of the same order of magnitude. We concluded that a higher resistance to CO2 influx in the mesophyll contributed to the lower net assimilation rates in ‘Peace’. (ii) Drought induced an important decrease in CO2 availability in ‘Robusta’ but not in ‘Peace’. This suggested that reduced CO2 influx was a major cause of the limitation of net CO2 assimilation during drought in ‘Robusta’, but not in ‘Peace’, where drought probably reversibly reduced the apparent carboxylation efficiency of Rubisco. Measurements of O2 evolution under saturating CO2 supported this view, as photosynthesis decreased in droughted plants of ‘Peace’ but not of ‘Robusta’. Moreover, estimates of the 13C/12C isotope ratio in the leaves of both cultivars showed drought-related decreases in discrimination, which, in the case of ‘Peace’, could only be explained by changes in activity of the photosynthetic carbon reduction cycle.

Apparent Km and Vmax for net NO3− uptake and short-term translocation patterns of recently absorbed N were compared in a stay-green mutant and wild-type selection line of Lolium perenne L. by means of a series of depletion studies using 15NO3−, performed over 12 d under conditions of progressively increasing N deprivation. In view of the greater retention of N in senescent leaves of the stay-green phenotype, it was predicted that NO3− uptake would be up-regulated relative to the normal line, and that a proportionally higher fraction of recently absorbed N would be allocated to young leaves. It was shown that the stay-green trait had significant phenotypic consequences for plant N relations, with higher ‘sink strength’ of shoots for recently absorbed N, and higher Vmax for NO3− uptake compared with those of normal plants. The stay-green mutation had no effect on the Km of the nitrate uptake system. Although the N-use efficiency might be expected to be lower in stay-green than in normal plants, there were no differences in rates of dry matter production.

Cell division was examined during leaf initiation in the slow-to-green mutant of Lolium temulentum L. to test the hypothesis that the cell cycle in the leaf primordium is a key regulator of the well characterized reduction in final leaf length in the mutant compared with that of the wild type. The cell doubling time (cdt, by colchicine method) was substantially longer in the youngest leaf primordium (YLP) of the mutant (107 h) than in the wild type (43 h) although the duration of the most rapid cell cycle (cc, by percentage labelled mitoses method) was between 18–20 h in each. As a consequence, the proportion of rapidly proliferating cells was only 20% in the mutant compared with 47% in the wild type. The size of the shoot apical meristem and the plastochron were similar between genotypes which indicates that the shoot meristem was largely buffered from the effects of the mutation. Mitotic cell area was also similar in the YLP of both genotypes. However, as the leaf elongated, mitotic cell area and interphase cell size were significantly larger in the mutant compared with the wild type. This change was coupled with a reduced number of cells per unit length of leaf in the mutant. The data are consistent in showing that the proportion of rapidly proliferating cells in the YLP (but not the rate of cell division) is a key parameter which influences growth of the leaf.

Unifoliated plants of Lolium temulentum L. cv. Ceres, a qualitative long-day grass, were induced to flower by one 24-h long day (LD) or by one 8-h short day (SD) advanced by 12 h in the normal regime, so-called ‘displaced short day’ (DSD). Standard light for SD and DSD was a mixture of fluorescence and incandescence at 400 μmol m−2 s−1 whereas the extension period of the 24-h LD was solely incandescence at 10–15 μmol m−2 s−1. The DSD system was first characterized by the timings of floral induction, stimulus translocation and apical development. Carbohydrates in the blade tissues and in leaf exudate were analysed comparatively in vegetative and induced plants.

Fructans were not detected in the leaf tissues whereas sucrose and starch were found to be present in similar amounts. In SD, their contents exhibited a diurnal fluctuation and were not in large excess. The common change observed during the two inductive treatments was that starch remained at a high level during the LD extension, even though the lighting was unsuitable for photosynthesis, and increased transiently in DSD.

Sucrose was the major sugar contained in the leaf exudate. Its content increased when flowering was induced, but not at the same time in the two systems. In LD, sucrose exudation rose when plants were returned to standard light after the inductive cycle, i.e. after the LD stimulus had left the leaf blade. By contrast, during the DSD, sucrose was transported at the same time as the floral stimulus. Results are discussed together with the methods used to time stimulus translocation and their implications.

The effects of long-term carbon dioxide enrichment on competition for nutrients and light in a ryegrass/clover association were determined for simulated swards of perennial ryegrass (Lolium perenne L. cv. Parcour) and white clover (Trifolium repens L. cv. Karina), which were grown as monocultures and in three mixtures (25/75, 50/50, 75/25), according to the replacement design, at two levels of nitrogen (N) supply (no additional N and 200 kg N ha−1) and at season-long ambient (380 ppm) and elevated (670 ppm) CO2 concentrations, in open-top chambers. Stands were cut four times, at about monthly intervals, to a height of 5 cm. Plant material was separated into different species, fresh and dry weights were determined and the content of macroelements (N, P, K, S, Mg) in both species was measured. In addition, plant height of both species at harvest dates and during several regrowth periods was monitored.

Results indicate that both species made demand on different resources and profited from growth in a mixed sward. CO2-related yield increase amounted to 16–42% for white clover whereas the effect of high CO2 on ryegrass yield ranged between −33% and +9% depending on N supply, mixture and year. As a result the contribution of white clover to total yield in mixed swards was significantly enhanced by CO2 enrichment at many harvests in both N supply treatments. Without additional N supply, shoot competition for light was intensified by CO2 enrichment to the disadvantage of ryegrass, since clover petioles grew longer and ryegrass was shorter at elevated CO2. With N fertilization, no marked effect of CO2 enrichment on interspecific competition could be observed. Since clover and total yield were increased by CO2 enrichment, nutrient requirements were also increased and potassium deficiency and increased intraspecific competition of clover for K was observed in the mixtures under elevated CO2 which had the highest nutrient withdrawal. Although white clover profited much more from CO2 enrichment in both N fertilization treatments, the suppression of ryegrass in mixed swards could only be observed under low N conditions.

Generally, the effect of N fertilization on competitive interference between both species was much greater than the effect of CO2 enrichment and it is suggested that the effect of elevated CO2 on the balance of species and the outcome of competition in a grass/clover sward is mainly dependent on the N status.

This study investigated the mechanisms which control the partitioning between roots and shoots in plants subjected to changes in environment. Two types of analyses were used: firstly, an examination of the cost and revenue associated with investment in different plant parts, and secondly, a test of the principle of functional equilibrium between roots and shoots, i.e. whether root dry matter × root specific activity balances shoot dry matter × shoot specific activity. Measurements were made on individual plants of Lolium perenne in sunlit controlled environments, grown from germination to canopy closure under optimal nitrogen supply. At the final harvest, increased air temperature (+4 °C above ambient) reduced whole-plant dry matter by 12% relative to the control, whereas elevated CO2 mole fraction (700 μmol mol−1) led to a 38% gain. The combined treatment yielded an intermediate result (+19%). Plants grown at +4 °C maintained balanced activity between roots and shoots throughout the experimental period, irrespective of CO2 concentration. This required enhanced allocation to roots in young plants to compensate for a strong negative effect of higher temperature on root specific activity, which suggests that plants conserve balanced activity by adjusting dry matter partitioning. The extra cost involved with the adjustment at +4 °C significantly enhanced the cost[ratio ]revenue ratio of plant investment. In ambient temperature, the balance between roots and shoots departed from equilibrium, slightly at ambient but substantially at elevated CO2: the plants accumulated excess carbon relative to nitrogen, and this imbalance increased with plant age. At elevated CO2, the cost[ratio ]revenue ratio increased in young plants but this was later reversed owing to loss of root specific activity, which explains the gradually declining CO2 stimulation with time. The strategies in equilibrating root and shoot functioning observed in the different treatments are discussed in the light of whole plant performance.

To assess adverse effects of ambient ozone on injury and yield, EDU (ethylenediurea) and non-EDU-treated plants of bean (Phaseolus vulgaris L. cv. Lit) were exposed to ambient air at four rural sites in the Netherlands in 1994. After 6 wk of exposure, the proportion of leaves injured in non-EDU-treated plants was higher than that in plants treated with the antioxidant. A maximal degree of 35% injury was observed. The highest ozone levels accumulated at each site for three consecutive days before injury clearly exceeded the proposed short-term critical level for injury development. Both injury in untreated plants and the protective effect of EDU differed between sites and were not related to the measured ozone levels. EDU increased the d. wt of dry (mature) pods after 11 wk of exposure but not of green (marketable) pods after 6 wk. The effect of EDU in enhancing dry pod yield did not differ between sites and was 20% on average. The proportional yield increases in EDU-treated plants compared with those of non-EDU-treated plants at the sites were not correlated with the measured ozone levels known to adversely affect bean yield. The available evidence, however, suggests that ambient ozone was involved in the observed yield increase by EDU. When the dry pods were harvested, EDU-treated plants had more leaves than non-EDU-treated ones. The effect of EDU in delaying senescence appears to be an important factor to consider when attempting to evaluate the effect of ambient ozone on pod weight in beans using the EDU approach.

Arabidopsis thaliana (L.) Heynh. was exposed to various concentrations of SO2 during almost the entire life cycle. No negative effects of SO2 on shoot biomass production were observed. There was a linear relation between the deposition of SO2 and the atmospheric SO2 concentration. Sulphur accumulated substantially in the shoots upon SO2 exposure. SO2 was either oxidized to sulphate or converted into extra organic sulphur compounds in a sulphate to organic sulphur ratio of 3[ratio ]1, independently of the atmospheric SO2 concentration. Exposure to SO2 resulted in an increase in water soluble non-protein sulphydryl content and a slight increase in the amount of glucosinolates, but both of these play a minor role as sinks for the excess sulphur. The ratio of total nitrogen to total sulphur decreased upon SO2 exposure as a result of the increased sulphate content. The organic nitrogen to organic sulphur ratio did not change upon SO2 exposure, indicating no changes in the composition of sulphur-containing compounds.

We tested the hypothesis that functional attributes of living leaves provide a basis for predicting the decomposition rate of leaf litter. The data were obtained from standardized screening tests on 38 British herbaceous species. Graminoid monocots had physically tougher leaves with higher silicon contents than did herbaceous dicots, and this corresponded with the lesser decomposibility of the former. Total base content of living leaves was a good predictor of litter decomposition rate, but the evolutionary and ecological basis for this relationship appeared to differ between graminoid monocots and herbaceous dicots. In the monocots, litter decomposition rate was strongly predicted by leaf potassium content, which appeared to reflect other growth-related plant attributes such as seedling maximum relative growth rate, foliar nitrogen and phosphorus content, specific leaf area and short leaf lifespan. In the dicots the relationship between total leaf base content and litter decomposition rate was not unambiguously explained by growth-related leaf attributes, possibly because of the considerable calcium uptake by dicots, which varies according to calcium availability in the soil.

Lipids were extracted from needles of Norway spruce plants of different provenances. The plants, seeds of which were originally collected from all over Europe, were grown at two sites in England at the same longitude but one 350 km north of the other. Significant correlations were observed between monogalactosyl diglyceride (MGDG) and digalactosyl diglyceride (DGDG) ratios on the one hand and, on the other, the lowest extreme winter temperature at the original site of seed collection (EWTS).

In the case of lipid changes, however, environmental differences due to site were more important than the inherent genotypic characteristics of the planted trees while provenances with the lowest EWTS values had more MGDG than DGDG. This implies that existing winter hardiness under mild climatic conditions does not come from changes in this ratio although DGDG formation from MGDG is known to be an important feature of winter hardening in Norway spruce elsewhere in Europe. However, closer analysis of the fatty acids in lipids from provenances at individual sites did reveal significant correlations between those provenances with low EWTS values and certain molar amounts or ratios of fatty acids in DGDG, some of which could be traced to genotypic expression. These partly involved the unusual Δ5 desaturations which are mainly confined to the fatty acids of those conifers like Norway spruce that harden to very low temperatures.

The presence of copper-binding proteins produced in response to added copper was examined in isolates of Laccaria laccata (Scop. ex Fr.) Cooke and Paxillus involutus (Batsch ex Fr.) Fr. taken from copper-contaminated and uncontaminated sites, and in a single isolate of Scleroderma citrinum Pers. from a contaminated site. Two isolates of Laccaria (GLac4 and ELac1) grew better in 1·5 mM and 2·5 mM copper than a third (Lac3G) and were considered to be more tolerant. Amongst five isolates of P. involutus, three (WJPax2R, GPaxRSp2 and Pax4) were capable of growth in media containing 4·0 mM copper and were regarded as tolerant. All isolates of both Laccaria and Paxillus were capable of some growth in 2·5 mM copper, but S. citrinum was much more copper-sensitive and the concentration had to be reduced at least 10-fold before any growth occurred. Tolerance of isolates was not related to whether they were taken from copper-contaminated or uncontaminated sites. Copper-binding proteins were detected in response to copper in the culture media in the two tolerant isolates of Laccaria (GLac4 and ELac1) but not in the least tolerant isolate. In Paxillus, similar proteins were found in two tolerant isolates (GPaxRSp2 and Pax4) but not in WJPax2R, which was also regarded as tolerant, nor in any of the less tolerant isolates. Copper-binding proteins were not detected in S. citrinum. The copper-binding protein purified from the Laccaria isolate ELac1 appeared as a single band in modified SDS–PAGE electrophoresis. Its molecular mass and spectral characteristics were consistent with it being a metallothionein.

Mycorrhizal mycelium was allowed to grow into a compartment from which roots were excluded. Ammonium or nitrate was supplied exclusively to the fungus and the respiration rate before and after nitrogen addition was measured. Mycelial respiration in the fungal compartment represented 11–29% of total fungal and root respiration. The mycelium in the fungal compartment received 20–29% of shoot net assimilation, and 43–64% of the carbon allocated to the mycelium was respired. The nitrogen-induced respiration increase ranged, in the fungal compartment, from 54 to 180%.

Little is known about non-mycorrhizal endophytic fungi in tropical orchids; still less is known about how endophytes vary within and between individual orchid plants. Fungal endophytes were isolated from roots and leaves of epiphytic and lithophytic orchids in the genus Lepanthes; seven species, from rainforests in Puerto Rico, were sampled. The endophytes observed most frequently were Xylaria species and Rhizoctonia-like fungi, found in 29% of roots and 19% of leaves, and 45% of roots and 31% of leaves, respectively. Five deuteromycete genera were also isolated, occurring in 19% of roots and 43% of leaves (combined). At least nine species of Xylaria were found, with several species sometimes occurring in a single plant. Differences between roots and leaves in frequency of Xylaria and Rhizoctonia isolates were not significant, although differences among orchid species in number and types of endophytes were. Heterogeneity of endophytes in single plants and plant organs was greater than differences between species. Many Lepanthes species are very restricted in distribution, and knowledge of their interactions with endophytes might be useful in species management.

Production of enzymes which degrade plant cell wall macromolecules has been studied in relatively few ericoid fungal isolates, although these polymers are a major component of the organic litter and an important source of nutrients for these fungi. Our aims were to investigate whether the ability to degrade the wall pectic component, only reported for one isolate, is a general feature of ericoid fungi. Of about 35 isolates from different geographic regions, all were capable of growing on pectin as the sole carbon source. Polygalacturonase (PG) activity was detected to a different degree in the culture filtrates and independently of the fungal growth rate. Solid and liquid isoelectric focusing allowed separation and identification of several polygalacturonase isoforms. Among the fungal isolates investigated, those from the northern hemisphere produced mostly acidic isoforms, whereas isolates from South Africa secreted more abundantly basic isoforms. However, purification and biochemical characterization of several PG isoforms from the different isolates revealed an optimal activity in the acidic pH range for all the PG enzymes tested. Polygalacturonase enzymes seem to be an important component of the enzymatic arsenal secreted by ericoid fungi during their saprotrophic life. In addition, they could also play a role during root colonization, since penetration across the plant cell wall is a prerequisite for the establishment of endomycorrhizal symbiosis.

The presence of extracellular carbonic anhydrase (CA) in relation to medium composition was investigated using cultures of the marine diatom Phaeodactylum tricornutum Bohlin. Large-volume cultures, with low initial cell inocula were grown on ASP-2 (no dissolved inorganic carbon (DIC), 550 μM NO3−), f/2 (2·0 mM DIC, 880 μM NO3−) and modified f/2 (2·0 mM DIC, 20 μM NO3− media. Cells growing on ASP-2 showed extracellular CA in the early stages of growth, whereas extracellular CA was not detected until partial depletion of total DIC in the stationary phase for cultures on f/2 or modified f/2. Both HCO3− and CO2 were important in carbon limitation, extracellular CA being present when the free-CO2 concentration fell below 5 μM, but the HCO3− concentration needed to be below 1 mM. When carbon-replete cells were transferred to carbon-limited conditions, extracellular CA was recorded within minutes, the process being light-dependent and completely inhibited by 3,3,4-dichlorophenyl-1,1-dimethylurea (DCMU). The addition of DIC to carbon-limited cells resulted in a rapid decrease in extracellular CA activity. The membrane-impermeable inhibitor of carbonic anhydrase, dextran-bound sulphonamide (DBS) was used to inhibit extracellular CA activity in relation to photosynthetic rate in carbon-replete and carbon-limited cells. At the lowest DIC concentration (0·10 mM), for cells with maximum external CA activity, DBS gave over 80% inhibition of the photosynthetic rate, demonstrating the key role of external CA in maintaining high photosynthetic rate under conditions of carbon limitation.

It is proposed that the key factor in the regulation of extracellular CA activity is the total flux of inorganic carbon (Ci) into the cell. This determines the Ci flux into the chloroplast and when this is inadequate to support the photosynthetic rate attained by a carbon-replete chloroplast at optimum photon flux density, extracellular CA is activated. This mechanism would explain the observed interaction of CO2 and HCO3− in the regulation of extracellular CA activity.

Re-analysis of data from Salisbury (1974) shows no significant relationship between seed size and habitat conditions, a relationship for which this paper is frequently cited as support. The discrepancy between the results reported here and those found in the original paper is due to Salisbury's ‘evidence’ being a table of congeneric pairs collected specifically because they possessed the target relationship. This paper emphasizes particularly the need for an unbiased sampling of species pairs in comparisons attempting to define ecological relationships. The analysis presented here represents the fourth such instance in which previously assumed robust examples of the relationship between seed size and habitat type are shown not to be so when the appropriate methods are used to account for relatedness among species.