For three decades, hypotheses relating to the occurrence and function of cyclic nucleotides in higher plants have been highly controversial. Although cyclic nucleotides had been shown to have key regulatory roles in animals and bacteria, investigations with higher plants in the 1970s and early 1980s were criticized on the basis of (i) a lack of specificity of effects apparently elicited by cyclic nucleotides, (ii) the equivocal identification of putative endogenous cyclic nucleotides and (iii) ambiguity in the identification of enzymes connected with cyclic nucleotide. More recent evidence based on more rigorous identification procedures has demonstrated conclusively the presence of cyclic nucleotides, nucleotidyl cyclases and cyclic nucleotide phosphodiesterases in higher plants, and has identified plant processes subject to regulation by cyclic nucleotides. Here we review the history of the debate, the recent evidence establishing the presence of these compounds and their role; future research objectives are discussed.

The tissue-specific and developmental expression of histone 2A was studied in onion (Allium cepa ‘Robusta’), using northern blots. Histone 2A expression was enriched in basal tissues, particularly in the inner, meristematically active parts of bulbs. The expression was assessed during a time course of bulb development, dormancy onset and post-harvest sprouting in field-grown material. During bulb development histone 2A expression in the inner bulb declined rapidly during bulb ripening, reaching a minimum with the onset of dormancy. During post-harvest storage, expression increased slowly, reaching a peak in the spring, coinciding with the first observed sprout emergence. It was concluded that in onion, as in other plant systems, histone 2A expression is linked to cell division and dormancy level, the peak in expression during post-harvest storage indicating the time of dormancy breakage. In cultivars where post-harvest sprouting occurred much earlier or much later than in ‘Robusta’, this expression peak occurred at about the same time of year, regardless of sprouting time. It was concluded that differences in storage longevity between cultivars were not due to differing times of dormancy breakage. Factors controlling the rate of sprout emergence post-dormancy are likely to be major determinants of storage capacity.

The resurrection plant Myrothamnus flabellifolia has the ability to recover from repeated prolonged and extreme desiccation cycles. During the dry state the inner walls of the xylem vessels seemed to be covered, at least partly, by a lipid film as shown by Sudan III and Nile Red staining. The lipid film apparently functioned as an ‘internal cuticle’ which prevented the adjacent parenchyma ray cells from complete water loss. The hydrophobic nature of the inner xylem walls was supported by the finding that benzene ascended as rapidly as water in the xylem of dry Myrothamnus branches. On watering, numerous lipid bodies were found in the water-conducting vessels, presumably formed from the lipid film and/or from lipids excreted from the adjacent living cells into the vessels. The presence of lipid bodies within the vessels, as well as the hydrophobic properties of the inner xylem walls, could explain the finding that the xylem pressure of hydrated, well watered plants (measured both under laboratory and field conditions with the xylem pressure probe) never dropped below c. −0.3 MPa and that cavitation occurred frequently at low negative xylem pressure values (−0.05 to −0.15 MPa). The xylem pressure of M. flabellifolia responded rapidly and strongly to changes in relative humidity and temperature, but less obviously to changes in irradiance (which varied between 10 and c. 4000 μmol m m−2 s−1). The morphological position of the stomata in the leaves could explain the extremely weak and slow response of the xylem pressure of this resurrection plant to illumination changes. Stomata were most abundant in the furrows, and were thus protected from direct sunlight. Simultaneous measurements of the cell turgor pressure in the leaf epidermal cells (made by using the cell turgor pressure probe) revealed that the xylem and the cell turgor pressure dropped in a ratio of 1:0.7 on changes in the environmental parameters, indicating a quite close hydraulic connection and, thus, water equilibrium between the xylem and cellular compartments. An increase in irradiance of c. 700 μmol m−2 s−1 resulted in a turgor pressure decrease from 0.63 to 0.48 MPa. Correspondingly, the cell osmotic pressure increased from 1.03 to 1.22 MPa. From these values and by assuming water equilibrium, the osmotic pressure of the xylem sap was estimated to be 0.25–0.4 MPa. This value seems to be fairly high but may, however, be explained by the reduction of the water volume within the vessels due to the floating lipid bodies.

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.

Chlorophyll fluorescence measurements were used to test whether externally applied abscisic acid (ABA) has an effect on the desiccation tolerance of lichens and their sensitivity to constant water saturation. A surplus of ABA did not decrease the time required to regain full photosynthetic capacity after prolonged dry periods. However, an effect of ABA could be observed when lichens were permanently hydrated at moderately high temperatures. Lichens suffered less from constant saturation if ABA was added to the incubation medium. These results suggest a positive effect of ABA on membrane function.

The relative rates of ammonium and nitrate-N uptake and assimilation by creeping bent (Agrostis stolonifera), were investigated for plants grown in soil and supplied with three different ratios of ammonium and nitrate-N. Following two preliminary defoliations, plants were supplied with the equivalent of 150 kg N ha−1, given as 15N-(differentially) labelled NH4+ and NO3−-N in three different ratios (20:80, 50:50 and 80:20), followed by sequential destructive harvests of shoots and roots at four points during a 35-d regrowth period. Maximum use of labelled nitrogen and ‘exhaustion’ of soil mineral nitrogen reserves occurred much earlier when plants were supplied with half or more of their nitrogen as ammonium, than occurred when they were supplied predominately with nitrate-N. The lack of consistency in the patterns of ammonium and nitrate-N absorption, however, implied that the plants had no specific preference for either nitrogen form. Supplying plants with different combinations of ammonium and nitrate produced distinctive differences in plant morphology. In the high nitrate treatment, plants preferentially partitioned resources into shoot and stolon formation, whereas in the high ammonium treatment, resources were preferentially partitioned into root production. These changes in plant morphology might be adaptations to aid species survival in environments associated with a predominance of either nitrogen form.

Leaching of nitrate through soil profiles is a major cause of soil acidification. Legume species differ in their ability to take up nitrate (NO3−) and in the degree to which soil NO3− impairs legume nodulation and N2 fixation. This pot experiment examined species variation in N2 fixation and acid production in grain legumes grown for 6 wk on a sandy soil with Ca(NO3)2 added at 0–56.7 mg N kg−1 soil. The eight legumes studied were Lupinus angustifolius (‘Gungurru’), Lupinus albus (‘Kiev mutant’), Lupinus luteus (‘Teo’), Cicer arietinum (Selection T1587), Pisum sativum (‘Dundale’ and ‘Wirrega’), Vicia faba (‘Fiord’) and Lathyrus sativus (Selection 453). While number of nodules was not significantly affected by treatment with NO3−, increasing supply of NO3− decreased nodule mass, with P. sativum ‘Wirrega’ being most sensitive and C. arientinum and V. faba least sensitive. The concentrations of nitrogen in plants generally decreased as the supply of NO3− increased. Increasing supply of NO3− decreased the percentage of N2 fixation, most significantly in L. sativus and least in L. albus. In the absence of added NO3−, the amounts of H+ produced per unit biomass (specific acid production) differed by as much as 2.2-fold among the species, with C. arientinum generating most H+, followed by L. angustifolius, L. luteus, L. albus, V. faba, L. sativus, P. sativum ‘Dundale’ and P. sativum ‘Wirrega’. This species variation was even greater at higher NO3− supply. In the presence of NO3− at up to 14.2 mg N kg−1, acid production by these legumes correlated well with uptake of excess cations but not with the concentration of nitrogen in plants. For all species, release of H+ declined with increasing supply of NO3−, decreasing most in L. sativus and least in C. arientinum. The results suggest that there might be scope for selecting grain legume species in order to minimize soil acidification by decreasing leaching of NO3−.

Microbial immobilization may decrease the inorganic nutrient concentrations of the soil to the extent of affecting plant nutrient uptake and growth. We have hypothesized that graminoids with opportunistic nutrient-acquisition strategies are strongly influenced by nutrient limitation imposed by microbes, whereas growth forms such as dwarf shrubs are less affected by the mobilization–immobilization cycles in microbes. By adding NPK fertilizer, labile C (sugar) and fungicide (benomyl) over a 5 yr period in a fully factorial design, we aimed to manipulate the sink–source potential for nutrients in a non-acidic heath tundra soil. After 2 yr, N and P accumulated in the microbial biomass after fertilization with no change in microbial C, which suggests that nutrients did not limit microbial biomass growth. After 5 yr, microbial C was enhanced by 60% in plots with addition of labile C, which points to C-limitation of the microbial biomass. Microbial biomass N and P tended to increase following addition of labile C, by 10 and 25%, respectively. This caused decreased availability of NH4+ and P, showing close microbial control of nutrient availability. The most common graminoid, Festuca ovina, responded to fertilizer addition with a strong increase, and to labile C addition with a strong decrease in cover, providing the first direct field evidence that nutrient limitation imposed by immobilizing microbes can affect the growth of tundra plants. Also in support of our hypothesis, following addition of labile C the concentrations of N and K in leaves and that of N in roots of F. ovina decreased, whilst the demand of roots for P increased. In contrast, the most common dwarf shrub, Vaccinium uliginosum, was only slightly sensitive to changes in resource availability, showing no cover change after 4 yr addition of labile C and fertilizer, and little change in leaf nutrient concentrations. We suggest that the differential responses of the two growth forms are due to differences in storage and nutrient uptake pathways, with the dwarf shrub having large nutrient storage capacity and access to organic forms of N through its mycorrhizal association. While the fungicide had no effect on ericoid mycorrhizal colonization of roots or symbiotic function inferred from plant 15N natural abundance, it decreased microbial biomass C and N after 2 yr. Throughout the fifth season, the availability of soil NO3− and inorganic P was decreased with no change in microbial biomass C, N or P, suggesting a negative impact of benomyl on N and P mineralization.

This study focuses on the following questions: (i) whether reductions in root:shoot ratio have a cost in terms of nutrient balance of the plant, and (ii) whether changes in resource-allocation patterns are proportional among different resources. Our approach was to analyse the variations in the allocation pattern induced by soil waterlogging. A pot experiment was conducted to analyse the effects of waterlogging on biomass, phosphorus (P) and nitrogen (N) accumulation of Paspalum dilatatum and Danthonia montevidensis, two waterlogging-tolerant grasses. When changing from oxic to anoxic conditions, a common response of these and other waterlogging-tolerant grasses is a reduction in allocation to below-ground resources. It was observed that (i) the reduction in root:shoot ratio caused by waterlogging did not have a cost in terms of capacity for nutrient uptake; (ii) resource partitioning within aerial parts was less sensitive to treatments than partitioning between roots and shoots; and (iii) biomass does not appear to be a useful currency for evaluating nutrient-allocation patterns, as the allocation of P and N was inadequately represented by biomass. The results presented here indicate that the existence of compensation mechanisms reduces the predictive value of the partition of resources for the capacity of plants to acquire resources. Data on the allocation of nutrients in relation to biomass suggest that the assumptions of independence in the allocation pattern between biomass and limiting nutrients under the effects of environmental factors can be extended.

To clarify the role of seasonal change, competitive response and nutrient availability in the competitive asymmetry of grassland species a competition experiment was conducted on Holcus lanatus, Anthoxanthum odoratum and Festuca ovina, which represent a successional sequence of decreasing nutrient availability. Seven harvests were taken over two growing seasons. At each harvest the dry weight of plant parts, dead leaves, leaf area and plant height were measured. Three key traits that determine the successional status of the species were studied: specific leaf area, specific shoot height, and dead leaf fraction.

The response of these traits to competition appeared to be limited and insufficient to change the competitive relations in the experiment. However, all three traits showed marked seasonal changes which resulted in superior growth and survival in winter of the species adapted to nutrient-poor environments. The findings support the theory that competitive asymmetry increases at higher nutrient levels. It is postulated that the directionality of light makes it possible for the dominant species to monopolize this resource more easily than nutrients.

Eight different species of the red algal genus Porphyra from different biogeographic regions in Europe, Africa, North America, Asia and Australia were collected over the course of the respective growth season and surveyed for the qualitative and quantitative occurrence of the low molecular weight carbohydrates, floridoside, D-isofloridoside and L-isofloridoside. Except for Porphyra saldanhae from South Africa, all species tested exhibited clear seasonal variations in their heteroside composition. Whereas Porphyra dioica and Porphyra umbilicalis from the North Sea showed highest carbohydrate concentrations in spring and summer, those of Porphyra leucosticta from Spain were highest in winter. The red algae studied exhibited highest concentrations of heteroside at different seasons, some in winter, others in spring or summer. The composition of the three compounds varied among the species studied. In P. columbina from Australia, L-isofloridoside was always quantitatively dominant, while floridoside was the major component in P. dioica. Usually D-isofloridoside was present in small concentrations, except in P. perforata from the Pacific coast of the USA where it occurred in equal concentrations with floridoside and L-isofloridoside. The results point to species-specific different enzymic activities of the underlying anabolic pathways. Correlations between the heteroside concentrations and various prevailing environmental parameters are detectable but do not reveal a general response of Porphyra. Data from the literature prove that numerous physical factors determine the growth patterns of various Porphyra species in different biogeographic regions. Of these, photoperiod, temperature and nutrients are the most important. Highest heteroside concentrations in the Porphyra species studied always coincided with enhanced growth, and this relationship determined that growth, though related to abiotic conditions, was also promoted by maximum carbohydrate content. The multiple physiological function of floridoside, D-isofloridoside and L-isofloridoside as osmolytes, compatible solutes and carbon reserves is discussed in relation to the environmental stresses that Porphyra species usually experience in their upper intertidal habitats.

Two cultivars of sorghum (CSH-1 and Ochuti) were grown in the presence and absence of the root hemiparasite Striga hermonthica in uniform conditions in the field in Kenya, Africa. S. hermonthica had a marked influence on growth and photosynthesis of ‘CSH-1’; however, ‘Ochuti’ showed a less severe response to infection and tolerance of the parasite. The variation in genotype response might be partly explained by later attachment of the parasite and a lower level of infection. Laboratory studies were used to determine the importance of both variables in determining host response to infection. Early infection by S. hermonthica had a more negative effect on the host than late infection. The level of parasite biomass supported by the host also influenced host productivity but the relationship was nonlinear. Low degrees of parasite infection had a proportionately much greater effect on host grain weight than at greater parasite loading. Early infection of ‘Ochuti’ in laboratory conditions resulted in lower stem dry weight than in uninfected plants but not in smaller total plant biomass or lower rates of photosynthesis. In conclusion, the time of parasite attachment affected host performance and might explain much of the variation in host sensitivity both within and between studies. The level of parasite infection affected host performance to a lesser extent. In addition, late attachment and low levels of infection might have implications for control management strategies.

Endogenous polyamine content of the ectomycorrhizal fungus Paxillus involutus, as well as the activity of its biosynthetic enzymes in relation to mycelia ageing were investigated in this work. Polyamines in free, PCA-soluble and insoluble conjugated forms, are present in Paxillus involutus mycelia in relatively high amounts and the ratio of putrescine to spermidine is age-dependent. Both arginine- and ornithine-decarboxylases are present, but putrescine biosynthesis proceeds mostly via ornithine decarboxylase and decreases with the age of mycelia. There was a large release of free polyamines from mycelia which showed age-dependent features. Clear polyamine uptake was observed in 2-wk-old mycelia and no competition between putrescine and cadaverine was detected. Putrescine uptake seems to reduce ornithine decarboxylase activity, but does not affect arginine decarboxylase.

Short-term phosphate uptake rates were measured on intact ectomycorrhizal and non-mycorrhizal Pinus sylvestris seedlings using a new, non-destructive method. Uptake was quantified in semihydroponics from the depletion of Pi in a nutrient solution percolating through plant containers. Plants were grown for 1 or 2 months after inoculation at a low relative nutrient addition rate of 3% d−1 and under P limitation. Four ectomycorrhizal fungi were studied: Paxillus involutus, Suillus luteus, Suillus bovinus and Thelephora terrestris. The Pi-uptake capacity of mycorrhizal plants increased sharply in the month after inoculation. The increase was dependent on the development of the mycobionts. A positive correlation was found between the Pi-uptake rates of the seedlings and the active fungal biomass in the substrate as measured by the ergosterol assay. The highest Pi-uptake rates were found in seedlings associated with fungi producing abundant external mycelia. At an external Pi concentration of 10 μM, mycorrhizal seedlings reached uptake rates that were 2.5 (T. terrestris) to 8.7 (P. involutus) times higher than those of non-mycorrhizal plants. The increased uptake rates did not result in an increased transfer of nutrients to the plant tissues. Nutrient depletion was ultimately similar between mycorrhizal and non-mycorrhizal plants in the semihydroponic system. Net Pi absorption followed Michaelis–Menten kinetics: uptake rates declined with decreasing Pi concentrations in the nutrient solution. This reduction was most pronounced in non- mycorrhizal seedlings and plants colonized by T. terrestris. The results confirm that there is considerable heterogeneity in affinity for Pi uptake among the different mycobionts. It is concluded that the external mycelia of ectomycorrhizal fungi strongly influence the Pi-uptake capacity of the pine seedlings, and that some mycobionts are well equipped to compete with other soil microorganisms for Pi present at low concentrations in soil solution.