Summary

I. INTRODUCTION – A HYPOTHESIS 374

II. EFFECTS OF CALCIUM ON PHYSIOLOGICAL PROCESSES 376

1. The chemical uniqueness of calcium 376

(a) Cytotoxicity 376

(b) Binding properties 376

(c) Stimulation/displacement potential 376

2. Calcium signaling and plant responses to environmental stress 377

(a) Control principles 377

(b) Carbohydrate metabolism 378

(c) Synthesis and function of membranes and cell walls 379

(d) Disease resistance and wound repair 380

(e) Cold tolerance 381

(f) Stomatal regulation 382

III. CALCIUM UPTAKE AND DISTRIBUTION AT THE WHOLE-PLANT LEVEL 382

1. Uptake at the root-soil interface 382

2. Transport and exchange in stems 384

3. Exchange of calcium by foliage 385

IV. ECOSYSTEM PROCESSES AND CALCIUM SUPPLY 387

1. Plant succession and soil acidification 387

2. Plant adaptations to nutrient deficiency 389

(a) Morphological adaptations 389

(b) Physiological adaptations 390

V. PLANT AND ECOSYSTEM RESPONSES TO HUMAN ALTERATIONS IN CALCIUM SUPPLY 391

1. Increased atmospheric inputs of acidity 391

(a) Reductions in soil cation pools 392

(b) Inhibition of calcium uptake and effects on root function 393

(c) Increased leaching of calcium from foliage 395

(d) Physiological indicators of altered forest function 397

(e) Wood chemistry, structure and function 402

2. Forest management 404

(a) Harvesting effects on nutrient supply 404

(b) Managing forest nutrient supply 405

VI. CONCLUSION 407

1. Whole-tree perspectives 407

2. Ecosystem perspectives 408

VII. EVALUATION OF THE HYPOTHESIS 410

Acknowledgements 411

References 411

Summary

Calcium occupies a unique position among plant nutrients both chemically and functionally. Its chemical properties allow it to exist in a wide range of binding states and to serve in both structural and messenger roles. Despite its importance in many plant processes, Ca mobility is low, making Ca uptake and distribution rate a limiting process for many key plant functions. Ca plays an essential role in regulating many physiological processes that influence both growth and responses to environmental stresses. Included among these are: water and solute movement, influenced through effects on membrane structure and stomatal function; cell division and cell wall synthesis; direct or signaling roles in systems involved in plant defense and repair of damage from biotic and abiotic stress; rates of respiratory metabolism and translocation; and structural chemistry and function of woody support tissues. Forest trees, because of their size and age capacity, have been examined for evidence of limitations imposed by the timing and level of Ca supply. Examination of Ca physiology and biogeochemical cycling for forested systems reveals many indications that Ca supply places important limitations on forest structure and function. These limitations are likely to be most significant with older trees, later successional stages, high levels of soil acidity and/or high canopy Ca leaching losses, or under conditions where plant competition is high or transpiration is limited by high humidity or low soil moisture. Evidence of structural and physiological adaptations of forests to limited Ca supply; indicators of system dysfunction at many levels under reduced Ca supply; and the positive responses of diverse indicators of forest vitality in liming experiments indicate that Ca is more important to forest function and structure than has generally been recognized. Lack of recognition of Ca limitations is due in part to that fact some important plant functions are controlled by changes in very small physiologically active pools within the cytoplasm, and whole-leaf Ca levels may not reflect these limitations. An additional aspect is the fact that Ca availability has declined significantly for many forests in just the past few decades. Additional research on the role of Ca supply in resistance of forests to disease, changes in structural integrity of woody tissues, restrictions on rooting patterns and function, and uptake of other nutrients, notably N, is needed. Increased understanding of the physiological ecology of Ca supply can be anticipated to provide important insights that will aid in future protection and management of both natural and commercial forest systems.

Frictional resistance to a penetrating body can account for more than 80% of the total resistance to penetration of soil. We measured the frictional resistance between growing root caps of maize and pea and ground and smooth glass surfaces, which was linearly correlated to load, allowing calculation of the coefficient of kinetic friction and adhesion. Coefficients of kinetic friction between the root caps and the ground and smooth glass surfaces were approximately 0.04 and 0.02, respectively, the first measurements of the frictional properties of root tips at rates approaching those of root elongation, and an order of magnitude smaller than those previously reported. Results suggest that roots are well designed for penetrating soil, and encounter only small frictional resistance on the root cap. These data provide important parameters for modelling soil stresses and deformation around growing root tips.

Environmentally induced maternal effects on offspring phenotype are well known in plants. When genotypes or maternal lineages are replicated and raised in different environmental conditions, the phenotype of their offspring often depends on the environment in which the parents developed. However, the degree to which such maternal effects are maintained over subsequent generations has not been documented in many taxa. Here we report the results of a study designed to assess the effects of parental environment on vegetative and reproductive traits, using glasshouse-raised maternal lines sampled from natural populations of Arabidopsis thaliana. Replicates of five highly selfed lines from each of four wild populations were cultivated in two abiotic environments in the glasshouse, and the quality and performance of seeds derived from these two environments were examined over two generations. We found that offspring phenotype was strongly influenced by parental environment, but because the parental environments differed with respect to the time of seed harvest, it was not possible to distinguish clearly between parental environmental effects and the possible (but unlikely) effects of seed age on offspring phenotype. We observed a rapid decline in the expression of ancestral environmental effects, and no main environmental effects on progeny phenotype persisted in the second generation. The mechanism of transmission of environmental effects did not appear to be associated with the quantity or quality of reserves in the seeds, suggesting that environmental effects may be transmitted across subsequent generations via some mechanism that generates environment-specific gene expression.

CH3Cl emissions by freshly harvested tubers of 61 potato cultivars ranged from less than 4 to 650 ng g−1 f. wt d−1. In experiments with Anna, the cultivar displaying the highest release rate, maximum emission of CH3Cl occurred at 20°C but the rate diminished after 48 h, a decrease due in part to inhibition of CH3Cl release by the accumulation of CH3Cl in the headspace. In 1995 and 1996, CH3Cl emission by tubers was first detectable 6 wk before the normal harvest date. If tubers were stored at 20°C immediately following harvest, release attained a maximum of 350 ng g−1 f. wt d−1 about 4 d after harvest, falling to less than 4 ng g−1 f. wt d−1 within 3 wk of harvest. On storage at 6°C post-harvest, maximum release of CH3Cl (c. 600 ng g−1) was delayed until 5 wk after harvest and release fell below 4 ng g−1 f. wt d−1 after 200 d. Release of CH3Cl was not significantly affected by cutting or bruising tuber tissue, and CH3Cl biosynthesis occurred in both core and superficial tissues of the tuber. The S-methyl group of either L- or D-methionine acted as a precursor of CH3Cl in sliced tubers, but the methylating system was specific for Cl−. An isotope dilution technique using C2H3Cl demonstrated that the amount of CH3Cl metabolized by the intact tuber was approximately 36% of the net release. However, investigations with 14CH3Cl showed that only a small proportion (7%) of the CH3Cl metabolized was fixed in involatile form in the tuber. In both skin and core tissues, fixed 14C was located primarily (80%) in the fraction soluble in phosphate buffer, but the specific activity of skin tissue was about twice that of core tissue. Autoradiography demonstrated that 14C fixation in the tuber was greatest within the lenticels, possibly indicating a microflora adapted to use the locally high concentrations of CH3Cl. Significant 14C fixation was also associated with the periderm but was not attributable to labelling of the O-methyl groups of the phenolic components of suberin. Within the core of the tuber, 14C fixation was located primarily in the phloem, pith and medullary rays.

Rice (Oryza sativa) plants were grown with their roots sandwiched between thin layers of phosphorus-deficient soil from which they were separated by fine mesh, and root-induced changes in the soil affecting phosphate solubility were measured. The concentrations of low molecular weight organic anions in the thin layers, particularly citrate, increased in the presence of the plants. Apparent rates of citrate excretion from the roots, calculated from the quantities in the soil and rates of decomposition calculated with a first order rate constant measured independently, varied from 337–155 nmol g−1 root f. wt h−1 over the course of plant growth, equivalent to 2–3% of plant d. wt. Rates of excretion were similar for NH4+ and NO3−-fed plants. The soil pH decreased from its initial value by up to 0.6 units for the NH4+-fed plants and increased by up to 0.4 units for the NO3−-fed ones. The contribution of organic anion excretion to the pH changes was small compared with that of the inorganic cation-anion balance in the plants. The extent to which the observed excretion of citrate and root-induced pH changes could account for the observed phosphate solubilization and uptake was assessed using a mathematical model. Previous work had shown that phosphate solubilization by rice in this soil could not be explained by enhanced phosphatase activity in the rhizosphere, and the roots were not infected with mycorrhizas. The model allows for the diffusion of the solubilizing agent (citrate or H+) away from the roots, its decomposition by soil microbes (citrate only); its reaction with the soil in solubilizing phosphate and diffusion of the solubilized phosphate to the roots. The model contains no arbitrary assumptions and uses only independently measured parameter values. The agreement between the measured time course of phosphorus uptake and that predicted for solubilization by citrate was good. Root-induced acidification by NH4+-fed plants resulted in additional solubilization, the acidification enhancing the solubilizing effect of citrate. However, the final phosphorus uptake by NH4+-fed plants was no greater than that of NO3−-fed plants, presumably because the acidification inhibited plant growth. The mechanism of solubilization by citrate involved formation of soluble metal-citrate chelates rather than displacement of phosphate from adsorption sites.

One alkaline invertase and two acid invertase activities were detected in the shoots of etiolated rice (Oryza sativa) seedlings. The alkaline invertase (AIT) was purified to homogeneity through steps of ammonium sulphate fractionation, concanavalin A-Sepharose affinity chromatography (non-retained), DEAE-Sephacel chromatography and preparative electrophoresis. The pH optimum of AIT was 7.0 and the molecular mass, determined by gel filtration, was 240 kDa. It is apparently a homotetrameric enzyme (subunit molecular mass 60 kDa). The isoelectric point was 4.4 by isoelectric focusing. The best substrate of the enzyme was sucrose, with a Km of 2.53 mM. The enzyme also hydrolysed raffinose, but not maltose or lactose, so it is a β-D-fructofuranosidase. It gave negative glycoprotein staining. Of the hydrolysis products, fructose was a competitive inhibitor and glucose was a non-competitive inhibitor. Treatment with an alkaline phosphatase could activate AIT, whereas other proteins such as BSA, concanavalin A and urease had no effect on the enzyme activity. The enzyme activity was inhibited by Tris, thiol reagents and heavy metal ions.

Numerous biochemical and physiological studies have demonstrated the importance of ascorbate (ASC) as a reducing agent and antioxidant in higher plant metabolism. Of special note is the capacity of ASC to eliminate damaging activated oxygen species (AOS) including O2−· and H2O2. N2-fixing legume nodules are especially vulnerable to oxidative damage because they contain large amounts of leghaemoglobin which produces AOS through spontaneous autoxidation; thus, ASC and other components of the ascorbate–reduced glutathione (ASC–GSH) pathway are critical antioxidants in nodules. In order to establish a meaningful correlation between concentrations of ASC and capacity for N2 fixation in legume root nodules, soybean (Glycine max) plants were treated with excess ASC via exogenous irrigation or continuous intravascular infusion through needles inserted directly into plant stems. Treatment with ASC led to striking increases in nitrogenase activity (acetylene reduction), nodule leghaemoglobin content, and activity of ASC peroxidase, a key antioxidant enzyme. The concentration of lipid peroxides, which are indicators of oxidative damage and onset of senescence, was decreased in ASC-treated nodules. These results support the conclusion that ASC is critical for N2 fixation and that elevated ASC allows nodules to maintain a greater capacity to fix N2 over longer periods.

Spatial distribution of cell turgor pressure, cell osmotic pressure and relative elemental growth rate were measured in growing tall fescue leaves (Festuca arundinacea). Cell turgor pressure (measured with a pressure probe) was c. 0.55 MPa in expanding cells but increased steeply (+0.3 MPa) in cells where elongation had stopped. However, cell osmotic pressure (measured with a picolitre osmometer) was almost constant at 0.85 MPa throughout the leaf. The water potential difference between the growth zone and the mature zone (0.3 MPa) was interpreted as a growth-induced water potential gradient. This and further implications for the mechanism of growth control are discussed.

Symptoms which are associated with die-back in Phragmites: growth inhibition, root and bud death, premature shoot senescence, blocked aeration and vascular systems, especially in rhizomes and roots, and abnormal surface and internal cell-wall lignification and suberization of roots were induced by each of three of the lower volatile organic acids, propionic, butyric and caproic. These acids were applied in nutrient media in concentrations similar to those previously associated with die-back sites and/or in sediments containing rotting rhizomes and roots of the plant. At concentrations of 1.4 and 0.56 mM, respectively, butyric and caproic acids were each found to be highly toxic at pH 4.5, but relatively innocuous at pH 6. Propionic acid, applied at a much higher concentration of 10.4 mM, was highly toxic at both pH 4.5 and 6. The results support previous findings that the undissociated forms of the organic acids are the more toxic. Rhizomes and roots, rotting in water or waterlogged sand, released cocktails of acids and produced pH in the range 4.8–5.4. Phragmites seedlings planted in these media died within 12 h. Overall, the results support the theory that die-back in Phragmites can be induced and/or perpetuated by organic acids released from the decaying underground parts of the plant or other sources of organic matter.

An improved method of cell fractionation allowed the extraction of soluble (sNR) and membrane-associated (mNR) forms of nitrate reductase (NR) from a dinoflagellate, even though in previous studies only mNR had been found in these algae. Both activities were assayed in cell-free extracts of Peridinium gatunense from Lake Kinneret, Israel, after disruption of the cells and differential centrifugation. In the cultures used, sNR showed much higher NO3−-reducing activity. Only a low proportion, 2.5–3% of NR activity, was found to be associated with mNR. Moreover, mNR comprised two forms as indicated by protein solubilization: a tightly membrane-bound and a more weakly attached NR. Ascorbate inhibited all NR activities, but that of mNR recovered after its removal. Polyvinyl pyrrolidone (PVP) and DTT also diminished sNR and mNR activities. For both enzymes, pH optima (7.65) and temperature optima (13–25°C) were similar, and agreed with those for optimum growth of P. gatunense both in culture and in the lake. The most efficient electron donor was NADH, though NADPH sustained low NR activities. Carboxylic anions such as succinate and malate did not support any reduction of NO3−, nor did they cause any stimulation of sNR or mNR activities. Both forms of NR showed a high affinity for their substrates: Km was c. 10 μM for NO3− and c. 5 μM for NADH. The high efficiency of NO3− assimilation by Peridinium seems to be limited mainly by energy under otherwise optimal nutritional conditions and, at low nitrate concentrations, the low Km may be one of the main reasons for the high competitivity of this alga in Lake Kinneret.

The influence of variation in the concentration of dissolved inorganic carbon (DIC) in the form of CO2 and HCO3− in the root media on the C and N metabolism of Lycopersicon esculentum cv. F144 was investigated under both saline and non-saline conditions. Tomato seedlings were grown in hydroponic culture (pH 6.5) with or without NaCl, and the root solution was aerated with either ambient CO2 (360 μmol mol−1) or CO2-enriched air (5000 μmol mol−1). Nitrate uptake and root tissue NO3− concentrations were increased slightly by elevated rhizosphere DIC concentrations in both control and salinity-treated plants. This was associated with 46% higher nitrate reductase activity in the roots of control plants supplied with elevated DIC than in those supplied with ambient DIC. The activity of phosphoenolpyruvate carboxylase (PEPc) in vitro in control and salinity-treated plants was unaffected by the supply of elevated rhizosphere DIC concentrations. However, PEPc activity in vitro was considerably higher than the rates of PEPc activity in vivo reported previously, indicating that PEPc activity was not in itself a limitation on the provision of anaplerotic C. Therefore elevated DIC concentration in the rhizosphere stimulated the uptake of NO3− and provided alternative C skeletons for the assimilation of the NH4+ resulting from NO3− reduction into amino acids within the roots. Salinity stimulated root glutamine synthetase (GS) activity up to double that in control plants. Furthermore, elevated DIC caused an increase in leaf and root GS activity of control plants while inhibiting GS activity in the roots of salinity-treated plants. Glutamine[ratio ]2- oxoglutarate aminotransferase (GOGAT) activity of salinity-treated plants was doubled by elevated rhizosphere DIC concentrations. These changes in GS and GOGAT activity must reflect changes in amino acid synthesis. Under saline conditions the xylem transport of NO3− is partly blocked and a larger root assimilation develops, requiring not only the transamination of 2-oxoglutarate to glutamate but also that of oxaloacetate to aspartate and the transamidation of aspartate to asparagine.

Seeds of the wetland legume, Lotus uliginosus, were germinated and grown in vermiculite which was either continuously flooded or well-drained. Plants from both treatments were infected by Mesorhizobium loti strain DUS341 via a ‘classical’ root hair pathway, although some flooded plants appeared to be infected via enlarged epidermal cells. Subsequent to infection by M. loti, nodule meristems, which had developed within the root outer cortex, were penetrated by infection threads that released bacteria into the meristematic cells. The infection threads and infection droplets were immunogold labelled with monoclonal antibodies (MAC265 and MAC236) that recognize epitopes (at approx. 155/170 and 170/210 kDa, respectively) on a glycoprotein component of the matrix that surrounded the bacteria within the threads or droplets. Although labelling of infection threads or infection droplets with MAC236 was stronger than that with MAC265, both antibodies strongly labelled material occluding intercellular spaces in the cortices of developing nodules that had not yet expressed nitrogenase (as determined by a lack of signal after immunogold labelling with an antibody raised against nitrogenase component II). After 60 d, nitrogenase activity, shoot and root dry weights, and nodule fresh weight per plant did not differ between the treatments. After a further 30 d submergence, the flooded stems developed extensive aerenchyma and there was profuse development of (nodulated) adventitious roots. Nodules also formed at the junction of adventitious roots and the subtending stem and these were connected vascularly to a small stalk of tissue which gave rise to both a nodule and an adventitious root. The flooded nodules had prominent lenticels, and possible air pathways from the atmosphere to the nitrogen-fixing bacteroids are discussed.

Nitrogen (N) allocated to leaf growth in forage grasses and legumes following severe defoliation is predominately mobilized from the remaining root and leaf sheath tissues, since both N uptake from the soil and N2 fixation are severely down-regulated for several days. The hypothesis that a low N reserve status at the time of defoliation limits N remobilization and leaf regrowth was tested with contrasting cultivars of Lolium perenne (cvs Aberelan and Cariad) in flowing solution culture. Plants were grown under ‘high’ or ‘low’ (uptake of N decreased by 50%) regimes of N supply for 10 d before a single severe defoliation. Labelling with 15N was used to assess the importance of N reserves, including putative vegetative storage proteins, relative to N translocated from concurrent uptake, as a source of leaf N during regrowth. Leaf regrowth, N uptake and N mobilization were all affected by previous N supply. Low plant N status at the time of defoliation increased regrowth dry weight of ‘Aberelan’ by 10% and translocation of N absorbed from the medium by 23%, while mobilization of N reserves was decreased by 56%. On the contrary, regrowth dry weight of ‘Cariad’ was decreased by 23%, and translocation of N absorbed by 21% in low plant N status, compared with high plant N status. Concentrations of soluble protein in roots and remaining leaf sheaths decreased after defoliation in plants only under optimal N supply. Analysis of soluble proteins in sheath material by SDS–PAGE suggested that three polypeptides (55, 36.6 and 24 kDa) might function as vegetative storage proteins, although they were of low abundance in plants, subjected to monthly harvests, grown in controlled conditions and in the field. The apparent antagonism between uptake of NH4+ or NO3− by roots and mobilization of N reserves is discussed together with evidence for functional vegetative storage proteins in L. perenne.

We previously found that the ethylene inhibitor Ag+ could overcome the inhibitory effect of nitrate on nodulation of soybean (Glycine max) cv. Bragg. The same treatment increased nodulation quantitatively under non-inhibitory conditions, strongly suggesting involvement of ethylene in the control of nodulation in this species. Supernodulation mutants that lack internal autoregulation of nodulation, however, had biosynthesis capacity similar to the wild type. In the present work, the effects of ethylene on nodulation of ‘Bragg’ and two separate, but allelic, supernodulating mutants (nts382 and nts1007) were compared. The nodulation process appeared much more sensitive than plant growth and development to ethylene, which reduced the number of nodules per plant, but nearly twofold more in the wild type than in the supernodulation mutants. The cause–effect relationship is established by the counteracting effect of Ag+ and the fact that the stronger the inhibition by ethylene, the higher the recovery of nodulation ability with the ethylene antagonist. This higher tolerance of or lower sensitivity to ethylene in nts382 persists even under low inoculum dose, where nodule number and mass could be decreased to wild-type levels. Differences between the mutant and the wild type in the triple response test do not appear to support differences in ethylene perception on a whole-plant basis. The results suggest that sensitivity of nodulation to ethylene might have been affected in supernodulation mutants.