Showing papers in "Plant Cell and Environment in 1990"

Effects of metals on enzyme activity in plants

Abstract: . Uptake of phytotoxic amounts of metal by higher plants or algae can result in inhibition of several enzymes, and in increase in activity (= induction) of others. Two mechanisms of enzyme inhibition predominate: (1) binding of the metal to sulphydryl groups, involved in the catalytic actionor structural integrity of enzymes, and (2) deficiency of an essential metal in metalloproteins or metal-protein complexes, eventually combined with substitution of the toxic metal for the deficient element. Metal accumulation in the cellular compartment of the enzyme is a prerequisite for enzyme inhibition in vivo. The induction of some enzymes is considered to play a significant role in the stress metabolism, induced by metal phytotoxicity. Peroxidase induction is likely to be related to oxidative reactions at the biomembrane; several enzymes of the intermediary metabolism might be stimulated to compensate for metal-sensitive photosynthetic reactions. The induction of enzymes and metal-specific changes in isoperoxidase pattern can be used as diagnostic criteria to evaluate the phytotoxicity of soils, contaminated by several metals. Lines for future research on metal phytotoxicity are proposed, involving the study of inhibition and induction of enzymes at the different cell membranes (especially the plasmamembrane) in vivo.

Changes in the concentration of ABA in xylem sap as a function of changing soil water status can account for changes in leaf conductance and growth

Abstract: . Maize seedlings (Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in l-m-long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20-d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as-yet unidentified inhibitor of stomatal opening.

Water‐stress‐induced xylem embolism in three species of conifers

Abstract: . The mechanism of water-stress-induced xylem embolism was studied in three species of conifers: Abies balsamea (L.) Mill., Picca rubens Sarg, and Juniperus virginiana L. Each species showed a characteristic relationship between xylem tension and the loss of hydraulic conductivity by air embolism. Abics balsamea and Picca rubens began to embolize at tensions between 2 and 3 MPa and were completely non-conducting between 3 and 4 MPa. Juniperus virginiana was least vulnerable, beginning to embolize at 4 and still retaining approximately 10% conductivity at 10 MPa. As with a previous study of the vessel-bearing Accr saccharum Marsh., a brief perfusion of branch segments with an oxalic acid and calcium solution (10 and 0.1 mol m−3. respectively) increased the vulnerability of the xylem to embolism; this was especially pronounced in Abies balsamea. In order to test whether embolism was caused by aspiration of air into functional tracheids from neighbouring embolized, ones (the ‘air-seeding’hypothesis), hydrated branch segments were injected with air at various pressures and measured for embolism. Results supported the air-seeding hypothesis because the relationship between injection pressure and embolism for both native and oxalic-calcium-treated segments was essentially the same as for embolism induced by xylem tension. Structural and experimental evidence suggested the air seeding occurred through inter-tracheid pit membranes when the thickened torus region of the membrane became displaced from its normal sealing position over the pit aperture. Thus, the embolism-inducing tension may be a function of pit membrane flexibility. This tension is of ecological significance because it reflects to some extent the range of xylem tensions to which a species is adapted.

Stomatal and hydraulic conductance in growing sugarcane: stomatal adjustment to water transport capacity*

Abstract: Stomatal conductance per unit leaf area in well-irrigated fiel- and greenhouse-grown sugarcane increased with leaf area up to 0.2 m 2 plant -1 , then declined so that maximum transpiration per plant tended to saturate rather than increase linearly with further increase in leaf area. (...)

Phytochrome, a family of photoreceptors with multiple physiological roles

Abstract: (...) This article reviews the regulation of development by phytochrome and discusses evidence from physiological studies of wild type, mutant and transgenic plants consistent with the proposal that different members of the phytochrome family have different photosensory roles. (...)

Plant response to atmospheric humidity

Abstract: . Plants growing in environments differing in prevailing humidity exhibit variations in traits associated with regulation of water loss, particularly cuticular and stomatal properties. Expansive growth is also typically reduced by low humidity. Nevertheless, there is little evidence in plants for a specific sensor for humidity, analogous to the blue light or phytochrome photoreceptors. The detailed mechanism of the stomatal response to humidity remains unknown. Available data suggest mediation by fluxes of water vapour, with evaporation rate assuming the role of sensor. This implies that stomata respond to the driving force for diffusional water loss, leaf-air vapour pressure difference. Induction of metabolic stomatal response to humidity may involve signal metabolites, such as abscisic acid, that are present in the transpiration stream. These materials may accumulate in the vicinity of guard cells according to the magnitude and location of cuticular transpiration, both of which could change with humidity. Such a mechanism remains hypothetical, but is suggested to account for feedforward responses in which transpiration decreases with increasing evaporative demand, and for the apparent insensitivity of stomatal aperture in isolated epidermis to epidermal water status. Other responses of plants to humidity may involve similar indirect response mechanisms, in the absence of specific humidity sensors.

Aerenchyma formation and associated oxygen movement in seminal and nodal roots of wheat.

Abstract: The present paper describes the effects of growth of roots of wheat (Triticum aestivum cv. Gamenya) in hypoxic nutrient solutions on acrenchyma formation and O2 movement from shoots to roots. Two types of roots were investigated: (1) seminal roots of 4–7-d-old seedlings, and (2) seminal and nodal roots of 10–28-d-old plants. Gas-filled porosity of seminal and nodal roots increased from 3 to 12% and from 5–7 to 11–15%, respectively, when the roots emerged in stagnant or N2-flushed solutions (0.003 mol m −3 O2) compared with growth in continuously acrated solutions (0.26 mol m −3 O2). However, neither root type increased in porosity when they were longer than 100–200 mm at the start of the exposure to these stagnant or N2-flushed treatments. A vernier microscope and cylindrical platinum-electrode were used to examine the relationship between root extension and transport of O2 from shoots to roots via the gas spaces. Measurements were made when the roots were in an anoxic medium and were dependent solely on O2 supplied from the shoots. For seminal roots of 5–7-d-old seedlings raised in stagnant solutions (90–100 mm), internal O2 transport was sufficient to support a rate of root elongation in the O2-free medium of between 0.03 and 0.17 mm h−1. When the O2 pressure around the shoots was increased from 20 to 100 kPa O2, the O2 concentrations at the walls of the expanding zone (2–7 mm from the tip) of these roots increased from 0.006 mol m−3 to between 0.04 and 0.26 mol m−3, and the rate of root extension increased five-fold. Oxygen transport to roots grown continuously in acrated solutions was considerably less than for roots raised in stagnant solutions; this difference was greater for seminal than for nodal roots. When the acrated seminal roots were longer than 100 mm and transferred to an O2-free root medium, O2 concentration became zero at the root tip causing elongation to cease. After 24 h of anoxia, none of these roots were able to resume elongation following a return to acrated solutions.

Natural deuterium and oxygen‐18 enrichment in leaf water of cotton plants grown under wet and dry conditions: evidence for water compartmentation and its dynamics

Abstract: Significant differences in leaf water oxygen and hydrogen isotopic composition were observed between cotton plants grown under wet and dry conditions The magnitude of the differences could be fully explained by the conventional model that describes the isotopic composition of an evaporating water pool under steady state conditions The results indicate that leaf water isotopic composition is strongly influenced by transpiration rate via its effects on relative humidity adjacent to the leaf surface and on the isotopic composition of the air moisture Our application of the model, however, provides evidence that leaf water must consist of a mixture of several isotopically distinct pools These pools are suggested to reside in the symplast, in the cell walls and intercellular spaces and in the veins A model is proposed suggesting that only the water residing in the cell walls and the intercellular spaces (the transpiration pool) interacts directly with the external environment The large symplastic pool responds to the external environment to a limited extent via its relatively slow exchange with water in the transpiration pool It is likely that the isotopic composition of water in the symplastic pool is strongly buffered against shortterm environmental variations, a possibility that would have important implications for the isotopic conditions under which organic matter biosynthesis occurs

Reflection signals and the perception by phytochrome of the proximity of neighbouring vegetation

Abstract: . Spectral photon distributions, red:far-red ratios (i.e. R:FR) and phytochrome photoequilibria (i.e. Pfr/Ptotal) were measured at various distances from artificial canopies composed of mustard or tobacco plants. Measurements were compared for radiation propagated predominantly vertically downwards and radiation propagated predominantly horizontally. Reflection signals from the artificial canopies were computed and shown to consist of a depletion of radiation over the 400–690 nm wavelength range, and an enhancement of radiation over the 690–800 nm range. R:FR and Pfr/Ptotal increased gradually with distance from the canopies, with significant depressions of both parameters evident at least as far as 30 cm from the vegetation stands. It is concluded that, in principle at least, detection of spectral quality differences by phytochrome would allow not only the presence but also the proximity of neighbouring plants to be perceived. Proximity perception is proposed as an ecologically valuable mechanism through which plants may be able to gauge their anticipatory responses to incipient shading according to the challenge posed by the nearness of neighbouring plants.

Sensing soil oxygen.

Abstract: . Under natural conditions where gaseous exchange between soil and atmosphere is restricted by excess water, the concentration of O2 in the rooting zone can become very low while reduced ions and organic compounds that are potentially phytoxic may accumulate. Mechanisms by which shoots and roots detect, and adjust to, this O2-deficient environment are reviewed. Injury to roots and their inability to function because of insufficient O2 is communicated to the shoot in a variety of ways, so that it adjusts physiologically. Roots may acclimate metabolically to a gradual fall in O2 supply, so that they either improve their tolerance of anoxia, or partially avoid O2-deficiency by structural changes that aid internal transfer of O2 to the roots from the shoot. Molecular mechanisms regulating such metabolic changes, including environmental cues, are discussed.

Ethylene‐promoted ascorbate peroxidase activity protects plants against hydrogen peroxide, ozone and paraquat

Abstract: In experiments where mung beans (Vigna radiata) and peas (Pisum sativum) have been pre-exposed to ethylene and afterwards treated with ozone, it has been shown that such ethylene-pretreated plants may become more resistant to ozone

Sensing of atmospheric CO2 by plants

Abstract: . Despite recent interest in the effects of high CO2 on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO2 concentration is relatively constant and because the conductance of the cuticle to CO2 is low, sensory mechanisms are likely to exist only for intercellular CO2 concentration. Therefore, responses of plants to changes in atmospheric CO2 will depend on the effect of these changes on intercellular CO2 concentration. Although a variety of plant responses to atmospheric CO2 concentration have been reported, most of these can be attributed to the effects of intercellular CO2 on photosynthesis or stomatal conductance. Short-term and long-term effects of CO2 on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO2. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO2. This may be because of genetic and environmental limitations to productivity or because plant responses to CO2 have evolved to cope with variations in intercellular CO2 caused by factors other than changes in atmospheric CO2.

Stable carbon isotope fractionation by marine phytoplankton during photosynthesis

Abstract: . In the marine environment, the range of values of carbon isotope fractionation between particulate tissue of phytoplankton and inorganic carbon can be more than 20‰ (− 35‰ < δ13C < − 14‰). This review considers the influence of seawater temperature, lipid content of phytoplanktonic cells, kinetic fractionation, and carbon pathway on δ13C values observed at sea. In order to study the contribution of carboxylases (RUBISCO and the β-carboxylases phosphoenolpyruvate carboxylase, phosphoenoplpyruvate carboxykinase and pyruvate carboxylase) to variations of particulate δ13C values at sea, we present results obtained simultenously on carboxylase activities and δ13C in various environmental conditions. The lowest δ13C values are clearly associated with predominance of ribulose-1.5-bisphosphate carboxylase activity, but it was more difficult to explain the high δ13C values. Different hypotheses are discussed.

Light-driven leaf movements*

Abstract: Mature leaves of many plants re-orientate their laminae photonastically in response to non-directional light signals, and/or phototropically in response to directional light signals, by flexing of pulvini, most commonly subtending their bases. (...)

Physiology of the interaction of angiosperm parasites and their higher plant hosts

Abstract: Caracteristiques physiologiques de plusieurs familles d'Angiospermes parasites: (Scrophulariaceae, Orobranchaceae...) concernant la germination de leurs graines, le developpement de sucoirs, et les reactions de leurs hotes (cereales). Description du phenomene de competition entre l'hote et le parasite, pour l'eau et les substances nutritives

Mechanism of soybean nodule adaptation to different oxygen pressures

Abstract: . Soybean nodules showed the ability to adapt to oxygen pressures above and below ambient levels and this adaptation involved a decrease in cortical intercellular air-spaces with increasing oxygen pressure. Nodules were grown in oxygen pressures from 4.7 to 75 kPa and the decrease in number and size of cortical intercellular spaces with increasing oxygen pressure was the result of a change in cell structure and the deposition of an electron dense material within intercellular spaces. Exposure to a saturating pressure of acetylene caused a similar inhibition of respiration and nitrogenase activity in nodules developed in oxygen pressures from 4.7 to 47 kPa, suggesting that putative acetylene-induced changes in oxygen diffusion resistance occur by a different mechanism than that involved in long-term adaptation to oxygen. However, in nodules grown at 75 kPa oxygen, the initial specific activities were lower and did not show an acetylene induced decline. The results are discussed in terms of the current theories of regulation of nitrogenase activity by oxygen availability.

Changes in starch and soluble sugars in relation to the acquisition of desiccation tolerance during maturation of Brassica campestris seed

Abstract: (...) Since the acquisition of desiccation tolerance is a long asynchronous process which took 9 d to be achieved, we determined criteria allowing us to separate freshly intact harvested seeds into desiccation intolerant and desiccation tolerant batches that differed in age by only 2 d. (...)

Photoinhibition in Vitis californica: interactive effects of sunlight, temperature and water status

Abstract: . In a series of factorial experiments with cultivated Vitis californica Benth. (California wild grape) growth outdoors in full sun, we examined the effects of sunlight, temperature and water status on net CO2 uptake and PSH chlorophyll fluorescence at 77K. Exposure to either high light or high temperature caused reductions in PSH activity followed by partial or complete overnight recovery. Upon simulataneous exposure to high light and high temperature, PSH inhibition was severe and persistent. The maximum chlorophyll fluorescence (FM) and the ratio of variable to maximum fluorescence (Fv/FM) differed in their responses to combinations of light and temperature. At both low and high light. FM declined with increasing temperature over a wide temperature range, while Fv/FM exhibited a similar sensitivity to temperature only at high light. Net CO2 uptake declined by mid-afternoon and recovered by the next morning in most leaves, regardless of incident light or temperature. However, high-light leaves exhibited severe and lasting declines if temperatures exceeded 45°C. Water-stressed leaves exposed to high light exhibited greater reductions of net CO2 uptake than water-stressed leaves exposed to low light. However, the degree of light-dependent decline in PSH fluorescence (FM and Fv/FM) did not vary with water status, indicating that reduced PSH activity was not a primary cause of reduced carbon gain during water stress.

Phototropism : mechanisms and ecological implications

Abstract: . Phototropism in seed plants, either etiolated or de-etiolated, is mediated by unidentified photoreceptor(s) sensitive to blue and near-UV regions of the light spectrum. Green plants may have an additional phototropic system sensitive to red light. Fluence-response studies of the blue light-sensitive phototropism, initially made on oat coleoptiles, have indicated the occurrence of multiple response types. Of those, two are found to be general: the first pulse-induced positive phototropism (fPIPP), or the so-called first positive curvature, and the time-dependent phototropism (TDP) or the second positive curvature. The fPIPP, elicited by a pulse stimulus shorter than a few minutes, is characterized by a bell-shaped fluence-response curve and the validity of reciprocity. The TDP, elicited by prolonged irradiation, is characterized by its dependence on the exposure time and the invalidity of reciprocity. Studies made on these two response types have revealed the following: (1) plants acquire directional light information for phototropism by sensing internal light gradients created by light scattering and absorption; (2) phototropism results from redistribution of growth, i.e. inhibition on the irradiated side and compensating stimulation on the shaded side; (3) lateral movement of growth regulators, the principle of the Cholodny-Went theory, can account for the growth redistribution, and auxin is clearly the mediating regulator in maize coleoptiles. This review further describes some mechanistic implications of fPIPP. Experimental results indicate that (1) fPIPP is mediated by a single step of photoreaction, (2) the responsiveness, reflected in the height of the fluenceresponse curve, is reduced by pre-irradiation with blue light and recovers gradually afterward, and (3) the light sensitivity, reflected in the position of the fluence-response curve along the log fluence axis, is also reduced by the pre-irradiation and recovers gradually. Analyses of these results, based on kinetic models, suggest that the bell-shaped fluence-response curve is caused by the difference in the amounts of a photoproduct between irradiated and shaded sides, and that fPIPP represents a mechanism of TDP. It is also indicated that phytochrome in the red-absorbing form exerts two separate effects on phototropism: reduction of the light sensitivity and enhancement of the responsiveness. Along with the discussion of the mechanisms of phototropism, their ecological implications are considered.

Some spectral properties of several soil types: implications for photomorphogenesis*

Abstract: . Light transmission through several sands, soils and size classes of glass beads was investigated. The glass beads served as a highly reflective model system for the sands. Light penetration was significantly better through sands than through either sandy or silty loams. For sandy soils, there was a shift in the spectrum leading to a change in the red-to-far-red photon ratio with increase in depth: this ratio dropped by more than 30% in the first few millimetres, and in some cases increased again with further increases in depth. The depths at which these changes occurred in the soil profile were specific for a given soil type. Similar spectral changes were obtained when sample depth for a sandy soil was held constant, and particle (ped) size of that soil was decreased. No such changes, either with sample depth or ped size, were observed with loams. Per cent transmittance in the blue (442 nm) was more than three orders of magnitude lower than in the red (632.8 nm) for sandy soils. When dry sandy soils were water-saturated, their transmittance increased by up to three orders of magnitude, and the red far-red ratios in the transmitted light increased in all cases. By contrast, water-saturated loams were essentially opaque. The possible significance of these spectral properties of soils in seed germination and seedling photomorphogenesis is discussed.

Growth, dry weight partitioning and wood properties of Pinus radiata D. Don after 2 years of CO2 enrichment

Abstract: Advanced selections (families 20010 and 20062) of P. radiata D. Don were exposed to either 340 or 660 μmol CO2 mol 1 for 2 years to establish if growth responses to high CO2 would persist during the development of woody tissues. The experiment was carried out in glasshouses and some of the trees at each CO2 concentration were subjected to phosphorus deficiency and to periodic drought. CO2 enrichment increased whole-plant dry matter production irrespective of water availability, but only when phosphorus supply was adequate. The greatest increase occurred during the exponential period of growth and appeared to be tied to increased rates of photosynthesis, which caused accelerated production of leaf area. The increase in whole-plant dry matter production was similar for both families; however, family 20010 partitioned larger amounts of dry weight to the trunks than family 20062. which favoured the roots and branches. Wood density was generally increased by elevated CO2 and for family 20010 this increase was due to thickening of the tracheid walls. Tracheid length was similar at both CO2 levels but differed between families. These results suggest that, as the atmospheric CO2 concentration rises, field-grown P. radiata should produce more dry weight at sites where phosphorus is not acutely deficient, even where drought limits growth; however, increases in wood production are likely only for genotypes which continue to partition at least the same proportion of dry weight to wood in the trunk.

Physiological responses of the mangrove Rhizophora mangle grown in the absence and presence of NaCl.

Abstract: . Propagules of the mangrove, Rhizophora mangle L., were precultivated for 9 months in a greenhouse. The young plants were transferred into unaerated nutrient solutions without and with 200 mol m 3 NaCl and subsequently their growth, their water relations and the photosynthetic properties of their leaves were studied. Growth of the salttreated plants was significantly increased, while the control plants gradually died off after finishing the experiments. The shoot water potential and the stomatal resistance of the leaves were lowered while the chlorophyll contents and the chlorophyll a/b ratio in the leaves of salt-treated plants were increased by NaCl, the net result being an enhanced rate of CO2 assimilation. The leaves of both sets of plants showed diurnal fluctuations in malic acid concentration which were more pronounced in the leaves of salt treated plants which, additionally, were more succulent. However, the plants showed no net CO2 fixation at night, indicating that Rhizophora mangle is a CAM-cycling plant. After 200 d of cultivation without or with NaCl, the Na+, Cl and K+ concentrations in tissues and vacuoles were measured. Energy-dispersive X-ray microprobe analyses on root vacuoles of control plants reveal Na+ preference, on those of salt treated plants a strong K+ preference. Vacuolar K+ concentrations are neither affected by NaCl nor do they vary across the root radius. High vacuolar Na+ and Cl concentrations are found in the hypodermis followed by a stepwise decrease towards the inner root cortex cells. Ion concentrations of the photosynthetically active leaf tissues seem to be regulated by (1) radial filtration across the root cortex: (2) ion exchange of the xlem parenchyma cells: and (3) sequestration of Na+ and Cl in the hypodermal water storage tissue of the leaves.

Does transpiration have an essential function in long-distance ion transport in plants ?

Abstract: . Long-term effects of transpiration on growth and on long-distance ion transport were investigated in maize over a whole growth cycle. Maize plants were grown with nutrients supplied at adequate levels in hydroculture or in soil at 50–60% and at >95% relative humidity. Although the amount of water lost by the plants under these conditions differed by a factor 2 to 3, there was neither a decrease in growth (fresh weight and dry weight) nor in ash content of the ‘humid’plants. This was also found when the upper part of the shoot (70–150 cm) was tested separately. It is suggested that transpiration is not essential for long-distance transport of mineral elements in plants. Alternatives are discussed.

Epidermal focussing and effects upon photosynthetic light‐harvesting in leaves of Oxalis

Abstract: . In Oxalis, epidermal cells on both the adaxial and abaxial surface of the leaf concentrated light within the leaf by a lens mechanism. Focal lengths of epidermal cells were estimated using two methods: they were calculated from radius of curvature measurements taken from individual epidermal cells, and were measured directly in agarose replicas of the leaf surface. In the three species of Oxalis examined, light that was incident upon the adaxial leaf surface was concentrated within the palisade, whereas light that was incident upon the abaxial leaf surface was concentrated within the spongy mesophyll. Using sensiometric analysis, theoretically maximal focal intesifications were measured in leaf replicas at the focal maximum and at intermediate positions corresponding to the mid-region of the palisade and spongy mesophyll tissues. Focal intensifications ranged from 2.2 to 10.4 times incident light at the focal maximum, and 1.3 to 4.5 in the palisade or spongy mesophyll layers. Elimination of epidermal focussing, by covering the leaf surface with a thin layer of mineral oil, strongly affected chlorophyll fluorescence induction curves resulting in a decrease of 10–40% in the initial (F0) and variable fluorescence (Fv). These results are consistent with the interpretation that the chloroplasts were adapted to their light microenvironment within the leaf and that focussing by the epidermis channelled light to a population of chloroplasts that were adapted to high light.

Poly(γ‐glutamylcysteinyl)glycines or phytochelatins and their role in cadmium tolerance of Silene vulgaris

Abstract: . The effect of cadmium on growth of Cd-tolerant and -sensitive plants of Silene vulgaris and on the production of metal-binding compounds in both types of plants was studied. The Cd-content of the roots and the Cd-root/shoot ratio was higher in Cd-tolerant plants. A Cd-binding compound (Cd-BC) with an apparent molecular mass of 14.5 kD was isolated from the roots of Cd-tolerant and -sensitive plants, grown in 40 mmol m−3 Cd for 21 d. More than 60% of the total Cd in the roots was associated with this compound. Determination of the amino-acid content of the purified Cd-containing compound from both types of plants showed that they possessed a similar amino-acid composition to that of phytochelatins. Only the bis- and tris-forms were present. The amount of Cd and sulphide associated with phytochelatin was greater in tolerant plants than in sensitive ones suggesting that an increased sulphide content of complexes containing peptide, sulphide and Cd may form the basis of evolved Cd-tolerance in Silence vulgaris.

Signal perception, differential expression within multigene families and the molecular basis of phenotypic plasticity

Abstract: . This Introduction to the Special Issue of Plant, Cell and Environment on ‘Sensing the Environment’is concerned with the molecular mechanisms that may link the perception of environmental signals with the evocation of those specific developmental responses that collectively are known as phenotypic plasticity. The significance of phenotypic plasticity at the evolutionary, developmental and ecological levels is outlined, and it is argued that the extent of an individual's adaptability to environmental conditions must be a reflection of the extent and sophistication of the controls over the synthesis and action of specific proteins. Reviewing evidence from a selected range of plant enzymes and regulatory proteins, it is proposed that differential regulation of the expression of members of multigene families may represent the molecular basis of phenotypic plasticity.

The relationship between the Rubisco reaction mechanism and models of photosynthesis

Abstract: . A new model of photosynthesis published recently in this journal (H. Farazdaghi & G. E. Edwards, Plant, Cell and Environment (1988) 11, 789–798; 799–809) clams to have a more complete mechanistic basis than currently used models based on the paper of G. D. Farquhar, S. von Caemmerer & J. A. Berry (Planta (1980) 149, 78–90). In this paper, we examine the validity of the new kinetic expression for the rate of CO2 fixation by Rubisco, and the derivation of an equation for photosynthetic CO2 assimilation as a function of light intensity and CO2 concentration presented in the new model. In addition, we compare measured response curve of photosynthesis to CO2 and light with simulated curves using alternative models. We conclude that the new model is mechanistically misleading and, empirically, overestimates the extent to which light and CO2 co-limit the rate of photosynthesis under most physiological conditions.

An automated greenhouse sand culture system suitable for studies of P nutrition

Abstract: (...) In this report, we describe a relatively simple and economical sand culture system capable of sustaining plant growth to maturity under controlled yet realistic P regimes. The system uses Al 2 O 3 as a solid-phase P buffer, and modern process control technology to control irrigation and addition of other mineral nutrients. (...)

An examination of the advantages of C3-C4 intermediate photosynthesis in warm environments

Abstract: . The photosynthetic responses to temperature in C3, C3-C4 intermediate, and C4 species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C3-C4 intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO2 compensation points at all temperatures examined in the C3-C4 intermediate, Flaveria floridana, compared to the C3 species, F. cronquistii. The C3-C4 intermediate, F. floridana, exhibited a C3-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C3 species, F. cronquistii. Using models of C3 and C3-C4 intermediate photosynthesis, it was predicted that by recycling photorespired CO2 in bundle-sheath cells, as occurs in many C3-C4 intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C3 plant. Without recycling photorespired CO2, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C3 plants, (1) intercellular CO2 partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO2, leaves of F. floridana appear to effectively concentrate CO2 at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO2-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C3-C4 intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C4 species, F. trinervia, than the C3 species, F. cronquistii. The C4-like pattern is probably related to the advanced nature of C4-like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C3-C4 intermediate plants create photosynthetic advantages at warm leaf temperatures that in C3 plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency.