Short communication. A steep Ca2[IMAGE]-dependence of a K[IMAGE] channel in a unicellular green alga

An increase of cytosolic Ca² in the unicellular green alga Eremosphaera viridis activities Ca²-dependent K channels causing a hyperpolarization of the plasma membrane. Data from parallel calcium, and potential measurements were combined with I/V relationships. This yielded a steep Ca²-dependence of K channels with a co-operativity of 4 and an affinity of 300 nM.Key words: Eremosphaera viridis, plasma membrane, Ca²-dependent K channel, co-operative binding.      

Photorespiratory glycine enhances glutathione accumulation in both the chloroplastic and cytosolic compartments

Transformed poplars overexpressing -glutamylcysteine synthetase (-ECS) in the chloroplast (Lggs) were used to investigate chloroplastic biosynthesis of glutathione (GSH). In Lggs leaves, GSH contents were enhanced by up to 3.7 fold. In general, the highest GSH contents were observed in lines with highest -glutamylcysteine (-EC) contents. These lines had relatively low glycine. In darkness, foliar GSH decreased and -EC increased. Illumination of pre-darkened Lggs in air resulted in a 5-fold decrease in the -EC : GSH ratio. This light-induced decrease was largely abolished if leaves were illuminated at high CO2. Consequently, the -EC : GSH ratio of illuminated leaves was much higher at high CO2 than in air. At high CO2 total foliar amino acids were higher, but glycine and serine were lower, than in air. These results suggest that photorespiratory glycine is used in chloroplastic GSH synthesis. Despite this net CO2 fixation was similar in Lggs to untransformed poplars. Pre-illuminated leaf discs from Lggs, and poplars overexpression -ECS in the cytosol (ggs), were incubated in darkness with a range of metabolites. After 15 h, discs for both types of transformant incubated on water had accumulated high levels of -EC and showed marked increases in the -EC : GSH ratio. Feeding glycine, serine, glycollate or phosphoserine, attenuated the dark-induced changes in the -EC : GSH ratio, whereas 3-phosphoglycerate (PGA), phosphoenolpyruvate, glycerate, and hydroxypyruvate did not. Glycine produced from glycollate was therefore required for maximal GSH accumulation in both the chloroplastic and cytosolic compartment. Production of glycine from PGA failed to meet the demand of increased GSH synthetic capacity.     

Alterations in the Components of Peanut Leaf Water Potential during Desiccation

Changes in components of leaf water potential during soil waterdeficits influence many physiological processes. Research resultsfocusing on these changes during desiccation of peanut (Arachishypogeae L.) leaves are apparently not available. The presentstudy was conducted to examine the relationships of leaf water_l, solute _s and turgor _p potentials, and percent relative watercontent (RWC) of peanut leaves during desiccation of detachedleaves and also during naturally occurring soil moisture deficitsin the field. The relationship of _p to _l and RWC was evaluated by calculating_p from differences in _l and _s determined by thermocouple psychrometryand by constructing pressure-volume (P-V) curves from the _land RWC measurements. Turgor potentials of ‘Early Bunch’and ‘Florunner’ leaves decreased to zero at _l of–1.2 to –1.3 MPa and RWC of 87%. There were no cultivardifferences in the _l at which _p became zero. P-V curves indicatedthat the error of measuring _s after freezing due to dilutionof the cellular constituents was small but resulted in artefactualnegative _p values. Random measurements on two dates of _l, _s, and calculation of_p from well-watered and water-stressed field plots consistingof several genotypes indicated that zero _p occurred at _l of–1.6 MPa. It was concluded that the relationships of _p,_l, _s, and RWC of peanut leaves were similar to leaves of othercrops and that these relationships conferred no unique droughtresistance mechanism to peanut.     

Optimization theory of stomatal behaviour II. Stomatal responses of several tree species of north Australia to changes in light, soil and atomospheric water content and temperature

In a companion paper several methods of calculating the marginal unit water cost of plant carbon gain (E/A) were tested to determine whether stomata were behaving optimally in relation to regulating leaf gas exchange. In this paper one method is applied to several tropical tree species when leaf-to-air vapour pressure difference (D), photosynthetic photon flux density, leaf temperature, and atmospheric soil water availability were manipulated. The response of leaves that had expanded during the dry season were also compared to that of leaves that had expanded in the wet season. Few differences in absolute value of E/A, or the form of the relationship, were observed between species or between seasons. In the majority of species, E/A increased significantly as either leaf-to-air vapour pressure difference increased, at a leaf temperature of either 33C or 38C, or as in photosynthetic photon flux density increased. In contrast, as leaf temperature increased at constant D, E/A was generally constant. As pre-dawn water potential declined, E/A declined. The relationship between E/A and D did not differ whether internal or ambient carbon dioxide concentration were kept constant. It is concluded that stomata are only behaving optimally over a very small range of D. If a larger range of D is used, to incorporate values that more closely reflect those experienced by tropical trees in a savanna environment optimization is incomplete.Key words: Stomatal optimization theory, marginal unit water cost.      

The role of solute accumulation, osmotic adjustment and changes in cell wall elasticity in drought tolerance in Ziziphus mauritiana (Lamk.)

Ber (Ziziphus mauritiana Lamk.) is a major fruit tree crop of the north-west Indian arid zone. In a study of the physiological basis of drought tolerance in this species, two glasshouse experiments were conducted in which trees were droughted during single stress-cycles. In the first experiment, during a 13 d drying cycle, pre-dawn leaf water (leaf) and osmotic () potentials in droughted trees declined from -0.5 and -1.4 MPa to -1.7 and -2.2 MPa, respectively, for a decrease in relative water content () of 14%. During drought stress, changes in sugar metabolism were associated with significant increases in concentrations of hexose sugars (3.8-fold), cyclitol (scyllo-inositol; 1.5-fold), and proline (35-fold; expressed per unit dry weight), suggesting that altered solute partitioning may be an important factor in drought tolerance of Ziziphus. On rewatering pre-dawn leaf and recovered fully, but remained depressed by 0.4 MPa relative to control values, indicating that solute concentration per unit water content had changed during the drought cycle.Evidence for osmotic adjustment was provided from a second study during which a gradual drought was imposed. Pressure-volume analysis revealed a 0.7 MPa reduction in osmotic potential at full turgor, with leaf at turgor loss depressed by 1 MPa in drought-stressed leaves. Coupled with osmotic adjustment, during gradual drought, was a 65% increase in bulk tissue elastic modulus (wall rigidity) which resulted in turgor loss at the same in both stressed and unstressed leaves. The possible ecological significance of maintenance of turgor potential and cell volume at low water potentials for drought tolerance in Ziziphus is discussed.Keywords: Ziziphus mauritiana, drought, solute accumulation, osmotic adjustment, proline.      

Mastoparan analogues stimulate phospholipase C- and phospholipase D-activity in Chlamydomonas: a comparative study

The G-protein activator mastoparan and its analogues are becoming popular tools for studying signalling in plants. Therefore the abilities of mastoparan, mas7, mas8, and mas17 to activate phospholipase C (PLC), PLD and to induce the deflagellation response in Chlamydomonas moewusii Gerloff were compared. The aim was to test whether their relative potencies in a plant system resemble those reported for bovine brain Go and Gi, as is generally assumed, and to determine at which concentrations cells become permeabilized, a known effect of higher concentrations. The concentrations at which 50% deflagellation was induced, were 2.0 M mastoparan, 3.0 M mas8, 3.6 M mas7, and 5.8 M mas17. Similar activities were found for the production of phosphatidic acid, which is the result of the combined activities of PLD and PLC (together with diacylglycerol kinase). PLD activity alone was measured in vivo by its ability to phosphatidylate n-butanol. Surprisingly, the concentrations that stimulated maximum activity were about 10-fold lower (1 M) than those that stimulated maximum PLC activity (10 M). Mas17 was an exception with both maxima above 10 M. All the compounds except mas17 permeabilized C. moewusii cells. The concentrations at which 50% of the cells were permeabilized to Evan's blue were 7.4 M mas8, 16.0 M mas7 and 22.4 M mastoparan. In conclusion, only mastoparan itself and the least active analogue mas17 induced maximum deflagellation, PLC and PLD activities without permeabilizing the cells.Keywords: Chlamydomonas, deflagellation, mastoparan, phospholipases C and D, phospholipid metabolism      

Natural abundance of 15N in amino acids and polyamines from leguminous nodules: unique 15N enrichment in homospermidine

The natural ¹⁵N abundance (¹⁵N value) in acetylpropyl derivatives of amino acids and in ethyloxycarbonyl derivatives of polyamines was determined using a gas chromatography/combustion/mass spectrometer-(GC/C/MS). ¹⁵N value determined for 12 amino acids and five polyamines by GC/C/MS were identical to those obtained by a direct combustion method using an automatic nitrogen and carbon analysis (ANCA) mass spectrometer, the difference being less than 1.0% in most cases. The GC/C/MS method was used to analyse ¹⁵N values in the amino acids and polyamines from root nodules of pea and faba bean and from stem nodules of Sesbania rostrata. The analysis of ¹⁵N values revealed that homospermidine had high ¹⁵N values, as much as +40%, while the amino acids investigated had ¹⁵N values between -3 and +6%, putrescine between +2 and +8%, cadaverine between +1 and +7%, spermidine between -2 and +4%, and spermine between 0 and +6%. The mechanism of ¹⁵N enrichment in homospermidine is discussed.     

Gene note. Characterization of a tomato karyopherin {alpha} that interacts with the Tomato Yellow Leaf Curl Virus (TYLCV) capsid protein

A karyopherin (LeKAP1) cDNA was isolated from tomato plants. The deduced LeKAP1 protein sequence of 527 amino acids showed similarity to other plant karyopherin proteins. When LeKAP1 was expressed in a yeast two-hybrid system together with the gene coding for the capsid protein (CP) of the tomato yellow curl leaf virus (TYLCV), it interacted directly with CP. Thus, LeKAP1 may be involved in the nuclear import of TYLCV CP and, potentially, the TYLCV genomes during viral infection of the host tomato cells.     

Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community

Periods of limited soil water availability are a feature of many temperate pasture systems and these have the potential to modify pasture plant and community responses to elevated atmospheric CO2. Using large pasture turves, previously exposed to elevated CO2 concentrations of 350 or 700 mol mol^-1 for 324 d under well-watered conditions the morphological and physiological responses of pasture species growing at these CO2 concentrations were compared when subjected to a soil moisture deficit-and to recovery from the deficit-with those that continued to be well watered.Net leaf photosynthesis of Trifolium repens (C3 legume), Plantago lanceolata (C3) and Paspalum dilatatum (C4) was increased by exposure to elevated CO2, but there was no consistent effect of CO2 on stomatal conductance. At low soil moistures, net photosynthesis declined and stomatal conductance increased in these three species. There was a strong CO2 x water interaction in respect of net photosynthesis; in Trifolium repens, for example, elevated CO2 increased net photosynthesis by approximately 50% under well-watered conditions and this increased to over 300% when soil moisture levels reached their minimum values. Similar values were recorded for both Paspalum dilatatum and Plantago lanceolata. Potential water use efficiency (net photosynthesis/stomatal conductance) was increased by both exposure to elevated CO2 and drought.Leaf water status was measured in three species: Trifolium repens, Paspalum dilatatum and Holcus lanatus (C3). Total leaf water potential (t) and osmotic potential () were decreased by drought, but CO2 concentration had no consistent effect. t and were highest in the C4 species Paspalum dilatatum and lowest in the legume Trifolium repens.In the wet turves, rates of leaf extension of the C3 grasses Holcus lanatus and Lolium perenne at elevated CO2 were frequently higher than those at ambient CO2, but there was no effect of CO2 concentration on the rate recorded in the C4 grass Paspalum dilatatum or the rate of leaf appearance in the legume Trifolium repens. Drought reduced leaf extension rate irrespective of CO2 in all species, but in Holcus lanatus the reduction was less severe at elevated CO2. Immediately after the dry turves were rewatered the leaf extension rate on tillers of Holcus lanatus and Lolium perenne were higher than on tillers in the wet turves, but only at ambient CO2. Consequently, despite the greater leaf extension rate during the soil moisture deficit at elevated CO2, because of the overcompensation after rewatering at ambient CO2, total leaf extension over both the drying and rewetting period did not differ between CO2 concentrations for these C3 grass species. Further investigation of this difference in response between CO2 treatments is warranted given the frequent drying and wetting cycles experienced by many temperate grasslands.     

Reduced aleurone {alpha}-amylase production in aged wheat seeds is accompanied by lower levels of high-pI [IMAGE]-amylase transcripts and reduced response to gibberellic acid

With the aim of determining the level at which ageing exerts its effect on the expression of -amylase, GA3 regulation of -amylase production was studied in isolated aleurone layers from aged wheat seeds. GA3-induced -amylase activity was lower in the tissue from aged seeds than in controls. However, the proportion of ³⁵S-methionine incorporated into -amylase was higher in the aged than in control tissue. The pattern of -amylase isoforms was resolved by isoelectric focusing and showed that two isogroups were present with the activity of the high-pI isogroup being higher in the control than in the aged lot. These apparently contradictory results may be explained in terms of differences in isozyme expression. Studies on the expression of -amylase genes indicated a reduction in the level of high-pI mRNA in aged tissue. Dose-response curves showed lower GA3-responsiveness of aleurone layers from aged seeds as compared to the controls. From these results, it is proposed that the diminished capacity of -amylase production in aleurone from aged seeds is apparently due to a decrease in the expression of the high-pI -amylase genes, and this reduction is associated with a decrease in the response to GA3.Key words: Seed ageing, wheat aleurone, gibberellic acid, -amylase isozymes, gene expression.      

Phytosulphokine-{alpha}, a peptidyl plant growth factor, stimulates somatic embryogenesis in carrot

Phytosulphokine- (PSK-) is the first chemically characterized peptide that acts as a plant growth factor. It stimulates the proliferation of asparagus and rice cells, but no information is yet available on its effects on plant morphogenesis. The effects of PSK- on somatic embryogenesis in carrot (Daucus carota L.) were examined. PSK-, when added to the induction medium for somatic embryogenesis, increased the number of somatic embryos. The chemical analogues [2-5]PSK- and tyrosine sulphate ester (Tyr-SO3 H), which have been used as negative controls in other systems, had no effect. Moreover the proliferation of cells during somatic embryogenesis was also enhanced by PSK- these results indicate that PSK- enhanced cell division and, as a consequence, stimulated carrot somatic embryogenesis. PSK- also stimulated the proliferation of embryogenic cells in medium that contained 2,4-dichlorophenoxyacetic acid (2,4-D), in which somatic embryos did not form, as well as the proliferation of non-embryogenic cells (cells that had lost the ability to form somatic embryos) in medium without 2,4-D. These results indicate that PSK- has a stimulatory effect on cell division generally in carrot cell cultures.Key words: Daucus carota, plant growth factor, somatic embryogenesis, sulphated peptide.      

Correlation of decreased calcium contents with proline accumulation in the marine green macroalga Ulva fasciata exposed to elevated NaCl contents in seawater

The involvement of Na⁺, K⁺, Cl^- or Ca²⁺ in the regulation of salinity stress-induced proline accumulation via the inhibition of the activity of proline dehydrogenase (PDH; EC 1.4.3.1), a catabolic enzyme of proline, was investigated in the marine green macroalga Ulva fasciata Delile. After 6 h of exposure to elevated artificial seawater (ASW) salinity, adjusted either by increasing the NaCl content in 30 ASW (a change in ion ratio) or by concentrating ASW (a constant ion ratio), the contents of Na⁺, K⁺ and Cl^- linearly accumulated with increasing salinity from 30-90 (parts per thousand); the accumulation pattern of each ion was similar between the two treatments. An increase in NaCl content in ASW induced proline accumulation, but decreased both the PDH activity and the total water-soluble Ca²⁺ contents, while concentrated ASW had no effect. As compared to a constant value at 30, both the contents of total and water-soluble CA²⁺ and the activity of PDH decreased 1 h after exposure to 60 (adjusted by increasing NaCl content in 30 ASW) and concomitantly the content of seawater Ca²⁺ increased, while proline accumulated after 3 h. The addition of 15 mM ethylene glycol-bis-(2-aminoethyl ether) N,N,N-tetraacetic acid (EGTA) in 60 ASW (adjusted by increasing the NaCl content in 30 ASW) enhanced both the proline accumulation and the decrease in the content of total and water-soluble cellular Ca²⁺ and the activity of PDH; the effects of EGTA were reversed by 10 mM CaSO4. These results indicate that a loss of cellular Ca²⁺ is associated with the NaCl induction of proline accumulation via an inhibition of PDH activity in U. fasciata.     

An Error in the Calibration of Xylem-water Potential against Leaf-water Potential

An error occurs in the calibration of xylem pressure potential() against leaf-water potential () when the calibration is madeusing plant material in which the water stress has been inducedartificially after excision. The impostion of water stress afterexcision affects the determination more than it affects , consequentlythe relationship between these two indices of water stress isaltered. Care should be exercised to ensure that identical proceduresare adopted during . calibrations and during susbsequent fieldmeasurements of with the pressure-chamber apparatus.     

The Meaning of Matric Potential

The commonly used equation, = P - + , which describes thepartitioning of plant water potential, , into components ofhydrostatic pressure, P, osmotic pressure, , and matric potential,, is misleading. The term , which is supposed to show the influenceof a solid phase on , is zero if a consistent definition ofpressure is used in the standard thermodynamic derivation. However,it can be usefully defined by = + _D, where _D is the osmoticpressure of the equilibrium dialysate of the system. The practicaland theoretical significance of this definition is discussed.     

Adaptation of the Euryhaline Charophyte Lamprothamnium papulosum to Brackish and Freshwater: Turgor Pressure and Vacuolar Solute Concentrations During Steady-State Culture and After Hypo-osmotic Treatment

The euryhaline charophyte Lamprothamnium papulosum (Wallr.)J. Gr. was adapted to media with decreasing salinities rangingfrom 550 to 0 mosmol kg^–1. Vegetative plants grown inmedia with osmotic pressures (⁰) in the range of 550 to 130mosmol kg^–1 maintained a constant turgor pressure () at309 + 7 mosmol kg^–1. The ions K+, Na+ and Cl–, werethe predominant solutes in the vacuole. Changes in their concentrationsaccount for the variation in internal osmotic pressure (¹) with,⁰. The divalent ions Mg²⁺, Ca²⁺ and were also present in significant amounts, but their concentrationsdid not alter with changes in, ⁰. In cells subjected to hypo-osmotic shock the regulation of was incomplete. The turgor pressure increased from 302 to 383mosmol kg^–1. The first rapid response to the sudden decreasein ⁰ was a loss of K⁺ and Cl^–. In contrast to the decreasein ionic concentrations an accumulation of sucrose occurredwhich could account for the increase of . The increase in sucroseconcentration started 24 to 48 h after the downshock and reachedits highest value after 3 to 4 weeks. The sucrose concentrationin the vacuole was up to 320 mol m^–3. During this timethe ionic content continued to decrease but did not counterbalancethe sucrose concentration sufficiently to regain the original. High sucrose levels accompanied by an enhanced were also observedduring the period of fructification (sexual reproduction: formationof antheridia and oogonia) in Lamprothamnium kept under conditionsof constant salinity. It is concluded that high sucrose content and elevated arecharacteristic of sexual reproduction in this charophyte. Lamprothamniumis able to tolerate different during various developmentalstages (e.g. vegetative and reproductive phases). Key words: Lamprothamnium papulosum, sucrose, turgor pressure     

Quantitative effects of the genes Lf, Sn, E, and Hr on time to flowering in pea (Pisum sativum L.)

Flowering time in pea (Pisum sativum L.) is determined by genetically controlled responses to photoperiod and temperature. To investigate these responses, 11 lines homozygous for the flowering genes Lf, Sn, E, and Hr were grown under contrasting semi-controlled photothermal environments and the durations (d) from sowing to first flower (f) were recorded. The effects of the four genes were quantified using a two-plane photothermal model which linearly relates the rate of progress from sowing to flowering (1/f) with the mean pre-flowering values of temperature (T) and/or photo-period (P), based on 1/fa + bT (when P is longer than the critical photoperiod, Pc) and 1/fa + bT + cP (when P<Pc). The main effect of Lf alleles was on temperature sensitivity (b) when P>Pc, which increased in the sequence Lf^d<Lf< lf<lf^a. Gene Hr, when together with Sn, increased photoperiod sensitivity (c) and reduced the intercept (a) when P<Pc. Allele sn determined a single plane response to temperature alone (i.e. a day-neutral response). Gene E, when present with lf Sn, increased 1/f in both the thermal (P<Pc) and photothermal (PPc) domains, mainly by increasing a and b, respectively. Variations in the coefficients of the thermal and photothermal responses determined that the critical photoperiod varied with temperature in all photoperiod-sensitive genotypes. A common base temperature of 0.2C was determined amongst Day-Neutral Class genotypes (sn) and thermal time from sowing to flowering increased in the sequence lf^a<lf< <:f<Lf^d. Intra-Class variations attributed to the Lf alleles were also detected in the Late (Sn hr) and Late High Response (Sn Hr) Classes. The linear photothermal model provided a sound basis for studying the quantitative effects of flowering genes in pea.     

Developmental Responses to Drought and Abscisic Acid in Sunflower Roots: I. ROOT GROWTH, APICAL ANATOMY, OSMOTIC ADJUSTMENT

Aeroponically grown sunflower seedlings (Helianthus annuus L.cv. Russian Giant) were droughted or treated with abscisic acid(ABA) for 7 d. Drought stress prompted a three-phase growthresponse in sunflower roots: an initial phase of increased rootelongation was followed by a period of almost complete inhibitionbetween about 6 h and 72 h; this was followed, in turn, by aphase of partial recovery in the rate of root elongation. Droughtdecreased the size of the apical meristem as cells in the proximalregion of the meristem vacuolated and elongated. Root-to-shootbiomass ratios (R:S) increased initially but declined after72 h. Drought stress decreased water potential () and osmoticpotential ( and increased turgor pressure _p in the apical 30mm of the roots. These initial changes were transitory, lastingabout 3 h. Thereafter, and began to rise; _p fell back to controllevels. In the later stages of treatment, fell as the stressgrew more severe, but fp was maintained by osmotic adjustment.Desiccation for 1 h increased turgor pressures in excised 30mm apical segments. The transitory increase in root elongationwas contemporary with the initial rise in _p in the root apices,while the periods of greatest inhibition and partial recoveryin root elongation were contemporary with the periods of declineand partial recovery in the length of the apical meristem respectively.The inhibition of root elongation and the anatomical changesin the root apices were not determined by loss of turgor orlack of photosynthate, but rather appeared to be an active responseby the meristem to a drop in external . Treatment with ABA triggeredmany of the same changes as drought stress: ABA promoted a three-phasegrowth response, increased R:S, triggered the same initial changesin , , and _p, increased _p in excised 3.0 mm apical segments,and induced the same pattern of anatomical changes in the rootapices as drought stress. It is proposed that ABA mediates drought-inducedchanges in the primary development of sunflower roots. Key words: Abscisic acid, apical meristem, drought, osmotic adjustmen     

Influence of UV-B radiation and Cd2+ on chlorophyll fluorescence, growth and nutrient content in Brassica napus

The possible interaction of two stresses, UV-B radiation and cadmium, applied simultaneously, was investigated in Brassica napus L. cv. Paroll with respect of chlorophyll fluorescence, growth and uptake of selected elements. Plants were grown in nutrient solution containing CdCl2, (0, 0.5, 2 or 5 M) and irradiated with photosynthetically active radiation (PAR, 400-700 nm, 800 mol m^-2 s^-1) with or without supplemental ultraviolet-B radiation (UV-B, 280-320 nm, 15 kJ m^-2 d^-1, weighted irradiance). After 14 d of treatment, the most pronounced effects were found at 2 and 5 M CdCl2 with and without supplemental UV-B radiation. Exposure to cadmium significantly increased the amount of Cd in both roots and shoots. In addition, increases occurred in the concentrations of Fe, Zn, Cu, and P in roots, while K was reduced. In shoots the S content rose significantly both in the presence and absence of UV-B radiation, while significant increases in Mg, Ca, P, Cu, and K occurred only in plants exposed to Cd and UV-B radiation. Manganese decreased significantly under the combined exposure treatment. The rise in S content may have been due to stimulated glutathione and phytochelatin synthesis. Cadmium exposure significantly decreased root dry weight, leaf area, total chlorophyll content, carotenoid content, and the photochemical quantum yield of photosynthesis. As an estimation of energy dissipation processes in photosynthesis, non-photochemical quenching (qNPQ) was measured using a pulse amplitude modulated fluorometer. The qNPQ increased with increasing Cd, while the combination of cadmium and UV-B reduced the qNPQ compared to that in plants exposed only to cadmium or UV-B radiation. The chlorophyll a:b ratio showed a reduction with UV-B at no or low Cd concentrations (0 M, 0.5 M CdCl2), but not at the higher Cd concentrations used (2 M, 5 M CdCl2). Thus in some instances there appeared to be a UV-B and Cd interaction, while in other plants response could be attributed to either treatment alone.Keywords: Brassica napus, cadmium, ultraviolet-B radiation.      

The effect of rhizosphere dissolved inorganic carbon on gas exchange characteristics and growth rates of tomato seedlings

The possibility that an enhanced supply of dissolved inorganic carbon (DIC=CO2+HCO3^-) to the root solution could increase the growth of Lycopersicon esculentum (L.) Mill. cv. F144 was investigated under both saline and non-saline root medium conditions. Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO2 concentrations in the range between 0 and 5000 mol mol^-1. The biomass of both control and salinity-stressed plants grown at high temperatures (daily maximum of 37C) and an irradiance of 1500 mol m^-2 s^-1 was increased by up to 200% by enriched rhizosphere DIC. The growth rates of plants grown with irradiances of less than 100 mol m^-2 s^-1 were increased by elevated rhizosphere DIC concentrations only when grown at high shoot temperatures (35C) or with salinity 28°C). At high light intensities, the photosynthetic rate, the CO2 and light-saturated photosynthetic rate (jmax) and the stomatal conductance of plants grown at high light intensity were lower in plants supplied with enriched compared to ambient DIC. This was interpreted as 'down-regulation' of the photosynthetic system in plants supplied with elevated DIC. Labelled organic carbon in the xylem sap derived from root DI¹⁴C incorporation was found to be sufficient to deliver carbon to the shoot at rates equivalent to 1% and 10% of the photosynthetic rate of the plants supplied with ambient- and enriched-DIC, respectively. It was concluded that organic carbon derived from DIC incorporation and translocated in the xylem from the root to the shoot may provide a source of carbon for the shoots, especially under conditions where low stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.