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.      

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.     

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.      

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.     

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      

Lectin-like activity of lupin seed conglutin {gamma}, a glycoprotein previously referred to as a storage protein

The lupin seed glycoprotein conglutin was capable of bindingvarious exogenous and endogenous N-glycosylated proteins andalso specifically bound to a Sephadex G-75 column. The bindingactivity of conglutin was abolished by either addition of 10mM EDTA or by enzymic deglycosylation and acid treatment. Inthe latter case activity could be restored by the addition ofMn and Ca ions. Immunological homology at polypeptide level of conglutin withother legume lectins was found. Key words: Conglutin , glycoprotein, legume lectin, Lupinus albus     

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.     

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.      

Alternative Methods of Analysing Water Potential Isotherms: Some Cautions and Clarifications: I. THE IMPACT OF NON-IDEALITY AND OF SOME EXPERIMENTAL ERRORS

In recent years alternative ways have been proposed to transformmeasurements of leaf water potential, , and relative water content,R^*, in order to derive values of osmotic pressure at full turgidityin leaves and shoots, ^o(when 0). Two types of transformationsare usually considered: 1/ versus R^* and versus 1/R^*, and linearregression is used to fit the data in the region where turgoris thought to be zero. It appears that when ^o is estimated bylinear extrapolation of 1/Psi; versus R^* then apoplastic watermight not influence the accuracy of ^o but when the versus \/R^*transformation is used apoplastic water causes an underestimateof ^o. We examine the accuracy of the estimate of ^o obtainedfrom the two transformations when there are random errors in, systematic errors in , and when the osmotic solutions arenon-ideal. The 1/ versus R^* transformation generally producesthe best estimate of ⁰ by linear extrapolation.     

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.      

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: II. GERMINATION AT REDUCED WATER POTENTIAL

Seed germination rates (GR =inverse of time to germination)are sensitive to genetic, environmental, and physiological factors.We have compared the GR of tomato (Lycopersicon esculentum Mill.)seeds of cultivar T5 to those of rapidly germinating L. esculentumgenotypes PI 341988 and PI 120256 over a range of water potential(). The influence of seed priming treatments and removal ofthe endosperm/testa cap enclosing the radicle tip on germinationat reduced were also assessed. Germination time-courses atdifferent 's were analysed according to a model that identifieda base, or minimum, allowing germination of a specific percentage(g) of the seed population (_b(g)), and a ‘hydrotime constant’(_H) indicating the rate of progress toward germination per MPa.h.The distribution of _b(g) determined by probit analysis was characterizedby a mean base (_b) and the standard deviation in _b among seeds(_b). The three derived parameters, _b, _b) and _H, were sufficientto predict the time-courses of germination of intact seeds atany . A normalized time-scale for comparing germination responsesto reduced is introduced. The time to germination at any (t_g())can be normalized to be equivalent to that observed in water(t_g(0)) according to the equation t_g(0)=[l–(/_b(g))]t_g().PI 341988 seeds were more tolerant of reduced and had a morerapid GR than T5 seeds due to both a lower _b and a smaller _H.The rapid germination of PI 120256, on the other hand, couldbe attributed entirely to a smaller _H. Seed priming (6 d in–1.2 MPa polyethylene glycol 8000 solution at 20 ?C followedby drying) increased GR at all >_b(g), but did not lower theminimum allowing germination; i.e. priming reduced _H withoutlowering _b. Removing the endosperm/testa cap (cut seeds) markedlyincreased GR and lowered the mean required to inhibit germinationby 0.7 to 0.9 MPa. However, this resulted primarily from downwardadjustment in _b during the incubation of cut seeds at low inthe test solutions. The difference in _b between intact and cutseeds incubated at high was much less (0.l MPa), indicatingthat at the time of radicle protrusion, the endosperm had weakenedto the point where it constituted only a small mechanical barrier.In the intact seed, endosperm weakening and the downward adjustmentin embryo _b ceased at < –0.6 MPa, while the reductionin _H associated with priming proceeded down to at least –1.2MPa. Based on these data and on the pressure required to pushthe embryos from the seeds at various times after imbibition,it appears that the primary effect of priming was to shortenthe time required for final endosperm weakening to occur. However,as priming increased GR even in cut seeds, priming effects onthe embryo may control the rate of endosperm weakening. Key words: tomato, Lycopersicon esculentum Mill., water potential, germination rate, seed priming, genetic variation     

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.      

The Derivation of the Cell Wall Elasticity Function from the Cell Turgor Potential

An equation is derived expressing average turgor pressure ofa leaf (_p) as a function of relative water content (RWC). Basedon this derivation, the relationships of the bulk elastic modulus(_v) and both RWC and _p, are formulated and discussed. The bulkelastic modulus (_v) becomes zero for _p = 0, that is at the turgorloss point for the leaf. At full water saturation the valueof ev is proportional to the water saturation turgor potential_p(max). The factor relating _P and _v (structure coefficient ,Burstrom, Uhrstr?m and Olausson, 1970) changes only very littlefor values of _p, which are not too close to zero. An exampleis given for the calculation from experimental data of the turgorpressure function, the structure coefficient function, and the_v function. Key words: Cell wall, Turgor pressure, Bulk elastic modulus     

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     

A simulation model to predict seed dormancy loss in the field for Bromus tectorum L

Bromus tectorum L. (cheatgrass) is an invasive winter annual whose seeds lose dormancy through dry after-ripening. In this paper a thermal after-ripening time model for simulating seed dormancy loss of B. tectorum in the field is presented. The model employs the hydrothermal time parameter mean base water potential (b(50)) as an index of dormancy status. Other parameters of the hydrothermal time equation (the hydrothermal time constant HT, the standard deviation of base water potentials b, and the base temperature Tb) are held constant, while b(50) is allowed to vary and accounts for changes in germination time-course curves due to stage of after-ripening or incubation temperature. To obtain hydrothermal time parameters for each of four collections, seeds were stored dry at 20C for different intervals, then incubated in water (O MPA) or polyethylene glycol (PEG) solutions (-0.5, -1.0, -1.5 MPa) at 15 and 25C. Germination data for the thermal after-ripening time model were obtained from seeds stored dry in the laboratory at 10, 15, 20, 30, 40, and 50°C for 0 to 42 weeks, then incubated at two alternating temperatures in water. Change in b(50) was characterized for each collection and incubation temperature as a linear function of thermal time in storage. Measurements of seed zone temperature at a field site were combined with equations describing changes in b(50) during after-ripening to make predictions of seed dormancy loss in the field. Model predictions were compared with values derived from incubation of seeds retrieved weekly from the field site. Predictions of changes in b(50) were generally close to observed values, suggesting the model is useful for simulating seed dormancy loss during after-ripening in the field.     

An Assessment of Gibberellin Structure-activity Relationships

Information on the biological activities of gibberellins (GAs)in the barley aleurone, Tangin-bozu dwarf rice, dwarf pea, lettucehypocotyl and cucumber hypocotyl bioassays is reviewed and discussedin the context of GA structure-activity relationships. The barley aleurone bioassay exhibits a limited response toGAs and it is suggested that this may be because the aleuronecells are able to carry out few GA interconversions. Consequentlyactivity is determined by the degree of compatibility betweenthe GAs and a receptor site. In this assay high biological activityis associated with GAs having a 3ß-hydroxy--lactonestructure. This activity is substantially enhanced by the additionalpresence of a 13-hydroxyl group. The substitution of a -lactoneor a -lactol for a -lactone results in reduced activity while3ß,13-dihydroxy GAs with either 20-carboxyl or 20-methylfunctions are completely inactive. The Tanginbozu dwarf ricebioassay responds to many more GAs than the barley aleuronesystem possibly because the rice seedlings can carry out extensiveGA interconversions. Under these circumstances GAs that areinactive per se can be metabolically converted to active forms.There is no interaction between the 3ß- and 13-hydroxyfunctions of GA molecules in the rice assay. Activity appearsto be determined by the degree oxidation of the C-20 group.The order of activity is usually -lactone > -lactol >-lactone > methyl > carboxyl. It is suggested this mayindicate that in rice seedlings C₂₀-GAs are converted to C₁₉-GAsvia a Baeyer-Villiger type oxidation. Activity in the dwarfpea bioassay is dependent upon GAs possessing both 3ß-and 13-hydroxyl groups and is again related to the state ofoxidation at the C-20 locus. In the lettuce bioassay activityis restricted to GAs with a -lactone function. In some instancesa -lactone, but not a -lactol, can substitute effectively. Thismay imply that the applied C₂₀-GAs are not converted to C₁₉-GAsand that the response to the -lactone results from the six-memberedring mimicking the -lactone at the receptor site. Only GAs havinga 19,10 or a 19,20 lactonic bridge show substantial activityin the cucumber bioassay. The additional presence of eithera 12- or a 13-hydroxyl group severely reduces activity.     

Some Methods for the Measurements of Tonoplast Resistance of Nitellopsis obtusa

Measurements of tonoplast resistance for unit area, T were carriedout by four methods. The T values obtained by two of these methods,when taking into account a correction for leakage current, were0.024 and 0.010 k cm². The other two methods, which made itpossible to neglect leakage current, gave the T values of 0.036and 0.021 k cm². Thus, the mean value of tonoplast resistancein the investigated cells of Nitellopsis obtusa was 0.023 kcm², while the measurements in which leakage current was nottaken into account gave the value 10.6 k cm². The results obtainedin this study indicate the fact that leakage current significantlyaffects measurements of tonoplast resistance and that the truevalue of that resistance is smaller by two or three orders ofmagnitude than that usually reported.     

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.