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.     

Hydrothermal time analysis of tomato seed germination responses to priming treatments

Controlled hydration of seeds followed by drying (seed priming) is used to break dormancy, speed germination, and improve uniformity of radicle emergence. To date, empirical trials are used to predict optimal priming conditions for a given seed lot. Since priming is based upon seed water relations, it was hypothesized that the sensitivity of germination to reduced water potential before priming might be mechanistically related to, and therefore predictive of, priming responsiveness. Analyses of germination of 13 tomato (Lycopersicon esculentum Mill.) seed lots at two temperatures (15C and 20C) and three water potentials (0, -0.28 and -0.43 MPa) showed that seed lot germination responses could be quantitatively characterized by parameters derived from thermal time, hydrotime, and hydrothermal time models (R²0.73-0.99). Six of the seed lots were primed at two temperatures (15°C and 20°C) and three water potentials (-1.0, -1.5 and -2.0 MPa) for various durations, dried, and their subsequent germination rates analysed according to hydropriming time and hydrothermal priming time models. The responses of germination rates to priming were characterized by hydropriming time (HP) and hydrothermal priming time (HTP) constants and the minimum water potential (min) and temperature (Tmin) for achieving a priming effect. The values of min and Tmin varied relatively little among tomato seed lots, and the generalized values of min=2.39 MPa and Tmin=9.10°C accounted for 74% (15°C), 57% (20°C), and 62% (across both temperatures) of the increase in germination rates following priming. Nonetheless, while the hydrothermal time models described germination patterns both before and after priming, there was relatively little predictive relationship between them.     

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.      

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.      

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.     

Pattern of respiration by intact inflorescences of the thermogenic arum lily Philodendron selloum

Inflorescences of the neotropical arum lily, Philodendron selloum, are strongly thermogenic for 2 d during anthesis. Continuous measurements of spadix temperature (Ts) and rate of oxygen consumption (VO2) were made outdoors in whole inflorescences attached to the plants. Some inflorescences were exposed to uncontrolled ambient temperature (Ta) while others were enclosed in clear water-jackets that produced nearly constant ambient conditions. A repeatable, diphasic pattern of heat production appeared, most clearly in water-jacketed inflorescences, and it comprised a short peak phase at sunset followed by a plateau phase that lasted until the following sunset. Regulation of Ta occurred in both phases, but at different levels. Peak phase Ta was regulated in the region of 38-42C, but plateau phase Ta was usually in the range of 25-36C. Both VO2 and total heat produced throughout anthesis increased at lower Ta. The data imply that the short peak phase is related to the enhancement of odour production that attracts a single species of large scarabaeid beetle in its native Brazil, and regulation of maximum Ta may prevent overheating. Thermoregulation in the long plateau phase produces equable temperatures inside the inflorescence that may facilitate the resident beetles' activities as a direct energetic reward.     

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.     

Switching off the heater: influence of ambient temperature on thermoregulation by eastern skunk cabbage Symplocarpus foetidus

The protogynous inflorescences of eastern skunk cabbage, Symplocarpus foetidus, are thermogenic and regulate spadix temperature (Ts) well above ambient temperature (Ta). Continuous records of oxygen consumption, carbon dioxide production, and temperatures of plants were made at a field site in Canada. At Ta between 3-24C, Ts ranged between 16-26C, and the warmest inflorescences were those in the receptive female or early pollen-bearing stages. Respiratory rates of the 2-g spadices increased with declining Ta, and reached a maximum of 0.54 mol O2s^-1 (0.73 ml min^-1), equivalent to 0.26 W of heat production. At Ta below 3°C, several inflorescences failed to maintain high Ts and abruptly switched Ts to near freezing. Some froze when Ta dropped to about -10°C. Those that did not freeze could quickly switch to the warm state if Ta rose above about 3°C. Switching was related to the balance between heat production and heat loss that tended to produce stable equilibria at either high or low Ts. Switching between warm and cool states resulted in a bimodal distribution of Ts in the field. A respiratory quotient of 1.0 showed that carbohydrate was the substrate for thermogenesis, and bomb calorimetry of florets confirmed that energy was imported from the root. Only 11 invertebrates, including only one flying insect, were found in 195 inflorescences, suggesting that heat production and temperature regulation are not closely associated with cross-pollination.     

Respiration and germination rates of tomato seeds at suboptimal temperatures and reduced water potentials

Suboptimal temperature (T) affects germination rates (reciprocalof time to radicle emergence) on a thermal time basis; thatis, the T in excess of a base or minimum temperature multipliedby the time to a given per cent germination [t_g) is a constant.Respiration rates are also sensitive to T, and proportionalrelationships are often found between respiration rates andgermination rates. Reduced water potential () delays seed germinationon a hydrotime basis (i.e. the in excess of a base water potentialmultiplied by t_g is a constant). It was tested whether respirationrates prior to radicle emergence vary in proportion to T and as expected from the thermal and hydrotime models. Respirationrates (C0₂ evolution) of cold-tolerant, rapidly germinating(PI 341988) and cold-sensitive, more slowly germinating (T5)tomato [Lycopersicon esculentum Mill.) seeds were evaluatedover a range of T and conditions. For both genotypes, respirationrates until the beginning of radicle emergence were relatedto T on a thermal time basis and increased approximately linearlywith above -2.0 MPa, consistent with the hydrotime model. Respirationrates were uniquely related to germination rates, regardlessof whether germination timing was affected by T, , or genotype.However, germination timing was unaffected when respirationrates were manipulated by varying 0₂ partial pressure. Thus,while both germination and respiration rates vary with T and consistent with thermal and hydrotime models of biologicaltime, respiration rates per se were not the limiting factorin germination timing of tomato seeds. Key words: Lycopersicon esculentum Mill., tomato, germination, respiration, temperature, water potential     

An improved strain-gauge device for continuous field measurement of stem and fruit diameter

An improved strain-gauge dendrometer was tested on apple (Pyrus malus L.) tree trunks and fruit to measure thickness dynamics. The sensor is similar to previous devices, but is more flexible allowing for a greater range of thickness change before it has to be physically reset. The practical resolution of the measurement system is given as one standard deviation about the mean of 51 observations taken on an Invar metal alloy in the field when temperatures ranged between 16C and 17C. The mean of these observations was 3.68356 mm with one standard deviation of 0.00947 mm. There is a positive correlation between temperature and sensor output having a slope of 2.96 m°C^-1. For a typical daily temperature change of 21°C, this causes an apparent change of 62 m. Stem thickness varied 0.7 mm diurnally with maximum values at night and minimum values during the day. Apple fruit diameter increased 3.3 mm over the 15 d period.Keywords: Strain gauge dendrometer, plant stem and fruit thickness sensor, plant water status, growth.      

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.     

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      

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.      

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.      

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.     

Apical abortion in calabrese is induced by periods of low temperature and results in premature differentiation of apical meristem cells

Apical abortion in calabrese (Brassica oleracea var. italica), a highly destructive disorder which occurs in overwintered transplants, has been investigated using a model system in which blindness (abortion of the apical meristem) can be reproducibly and predictably induced. An initial experiment examined the susceptibility of 12 cultivars to apical abortion when grown throughout a winter period under commercial conditions. Three of those varieties showed very high levels of blindness (100%). Subsequently, plants of the susceptible cultivar PETO 7204 were subjected to an inductive period of low light intensity (30 mol m^-2 s^-1) and low temperature (4 C). Apical meristematic cells of all plants ceased mitotic activity within 3 d of being transferred to a regime comprising higher light intensity (100 mol m^-2 s^-1) and temperature (15 C). Using this system the structures of normal apices were compared with those which became blind. Blindness was characterized by a cessation of leaf primordium production by the vegetative apex, the last formed primordium growing on in some cases to form a mature normal leaf, or in others, a deformed structure known as a whip-tail. The inactive apical bud became embedded in the tissues of this last-formed structure. The cells of the inactivated apical bud remained alive, but lost their meristematic capability, becoming enlarged, highly vacuolated parenchyma cells with amyloplasts.Keywords: Apical abortion, apical meristem, blindness, calabrese.      

Potassium Transport Across the Membranes of Chara: II.42K FLUXES AND THE ELECTRICAL CURRENT AS A FUNCTION OF MEMBRANE VOLTAGE

Smith, J. R. 1987. Potassium transport across the membranesof Chara. II. ⁴²K fluxes and the electrical current as a functionof membrane voltage.—J. exp. Bot. 38: 752–777. The current required to clamp the trans-membrane voltage ofinternodal cells of Chara australis at different levels wasmeasured simultaneously with either the ⁴²K influx or efflux.Examination of the voltage-dependence of the ratio of the electricalcurrent to the unidirectional tracer fluxes yielded no evidenceof any amplification of the electrical driving force on theK⁺ ions. There was thus no evidence for the interaction of K⁺ions with themselves or any other species during their passageacross the membrane. These measurements allow the determinationof , the fraction of the electrical current carried by K⁺ ions.When the external [K⁺] = 10 mol m^–3, the average valueof was 0?85 for V_m > –125 mV and 07?5 for V_m <–150 mV. When the external [K⁺] = 0?1 mol m^–3, was 0?6 for V_m < –80 mV and 0?1 for V_m > –250mV. It was also found that the conductance associated with K⁺transport was inhibited by hyperpolarization. Key words: Potassium, conductance, flux-ratio     

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.     

High-light damage in air-dry thalli of the old forest lichen Lobaria pulmonaria-interactions of irradiance, exposure duration and high temperature

High-light damage in air-dry thalli of Lobaria pulmonaria were measured in the laboratory as reductions in maximal PSII efficiency (Fv/Fm) after a 48 h recovery in a hydrated state at low light to account for permanent damage. Thalli treated with the lowest light dose (90 mol photons m^-2) recovered normal Fv/Fm-values with increasing irradiances (400-700 nm) up to 1000 mol photons m^-2 s^-1. Doubling this dose lowered the threshold level for damage from 1000 to 320 mol photons m^-2 s^-1, and reduced Fv/Fm at 1000 mol photons m^-2 s^-1 by more than 50%. A second doubling of the dose to 360 mol photons m^-2 caused damage at 200 mol photons m^-2 s^-1, and a nearly complete cessation of PSII efficiency occurred at 1000 mol photons m^-2 s^-1. No reciprocity of irradiance and duration of illumination for PSII function was found. The measured time-dependent decrease in Fv/Fm was remarkably similar for the naturally coupled, but artificially separated, light and temperature factors. Therefore, the damage of high light on desiccated L. pulmonaria seemed to be an additive effect of high irradiance and high temperatures. Air-dry thalli were highly heat susceptible, being affected already at temperatures around 40C. Logging operations in forests are likely to raise the solar radiation at remaining lichen sites to destructive levels.Keywords: Lichens, high-light damage, heat stress, poikilohydric organisms, reciprocity.      

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.