Showing papers in "Journal of Experimental Botany in 1980"

The Transport of Water from Soil to Shoot in Wheat Seedlings

Abstract: A technique is described for precisely measuring the drop in water potential, Ay/, between the soil and the leaf xylem of wheat seedlings. The technique was used to explore the relation between transpiration rate and Ay at various times during the monotonic drying of the soil in which the plants were growing. When the soil was wet, the relation was linear, but, as the soil dried, nonlinearities appeared which were, in the main, explicable in terms of simple soil physical models describing the flow of water through the soil to the roots. There was no sign of the major hydraulic resistance at the root: soil interface that other people have recently found. I N T R O D U C T I O N When a soil is wet it has little direct influence on the uptake of water from it by a plant. When it is dry it has a large influence. When it is neither wet nor dry the extent of its influence is a matter of controversy. Some have argued that the conductivity of soil to water is often so small that the transport of water to the plant's roots may limit uptake even when much available water remains in the soil (e.g. Gardner, 1960; Carbon, 1973). Others have argued that the properties of the plant and of its aerial environment dominate the uptake unless the soil is so dry that there is virtually no available water left in it (e.g. Newman, 1969; Lawlor, 1972). Others again have argued that it is neither the soil nor the plant, but the interface between the two that dominates (Herkelrath, Miller, and Gardner, 1977; Faiz and Weatherley, 1978). Tinker (1976) has comprehensively reviewed many of the issues involved. The controversy may have arisen for many different reasons. (1) There is much confusion about the measurement of leaf water potential, y/L (Neumann, Thurtell, and Stevenson, 1974; Bunce, 1978). Since y/L is often used in calculating the gradients of soil water suction near the roots, these calculated gradients may be grossly wrong. (2) Few experimenters have taken the trouble to measure carefully the hydraulic properties of their soils. Hydraulic conductivity decreases by several orders of D ow naded rom http/academ ic.p.com /jxb/article-ct/31/1/333/473180 by gest on 03 D ecem er 2018 334 Passioura—Transport of Water in Wheat magnitude as a soil dries, and does so particularly rapidly in the light soils that many have used. The popular technique of measuring saturated conductivity and using it to calculate unsaturated conductivity (e.g. Hansen, 1974) may be too crude for our purposes. (3) Salts build up at the interface between root and soil when their rate of uptake by the root is less than their convective inflow from the soil (Tinker, 1976). Such a build-up may vary widely between experiments, and, if large, could lead to large decreases in soil water potential close to a root even when none is expected from calculations based on the flow rate of water and the hydraulic properties of the soil. (4) Roots may shrink when water-stressed (Huck, Klepper, and Taylor, 1970), thereby losing much of their hydraulic contact with the soil. A large difference in water potential presumably develops between root and soil (Cowan and Milthorpe, 1968). The shrinkage will depend on many factors, and may depend especially on

Osmotic Adjustment in the Spikelets and Leaves of Wheat

Abstract: The response of osmotic potential to changes in water potential in the spikelets and leaves of wheat genotypes was studied in a controlled environment. While there was evidence of osmotic adjustment in the leaves of only some genotypes, it seemed to be present in the spikelets of all genotypes. Spikelets also differed from leaves in that osmotic potentials at full turgor were usually higher. During grain growth, levels of osmotic potential were observed in the spikelets of some genotypes which were up to 1-1 MPa greater than the corresponding water potential. Osmotic responses in both spikelets and leaves are discussed in terms of responses expected from concentration effects only.

Salt Tolerance in the Halophyte Suaeda maritima L. Dum.: Evaluation of the Effect of Salinity upon Growth

Abstract: The effects of the concentrations and species of ions in the external medium upon the growth of Suaeda maritima, a coastal halophyte, have been investigated. Plants grown in highly saline conditions (340 mol m−3 NaCl) had larger fresh and dry weights than plants grown in a standard culture medium (13 mol m−3). However, differing contributions of water and inorganic ions meant that these increases in fresh and dry weights greatly exaggerated the increase in growth, i.e. organic dry matter production. Enhanced growth in high salt concentrations was evident only after prolonged exposure and was attributable to increased cell size. Growth increased smoothly with increasing mineral concentration in the external medium and was maximal at 170 mol m−3 added NaCl, although 90% maximal growth was obtained at only one tenth of this concentration. Ion-specific growth enhancement was not apparent at low concentrations of NaCl. Growth at high concentrations (340 mol m−3) was maximal in NaCl and the effects of other salts ranged between this and toxicity in KC1. It is suggested (1) that the description of growth stimulation in halophytes by salt is somewhat arbitrary, (2) that increased organic growth with increasing salt concentration results from the relationship between turgor pressure and extension growth, and (3) that tolerance to extreme salinities is a separate phenomenon from the growth response.

Chloroplast DNA in Expanding Spinach Leaves

Abstract: The proportion of chloroplast DNA in total DNA from spinach leaves has been measured using the second order reassociation kinetics of a 3H-labelled chloroplast DNA probe in total DNA extracts. There was no significant difference between the proportion of chloroplast DNA in the basal and distal halves of 2 cm leaves and in the distal halves of 5, 8, and 10 cm leaves. The mean of all the observations was 21-1 ± 0-7%. There was little change in the average total DNA content of cells from any of the leaves but cells from larger leaves contained 130-170 chloroplasts while cells from the basal half of 2 cm leaves contained about 20 chloroplasts which were smaller than those from the larger leaves. Consequently the average number of copies of the plastome per chloroplast in large leaves was about 30 (5 x 10~13 g DNA) and in the smaller chloroplasts in the base of 2 cm leaves was 200 (32 x 10~15 g DNA). Staining with the DNA fluorochrome 4,6-diamidino-2 phenyl indole (DAPI) showed 10-15 plastid nucleoid areas in chloroplasts of larger leaves, suggesting there are 2-3 copies of the plastome per plastid nucleoid.

Effect of Anaerobiosis on Rice Seedlings: Growth, Metabolic Rate, and Fate of Fermentation Products

Abstract: When 3 d old aerobic rice seedlings are subjected to 48 h of anaerobiosis a strong alcoholic fermentation system is induced which operates at a constant rate during the treatment. Ethanol accumulates in seedling tissues during the first hours of anaerobiosis to reach a limit value which is maintained thereafter. Of the total ethanol production during the anaerobic treatment, only 2% is accounted for by ethanol stored in tissues, the remaining 98% is found in the growth medium. Concomitant effects of anaerobiosis on seedling growth, consumption of endosperm reserves, and variations in the level of activity of alcohol dehydrogenase (ADH) and soluble proteins are reported.

The Extraction, Solubility, and Characterization of Two Groups of Barley Storage Polypeptides

Abstract: This paper reports further studies on the characteristics of the storage protein fraction (hordein) of barley. Hordein consists of two groups of polypeptides (termed 'B' and 'C') coded by two separate but linked loci. Whereas the 'C' polypeptides are readily soluble and extracted in 60% (v/v) ethanol at room temperature, the 'B' group is more soluble in, and therefore more efficiently extracted by, 50% (v/v) propan-l-ol or 45% (v/v) propan-2-ol at elevated temperatures and in the presence of 2-mercaptoethanol. However, the most efficient conditions for hordein extraction (50% propan-l-ol + 2% (v/v) 2-mercaptoethanol at 60 °C) also extract some contaminating non-hordein polypeptides resulting in an apparently increased lysine content of the hordein fraction. Amino acid analysis of the purified 'B' and 'C' hordein groups shows that, whereas 'C' hordein contains more glutamate + glutamine, proline, and phenylalanine than 'B' hordein, it contains only traces of lysine and sulphur amino acids in contrast to 'B' hordein which contains 0-5% lysine 0-6% methionine, and 2-5% cysteine. Equilibrium sedimentation analyses carried out on the purified 'B' and 'C' groups indicates that the preparations were reasonably monodisperse with molecular weights of approximately 32 000 and 52 000 respectively. These values are considerably lower than those previously determined by SDS-PAGE.1

Effects of Mechanical Impedance on Root Growth in Barley (Hordeum vulgare L.) III. OBSERVATIONS ON THE MECHANISM OF RESPONSE

Abstract: The effects of mechanical stress on whole root systems was investigated using beds of solid glass spheres (ballotini) continuously supplied with aerated nutrient solution. As noted in earlier experiments, increased mechanical impedance slowed root extension and altered cell size and number; it also caused distortion of the root apex, stimulated growth of lateral shoot meristems, and induced the formation of nodal roots. The development of lateral branch roots was enhanced and where root axes curved around ballotini lateral roots formed preferentially on the outer (convex) side whereas root hairs developed on the inner (concave) side. After roots were relieved from mechanical stress at least 3 d elapsed before the rate of extension growth equalled that of unimpeded plants. When intact Zea mays root apices first made contact with ballotini their elongation was slowed by 70% for about 10 min; where root caps were removed before the roots made contact, no such effects were seen. We discuss the general nature of the mechanism of response to mechanical stress.

The Effects of Salinity upon Galactosyl-Glycerol Content and Concentration of the Marine Red Alga Porphyra purpurea (Roth) C.Ag.

Abstract: Changes in the major alcohol-soluble, low molecular weight carbohydrates of P. purpurea, O a-D-galactopyranosyl-(l-»2)-glycerol ('floridoside') and O-a D galactopyranosyl-(l->l)-glycerol ('isofloridoside'), have been examined in response to salinity variation. 'Floridoside' is shown to vary in absolute amount, increasing in hypersaline and decreasing in hyposaline media. 'Isofloridoside' content per cell does not change in a similar manner. Responses are almost identical under light or dark conditions, 'floridoside' changes being complete within 24 h. Decreasing the external water potential using ionic and non-ionic solutes has the same effect upon galactosyl-glycerol content. The amount of 'floridoside' synthesized and degraded under hypersaline and hyposaline conditions respectively is shown to be insufficient to restore cell volume to its original value. It is therefore suggested that the primary function of 'floridoside' increases in concentrated sea-waters is that of a compatible solute, serving to protect the cell during periods when the external salt content is increased dramatically.

Stomatal Responses to Sulphur Dioxide and Vapour Pressure Deficit

Abstract: Stomatal conductances (gj of plants of Vicia faba, Raphanus sativus, Phaseolus vulgaris, Helianthus annuus, and Nicotiana tabacum were measured in chambers containing either clean air or air containing between 18 and 1000 parts 10~9 S02 at water vapour pressure deficits (vpd) ranging from 1 -0 to 1 • 8 kPa When vpd was low ( 1 part 10~6) may close stomata (Bonte, de Cormis, and Louguet, 1977; Caput, Belot, Auclair, and Decourt, 1978; Kondo and Sugahara, 1978) Environmental variables such as sulphur supply to the soil and atmospheric humidity can also influence stomatal responses to S02 Cowling and Lockyer (1976) reported a reduction in the coefficient of transpiration in ryegrass plants exposed to 17-5 parts 10~9 S02 for 198 d when the supply of soil sulphur was inadequate, but no difference was observed between water use of polluted and unpolluted plants if soil sulphur was not limiting Mansfield and Majernik (1970) found that stomatal apertures of Vicia faba increased following exposure to 0-25-1 part 10~6 S02 at relative humidities above 40% but decreased when humidity was below 40% at 18 °C These varied reports demonstrate that stomatal response to S02 are complex and must be studied in well-defined environments Because such responses influence rates of transpiration and C02 exchange and modify the absorption rates of S02 at metabolic sites within the leaf, they have important implications for growth, development, and yield of crop plants The objectives of this investigation were therefore (i) to monitor continuously the responses of stomata in plants exposed to filtered air or air containing a range of S02 concentrations greater than 17-5 parts 10~9 (50 pg rrr3) throughout several photoperiods and (ii) to study the influence of atmospheric humidity on stomatal responses to S02 METHODS Plants were grown in pots in compost containing adequate sulphur in growth rooms and well watered Three-week old plants, with pots sealed in polythene bags to prevent water loss, were transferred to two exposure chambers in which environmental conditions were comparable to those in the growth room (189 W m~2 and 22 ± 0-5 °C day and 16 ± 0-5 °C night temperatures) The plants were then left to acclimatize in charcoal-filtered air (< 2 parts 10~9 S02) for one photoperiod before making measurements or imposing an experimental treatment The following day, both chambers were again supplied with clean air and continuous measurements of transpiration rates and stomatal conductances were made on both sets of plants using a sensitive monitoring system and methods of analysis described previously (Black and Unsworth, 1979) Since stomata are extremely sensitive to small changes in environmental conditions the temperature, humidity, and air movements of both exposure chambers was precisely controlled On the third day, when stomata were fully open, a known concentration of S02 was introduced into one chamber and the responses of polluted and control plants were monitored while water vapour pressure deficit (vpd) (defined here as the difference bel ween saturation vapour pressure at mean leaf temperature and the vapour pressure in the chamber) was varied from 1-0 to 1-8 kPa Temperature and irradiance were kept constant In a separate series of experiments, Vicia faba L (cv Dylan) plants were fumigated with S02 for several hours in the dark when stomata were closed or when they had been prevented from closing by exposure to low C02 concentrations (50 parts 10-6 C02) The stomatal conductance of these plants was monitored using a water vapour infrared gas analyser in the dark period before introduction of S02, during exposure to S02 concentrations which ranged from 17-5 to 1000 parts 10~9, and in the subsequent light and dark periods following removal of the pollutant Checks on the accuracy of stomatal conductance values calculated from infrared gas analysis measurements were made using a diffusion porometer (Delta-T Devices Mark II) Agreement between the two sets of conductance values was extremely good (Black and Black, 1979) Both these methods estimate leaf conductance and not stomatal conductances However since leaf This content downloaded from 207461315 on Fri, 26 Aug 2016 05:36:03 UTC All use subject to http://aboutjstororg/terms Black and Unsworth—S tomatal Responses to S02 669 conductance was small (0-02 cm s ') when plants were in the dark, stomatal conductances were assumed to be equal to the measured and calculated values of leaf conductance RESULTS In the results to be described, although each experiment was repeated at least four times it is not always appropriate to describe the change in stomatal conductance by a mean value and standard error since stomatal aperture is influenced by a variety of factors including plant age, plant water status, time of day, etc Therefore Figs 1 and 2a demonstrate the typical responses of single sets of plants exposed to either filtered or polluted air whereas Fig 2b describes the results of duplicate experiments on different sets of plants Figures 3 and 4 show the data obtained from at least four different sets of plants Figure 1 shows the stomatal conductance of Vfaba plants exposed to filtered air and air containing 17-5 and 87-5 parts 10~9 (200 pg m~3 and 1000 jug m~3) S02 '2 when vapour pressure deficit was 1-3 kPa at 22 °C The figure demonstrates the typical 20-30% increase in conductance induced by exposure to S02 in the light Exposure to higher concentrations (up to 350 parts 10~9) resulted in the same proportional increase in conductance irrespective of S02 concentration In the dark, stomatal conductances of polluted plants were considerably larger than those of control plants Consistent with differences in stomatal responses, treated plants transpired faster than controls Figure 1 also shows that stomata responded within 15 min after addition of Lights on Lights off Lights on Lights off Lights on Lights off I \ so2 I I I J 09

Regulation of Ribulose Bisphosphate Carboxylase Activity in Intact Wheat Leaves by Light, CO2, and Temperature

Abstract: The activity of the enzyme ribulose bisphosphate carboxylase (RuBPCase) was estimated after rapidly extracting it from intact wheat leaves pretreated under different light and C02 levels. No HCOj was added to the extraction buffer since it is shown to inhibit RuBPCase. The activity increased as light intensity or C02 concentration during pretreatment was increased. Enzyme activity increased as temperature during pretreatment was decreased. Light activation did not affect the affinity of RuBPCase for C02. A Km of 30 //M C02 under air level 02 was determined. C02, light and temperature are three main limiting factors of photosynthesis. It seems that the activity of RuBPCase is regulated by these factors according to the requirements for C02 fixation.

The Meaning of Matric Potential

Abstract: The commonly used equation, y — P — n + z, which describes the partitioning of plant water potential, y, into components of hydrostatic pressure, P, osmotic pressure, n, and matric potential, r, is misleading. The term r, which is supposed to show the influence of a solid phase on y, is zero if a consistent definition of pressure is used in the standard thermodynamic derivation. However, it can be usefully defined by r = y + 7iD, where nD is the osmotic pressure of the equilibrium dialysate of the system. The practical and theoretical significance of this definition is discussed.

The Relationship between Sucrose and Starch during ‘Dark’ Export from Leaves of Uniculm Barley

Abstract: Uniculm barley plants were grown in 8 h photoperiods at a quantum flux density of 655 /rE m~2 s_1. Groups of plants were transferred to four different light environments for one 8 h photoperiod (106, 270, 665, and 975 ,uE rrr2 s"1) and harvested at intervals throughout the succeeding dark period for subsequent carbohydrate analysis of the youngest mature leaf. Sucrose was the predominant carbohydrate in the leaves (attaining a level of c. 100 mg dm-2 after 8 h at 975 fiE m"2 s"1) but starch was also of significance (20 mg dm"2 after 8 h at 975 fiE m"2 s_1). During the dark period, following a photoperiod at the three highest light levels (270, 665, and 975 fiE m"2 s"1), sucrose was exported first while the starch level remained fairly constant. When the sucrose level fell to 15-20 mg dm-2 starch degradation began. This critical sucrose level was reached earlier in those plants subjected to lower quantum flux densities during the preceding photoperiod. The delay in the remobilization of starch suggests an important regulatory mechanism which may be dependent upon the sucrose level. At 106 /iE m"2 s"1 the sucrose level rose to only 10 mg dm"2. Here there was no discernible delay in the depletion of sucrose or starch.

A Model for C3 Leaves Describing the Dependence of Net Photosynthesis on IrradianceII. APPLICATION TO THE ANALYSIS OF FLAG LEAF PHOTOSYNTHESIS

Abstract: Measurements of the photosynthesis-light response of flag leaves in a winter wheat crop were made during the period from maximum elongation until complete senescence. Immediately following leaf elongation, the maximum rates of photosynthesis and the values of efficiency at low light were in the range 2-8-3-6 g C02 m~2 h-1 and 8-11 //g C02 J-1 respectively. The shape of the photosynthesis light response remained constant throughout and was close to a 'Blackman' type response rather than a rectangular hyperbola. The results were analysed, therefore, using a more recent model which is a non-rectangular hyperbola. Stomatal and internal resistances were equally important in limiting the maximum rate of photosynthesis. INTRODUCTION The model, derived by Marshall and Biscoe (1980), describes the photosynthesis light response (PLR) as a non-rectangular hyperbola which has four parameters: Pn ma* (maximum rate of net photosynthesis), Rd (rate of dark respiration), a (photochemical efficiency of photosynthesis at low light intensity), and d (ratio of physical to total diffusion resistance to C02). In this paper the effect of age on flag leaf photosynthesis is assessed in terms of these individual parameters. Com parisons and investigations need not be limited to effects of age alone but may also include effects of treatments, for example, differing levels of nitrogen fertilization or water stress. However, such comparisons are valid only if the calculated values of the parameters are reliable and representative of the actual measurements. This was shown to be the case in Marshall and Biscoe (1980) and further proof is given in the Results section of this paper. Having applied the model as an analytical tool to summarize information about the PLRs, the causes of changes with age in calculated parameter values are then investigated with reference to the physical and chemical processes they represent. 1 Present address: Broom's Barn Experimental Station, Higham, Bury St. Edmunds, Suffolk, IP28 6NP. This content downloaded from 207.46.13.15 on Fri, 26 Aug 2016 05:36:03 UTC All use subject to http://about.jstor.org/terms 42 Marshall and Biscoe—Flag Leaf Photosynthesis MATERIALS AND METHODS Using an infrared gas analysis system (Marshall and Biscoe, 1977), detailed measurements of PLRs were made on the flag leaf in a winter wheat crop in 1977, from maximum elongation (5 June 1977) until complete senescence, a period of 65 d. The crop underwent normal farm management and the season was both cooler and wetter than average. The model was fitted to all sets of measurements with the aid of a computer program from the NAG library (fitting routine E04JAF) as described in Marshall and Biscoe (1980). RESULTS AND DISCUSSION Photosynthesis-light response curves Figure 1 shows three typical sets of flag leaf PLR measurements made 17, 31, and 51 d after maximum elongation (DAME). These curves show that as the leaf ages the maximum rate of net photosynthesis decreases but there appears to be relatively little change in the initial slope at low light intensity, consistent with other results where detailed PLR measurements have been presented (Ludlow and Wilson, 1971; McPherson and Slatyer, 1973). The solid lines in Fig. 1 are non-rectangular hyperbolae and it can be seen that they are an acceptable fit. The calculated values of the parameters are shown in Table 1.

Change in Fatty Acid Content of Polar Lipids during Ageing of Seeds of Peanut (Arachis hypogea L.)

Abstract: Seeds of two cultivars of peanuts (Arachis hypogea L.) were harvested in different seasons and stored in 5 °C for different periods (2 and 38 months). The seeds stored for the longer period had a lower germination, leached more electrically conducting material when soaked in distilled water for 24 h, and had lower contents of fatty acids in polar lipid fractions. The fatty acid compositions of neutral lipid, glycolipid, and phospholipid fractions were similar in the differently aged seeds. Hydroperoxides could not be detected. Seeds of one cultivar were rapidly aged in an environment of 38 °C and 90% relative humidity. During 28 d, germination fell from 95% to 15%, leaching of electrically conducting material increased about 5-fold, and the content of fatty acids in polar lipid fractions fell to almost 10% of the original. The negative relationship between leaching and germination approximated closely to a straight line. Most of the decline in content of fatty acids from polar lipids preceded most of the change in germination and leaching. The fatty acid compositions of the different lipid fractions were almost unchanged. This, together with the actual fatty acid compositions, proved that some factor other than peroxidation was involved in the lipid deterioration, and peroxidation was probably unimportant. Loss of control over subcellular compartmentation or intracellular concentrations of metabolites, resulting from loss of membrane lipids, may have been the cause of the loss of viability.

The effect of light-intensity and relative-humidity on growth-rate and root respiration of plantago-lanceolata and zea-mays

Abstract: Received 19 July 1979ABSTRACTPlants of Plantago lanceolata L. and Zea mays L., cv. 'Campo' were grown at two levels of lightintensity. Especially in the roots, the rate of dry matter accumulation decreased at low lightintensity. The carbohydrate content of both roots and shoots of P. lanceolata was not affected bylight intensity. The relative contribution of SHAM'-sensitive respiration, the alternative chain, tototal root respiration of both P. lanceolata and Z. mays, was not affected by light intensity duringthe daytime. The alternative pathway was somewhat decreased at the end of the dark period, but notin the root tips (0-5 mm) where it still contributed 56% in respiration. It was, therefore, concludedthat photosynthesis is not a major factor in regulation of root growth in the species investigated.To see whether the effect of light intensity on root growth rate was via transpiration, plants of Z.mays were grown at different air humidities. Both high humidity and low light intensity affected theroot morphology in such a way that the distance between the apex and the first laterals on theprimary root axis increased. It is suggested that this effect on root morphology is due totranspiration and the subsequent removal of root-produced inhibitors of lateral root growth;although light intensity also affected the rate of dry matter accumulation of roots and the rate wasnot affected by the humidity of the air. It is, therefore, concluded that the effect of light intensityon the rate of dry matter accumulation of roots of Z. mays is not via an effect on transpiration.INTRODUCTION

Senescence-dependent increase in the permeability of liposomes prepared from bean cotyledon membranes.

Abstract: Liposomes containing 79 mM Tris-acetate and 50 mM KC1 were prepared from the total lipid extracts of smooth microsomal membranes isolated from 2, 4, 7 and 9 d old bean cotyledons. Permeability to glycerol was determined by spectrophotometric measurements of osmotic swelling when the liposomes were placed in either isotonic or slightly hypotonic glycerol. For liposomes from 2 and 4 d old membrane there was a time-dependent decrease in absorbance at 450 nm from which initial swelling rates reflecting the influx of glycerol and water were calculated. At 25 °C these rates were not significantly different. For liposomes from 7 and 9 d old membrane there was no change in absorbance with time at 450 nm signifying that these older liposomes were equally permeable to electrolytes and non-electrolytes, and therefore incapable of swelling. Permeability to glucose was determined by preparing the liposomes in a solution of the sugar, passing them through a Sephadex column to eliminate unsequestered glucose, and quantifying sugar leaked from the liposomes over time by measuring NADPH formation through the tandem actions of hexokinase and glucose-6-phosphate dehydrogenase. The rate constants for glucose leakage from 2 and 7 d old liposomes were 0-55 and 3-94 respectively, again indicating a dramatic increase in permeability with advancing age. These changes in permeability correlate temporally with the appearance of gel phase lipid in both liposomes and the membranes from which they were derived, suggesting that the coexistence of discrete liquid-crystalline and gel phase lipid domains renders membranes leaky and contributes to loss of intracellular compartmentation in senescing tissue.