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     

The Role of Glutamate Decarboxylase and {gamma}-Aminobutyric Acid in Germinating Barley

The variations in glutamate decarboxylase activity and in glutamicacid and -ABA concentration have been measured in barley embryosduring the uptake of water and in the roots and shoots for upto 6 days of growth. Glutamate decarboxylase activity was relativelysteady in the embryos during soaking but rose rapidly once growthbegan. This development paralleled an increase in the concentrationof glutamic acid in both roots and shoots at a time when theconcentration of -ABA was falling. During soaking in aeratedwater, the -ABA content of the embryos rose for 36 h, at whichpoint it accounted for 35 per cent of the soluble amino acids.-ABA was found to be a major free amino acid in roots but notin shoots. Experiments in vivo involving ¹⁴C-labelled glutamicacid and -ABA indicated that carbon from -ABA passed very rapidlyinto the citric-acid cycle intermediates and also that, throughoutthe period studied, -ABA was formed from glutamic acid despitethe alterations in relative concentrations of these amino acidsin the growing tissue.     

Axial Distribution of Glycosidases in Relation to Cellular Growth and Ageing in Pisum sativum Root

Tanimoto, E. 1985. Axial distribution of glycosidases in relationto cellular growth and ageing in Pisum sativum root.—J.exp. Bot. 36: 1267–1274. Eight glycosidase activities were measured in relation to cellulargrowth and ageing along the axis of pea roots. The highest activities of ß-D-fucosidase and ß-D-glucosidasewere in the apical 1.0 mm segment containing the root cap andmeristem. Activities of -L-arabinosidase, -D-mannosidase, ß-D-galactosidase, -D-glucosidase and ß-D-xylosidase werehighest in the segment between 1 and 2 mm from the tip and containingyoung elongating cells with high growth potential. The activityof -D-galactosidase was high between 1 and 2 mm from the roottip, decreasing to a minimum 4-5 mm from the tip and increasingagain up to the base of root. This distribution of enzyme activitiesrelative to the root tip remained unchanged during 28 h whilethe root length doubled. No -L--fucosidase, ß-L-fucosidaseand -D-xylosidase activities were detected. Key words: Pisum sativum, root growth, glycosidase, galactosidase, mannosidase     

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 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.     

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.      

Water Movement from Soil to Root Investigated through Simultaneous Measurement of Soil and Stem Water Potential in Potted Trees

Legge, N. J. 1985. Water movement from soil to root investigatedthrough simultaneous measurement of soil and stem water potentialin potted trees.—J. exp. Bot. 36: 1583–1589. Osmotic tensiometers implanted in the stems of three mountainash (Eucalyptus regnans F. Muell.) saplings growing in largeplastic bins recorded stem water potential, _w, while soil waterpotential, _w, was simultaneously recorded by instruments nearthe trees' roots and in the surrounding root-free soil Earlyin a drying cycle, with the soil still wet, the diurnal variationin ₁, was often slight, despite diurnal variations in _u approaching2.0 M Pa. Late in a drying cycle the diurnal fluctuations in₁, and _u were very similar although changes in ₁, still laggedup to 1.5 h behind changes in _u. ₁values at this time occasionallyreached –3.0 MPa with no apparent damage to the treesWatering the bins in daytime led to a response in ₁, valueswithin about 5 min, whereas _u, values did not respond for afurther 20 min. _u values then rose rapidly but after only 1h began to decline again, while ₁, values remained at or nearsaturation for the rest of the day. Water uptake hypotheseswhich attribute an important role to a soil-root interface resistanceare not supported by these data Key words: —Soil water potential, penrhizal gradients     

The Transport of Water from Soil to Shoot in Wheat Seedlings

A technique is described for precisely measuring the drop inwater potential, , between the soil and the leaf xylem of wheatseedlings. The technique was used to explore the relation betweentranspiration rate and at various times during the monotonicdrying of the soil in which the plants were growing. When thesoil was wet, the relation was linear, but, as the soil dried,nonlinearities appeared which were, in the main, explicablein terms of simple soil physical models describing the flowof water through the soil to the roots. There was no sign ofthe major hydraulic resistance at the root: soil interface thatother people have recently found.     

Electrochemical Relations in the Transfer of Ions to the Xylem Sap of Maize Roots

The difference in electrical potential between the xylem exudateof isolated roots of 1-week-old maize seedlings and the culturesolution surrounding the roots has been determined togetherwith the concentrations of potassium, calcium, chloride, andsulphate in the xylem sap. The difference in electro-chemicalpotential (µ) for these ions has been calculated fromthe measurements. The effects on µ of varying the saltstatus of the roots, the composition of the culture solutionand of 2-4-dinitrophenol have been examined. µ for potassiumand chloride was always positive, implying that movement ofthese ions to the xylem sap was under metabolic constraint.However, pretreatment of the maize seedlings with potassiumchloride and increasing levels of dinitrophenol in the culturesolution over the exudation period caused little or no significantchange in µ for potassium although the rate of movementof ions to the xylem was substantially reduced. For calcium,µ was negative with roots in dilute culture solutionsin the absence of dinitrophenol, implying that calcium couldenter the xylem by passive diffusion. Addition of dinitrophenolchanged the sign of µ and thus brought about an apparentlyactive transport of calcium.Only in the absence of competinganions was the rate of entry of chloride significantly correlatedwith µ, and no such relationship was found for potassiumor calcium individually. These results encourage doubt as towhether measurements of µ provide a valid basis for decidingto what extent the movement of individual ions depends on specific‘active’ or ‘passive’ transport processes.Thedifference in electrical potential appears to be a characteristicof the living root and to depend on the total concentrationof ions in the external solution and on their rate of transferinto the xylem sap.     

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.     

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.      

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     

Interspecific Grafts Demonstrate Root System Control of Leaf Water Status in Water-Stressed Phaseolus

We examined the importance and the mechanisms of the root systems'effect on leaf water status in two bean species: Phaseolus vulgarisL. cv. Redcloud (Pv) and P. acutifolius Gray MN cultivatedaccession 258/78 (Pa). Pa maintains a higher leaf water potential(₁) than Pv. We used reciprocal grafts between the two species.We grew four plants (one of each graft combination) in one potso they experienced the same soil water potential. Shoot genotypedetermined ₁ of well-watered plants. Root genotype determined₁ of the most stressed plants. Stressed Pa root systems increased₁ of Pv shoots by 0·1 MPa over Pv shoots on Pv roots.Pa roots did not maintain by affecting stomatal conductancenor by simply having more dry weight. Pa roots may have greaterhydraulic conductivity than Pv roots. Key words: Phaseolus acutifolius, Phaseolus vulgaris, leaf water potential, root-shoot communication     

Leaf Water Potential and Control of Water Loss in Droughted Sitka Spruce Seedlings

Sitka spruce seedlings were subjected to drought in experimentsin a growthroom, a greenhouse, and out of doors. The plantswere grown in a double chamber with the bulk of the roots inthe upper part where they dried out the soil when water waswithheld. A few new roots penetrated into the lower part inwhich the soil remained moist. The double chamber system enabledthe plant to attain a high water psotential by night and theshoot was only periodically under mild water stress. Measurementswere made on soil water potential (_solt), leaf water potential(₁), transpiration (E), and stomatal conductance (k_s). As soildecreased over a period of 4.5 d, E and k_s decreased progressively.The decline in E and k_s which indicated stomatal closure, occurredat a higher ₁ than has been reported for Sitka spruce. The behaviourof the stomata appeared to be modified by conditions at theroot, and it is proposed that differences in the response to₁,depend on Whether the latter is reduced by resistances in thexylem between root and leaf, as is known to occur in large treesin moist soil, or by stresses at the root itself.     

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.     

Regulation of Electrogenic Pumping in Barley by pH and ATP

The relationship of the electrogenic pump to the ATP concentrationin barley roots was examined. Excision, salt accumulation andchanges in temperature produced changes in the ATP concentrationwhich did not correlate with changes in membrane potential (_m).Illumination of seedlings prior to excision of the root elevatedthe ATP level and caused _m to hyperpolarize. Metabolic inhibitionby sodium azide resulted in a fall in ATP concentration andmembrane depolarization. With treatment in azide for longerthan 10 min there was a linear relationship between ATP concentrationand _m. Time-courses of the effects of azide and carbon monoxideshowed that this relationship did not hold at short treatmenttimes because the ATP concentration fell more rapidly than thedecay of _m. Application of butyrate or fusicoccin produced littleor no change in the ATP concentration but both caused significantchanges in _m. The effects on _m of butyrate, fusicoccin, external pH and metabolicinhibitors were considered to be consistent with regulationof electrogenic pumping by cytoplasmic pH. Key words: Cytoplasmic pH, Electrogenic pumping, Fusicoccin, Butyrate     

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 Structure and Functions of Xyloglucan

Xyloglucan is a polysaccharide found in the primary cell wallsof all higher plants examined. Its cellulose-like backbone,which is about 0.15 to 1.5 µm long, consists of 300 to3 000 ß-(14)-linked D-glucopyranose residues. About60–75% (or, in grasses, about 30–40%) of the glucoseresidues have side-chains attached to position 6. The majorside-chains are: D-xylopyranosyl--1 -, D-galactopyranosyl-ß-(12)-D-xylopyranosyl--I , L-arabinofuranosyl-(1 -2)-D-xylopyranosyl--1-, and (except in grasses) L.-fucopyranosyl--(1 -2)-D-galactopyranosyl-ß-(1-2)-D-xylopyranosyl--1-. There is some regularity in the distribution of these side-chainsalong the backbone. Xyloglucan plays two very different r?les in the control ofcell growth: (a) as a major building material of the wall [concentrationof xyloglucan in the wall in vivo 10% (w/v)] it probably directlydictates wall extensibility and, therefore, the rate of cellexpansion and (b) it can be broken down to a fucose-containingoligosaccharide which [at a concentration of 0.0000001% (w/v)]exerts a hormone-like anti-auxin effect on growth. In addition,xyloglucan lacking fucose is used by certain dicotyledonousseeds as a food reserve which is mobilized after germination.Xyloglucan is, therefore, the subject of considerable currentinterest in several apparently disparate areas of botany. Key words: Xyloglucan, ‘oligosaccharin’, hemicellulose, auxin, anti-auxin, growth, cell walls, reserve carbohydrate