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.     

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     

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 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     

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.     

Physiological Responses to Drought of Lolium perenne L.: Measurement of, and Genetic Variation in, Water Potential, Solute Potential, Elasticity and Cell Hydration

Thomas, H. 1987. Physiological responses to drought of Loliumperenne L.: Measurement of, and genetic variation in, waterpotential, solute potential, elasticity and cell hydration.—J.exp. Bot. 38: 115–125. Clonally-replicated genotypes of Loiium perenne L. were grownin a controlled environment. Leaf water potential (_w) osmoticpotential (_s), turgor potential (_p = _w – _s), elasticity(E), leaf hydration (g water per g dry matter, H) and numberof green leaves per tiller (N_GL) were measured before and duringa 42 d drought treatment. A simplified method of estimating E (at _w < 1?0 MPa) usingonly six measurements was developed to permit a measurementrate of 8 leaves per hour. Measurement errors in all characterswere 3% or less. During drought, _w and _s (at _w = 0?5 MPa) decreased significantly,_p and E increased significantly, and H decreased slightly. Plantsize during drought was negatively correlated with _s, and ^Hand positively correlated with _p, osmotic adjustment, E andN_GL. Measurements made on the genotypes before draughting didnot give a reliable indication of their physiological conditionafter adaptation to drought. Genetically controlled variation (‘broad sense heritability’)of drought-adapted plants for E was 15%, _w 23%, _s, 34%, _p, 35%,H 34% and N_GL 64%. The possibilities for, and effectivenessof, divergent selection of genotypes with high and low expressionof the characters are discussed. Key words: Water relations, Lolium, genetic variation     

Derivation of Pressure-Volume Curves by a Non-Linear Regression Procedure and Determination of Apoplastic Water

Data from pressure-volume (PV) analysis may be submitted totransformation I [i.e. leaf water potential (₁) versus inverserelative water content (1/R)] or to transformation II (i.e.1/₁ versus R). This may cause an essential distortion of theerror structure especially in transformation II due to the relativelylarge range which is to be covered by the 1/₁ ratio. Similarly,logarithmic transformation of leaf turgor potential (_P) whenderiving the sensitivity factor of elasticity (ß)by linear regression from values of In _p and 1/R may distortthe error structure. In order to investigate the magnitude ofthe distortion effect on parameters derived from PV analysisby regression a non-linear regression procedure was comparedwith the common linear procedure when calculating _p from ßin the turgid region and leaf osmotic potential (_P) in boththe turgid and non-turgid region. As test plants we used fieldgrown species of spring barley (Hordeum distichum L., cvs Gunnarand Alis). The results show that transformations and applicationof linear regression procedures distort the error structureof _p more than the error structure of _', which was only slightlyaffected. However, we recommend the use of the non-linear procedurein both cases. Furthermore, from PV analysis, obtained by thermocouple hygrometryon living and killed leaf tissue, respectively, we derived themathematical basis for calculating the apoplastic water fraction(R_a). R_a was 0.15 at R= 1 and decreased with dehydration. The equations describing the relation between and R and between_p and R were extended to take into account the apoplastic waterfraction. Key words: Apoplastic water, distortion errors, non-linear regression, pressure-volume curves     

Characterization of Spring Wheat Genotypes Differing in Drought-Induced Abscisic Acid Accumulation: II. LEAF WATER RELATIONS

Aspects of the water relations of spring wheat (Triticum aestivumL.) are described for cultivars Highbury (low ABA) and TW269/9(high ABA), and low and high ABA accumulating F⁶selections derivedfrom a cross between them. In a pot experiment, pressure-volume (P-V) curves were constructedfor main stem leaf four (MSL4) of well-watered plants of Highburyand TW269/9. Estimates of solute potential (²) from these curveswere similar for the two cultivars, but varied with the timeof sampling and the time allowed for hydration in dim light. In a field experiment with four low and four high ABA F⁶lines,P-V curves for flag leaves from both droughted and irrigatedplants gave at both zero turgor (^p) and zero water potential(¹) which differed with degree of stress, sampling time andgenotype. ¹was strongly dependent on the initial_L of the leafand was reduced on average by c. 0.4 MPa per MPa decline ininitial L.5, was lower (more negative) by c. 0.1-MPa in theafternoon than in the morning. Overall, was also 0.1 MPa lowerin low ABA lines than in high ABA lines. In another field experiment, flag leaves of five low and fivehigh ABA F⁶lines were sampled over a 4 week period from droughtedplots and _L and 5, measured (the latter by osmometry with expressedsap). For these leaves 5, at zero p or zero _L was consistentlylower by 0.3–0.5 MPa than estimates of 5, from the P-Vcurves with flag leaves. However, data for the low ABA lineswere again lower (by c. 0.1 MPa) than those for high ABA lines. The consequences of these differences in 1 are discussed inrelation to the stimulation of ABA accumulation in low and highABA selections. Key words: Water potential, Solute potential, P-V curves, Wheat (Triticum aestivum), Drought stress     

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.     

Analysis of Pressure-Volume Curves of Leaves of Rosa hybrida cv. Sonia

By analysing the relationship between inverse water potential(^–1), and relative water content (RWC) measured on leavesof roses (Rosa hybrida cv. Sonia), grown soilless, it was foundthat a non-linear (NL) model was better suited than a linearmodel to reproduce values observed in the non-turgid region.To explain this apparent curvature, it is assumed that a reductionof the non-osmotic water fraction (A_p) takes place when decreases.Osmotic potentials () measured on fresh and frozen leaf discstend to support this hypothesis. A method for exploiting PVcurves, which takes into account the variation of A_p, is described.It delivers values for the turgor pressure (_p), the relativeosmotic water content, and the mean bulk volumetric elasticitycoefficient, lower than those given by the linear model. Onthe other hand, it gives higher estimates for A_p and for . Whenapplying the traditional model to obtain estimates for waterrelations characteristics of rose leaves, and comparing resultsfrom two distinct salinity treatments (electrical conductivitiesof 1·8 mS cm^–1 and 3·8 mS cm^–1, respectively),one deduces a significant reduction of at turgor-loss in thehigh salinity treatment. The NL method is, in addition, ablesimultaneously to reveal a reduction of and a significant increasein _p at RWC=100% this proves that soilless–grown roseplants are able to osmoregulate when subjected to a constantand relatively high degree of salinity. Key words: Apoplastic water, non-linear regression, pressure-volume curves, tissue-water relations     

Water Relations of Chromium VI Treated Bush Bean Plants (Phaseolus vulgaris L. cv. Contender) under both Normal and Water Stress Conditions

The effect of Chromium VI on leaf water potential (^w), solutepotential (^a), turgor potential (^p) and relative water content(RWC) of primary and first trifoliatc leaves of Phaseolus vulgarisL. was studied under normal growth conditions and during anartificially induced water stress period in order to establishthe possible influence of this heavy metal on the water stressresistance of plants. Plants were grown on perlite with nutrientsolution containing 0, 1•0, 2•5, 5•0 or 10•0µg cm^–3 Cr as Na₂Cr₂O₇.2H₂O. The effect of Cr onwater relations was highly concentration dependent, and primaryand first trifoliate leaves were affected differently. The growthreducing concentrations of Cr (2•5, 5•0 and 10•0µg cm^–3) generally decreased _s and _w and increased_p in primary leaves. The 1•0 µg cm^–3 Cr treatmentdid not affect growth, but altered water relations substantially:in primary leaves _w and _p were increased and _s decreased, whilein trifoliate leaves the effect was the opposite. All Cr treatedplants resisted water stress for longer than control plants.The higher water stress resistance may be due to the lower _sand to the increased cell wall elasticity observed in Cr VItreated plants. Key words: Phaseolus vulgaris, Chromium VI, water stress, Richter plot     

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     

The Relationship Between Transpiration Rate, Water Potential, and Resistances to Water Movement in Sunflower (Helianthus annuus L.)

The effects of transpiration rate on the vertical gradientsof leaf and stem xylem water potential ( and ) were examinedusing hydroponic sunflower plants. Transpiration was variedby stepwise alterations of environmental conditions. The gradientsof and were relatively small (2.3 and 0.8 x 10⁵ Pa m^–1)when transpiration rates approached zero, but increased sharplyto 5.4 and 2.3 x 10⁵ Pa m^–1 as transpiration increased.However, the gradients were independent of transpiration ratesabove 0.4 g dm^–2 h^–1 owing to variability of theplant resistance. The gradients of _I were usually less thanhalf those of _I. ₁ in individual leaves remained constant over a wide range oftranspiration rates (0.4—2.4 g dm^–2 h^–1) andeach leaf possessed a characteristic plateau value related toits elevation. _I responded similarly but was approximately 2.0x 10⁵ Pa higher than _I at the same elevation. Identical resultswere obtained regardless of the procedure employed to vary transpiration. The drop in water potential between stem and leaf implies thatthe leaf resistance is appreciable. This was confirmed usingrapidly transpiring excised leaves freely supplied with water._I increased by 2.0–2.5 x 10⁵ Pa following removal of theroot resistance but remained 2 x 10⁵ Pa lower than similar excisedleaves in darkness. Furthermore, ^I in excised leaves remainedconstant over a wide range of transporting rates, demonstratingthat the leaf resistance is also variable. The results are discussed in relation to previous reports.     

Analysis of the diurnal change in osmotic potential in leaves of Fraxinus excelsior L.

The daily cycle of the transpiration rate, stomatal conductance,water potential, and the concentration of the main osmoticumidentified in ash leaves, malate and mannitol, were monitoredin a field on the Isere river plain. On sunny days, the stomatalconductance tends to remain close to its maximun value allowinga high transpiration rate and diurnal variations in leaf waterpotential, _w, which may fall as low as –2 MPa at solarnoon. These variations of _w are closely correlated with changesin malate, mannitol and the concentration of the well-knownosmoticum K⁺, which agree with the involvement of an osmoticadjustment to counteract the evaporative demand during daylighthours. How malate, mannitol and K⁺ contribute to the osmoticadjustment was analysed subsequently by comparing the solutepotential _s, evaluated by the Boyle-Van't Hoff relation, tothe osmotic potential measured by thermocouple psychrometry.These experiments have led us to suspect some errors in themeasurement of , presumably due to experimental artefacts andthe ability of Ca ²⁺ , present in high levels in leaves, toform chelates with malate once the cells have been decompartmentedby freezing and thawing. Since significant changes of Ca²⁺ alsooccurred during the diurnal variations of _w, the possible mechanismsby which Ca²⁺ may be implicated in controlling the water statusof the tree are discussed. Key words: Fraxinus excelsior L, osmotic adjustment, thermocouple psychrometry, malate, calcium     

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.     

Genotypic Differences in Leaf Water Relations between Brassica juncea and B. napus

Various kinds of measurement of tissue water status were madeseveral times during water stress and recovery in Brassica juncea(cv Canadian Black) and B napus (cv Drakkar) Unstressed plantsof the two species had similar leaf water potentials (_w), solute(_s) and turgor potentials (_p) Values of relative water content(RWC) and the slope of the linear relationship between _p andRWC (_p/RWC) were greater in B napus than in B juncea Statistical correlations of pooled data for the watered andstressed treatments differentiated the relationships among RWC,_w and its components in the two species The major statisticaldifference was that _p/RWC was related to RWC in B napus andto _w and _s in B juncea A decline in _p/RWC with decreasing _sin B juncea may be a mechanism for maintaining _p at low soilwater potentials through maintenance of more elastic cell walls. Brassica juncea, Brassica napus, osmotic adjustment, tissue elasticity, water relations     

Molecular dynamics simulations of oligosaccharides and their conformation in the crystal structure of lectin-carbohydrate complex: importance of the torsion angle {psi} for the orientation of {alpha}1,6-arm

The conformation of the heptasaccharide Man-1,6-(Man-1,3)(Xyl-ß1,2)-Man-ß,4-GlcNAc₂-ß1,4-(L-Fuc-1,3)-GlcNAc₁,the carbohydrate moiety of Erythrina corallodendron lectin (EcorL),the hexasaccharide Man-1,6-(Man-1,3) (GlcNAc-ß1,4)-Man-ß1,4-GlcNAc-ß1,4-GlcNAcand their disaccharide fragments have been studied by moleculardynamics (MD) simulations for 1000 ps with different initialconformations. In the isolated heptasaccharide, the most frequentlyaccessed conformation during MD has a value of 180° aroundMan-1,6-Man linkage. This conformation is stabilized by theformation of a hydrogen bond between the carbonyl oxygen ofGlcNAc₂ with the O3/O4 hydroxyls of the 1,6-linked mannose residue.The conformation of the heptasaccharide found in the crystalstructure of the EcorL-lactose complex (Shaanan et al., Science,254, 862, 1991), that has a value of 76° around Man-1,6-Manlinkage, is accessed, although less frequently, during MD ofthe isolated oligosaccharide. The ,, = 58°,–134°,–60°conformation around Man-₁,6-Man fragment observed in the crystalstructure of the Lathyrus ochnrs lectin complexed with a biantennaryoctasaccharide (Table I in Homans,S.W., Glycobiology, 3, 551,1993) has also been accessed in the present MD simulations.These values for the 1,6-linkage, which are observed in theprotein-carbohydrate crystal structures and are accessed inthe MD simulations, though occasionally, have not been predictedfrom NMR studies. Furthermore, these different values of leadto significantly different orientations of the 1,6-arm for thesame value of . This contrasts with the earlier predictionsthat only different values of can bring about significant changesin the orientation of the ₁,6-arm. The MD simulations also showthat the effects of bisecting GlcNAc or ß1,2-xyloseare very similar on the ₁,3-arm and slightly different on the₁,6-arm. bisecting GlcNAc carbohydrates glycoprotein lectinsaccharide complex     

A Transient Stimulation of Net Photosynthesis of Rye Leaves by {alpha}-Hydroxy-2-pyridinemethanesulphonic Acid ({alpha}-HPMS) due to Inhibition of Photorespiratory CO2 Release

The effects of -hydroxy-2-pyridinemethanesulphonic acid (-HPMS)upon net photosynthesis (P_n, the CO₂ compensation point (),post-lower illumination burst of CO₂ (PLIB) and post-lower temperatureburst of CO₂ (PLTB) in detached rye (Secale cereale L.) leaveswere investigated. At low concentrations ( 0.5 mol m^–3),-HPMS initially stimulated P_n and decreased the magnitude ofboth PLIB and PLTB. The decreased at all concentrations of-HPMS (0.05–5.0 mol m^–3. The effects of -HPMS onP_n and were time-dependent and, after a few minutes, the P_nwas inhibited while values increased considerably. At a higherconcentration (5.0 mol m ^–3), the transient effects of-HPMS were shorter () or not observed at all (P_n. Both PLIBand PLTB, when expressed in relation to P_n, increased at higherlevels of this compound. Similar data with respect to the effectsof -HPMS on PLIB and PLTB were found for leaves of dandelion(Taraxacum officinale L.). The results suggest that -HPMS may stimulate P_n by inhibitingphotorespiration, as originally suggested by Zelitch (1966),but only at low concentrations and over a short time span. Thedecrease of PLIB and PLTB values at low -HPMS levels is consistentwith these processes being a residual activity of the glycolatepathway. Key words: CO₂ compensation point, -hydroxy-2-pyridinemethanesulphonic acid, photorespiration, photosynthesis     

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     

Wilting of Leaves in Vicia faba L.

Wilting of leaves of Vicia faba L., which occurs when the pressurepotential (_p) is zero, and the leaf-water potential () at wiltingboth depend entirely upon the solute potential at incipientplasmolysis (_so) and not on soil-water status. Wilting in V.faba is acropetal; this is consistent with the hypothesis thatthere is a gradient of decreasing _so up the plant and that wateris transferred from the lower to the upper leaves, hasteningthe overall water loss from the lower leaves to the point when_p is zero. The gradient in _so up the plant is of the order of3–8 bar. It is proposed that wilting when _p>0 (i.e. > _so) shouldbe ‘apparent wilting’ and that when _p0 (i.e. _so),‘true wilting’.