Field Studies of Cereal Leaf Growth: III. BARLEY LEAF EXTENSION IN RELATION TO TEMPERATURE, IRRADIANCE, AND WATER POTENTIAL

A mechanical auxanometer, suitable for measuring hourly ratesof leaf extension of grass and cereal crops, is described. Severalof these instruments were used to monitor leaf extension rates(R_E) of a spring barley crop. R_E of main stem and first leaftillers responded similarly to environmental factors. DuringMay, when soil water deficits were less than 50 mm, and on dulldays later in the season, R_E was directly related to meristemtemperature with night and day measurements responding similarly.During the central 10 h of bright days in late May and June,R_E was unrelated to temperature but slowed during bright sunshineand accelerated at the start of cloudy periods. Pressure chambermeasurements of total leaf water potential () showed that brightsun caused to decrease rapidly and that this was associatedwith slow R_E. Analysis of 2 h mean values of and R_E indicatedthat, at any given temperature, R_E slowed in direct proportionto decrease of .     

Stomatal Response to Water Stress and its Relationship to Bulk Leaf Water Status and Osmotic Adjustment in Pearl Millet (Pennisetum americanum [L.] Leeke)

The water potential () at which stomata completed closure (_8Lmin)was determined for pearl millet (Pennisetum americanum [L.]Leeke) at two growth stages by monitoring changes in leaf conductance(g_L) and following shoot detachment. Leaf water status wasevaluated concurrently using a pressure-volume (P-V) technique. In a pot experiment with young vegetative plants, _8Lmin closelyapproximated to the estimated at zero turgor (_u) both for controland for drought-conditioned plants which had osmotically adjusted.However, for penultimate leaves of field-grown flowering plants,_8Lmin was found to be 0.61 (irrigated plants) and 0.87 (droughtedplants) MPa below _u. In drought-stressed field-grown plants,osmotic adjustment (characterized by a decrease in solute (osmotic)potential (_s ) at both full hydration and zero turgor) was insufficientto maintain a positive bulk leaf turgor potential (_p) once had declined to below about -1.5 MPa. It is suggested that localizedadjustment by the stomatal complex in response to environmentaldifferences, leaf ageing and/or ontogenetic change, is responsiblefor the uncoupling of stomatal from bulk leaf water status. Key words: Stomata, Water stress, Pennisetum americanum     

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.     

Stomatal Behaviour and Water Relations of Chilled Phaseolus vulgaris L. and Pisum sativum L.

Leaf diffusion resistance interpreted as stomatal resistance,leaf water potential (_w), solute potential (_s) and leaf turgorpotential (_p) of the chilling sensitive species Phaseolus vulgariswere determined during chilling at 4 °C in the light. Bothchill-hardened and non-hardened plants were used. For comparison,the chilling resistant species Pisum sativum was also used. The results for chilled P. sativum were similar to those obtainedfor chill-hardened P. vulgaris plants receiving a chilling treatment.In both cases a reduction in stomatal aperture and the maintenanceof a positive leaf turgor were the responses to chilling. Leavesof chilled but non-hardened P. vulgaris plants were found tomaintain open stomata throughout the chilling treatment despitea severe wilt developing after 7 h at 4 °C. This was incontrast to the chill-resistant P. sativum. which showed a rapidclosing and subsequent re-opening of the stomata to a new reducedaperture. During the first 12 h of chilling _wof P. vulgaris leaves changedfrom –0.47 MPa to –1.24 MPa. On more prolonged chilling_w tended to return to pre-chilling values. In addition. _p decreasedfrom 0.42 MPa to zero after only 9 h of chilling, and remainedat this value for the remainder of the chilling period, _s, changedrapidly from –0.89 MPa to –1.35 MPa in the first7.5 h, and after 9 h. _w and _s, were equal, i.e. zero _p. In contrast,the chilling resistant plant P. sativum maintained a positive_p throughout the chilling period, and there was little differencebetween values of _w, and _s in control and chilled leaves. Key words: Chilling, Stomata, ater relations, Phaseolus vulgaris, Pisum sativum     

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.     

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.     

Osmotic Adjustment to Water Stress in Pearl Millet (Pennisetum americanum (L. ) Leeke) in a Controlled Environment

A pressure-volume (P-V) and an expressed sap (cryoscopic) techniquewere compared for assessing osmotic adjustment to water stressby pearl millet (Pennisetum americanum (L. ) Leeke) plants grownin a controlled environment cabinet. For leaf water potentials( ) above the point of zero turgor, there was good agreementbetween estimates of solute potential ( _s)and turgor ( _p) obtainedby the two methods. Reductions in pre-dawn leaf to –1.8 MPa over 5–6d resulted in net solute accumulation as indicated by a fallin s at full hydration of about 0.3 MPa. The degree of osmoticadjustment increased linearly with the decrease in pre-dawn. Adjustment in cv. BJ 104 was significantly (P < 0.05) lessduring a second drought than during a first, and cv. Serere39 was significantly (P < 0.05) less able to adjust osmoticallythan BJ 104. Adjustment was greater in leaves which were undergoing extensiongrowth during the drought than in leaves already fully extendedbefore drought started. Much of the adjustment was lost within24 h following rewatering, the loss being most complete in theolder, fully extended leaves.     

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     

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.     

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     

Solute Accumulation and Growth in Plants of Pearl Millet (Pennisetum americanum [L.] Leeke) Exposed to Abscisic Acid or Water Stress

Henson, I. E. 1985. Solute accumulation and growth in plantsof pearl millet (Pennisetum americanum [L.] Leeke) exposed toabscisic acid or water stress.—J. exp. Bot. 36: 1889–1899.Experiments were conducted to investigate whether abscisic acid(ABA) elicits the accumulation of solutes and lowering of osmotic(solute) potential (₂) which occurs in leaves of pearl millet(Pennisetum americanum [L.] Leeke) exposed to water stress.When (?)–ABA was injected into the base of the shoot of15–d–old plants, ₂ of the fifth leaf was reducedsignificantly below controls 27–72 h after treatment.The reductions, however, were small (< 0.10 MPa) and wereaccompanied by a significant inhibition of shoot growth. Incontrast, no significant reduction in ₂ or in growth occurredwhen the hormone was introduced directly into leaf five viaits mid–rib. ABA concentrations in leaf five were highshortly after direct injection, but declined to control levelswithin 48 h. Injecting ABA into the shoot base resulted in lowerleaf five ABA concentrations. Hence, the ABA concentration inthe leaf was not the most critical factor for its effect on₂. ABA also reduced ₂ of shoots when applied to seedlings 48h or 72 h after sowing via the roots. As with older plants,the effects of ABA on ₂ were small ( 0–2 MPa) and wereaccompanied by inhibition of shoot growth. A water stress treatmentand an ABA treatment were compared. Although both treatmentsresulted in a similar degree of growth inhibition, the stresstreatment was much more effective than was ABA in reducing ₂. Key words: Pennisetum americanum [L.], pearl millet, abscisic acid, water stress, osmotic     

Diurnal Changes in Endogenous Abscisic Acid in Leaves of Pearl Millet (Pennisetum americanum (L. ) Leeke) under Field Conditions

Diurnal variation in leaf abscisic acid (ABA) content was investigatedin pearl millet (Pennisetum americanum (L. ) Leeke) growingin the field in the semi-arid tropics and subjected to varyingdegrees of water stress. There was a two- to three-fold change in ABA content duringthe photoperiod in three groups of ‘severely’ stressedplants of the genotype BJ 104. Maximum ABA occurred mid-morning(1030 h). ABA levels then declined to a minimum at 1500 h. Changesin ABA content of ‘moderately’ stressed and fullyirrigated plants were smaller, but still significant. Though,when averaged over the day, levels of ABA of the five groupswere positively related to the degree of water stress, relationshipsbetween ABA concentration and total water () or turgor (_p) potentialsvaried considerably with time of sampling. Within groups, changesin ABA contents during the day were not always accounted forby changes in or _p. Temporal changes in leaf ABA content similar to those foundin BJ 104, and largely unrelated to , were observed in the genotypesSerere 39 and B282 in a subsequent year. Leaf ABA content of droughted plants (BJ 104) did not declineappreciably overnight despite a marked increase in . However,a large reduction in ABA content with increase in did occurfollowing heavy rainfall. Diurnal changes in stomatal conductance (g₁) of BJ 104 couldnot be simply accounted for by temporal changes in total leafABA content, even when allowance was made for effects of irradianceand other environmental variables on g₁. It is suggested thatthe sensitivity of stomata to ABA, or accessibility of the hormoneto the stomatal complex, changes during the day.     

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     

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     

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     

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.     

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     

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     

Plant Growth in Polyethylene Glycol Solutions in Relation to the Osmotic Potential of the Root Medium and the Leaf Water Balance

The changes in leaf extension, plant dryweight, leaf area, netassimilation rate (E), relative growth-rate (R_W), and relativeleaf growth-rate (R_L), have been studied for four species grownfor 2 weeks in solutions of polyethylene glycol 4000 of controlledosmotic potentials. All aspects of growth were decreased bydecreasing the osmotic potential (_sol) of the root medium andthe leaf water potential (), and ceased when _/ was greater than— 10 bars in bean, cotton, maize. These plants were moresusceptible than ryegrass to water stress. Growth of bean stoppedat equal to about —6 bars, but even at —10 barsryegrass was capable of some growth. Slight decrease in fromthe values in the control plants decreased growth during thefirst week but partial recovery was apparent during the secondweek's growth in solution culture, when leaf extension, E, R_Land R_W increased in plants subjected to stress. Examinationof the water balance, water potential, osmotic potential andturgor of the leaf in relation to relative water content suggeststhat recovery was related to increased turgor and that the abilityof the plants to grow at reduced values of the osmotic potentialof the root medium and of the leaf water potential depend onthe maintenance of turgor.     

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