Measurement of a growth-induced water potential gradient in tall fescue leaves

Spatial distribution of cell turgor pressure, cell osmotic pressure and relative elemental growth rate were measured in growing tall fescue leaves ( Festuca arundinacea ). Cell turgor pressure (measured with a pressure probe) was c . 0.55 MPa in expanding cells but increased steeply (+0.3 MPa) in cells where elongation had stopped. However, cell osmotic pressure (measured with a picolitre osmometer) was almost constant at 0.85 MPa throughout the leaf. The water potential difference between the growth zone and the mature zone (0.3 MPa) was interpreted as a growth-induced water potential gradient. This and further implications for the mechanism of growth control are discussed.     

Determination of Water, Solute and Turgor Potentials of Mycelium of Various Basidiomycete Fungi causing Wood Decay

Using thermocouple psychrometry, the water, solute and turgorpotentials of hyphae of Phallus impudicus and Serpula lacrimans,cords of P. impudicus, Phanerochaete velutina and S. lacrimansand rhizomorphs of Armillaria mellea were determined. A comparisonwas made between laboratory- and field-grown cords of P. impudicusand P. velutina. Comparison was made also between the waterrelations of the hyphae at the growing front, and hyphae ofthe more distal, basal regions. Similarly, the growing regionsof cords/rhizomorphs were compared with the older, non-extendingbasal regions of the same structures. In all cases gradientsof water and turgor potentials were found from basal, olderregions to younger, extending regions. These results are discussedin relation to the mechanism of translocation in fungi. Key words: Mycelium, Basidiomycotina, Turgor gradients, Translocation     

Physiological Integration among Clonal Ramets during Invasion of Disturbance Patches in a New England Salt Marsh

Resource sharing between ramets growing across environmentalresource gradients may have important consequences for clonalplant populations and community dynamics. As the clonal saltmarsh grasses, Spartina patens and Distichlis spicata, vegetativelycolonize disturbance-generated bare patches, they span steepgradients in soil salinity and available sunlight. Examinationof water relations and carbon translocation in the field andgreenhouse revealed that connected ramets of these marsh grassesshare water and carbon in response to gradients in resourceavailability. Ramets colonizing disturbance patches rely uponphysiological integration with connected parent ramets to overcomewater stress associated with hypersaline patch environments.In addition, upon establishment inside a bare patch, daughterramets may translocate carbon back to shaded parent ramets inthe surrounding vegetation outside of patches. Physiological integration of ramets colonizing disturbance-generatedbare patches and parent ramets outside of patches may explainthe predominance of vegetative invasion over sexual recruitmentin marsh succession. Hypersaline soil conditions, which inhibitseedling recruitment into patches, have little effect on thesuccess of clonal colonizers that can import water from parentramets. This success appears to be due to the ability of clonalmarsh grasses to translocate water and carbon products betweenramets growing across opposing gradients in resource availability.Copyright1995, 1999 Academic Press Clonal integration, Distichlis spicata, halophytes, salt marsh ecology, secondary succession, Spartina patens     

Water Potential, Translocation and Assimilate Partitioning

Lang, A. and Thorpe, M. R. 1986. Water potential, translocationand assimilate partitioning.—J. exp. Bot. 37: 495–503. The effect of water status upon translocation and assimilatepartitioning is examined both from theory and in an experimentwith young Phaseolus plants. Theory predicts that translocationis unlikely to be directly affected by water status. However,water potential differences within plants should influence translocationflow, with regions at lower potentials attracting disproportionatelylarge shares of assimilate. This prediction is supported in the experiment with Phaseolusin which the pattern of partitioning in the root changed rapidlyin response to bathing portions of it in solutions of differentosmolarity. The relevance of these findings to the growth of plants undernatural conditions is considered and evidence is presented thatwater potential gradients may be an Important factor in thecontrol of partitioning Key words: Phloem translocation, xylem transport, partitioning, water potential, control, osmotic potential     

Effects of Water and Nutrient Availability on Water Relations, Gas Exchange and Growth Rate of Mature Plants and Resprouts of Arbutus unedo L.

This study examines the effects of water supply and nutritionon the water status, gas exchange and growth of mature plantsand resprouts of Arbutus unedo, a Mediterranean evergreen shrubadapted to drought and poor nutrition. Mature plants of A. unedorespond to irrigation with increased leaf water potential duringsummer drought, but they show a very conservative use of waterand they do not increase leaf conductance. There is also a verysmall increase in net photosynthesis and growth, which doesnot significantly increase productivity. Resprouts of A. unedo increase water potential, leaf conductance,transpiration rate, net photosynthesis and growth rate in responseto watering, showing a less conservative use of water than matureplants. Increased growth rates, both in mature plants and resprouts,are likely to be due to the higher cell turgor caused by improvedleaf water potential, rather than to increased photosynthesis. The only effect of nutrient addition on mature plants is anincrease in leaf nutrient content, and other aspects of thephysiology and growth of resprouts were unaffected. We thereforeconclude that water is a more limiting factor than nutrientsfor mature plants and resprouts of A. unedo growing in the studyarea. These results support previous data which indicate thathigher growth rates in resprouts than in mature plants of A.unedo are mainly the result of a higher water availability.Copyright1994, 1999 Academic Press Arbutus unedo L., strawberry tree, resprouts, water stress, nutrient availability, water relations, gas exchange, growth rate, regeneration     

Growth-induced Water Potentials in Plant Cells and Tissues

A physical analysis of water movement through elongating soybean (Glycine max L. Merr.) hypocotyls was made to determine why significant water potentials persist in growing tissues even though the external water potentials were zero and transpiration is virtually zero. The analysis was based on a water transport theory modified for growth and assumed that water for growing cells would move through and along the cells in proportion to the conductivity of the various pathways.

Water potentials calculated for individual cells were nearly in local equilibrium with the water potentials of the immediate cell surroundings during growth. However, water potentials calculated for growing tissue were 1.2 to 3.3 bars below the water potential of the vascular supply in those cells farthest from the xylem. Only cells closest to the xylem had water potentials close to that of the vascular supply. Gradients in water potential were steepest close to the xylem because all of the growth-sustaining water had to move through this part of the tissue. Average water potentials calculated for the entire growing region were −0.9 to −2.2 bars depending on the tissue diffusivity.

For comparison with the calculations, average water potentials were measured in elongating soybean hypocotyls using isopiestic thermocouple psychrometers for intact and excised tissue. In plants having virtually no transpiration and growing in Vermiculite with a water potential of −0.1 bar, rapidly growing hypocotyl tissue had water potentials of −1.7 to −2.1 bars when intact and −2.5 bars when excised. In mature, nongrowing hypocotyl tissue, average water potentials were −0.4 bar regardless of whether the tissue was intact or excised.

The close correspondence between predicted and measured water potentials in growing tissue indicates that significant gradients in water potential are required to move growth-associated water through and around cells over macroscopic distances. The presence of such gradients during growth indicates that cells must have different cell wall and/or osmotic properties at different positions in the tissue in order for organized growth to occur. The mathematical development used in this study represents the philosophy that would have to be followed for the application of contemporary growth theory when significant tissue water potential gradients are present.

     

Partitioning Control by Water Potential Gradient: Evidence for Compartmentation Breakdown in Grape Berries

Xylem exudate was obtained from berries of Riesling grapes atdifferent stages of development after the onset of ripeningusing a pressure bomb technique. The osmotic potential of theexudate bore a 1:1 relationship to that of juice from the sameberries which were afterwards crushed and centrifuged. Thisresult provides the first direct evidence of compartmentationbreakdown in grape berries after the onset of ripening. Changesin berry deformability which occur at the same time and measurementsof the dynamics of exudation flow lead to the same conclusionregarding compartmentation breakdown. The breakdown in compartmentation occurs at the same time asthe rate of phloem translocation to the fruit suddenly increases.A mechanism was recently proposed to account for this increase.It required the existence of a water potential difference betweensource and sink such as would result from compartmentation breakdownin the sink tissues. The results, therefore, may be taken toindicate that this mechanism is indeed involved in the controlof assimilate partitioning in Vitis. Evidence in other publicationssuggests that the mechanism may be reasonably widespread inplants. Key words: Assimilate partitioning, phloem translocation mechanism, Vitis vinifera L., water potential gradients     

Response Breadths on Environmental Gradients of Germination and Seedling Growth in Two Dominant Forest Tree Species of Central Himalaya

The responses of Pimis roxburghii (chir pine), an early successionalspecies, andQuercus leucotrichophora(banj oak), a late successionalspecies were compared in terms of seed germination and seedlinggrowth. Seed germination was observed on single-factor gradientsof temperature, water stress, seed desiccation and light, andseedling growth on gradients of shade and soil moisture. Thesuppression of germination caused by continuous dark and far-redwas greater in Pinus than in Quercus. Pinus seed germinationshowed wider response breadth on gradients of water stress andseed desiccation, and narrower response breadth on the gradientof light quality, compared withQuercus. In terms of seedlinggrowth, the response breadth of Pinus was wider on the gradientof soil moisture and narrower on the gradient of shade comparedwith Quercus. Under higher soil moisture stress, the seedlingsof both species attained similar heights, butQuercus seedlingsachieved higher dry weights, a higher root: shoot ratio andlower leaf weight ratio than Pinus.     

Different growth responses of C3 and C4 grasses to seasonal water and nitrogen regimes and competition in a pot experiment

Understanding temporal niche separation between C₃ and C₄ species(e.g. C₃ species flourishing in a cool spring and autumn whileC₄ species being more active in a hot summer) is essential forexploring the mechanism for their co-existence. Two parallelpot experiments were conducted, with one focusing on water andthe other on nitrogen (N), to examine growth responses to wateror nitrogen (N) seasonality and competition of two co-existingspecies Leymus chinensis (C₃ grass) and Chloris virgata (C₄grass) in a grassland. The two species were planted in eithermonoculture (two individuals of one species per pot) or a mixture(two individuals including one L. chinensis and one C. virgataper pot) under three different water or N seasonality regimes,i.e. the average model (AM) with water or N evenly distributedover the growing season, the one-peak model (OPM) with morewater or N in the summer than in the spring and autumn, andthe two-peak model (TPM) with more water or N in the springand autumn than in the summer. Seasonal water regimes significantlyaffected biomass in L. chinensis but not in C. virgata, whileN seasonality impacted biomass and relative growth rate of bothspecies over the growing season. L. chinensis accumulated morebiomass under the AM and TPM than OPM water or N treatments.Final biomass of C. virgata was less impacted by water and Nseasonality than that of L. chinensis. Interspecific competitionsignificantly decreased final biomass in L. chinensis but notin C. virgata, suggesting an asymmetric competition betweenthe two species. The magnitude of interspecific competitionvaried with water and N seasonality. Changes in productivityand competition balance of L. chinensis and C. virgata undershifting seasonal water and N availabilities suggest a contributionof seasonal variability in precipitation and N to the temporalniche separation between C₃ and C₄ species. Key words: Chloris virgata, competition, growth, Leymus chinensis, nitrogen seasonality, water seasonality Received 19 November 2007; Revised 29 January 2008 Accepted 4 February 2008     

Water Flow Across the Sieve Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. IV. Root Tips and Seed Coats

In the present paper, the theory developed in Part I of thisseries is applied to seed coats of Phaseolus vulgaris and somecombined data on root tips of Hordeum distichum and Hordeumvulgare. Because of the large back-pressures implied, it isconcluded that phloem transport into these primary sinks wouldbe physiologically impossible in the absence of a symplasticpathway for the unloading of water from sieve elements. In thiscase, unloading of water and sucrose will occur predominantlyas a pressure-driven flow of solution through plasmodesmata,although diffusion can contribute significantly to the plasmodesmatalsucrose flux. At least 20% of the plasmodesmata connecting sieveelements and adjacent cells must be unobstructed if large changesin turgor and osmotic pressure are to be avoided. Dependingon the membrane area available for water fluxes, it is possiblethat the difference in water potential across the sieve-tubeplasmalemma can lead to significant errors when axial turgorgradients are estimated from gradients of osmotic pressure andexternal water potential. The magnitude and even the sign ofthese errors is uncertain, but it is possible that sieve-tubeturgor pressures will be significantly underestimated in primarysinks Phloem, turgor, osmotic pressure, plasmodesmata, Munch hypothesis, Phloem unloading     

Effects of Waterlogging on Water Relations of Actively-growing and Dormant Sitka Spruce Seedlings

Two-year-old Sitka spruce [Picea sitchensis (Bong.) Carr.] seedlings,either actively growing or dormant, were waterlogged in a growthroom at 15 °C. Shoot and root growth, transpiration andleaf water potential were observed. In actively-growing plants shoot extension continued after waterlogging,though at a reduced rate, and shoots of dormant plants brokebud and extended during the waterlogging period. Root growthwas suppressed by waterlogging in both types of plant. The 22day waterlogging treatment eventually killed the actively-growingplants but plants which were dormant at the time of waterloggingwere more tolerant. Changes in plant water relations after waterloggingwere entirely different depending on the condition of the plantswhen the soil was flooded. Dormant plants showed a gradual reductionin transpiration and increased water stress over the waterloggingperiod; after the soil was drained leaf water potential increasedto equal the value of control plants which had been maintainedin a freely drained condition, but transpiration did not increaseuntil root growth began. Actively-growing plants exhibited amore complex behaviour, characterized by a very rapid reductionin transpiration after waterlogging, accompanied by a briefperiod of water stress, followed by a period of increasing transpirationrate in the absence of water stress. Finally a second reductionin transpiration occurred and water stress increased as theseedlings died. The importance of the stage of activity of theroot system to the response of plants to waterlogging is discussed. Picea sitchensis (Bong.) Carr., Sitka spruce, waterlogging, water relations, dormancy, transpiration, water potential     

Cell turgor, osmotic pressure and water potential in the upper epidermis of barley leaves in relation to cell location and in response to NaCl and air humidity

Previous single-cell studies on the upper epidermis of barleyleaves have shown that cells differ systematically in theirsolute concentrations depending on their location relative tostomatal pores and veins and that during NaCl stress, gradientsin osmotic pressure () develop (Fricke et al., 1995, 1996; Hinde,1994). The objective of the present study was to address thequestion to which degree these intercellular differences insolute concentrations and it are associated with intercellulardifferences in turgor or water potential (). Epidermal cellsanalysed were located at various positions within the ridgeregions overlying large lateral or intermediate veins, in thetrough regions between those veins or in between stomata (i.e.interstomatal cells). Turgor pressure of cells was measuredusing a cell pressure probe, and of extracted cell sap wasdetermined by picolitre osmometry. For both large and intermediatelateral veins, there were no systematic differences in turgorbetween cells located at the base, mid or top of ridges, regardlessof whether plants were analysed at low or high PAR (10 or 300–400µmol photons m^–2 s^–1). However, turgor withina ridge region was not necessarily uniform, but could vary byup to 0.14 MPa (1.4 bar) between adjacent cells. In 60 out of63 plants, turgor of ridge cells was either slightly or significantlyhigher than turgor of trough (lowest turgor) or interstomatalcells (intermediate turgor). The significance and magnitudeof turgor differences was higher in plants analysed under highPAR or local air flow than in plants analysed under low PAR.The largest (up to 0.41 MPa) and consistently significant differencesin turgor were found in plants treated for 3–9 d priorto analysis with 100 mM NaCl. For both NaCl-treated and non-treated(control) plants, differences in turgor between cell types weremainly due to differences in since differences in were negligible(0.01–0.04 MPa). Epidermal cell , in NaCl-treated plantswas about 0.38 MPa more negative than in control plants dueto higher . Turgor pressures were similar. Following a suddenchange in rooting-medium or air humidity, turgor of both ridgeand trough cells responded within seconds and followed the sametime-course of relaxation. The half time (T_1/2) of turgor relaxationwas not limited by the cell's T_1/2 for water exchange. Key words: Barley leaf epidermis, cell turgor, heterogeneity, NaCl stress, osmotic pressure, water potential     

Cessation of cell elongation in rye coleoptiles is accompanied by a loss of cell-wall plasticity

The mechanism by which endogenous cessation of coleoptile elongationafter emergence of the primary leaf is brought about was investigatedin rye seedlings (Secale cereale L.) that were either grownin darkness or irradiated with continuous white light. In 3-d-oldetiolated (growing) coleoptiles a turgor pressure of 0.59 MPawas measured. In 6-d-old coleoptiles, which had ceased to elongate,cell turgor was 0.51 MPa and thus only 13% lower than in therapidly growing organ. Hence, the driving force for growth (turgor)is largely maintained. Cell-wall plasticity (E_pl) and elasticity(E_Ql were determined with a constant load extensiometer bothin vivo (turgid coleoptile segments) and in vitro (frozen-thawedsamples). Cessation of coleoptile elongation was correlatedwith a 95% reduction in E_pl9 whereas E_Ql was only slightly affected.Extension kinetics were measured with living and frozen-thawedsegments cut from growing and non-growing coleoptiles. The correspondingstress-strain (load-extension) curves indicate that the cellwall of the growing coleoptile behaves like an elastic-plasticmaterial whereas that of the non-growing organ shows the behaviourof an elastic solid. These data demonstate that E_pl representsa true plastic (irreversible) deformation of the cell wall.It is concluded that cessation of coleoptile growth after emergenceof the primary leaf is attributable to a loss of cell-wall plasticity.Hence, a mechanical stiffening of the cell wall and not a lossof turgor pressure may be responsible for the deceleration ofcell elongation in the rye coleoptile. Key words: Extension growth, rye coleoptile, cell-wall extensibility, turgor pressure     

Aspects of the Water Relations of Ileostylus micranthus (Hook. f.) Tieghem, a New Zealand Mistletoe

Leaf water potentials in the mistletoe, Ileostylus micranthusgrowing outdoors decreased rapidly during the early part ofthe day but remained relatively steady in the early afternoondespite increases in atmospheric vapour pressure deficit (vpd).Minimum water potentials of the mistletoe were relatively constant.They were held at values lower than those of hosts when thelatter maintained high water potentials but approached or evenexceeded those of hosts when they developed low water potentials.In contrast, cut shoots of Ileostylus usually maintained higherwater contents and leaf water potentials than those of its hostswhen both were desiccated separately in the laboratory. Pressure-volumeanalyses indicated that Ileostylus had lower water potentialat full turgor, a lower water potential but higher relativewater content at turgor loss, and a higher bulk modulus of elasticitythan the following four hosts: the native Kunzea ericoides andCoprosmapropinqua, and the introduced Ribes sanguineum and Teline monspessulana.Water potential at turgor loss (_tlp) was strongly correlatedwith the minimum field water potential of both mistletoes andhosts. When _tlpof mistletoe and host is similar (as on Kunzeaand Ribes) field water potentials are also similar, but when_tlpis lower in the mistletoe (as on Coprosma and Teline), thefield water potential of the mistletoe is lower than that ofits host. Consequently, I. micranthus is likely to be more frequenton hosts that maintain high field water potentials than on hoststhat develop low water potentials. Copyright 1999 Annals ofBotany Company Water relations, water potential, osmotic potential, pressure-volume, Ileostylus micranthus , mistletoe, New Zealand.     

Gradients of turgor, osmotic pressure, and water potential in the cortex of the hypocotyl of growing ricinus seedlings : effects of the supply of water from the xylem and of solutes from the Phloem

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe. Because the osmotic pressure of individual cells (πⁱ) was also determined, the water potential (ψ) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of well-watered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor ≈ πⁱ and ψ ≈ 0 across the cortex, there were also no gradients of ψ across the tissue. Gradients of cell turgor and πⁱ increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cutting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCl₂ solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.     

Shoot growth and morphometric analyses of Serengeti graminoids

Nine species from the Serengeti National Park, Tanzania, were studied to provide morphometric and growth rate parameters for a simulation model of their growth (Coughenour, McNaughton & Wallace, 1984). Short grasses had a much greater capability than taller species for packing a large number of shoots with a high proportion of lamina on a given crown biomass. Absolute growth rates varied with species height group, shoot type and growth phase. The frequency distribution of growth rates indicated that maximum growth rates were similar but that modal growth rates decreased with height. This may be an important trait allowing short grasses both to endure unfavourable conditions and to effectively exploit transient showers and resultant water availability. The ability of smaller-statured specie's to concentrate a high number of shoots with a large proportion of blade on a given crown area is an effective mechanism for increasing productivity early in the growing season. Taller grasses could achieve the same production rates as short grasses only with greater individual growth rates of fewer shoots. These patterns may help to explain gradients of graminoid stature along rainfall and growing season gradients.     

The Effects of Shaking on the Growth and Water Relations of Festuca arundinacea Schreb

The extension growth of Festuca arundinacea was reduced by shaking,and the leaves formed during shaking were thinner and narrowerthan the controls. The response to shaking was modified by thenutritional status of the plant — shaken plants took upmore phosphorus from a phosphorus deficient soil than the controlsand this advantage seemed to offset the influence of shaking. Shaken plants displayed a higher stomatal conductance and alower water potential than the controls, though it was consideredthat the effect of shaking could not be attributed entirelyto differences in water potential. Festuca arundinacea Schreb., water potential, stomatal conductance, phosphorus, shaking     

Water Relations of the Mistletoe Amyema fitzgeraldii and its Host Acacia acuminata

Water relations of the mistletoe Amyema fitzgeraldii and itshost Acacia acuminata were studied near Geraldton, Western Australia.Transpiration rates of host and parasite under unstressed winterconditions varied more than 300–fold between day and nightwhile leaf water potential gradients between the partners remainedwithin the range 0·4–0·6 MPa. Plots of transpirationagainst leaf water potential indicated closely similar fluidphase resistances in host and parasite during daytime but divergentbehaviour at night due to an apparently large increase in resistanceof the haustorial junction between the partners. Data for summerand winter studies across a full range of light intensitiesshowed the parasite to transpire, on average, 1·4 timesfaster and to exhibit noticeably lower water use efficienciesthan its host. Experiments following restorative changes atnight in leaf water potentials of host and parasite on detachedhost branches supplied through their cut ends with water indicatedthat the haustorium offered a major resistance to water uptakeby the parasite. Restoration of leaf water potentials by theparasite lagged markedly behind that of the host, especiallyduring winter, leading to a rapid build up in potential gradientbetween partners. A phase of rapid flow into the parasite thenfollowed, presumably motivated by lowering of the haustorialresistance. Reversal of the potential gradient between hostand parasite was recorded in a night-time study involving abagged (non-transpiring) mistletoe attached to a host branchfrozen at the base to prevent further water uptake. Mechanismsare proposed to account for the apparently highly variable natureof the resistance of the haustorium. Key words: Mistletoe, transpiration, haustorial resistance, Amyema fitzgeraldii, Acacia acuminata     

An Electrophysiological Study of the Stomatal Complex of Tradescantia virginiana

Measurements of electrical potential difference (PD) and potassiumactivity were carried out on the intact leaf of Tradescantiavirginiana. PD gradients across the stomatal complex were observedwith both open and closed stomata. The guard cell PD appearedto be linearly related to stomatal aperture. With the stomataopen a gradient of potassium activity across the stomatal complexwas observed which became reversed on stomatal closure. Calculationof the driving forces on potassium suggested that it was distributedpassively between the vacuoles of the cells of the stomatalcomplex. The electrophysiological data obtained from this investigationenabled potassium activity in the apoplast to be calculated.The results showed that on stomatal closure there was a massiveincrease in the potassium activity in the guard cell wall. Key words: Stomata, Ionic gradients, Electrical potentials     

Control of crops leaf growth by chemical and hydraulic influences

Three species of forage grasses (Festuca arundinacea, Eragrostiscurvula, Sporobolus stapfianus) commonly grown in the Mediterraneanregion were subjected to a soil drying treatment. Leaf growthrate in F. arundinacea was highly sensitive to soil drying andlow growth rates were associated with high laminar turgors.The production of ABA was stimulated by soil drying and therewas a clear relation between increasing ABA accumulation andreduction in leaf growth. Leaf growth of E. cutvula, a C⁴ warmseason grass, was relatively insensitive to soil drying whichwas not accompanied by a substantial increase in leaf ABA content.S. stapfianus, a resurrection plant, was highly sensitive todecreasing soil water availability. In these two latter species,leaf growth was substantially restricted before ABA accumulationoccurred. It is suggested that reductions in laminar turgorof E. curvula and S. stapfianus may be limiting leaf growthas soil dries. The results indicated a different mechanism ofsensing and responding to reduction in soil water availabilityfor the three species studied. The relative importance of thechemical and hydraulic control of leaf growth is discussed. Key words: Leaf growth, water relations, abscisic acid, Festuca arundinacea, Eragrostis curvula, Sporobolus stapfianus