Nitrate Supply and the Biophysics of Leaf Growth in Salix viminalis

The influence of nitrogen on leaf area development and the biophysicsof leaf growth was studied using clonal plants of the shrubwillow, Salix viminalis grown with either optimal (High N) orsub-optimal (Low N) supplies of nitrate. Leaf growth rate andfinal leaf size were reduced in the sub-optimal treatment andthe data suggest that in young rapidly growing leaves, thiswas primarily due to changes in cell wall properties, sincecell wall extensibility (% plasticity) was reduced in the LowN plants. The biophysical regulation of leaf cell expansion also differedwith nitrogen treatment as leaves aged. In the High N leaves,leaf cell turgor pressure (P) increased with age whilst in theLow N leaves P declined with age, again suggesting that foryoung leaves, cell wall plasticity limited expansion in theLow N plants. Measurements of cell wall properties showed thatcell wall elasticity (%E) was not influenced by nitrogen treatmentand remained constant regardless of leaf age. Key words: Salix, cell wall extensibility, nitrogen nutrition, biophysics of leaf growth     

Increased Synthesis of ABA in Partially Dehydrated Root Tips and ABA Transport from Roots to Leaves

Zhang, J. and Davies, W. J. 1987. Increased synthesis of ABAin partially dehydrated root tips and ABA transport from rootsto leaves.—J. exp. Bot. 38: 2015–2023. Isolated root tips of pea (Pisum sativum L. cv. Feltham First)and Commelina communis L. were air-dried until they lost between10% and 40% of their fresh weight, followed by a period of incubationat these reduced water contents. These treatments resulted inincreased ABA production, suggesting that root tips of bothspecies have the capacity to synthesize ABA in increased amountswhen water deficits develop in the root. The ABA concentrationin pea roots increased linearly as turgors fell below about0·15 M Pa and relative water contents (R WC) fell below90%. Commelina roots produced more ABA when RWC fell below asimilar value but the threshold turgor for increased ABA productionin Commelina roots was around 0·30 MPa. Roots of intact plants loaded with ABA as a result of incubationin solutions of varying concentrations provided ABA to leaveswhich resulted in increased ABA concentrations in the leaveswhen these were assayed several hours later. This occurred whenthese roots were not contributing substantially to transpirationalflux. Leaves on shoots that were enclosed and darkened and thereforenot transpiring, did not accumulate ABA from ‘loaded’roots. A role for root-sourced ABA in root-to-shoot communication ofthe effects of soil drying is discussed. Key words: ABA, roots, water relations     

Effects of the fungal endophyte, Neotyphodium lolii, on net photosynthesis and growth rates of perennial ryegrass (Lolium perenne) are independent of In Planta endophyte concentration

• Background and Aims Neotyphodium lolii is a fungal endophyteof perennial ryegrass (Lolium perenne), improving grass fitnessthrough production of bioactive alkaloids. Neotyphodium speciescan also affect growth and physiology of their host grasses(family Poaceae, sub-family Pooideae), but little is known aboutthe mechanisms. This study examined the effect of N. lolii onnet photosynthesis (P_n) and growth rates in ryegrass genotypesdiffering in endophyte concentration in all leaf tissues. • Methods Plants from two ryegrass genotypes, Nui D andNui UIV, infected with N. lolii (E+) differing approx. 2-foldin endophyte concentration or uninfected clones thereof (E–)were grown in a controlled environment. For each genotype xendophyte treatment, plant growth rates were assessed as tilleringand leaf extension rates, and the light response of P_n, darkrespiration and transpiration measured in leaves of young (30–45d old) and old (>90 d old) plants with a single-chamber openinfrared gas-exchange system. • Key Results Neotyphodium lolii affected CO₂-limited ratesof P_n, which were approx. 17 % lower in E+ than E– plants(P < 0·05) in the young plants. Apparent photon yieldand dark respiration were unaffected by the endophyte (P >0·05). Neotyphodium lolii also decreased transpiration(P < 0·05), but only in complete darkness. There wereno endophyte effects on P_n in the old plants (P > 0·05).E+ plants grew faster immediately after replanting (P < 0·05),but had approx. 10 % lower growth rates during mid-log growth(P < 0·05) than E– plants, but there was noeffect on final plant biomass (P > 0·05). The endophyteeffects on P_n and growth tended to be more pronounced in NuiUIV, despite having a lower endophyte concentration than NuiD. • Conclusions Neotyphodium lolii affects CO₂ fixation,but not light interception and photochemistry of P_n. The impactof N. lolii on plant growth and photosynthesis is independentof endophyte concentration in the plant, suggesting that theendophyte mycelium is not simply an energy drain to the plant.However, the endophyte effects on P_n and plant growth are stronglydependent on the plant growth phase.     

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     

A Simple Instrument for Measuring Leaf Extension in Grasses, and its Application in the Study of the Effects of Water Stress on Maize and Sorghum

The design of a simple instrument to monitor leaf expansionin grasses is described. The instrument was used to comparethe effects of water stress on leaf extension of two cultivarsof maize and sorghum. The effect of withholding water for 3days was an appreciable reduction in the rate of leaf expansionin both plants, particularly during the light period. In well-wateredplants of both species, leaf extension continued at a steadyrate even when leaf turgor fell to around 0.1 MPa. In water-stressedmaize plants, leaf turgor during the light period fell to zeroand leaf growth ceased. When turgor was restored, followingstomatal closure, leaf extension resumed at a slow rate. Inunwatered sorghum plants, leaf turgor remained at a value greaterthan 0.1 MPa but the rate of leaf extension was significantlyreduced. The reduction in leaf turgor in the unwanted plantsresulted partly from an increase in solute potential. Zea mays L, maize, Sorghum bicolor L, leaf expansion, leaf turgor, water stress     

Acclimation to Drought in Acer pseudoplatanus L. (Sycamore) Seedlings

A glasshouse experiment was conducted with well-watered andwater-stressed seedlings of sycamore (Acer pseudoplatanus L.)grown in soil columns. Water was withheld when the seedlingswere 82-d-old. Effects of soil drying on stomatal behaviour,water relations, xylem cavitation, and growth of leaves androots were evaluated. Stomatal conductance declined well before any observable changein bulk leaf water potentials, and was correlated with soilwater status. At seven weeks, osmotic potential had declinedby 0·51 MPa and 0·44 MPa at full and zero turgor,respectively. Drought significantly increased both bulk elasticmodulus and leaf dry weight to turgid weight ratio of water-stressedplants. Drought had no effect on relative water content at zeroturgor. Water cavitation in the xylem was detected as ultrasonic acousticemissions (AE). Water-stressed plants displayed significantlyhigher rates of AE than well-watered plants. Maximum rate ofAE coincided with the minimum level of stomatal conductanceand apparent rehydration of the leaves. Drought caused changes in the root distribution profile andit increased the root weight. The increase in root weight wasmainly due to a substantial shift in assimilates allocated infavour of roots with total biomass being unaffected. Leaf growthwas maintained for six weeks without any significant declinein expansion rate. However, the development of severe waterstress reduced both leaf production and expansion.     

Control of leaf cell elongation in barley. Generation rates of osmotic pressure and turgor, and growth-associated water potential gradients

In a previous study on the effects of N-supply on leaf cell elongation, the spatial distribution of relative cell elongation rates (RCER), epidermal cell turgor, osmotic pressure (OP) and water potential (Ψ) along the elongation zone of the third leaf of barley was determined (W. Fricke et al. 1997, Planta 202: 522–530). The results suggested that in plants receiving N at fixed relative addition rates (N-supply limitation of growth), cell elongation was rate-limited by the rate of solute provision, whereas in plants growing on complete nutrient solution containing excessive amounts of N (N-demand limitation), cell elongation was rate-limited by the rate of water supply or wall yielding. In the present paper, these suggestions were tested further. The generation rates of cell OP, turgor and Ψ along the elongation zone were calculated by applying the continuity equation of fluid dynamics to the previous data. To allow a more conclusive interpretation of results, anatomical data were collected and bulk solute concentrations determined. The rate of OP generation generally exceeded the rate of turgor generation. As a result, negative values of cell Ψ were created, particularly in demand-limited plants. These plants showed highest RCER along the elongation zone and a Ψ gradient of at least −0.15 MPa between water source (xylem) and expanding epidermal cells. The latter was similar to a theoretically predicted value (−0.18 MPa). Highest rates of OP generation were observed in demand-limited plants, with a maximum rate of 0.112 MPa · h⁻¹ at 16–20 mm from the leaf base. This was almost twice the rate in N-supply-limited plants and implied that the cells in the leaf elongation zone were capable of importing (or synthesising) every minute almost 1 mM of osmolytes. Potassium, Cl⁻ and NO₃ ⁻ were the main inorganic osmolytes (only determined for demand-limited plants). Their concentrations suggest that, unlike the situation in fully expanded epidermal cells, sugars are used to generate OP and turgor. Anatomical data revealed that the zone of lateral cell expansion extended distally beyond the zone of cell elongation. It is concluded that leaf cell expansion in barley relies on high rates of water and solute supply, rates that may not be sustainable during periods of sufficient N-supply (limitation by water supply: Ψ gradients) or limiting N-supply (limitation by solute provision: reduced OP-generation rates). To minimise the possibility of growth limitation by water and osmolyte provision, longitudinal and lateral cell expansion peak at different locations along the growth zone. Received: 15 October 1997 / Accepted: 12 March 1998     

Nitrate Accumulation and its Relation to Leaf Elongation in Spinach Leaves

The leaf elongation rate (LER) of spinach leaves during theday was twice that during the night when grown at a photon fluxdensity of 145 µmol m^–2 s^–1. All leaves showedthe same LER-pattern over 24 h. Due to low turgor, LER was lowin the afternoon and in the first hours of the night until wateruptake restored full turgor. Osmotic potential remained constantdue to increased nitrate uptake and starch degradation in thisperiod. LER increased to high rates in the second part of thenight and in the morning. The lower rate in the dark comparedto the light was not caused by the lower night temperatures,as increased photon flux density during growth resulted in equalrates in the light and the dark. Increased relative humiditydecreased LER and afternoon rates were most sensitive to waterstress. A ‘low light’ night period did not changeLER-pattern during the night or on the following day. We concludethat nitrate is not an obligatory osmoticum during the nightand can be exchanged for organic osmotica without decreasingLER. During the night the turgor is first restored by increasingwater uptake, nitrate uptake and starch degradation. This resultedin increased leaf fresh weight in this period. Thereafter, elongationincreased by simultaneous uptake of nitrate and water. Nitrateconcentration was, therefore, constant in the older leaves.In the younger leaves nitrate concentration increased to replacesoluble carbohydrates. The vacuoles of the old leaves were filledwith nitrate before those of the young leaves. Key words: Spinacia oleracea L., nitrate accumulation, osmotic potential, organic acids     

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     

Turgor Pressure and Phototropism in Sinapis alba L. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropismin Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls.Phototropism was induced by adding 0.27 µmol m^–2s^–1 unilateral blue light to a background of low pressuresodium (SOX) lamp light. Curvatures of some 6° from thevertical were reached by 60 min, the curvature rate between20 min and 60 min being 0.14° min^–1. From the axialgrowth rate and tissue geometry the local growth rates of illuminatedand shaded sides of the hypocotyl were calculated to be 1.5and 4.5 µmin^–1 respectively. Turgor pressures ofexpanding cells in control plants and in the shaded and illuminatedsides of the blue light illuminated hypocotyls were measuredto be 0.40-0.55 MPa with a pressure probe. No changes in turgorpressure were observed on initiation of curvature. The decayof pressure in the cells of non-transpiring plants followingexcision indicated that the yield stress threshold of the tissuemay be as low as 0.1 MPa. These results indicate that the phototropicgrowth response in this tissue is not mediated by changes inturgor pressure. Key words: Sinapis alba L., phototropism, turgor pressure     

Response to Saturation Deficit of Leaf Extension in a Stand of Pearl Millet (Pennisetum typhoides S. & H.)II: II. DEPENDENCE ON LEAF WATER STATUS AND IRRADIANCE

This paper describes how the dominant relation between leafextension and temperature in pearl millet is modified by atmosphericsaturation vapour pressure deficit (SD) and irradiance. Standsof plants were grown at two levels of SD and soil moisture content.Leaf extension, water potential (₁) and stomatal conductancewere all reduced at high SD, ₁ was more closely related to transpirationrate than to SD itself. Leaf extension rate (R) was poorly correlatedwith ₁, even after correction for temperature differences, owingto variation in solute potential between leaves. However, Rin individual leaves was linearly related to turgor potential,except after periods of low irradiance. The thermal time conceptwas modified to incorporate turgor potential and used to showthat the ‘turgor thermal rate of extension’ decreasedsharply at low irradiances, presumably due to assimilate shortage. Key words: Extension, Saturation deficit, Millet     

Leaf illumination and root cooling inhibit bean leaf expansion by decreasing turgor pressure

Phaseolus vulgaris plants with expanding primary leaves weresubjected to dark-light or light-dark transition at a root temperatureof 25 °C, or to root cooling to 10 °C. Illuminationor darkening caused rapid changes in water flux through theplants and in epidermal turgor pressure when analysed by pressureprobe. However, these were not concurrent with variations inbulk leaf water potential and turgor pressure as determinedby the pressure chamber method. In addition, the turgor pressureof epidermis measured with the pressure probe was invariably0.05 to 0.15 MPa lower than that measured in bulk tissue withthe pressure chamber. Cooling roots to 10°C induced waterstress and wilting. Both techniques indicated a decrease ofturgor pressure, but a 20-30 min lag was observed with the pressurechamber. Due to stomatal closure and decreased transpiration,root-cooled plants regained cell turgor after 5-7 h of cooling,but bulk tissue and epidermal turgor (as well as leaf growthrate) remained significantly lower than control levels. Thesefindings indicate that changes in turgor pressure as the resultof hydraulic signalling are sufficient to explain the rapidchanges in growth rate following illumination or cooling reportedin earlier work (Sattin et al 1990). They also indicate thatdata obtained by use of the pressure chamber must be treatedwith caution. Key words: Phaseolus vulgaris, expansion growth, water relations, hydraulic signalling, pressure probe, pressure chamber     

Role of light, carbon dioxide and nitrogen in regulation of buoyancy, growth and bloom formation of Anabaena flos-aquae

The availability of light, CO₂ and NH₄-N interacted to controlbuoyancy and growth of the gas vacuolate blue-green alga, Anabaenaflos-aquae. At high light intensities algal growth rates werehigh; however, the alga was non-buoyant regardless of the availabilityof CO₂ or NH₄-N. The mechanism for buoyancy loss involved increasedcell turgor pressures at higher light intensities which resultedin collapse of gas vacuoles. At lower light intensities algalgrowth rates and cell turgor pressures were reduced and buoyancywas controlled by the availability of CO₂ and inorganic nitrogen.Carbon dioxide limitation increased buoyancy, while reducedinorganic nitrogen availability reduced buoyancy. Mechanismsfor buoyancy regulation at low light intensities involved changesin cellular C/N ratios which appeared to affect the rate ofsynthesis and accumulation of protein-rich gas vacuoles. Algalspecific growth rates were combined with buoyancy data to forma single index (µ_bloom) to the rate of surface bloom formationof A.flos-aquae as a function of the availability of light,CO₂ and NH₄-N. The bloom formation index was enhanced with decreasedavailability of light and CO₂, and increased availability ofNH₄-N.     

Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L.--Nature and Cellular Location of Turgor-Sensitive Unloading

Patrick, J. W., Jacobs, E., Offler, C. E. and Cram, W. J. 1986.Photosynthate unloading from seed coats of Phaseolus vulgarisL.—Nature and cellular location of turgor-sensitive unloading—J.exp. Bot. 37: 1006–1019. Unloading rates of ¹⁴C-Photosynthates from excised seed-coathalves of Phaseolus vulgaris L. plants were sharply increasedat cell turgor potentials in excess of 5 ? 10⁵ Pa. Turgor-sensitiveunloading occurred in the absence of any change in the passivepermeability of, and active sucrose influx across, the plasmalemmaand tonoplast membranes. The proton ionophore CCCP, and lowtemperature significantly slowed turgor-sensitive unloadingwhile PCMBS, a non-permeating sulphydryl-modifying compound,was without effect. Turgor-sensitive unloading significantlydepressed the ¹⁴C-Photosynthate content of the ground and branchparenchyma, but had no effect on the ¹⁴C-Photosynthate levelsin the vascular tissues. Cycling of cell turgor potentials aboveand below 5 ? 10⁵ Pa elicited reproducible responses in theunloading rate of ¹⁴C-Photosynthates. Increasing turgor above5 ? 10⁵ Pa resulted in a burst of ¹⁴C-Photosynthate unloading.Reversal to turgors less than 5 ? 10⁵ Pa caused a rapid depressionin unloading rate. It is proposed that turgor-sensitive unloadingis facilitated by a specific turgor-sensitive porter locatedon the plasmalemma of the ground and/or branch parenchyma cellsof bean seed coats. Key words: Bean, seed coat, turgor-sensitive unloading, phloem     

Xyloglucan Endotransglycosylase Activity, Microfibril Orientation and the Profiles of Cell Wall Properties Along Growing Regions of Maize Roots

A series of physical and chemical analyses were made on theexpanding zone of maize seedling roots grown in hydroponics.Comparison of longitudinal profiles of local relative elementalgrowth rate and turgor pressure indicated that cell walls becomelooser in the apical 5 mm and then tighten 5–10 mm fromthe root tip. Immersion of roots in 200 mol m^–3 mannitol(an osmotic stress of 0·48 MPa) rapidly and evenly reducedturgor pressure along the whole growing region. Growth was reducedto a greater extent in the region 5–10 mm from the roottip than in the apical region. This indicated rapid wall-looseningin the root tip, but not in the more basal regions. Following 24 h immersion in 400 mol m^–3 mannitol (an osmoticstress of 0·96 MPa) turgor had recovered to pre-stressedvalues. Under this stress treatment, growth was reduced in theregion 4–10 mm from the root tip, despite the recoveryof turgor, indicating a tightening of the wall. In the rootapex, local relative elemental growth rate was unchanged incomparison to control tissue, showing that wall properties herewere similar to the control values. Cellulose microfibrils on the inner face of cortical cell wallsbecame increasingly more parallel to the root axis along thegrowth profile of both unstressed and stressed roots. Orientationdid not correlate with the wall loosening in the apical regionof unstressed roots, or with the tightening in the region 5–10mm from the root tip following 24 h of osmotic stress. Longitudinal profiles of the possible wall-loosening enzymexyloglucan endotransglycosylase (XET) had good correspondencewith an increase in wall loosening during development. In thezone of wall tightening following osmotic stress, XET activitywas decreased per unit dry weight (compared with the unstressedcontrol), but not per unit fresh weight. Key words: Osmotic stress, turgor, growth, cell wall properties, microfibrils, XET     

The Effect of Water Stress upon Polyamine Levels in Barley (Hordeum vulgare L.) Leaves

The effect of water stress on leaf polyamine content of fourHordeum vulgare varieties, Alger/Ceres, Palmella Blue, Rihaneand Roho, with different drought characters was studied After6 d without water Alger/Ceres, Palmella Blue and Rihane hadaccumulated putrescine, although only in concentrations up totwice those found in the controls, but Roho had a decreasedputrescine content. However, one common response was identified;the accumulation and subsequent loss of putrescine was dependentupon the maintenance and loss of leaf turgor respectively. Consequentlyvarietal differences in putrescine accumulation were relatedto water consumption rates and the extent of osmotic adjustment.Spermine behaved in a similar manner to putrescine but spermidinelevels always decreased. Polyamine levels were never high enoughto be an important component of solute accumulation. Prolinelevels were 150-fold higher and glycine-betaine levels 50-foldhigher than polyamines in stressed plants. Proline and glycine-betaineaccumulation occurred once a threshold turgor was reached, whichin Roho appeared to be a reduction by 0•2–0•25MPa or 30–40%. The importance of polyamine accumulationduring water stress is discussed. Key words: Hordeum vulgare, polyamine, water stress     

Auxin Changes Both the Extensibility and the Yield Threshold of the Cell Wall of Vigna Hypocotyls

Elongation of plant stem is governed by two simultaneous processes:irreversible yielding of the cell wall and uptake of water.Among many candidates for the parameters that regulate and/or restrict growth, we focused on the mechanical propertiesof the cell wall and determined those parameters that governthe process of IAA-induced growth by means of the pressure-jumpmethod combined with the pressure-probe technique. The elongation growth of segments excised from the elongationzone of Vigna hypocotyls was accelerated by xylem perfusionwith 10^–4 M IAA. During the promotion of growth, boththe extensibility () of the cell wall and the effective turgor(Pⁱ–Y) increased while only a little or no change in theintracellular pressure (Pⁱ) occurred. These results indicate that IAA increases not only the extensibilityof the cell wall but also the effective turgor, i.e., the drivingforce for yielding of the cell wall. However, the driving forceis not increased by the increase in Pⁱ but by the decrease inthe yield threshold (Y). These results suggest that Y is adjustableduring the regulation of growth. ¹Present address: Department of Biology, Faculty of Science,Okayama University, Okayama, 700 Japan (Received September 20, 1990; Accepted November 27, 1990)     

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     

Cell elongation, turgor and osmotic pressure in developing sunflower hypocotyls

The relationship between cell elongation, change in turgor andcell osmotic pressure was investigated in the sub-apical regionof hypocotyls of developing sunflower seedlings (Helianthusannuus L.) that were grown in continuous white light. Cell turgorwas measured with the pressure probe. The same hypocotyl sectionswere used for determination of osmotic pressure of the tissuesap. Acceleration of cell elongation during the early phaseof growth was accompanied by a 25% decrease in both turgor andosmotic pressure. During the linear phase of growth both pressuresremained largely constant. The difference between turgor andosmotic pressure (water potential) was –0.10 to –0.13MPa. Excision of one cotyledon had no effect on growth, turgorand osmotic pressure. However, after removal of both cotyledonscell elongation ceased and a substantial decrease in both pressureswas measured. In addition, we determined the longitudinal tissuepressure in seedlings from which one or both cotyledons hadbeen removed. Tissue pressure and turgor were very similar quantitiesunder all experimental conditions. Our results demonstrate thatturgor and cell osmotic pressure show a parallel change duringdevelopment of the stem. Cessation of cell elongation afterremoval of the cotyledons is attributable to a decrease in turgor(tissue) pressure, which provides the driving force for growthin the hypocotyl of the intact plant. Key words: Cell elongation, Helianthus annuus, osmotic pressure, tissue pressure, turgor     

The Importance of Light Intensity for Pollen Tube Growth and Embryo Survival in Wheat x Maize Crosses

The success of Triticum aestivumxZea mays crosses, used to producewheat doubled haploids, is influenced by light intensity. Toexamine the basis for this response, pollen tube growth, embryosurvival and indicators of photosynthetic rate were measuredin two wheat cultivars (‘Karamu’ and ‘Kotuku’)crossed with maize at two irradiance levels (250 or 750 µmolm^-2s^-1, PAR). Pollen tube growth was significantly affectedby light intensity in ‘Karamu’ plants but not in‘Kotuku’ plants, despite both cultivars being pollinatedby the same maize source. The percentage of pollen tubes reachingthe cavity between the ovarian wall and integuments, or in themicropyle of ‘Karamu’ plants at high light intensity(65%) was nearly three-times greater than that at low lightintensity (22%). Thus, either low light intensity can affectthe maternal wheat plant in a way that inhibits pollen tubegrowth and/or high light intensity may promote pollen tube growthin ‘Karamu’ plants. Significant differences in ratesof electron transport in plants grown at the two light intensitiesindicated that the rate of photosynthesis may also have an effecton pollen tube growth. These results have importance for improvingthe efficiency of wheat x maize crosses and other wide cerealcrosses. Copyright 2001 Annals of Botany Company Intergeneric hybridization, light intensity, pollen tube growth, embryo survival, Triticum aestivum, wheat,Zea mays , maize