A Leaf Lamina Compression Method for Estimating Turgor Pressure

8/6/2019 A Leaf Lamina Compression Method for Estimating Turgor Pressure

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Proof OnlyHORTSCIENCE 45(3):16. 2010.A Leaf Lamina Compression Methodfor Estimating Turgor PressureAdonai Gimenez Calbo1, Marcos David Ferreira, andJose Dalton Cruz Pessoa

Embrapa Agricultural Instrumentation, Brazilian Agricultural ResearchCorporation, rua Quinze de Novembro, 1452, rua Quinze de Novembro,

2723, Sao Carlos, SP, 13560-970, Brazil

Additional index words. Brassica oleracea, Chichorium endivia, Lactuca sativa, irrigation

scheduling, turgor pressure, vegetables, wiltmeter

Abstract. A portable wiltmeter instrument to estimate leaf turgor pressure according to anadaptation of the flattening method was developed. In the instrument, a flexible inflatingmembrane presses the leaf against a flattening plate having small orifices surrounded bya finely engraved network of obtuse indentations through which air flow is delivered.During a measurement, as the compression builds up, the leaf is progressively moldedagainst the flattening plate, and as a consequence, the air flow (x) crossing the plate isreduced toward zero. The smallest leaf compression (p0) that blocks the air passage is anestimate of the leaf turgor. Wiltmeter measurements were compared with pressure probemeasurements of cell turgor pressure in detached leaves of lettuce (Lactuca sativa L.), kale(Brassica oleracea L. var. Acephala), and chicory (Chichorium endivia L.), which wereallowed to suffer diverse levels of wilting caused by transpiration. Such observed wiltmeterreadings were a little lower than the cell turgor pressure measured with a pressure probe;the regression coefficients between these methods were: 0.862 for lettuce, 0.885 for kale,and 0.962 for chicory. This portable quantitative procedure to measure leaf firmness haspotentially valuable applications related to postharvest and field plant physiology studies.

Leaf cell turgor pressure is a water statusvariable related to firmness, growth, and withthe consumer perception of wilting and fresh-ness. Turgor pressure measurements are usu-ally made at a laboratory using laborious

procedures. With the pressure probe technique,for example, cell turgor pressure is estimatedafter impaling the probe capillary into plant

cells within plant tissues (Husken et al., 1978; Nonami et al., 1987). Typically a pressureprobe is made of an oil-filled microcapillaryconnected to a coupling chamber having a

pressure transducer. In a measurement, afterthe capillary is impaled into a cell, the low-viscosity oil transmits cell fluid pressure to the

pressure transducer. Next, using a micrometri-cal piston system (Nonami et al., 1987) orusing a thermoelastic pressurization system(Pessoa and Calbo, 2004), the oil/water menis-cus is returned to the preimpalement positionand, at this point, the transduced cell turgor

pressure is measured.In the field, plant water status is fre-

quently accessed using the pressure chamber(Scholander et al., 1964). The variable mea-sured by this instrument, however, is not turgor

pressure but the complementary air pressure,which is needed to extract sap water out of the

petiole of leaves subjected to an air pressure

ramp. In other words, the pressure chambermeasures the leaf water tension state, which inseveral applications can be taken as an estimateof the leaf water potential (Boyer, 1985). The

pressure chamber method, however, is invasive,and it involves measurements that have to bemade in detached leaves. Additionally, thesemeasurements became very time-consuming

when used to estimate leaf turgor pressure,because the procedure is indirect and requiresa series of points relating applied air pressurewith extracted sap volume of the individualleaves, in which turgor pressure is being esti-mated (Calbo and Moraes, 1997).

A specific portable instrument to measureleaf turgor firmness index was developed byHeathcote (Heathcote et al., 1979). In thisinstrument, a leaf resting over a cavity is

pressed by a central rod and the deformationread with a micrometer is taken as a cell turgor-dependent index. This portable leaf strain

probe, however, is sensitive to leaf thicknessand leaf venation, even when measuring dif-ferent leaves from a single plant (Turner andSobrado, 1983). More recently, a leaf patchclamp pressure probe for field use was de-veloped (Zimmermann et al., 2008). In thisinstrument, an electric pressure signal is gen-erated as a function of the changes in leafvolume and turgor pressure. For measurements,the leaf is clamped between two planar pads;the first pad is a rigid support and the secondone is a soft silicone sensor pad, in which anencased pressure transducer reads a signal thatis always a fraction of the applied leaf com-

pression. According to the authors, this in-strument enables continuous data acquisitionfrom leaves with different thicknesses. The leaf

cell turgor pressure response in the patch clampis approached with aid of an intricate mathe-matical approximation applied to a data set that

presents a delayed time line response that canbe as large as 4 h on sunny days.

For convex-shaped organs such as manyfruits, a flattening method is being used toestimatethe turgor-dependent pressure firmnessfor several applications, including some newfruit firmness half-life determinations (Caronet al., 2003; Kluge et al., 1999; Nizio et al.,

2008). The flattening method was developedinitially to model a grape berry as if it were-a thin-walled balloon filled with pressurizedwater (Berstein and Lustig, 1981; Bernsteinand Lustig, 1985). According to this model, anexternal force applied with a transparent plateflattens a fruit surface area in which the value isequal to the turgor pressure multiplied by thefruit flattened area. More recently (Lintilhacand Outwater, 1998), an analogous thin-walled

balloon approach was used to measure epider-malcells under themicroscope with a procedurenamed ball tonometry.

Using a more general plant physiologyreasoning, the flattening procedure was ex-

tended to estimate cell turgor pressure of otherconvex fruits and vegetables covered by softdermal tissues (Calbo and Calbo, 1989; Calboand Nery, 1995) composed of thin-walled

poliedric cells that cover the internal plantcellular structure made out of parenchymatouscells having deformable intercellular air vol-umes. Accordingly, for some regular cellularlattices, it was demonstrated that there is asimple mathematical relation between theflattening pressure and cell turgor (Calbo and

Nery, 2001). In these lattice models, the flat-tening pressure and the cell turgor pressureare related by a cell compression ratio, whosemagnitude ranges from zero to one depending

on remaining intercellular air volume fractionduring mechanical axial compression assays.A first attempt to develop a portable in-

strument to measure the leaf turgor using theflattening method involved a setup having a

plain rigid base over which the leaf was com-pressed by a piston flattening plate, whose sur-face was finelyindented around a few smallairflow outlet orifices (Calbo, 1991). For a mea-surement, the leaf was progressively com-

pressed by this piston and the reduction ofapplied air flow, filtered between the leaf andthe flattening plate, was used as a criterion ofleaf flattening that was used to estimate theleaf turgor. The instrument was simple andquantitative, but the piston borders caused leafdeformation marks, especially in thicker leafs,and these marks were considered to be a po-tential cause of leaf turgor underestimation inthick leaves.

In this communication, a portable instru-ment that makes use of the flatteningmethod toestimate the leaf turgor pressure status, withoutcausing leaf indentation marks, is presented for

possible use in postharvest and field-oriented plant physiology studies. This wiltmeter in-strument performance was then consideredmainly with reference to the pressure probemethod to measure cell turgor in leaves ofvegetable crops.

Received for publication 3 Sept. 2009. Acceptedfor publication 9 Dec. 2009.We are indebted to Mr. Joao Batista Gomes fromEmbrapa Vegetables for building preliminarymodels of the portable instrument used to measureleaf firmness.1To whom reprint requests should be addressed;e-mail [email protected].

HORTSCIENCE VOL. 45(3) MARCH 2010 1

POSTHARVEST BIOLOGY AND TECHNOLOGY


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Proof OnlyMaterials and MethodsThe instrument inF1 Figure 1 enables the useof a leaf lamina compression method adapta-tion to measure the leaf turgor-dependent

firmness pressure (Calbo and Pessoa, 2008 AU).This wiltmeterwas assembled with a flattening

plate [Fig. 1 (1)] having a few (greater thanfive) nearly centralized orifices [Fig. 1 (2)]; anair flow source; a flowmeter [Fig. 1 (4)]; and

a membrane hydraulic leaf compressing mech-anism. The air flow source was composed ofan air compressor [Fig. 1 (9)], an air escape

pressure regulator [Fig. 1 (10)], and an inlet airflow restriction [Fig. 1 (11)]. The membranehydraulic leaf compressing mechanism, on theother hand, wasmade with a sandwiched mem-

brane [Fig. 1 (6)] fixed at the instrument baseand works with water introduction to pressthe leaf against the flattening plate while thisapplied pressure is measured in a manometer

[Fig. 1 (7)].The air flow source delivers an air inlet

pressure (Dp0) of %6 kPa. Of this pressure,while the instrument is open, %4 kPa isdissipated at the inlet air restriction [Fig. 1(11)] R1 and 2.0 kPa at the flow meter re-striction [Fig. 1 (3)] R2. At this condition, theflowmeter reading is 200 mm for an air flowof 90 mLmin1. For practical use, this inlet air

pressure is obtained by simple adjustment ofthe pressure regulator knob [Fig. 1 (10)] at 200mm in the flowmeter manometer of the openedwiltmeter.

Measurement involves fixing the leaf [Fig.1 (5)] with the screw nut [Fig. 1 (12)] and

subjecting the leaf to a progressive compres-sion against the flattening plate [Fig. 1 (1)]with the syringe [Fig. 1 (8)]. As the applied

pressure increases, the leaf is progressivelymolded against the flattening plate while theair flow is attenuated down to zero in this aircompression ramp-up assay.

Theoretical considerations. A quadraticmodel [Eq. (1)] generates an approximationfor the nonlinear relation between the appliedleaf compression (p) and the air flow (x) forxvalues close to zero F(Fig. 2).

p = p0 A x + B x2 [1]

In this equation, p is the applied pressure,

x is the air flow, p0 is the estimated applied

Fig. 1. Scheme of a wiltmeter instrument to measure leaf turgor pressure using the flattening methodimplemented with aidof an airflow attenuation procedure. Thesystem is composed of a flatteningplatewith a slightly granular base(1) having centralized microair inlet orifices (2); an air restriction (3) flowmeter in which airflow is read in a U tube manometer (4) where the progressof the leaf (5) flattening isfollowed; a flexible membrane fixed in the instrument base (6) is the element used to compress the leafagainst the flattening plate while pressure, read in the manometer ( 7), is being applied with a water-filled syringe (8). The air flow needed for this flattening attenuation mechanism is fed by an aircompressor (9) coupled to a pressure regulator (10) and an inlet air restriction (11). During themeasurement, the leaf remains clamped with aid of a bolt screw nut ( 12), while the spring (13) eases

instrument opening and leaf freeing by forcing the flattening plate movement around the axle (14).

Fig. 2. Typical curves relating applied leaf compression versus the attenuated or filtrated air flow between a kale leaf and the flattening plate obtained duringa wiltmetermeasurement. (A) Applied leaf compression (p) versusread air flow (x). (B) Linearization using theinverse of theappliedleaf pressure (1/p) versusread air flow (x).

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Proof Only pressure at the intercept (x0 = 0), whereas Aand B are fitting parameters. If experimentaldata could be taken even closer to zero, then alinear approximation would also be valid andin this case, in the quadratic term of Eq. [1],could be regarded as negligible.

Another simple instrument working ap-proximation to estimate the applied pressureat the intercept (p0) is by an inverse lineari-zation procedure (Calbo et al., 1989); in thiscase, the air flow (x) and applied leaf pressure

( p) are given by Eq. [2].1=p = ax=pair + 1=p0 [2]

where p is the applied pressure, x is the airflow, p0 is the estimated applied pressure atthe intercept (x0 = 0), a is a leaf strain para-meter, and pair is the wiltmeter adjusted inletair pressure.

In a more simplified way, Eq. [2] can alsobe written as:

1=p = Cfx + 1=p0 [3]

where Cfis:

Cf = a=pair [4]

Eqs. [3] and [4] are linearization approxi-mations of the actual nonlinear relation be-tween p andx, which are valuable forx valuesmuch smaller than the instrument open airflow x0.

x


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Proof Onlypressure probe capillary replacement followed by measurements with a pressure probe andwiltmeter in new leaf samples.Results and Discussion

During readings in lettuce, kale, and chic-ory, the wiltmeter instrument caused no leafdamage, except some eventual small marks inlarger veins. This portable instrument is suit-able for field use, where it can be used to read

clean leaf surfaces. As a consequence, in somedusty environments, it is usually necessary toremove leaf-adheredparticleswith a softtowel.Similarly, in moist environments, it is neces-sary to remove carefully all the leaf surface-free water, because the flattening plate needs to

be dry and clean to generate reliable readings.In preliminary assays, not presented, moistenedleaves caused unduly low leaf flattening pres-sure results in comparison with these sameleaves measured again, 1 or 2 min later after

being surface-dried with a paper towel.Air permeation as a water status measuring

tool. Air flow (x) across the remaining airvolumes contained between the leaf and the

pressed flattening plate reduces nonlinearly asthe applied compression (p) increases (Fig. 2A)and this air flow attenuation enabled the adap-tation of the flattening method to this wiltmeter

portable instrument. The instrument basic pro-cedure ofreducing the air flow (x) down to zeroto estimate p0, however, does not work equallywell in all cases and for this reason, it isfrequently preferable to estimate p0 with somesuitable approximation procedure. The chosenapproximation method used in this work wasthe linearization obtainedby plotting 1/p versusx (Fig. 2B). The use of this extrapolationmethod, however, is time-consuming for most

postharvest or plant physiology applications.

Consequently, in this study, the linearizationmethod was used only as a tool to calculate thew coefficient of each leaf vegetable, which wasapplied to estimate p0 using a single rapidreading along with the coefficient-correctedapproximation. For these leaf vegetable crops,the average observed coefficient (w) was 1.06for crisp lettuce, 1.06 for kale, and 1.03 forchicory. The coefficient-corrected procedureto estimate p0 is simple and can be used evenin field-oriented studies, because it involvesonly the rapid reading made at xj = 4 mm (1.8mLmin1). The estimate of p0 is then madelater with aid of the previously determinedw coefficient. Observed w values close to onewere considered a favorable result because itindicates that for most field applications, therapid procedure, even without coefficient cor-rection adjustment, is a valuable estimation thatcan be used as a leaf wilting index orevenas anauxiliary irrigation scheduling index.

It is important to point out that a proce-dure such as the linearization, or the quadratic,method may become one practical way toimplement new portable microprocessed wilt-meters, in which p0 will be calculated immedi-ately and more accurately with aid of real-timedata acquisition procedures.

The instrument air flow control. When airpassage orifices [Fig. 1 (2)] in the flattening

plate (Fig. 1) are closed, the air flow reduceswith a response time (t1/2) of%2 s, which is arelatively rapid instrument response time. Dur-ing a measurement, the instrument responsetime may be a little more complex variable to

be measured, and eventually, it could be a littlelonger, because it depends on the time theleaf flattening strain occurs under the appliedcompression stress. With the response timerelated to the air flow, it is also directly relatedto the used inlet air pressure (pair) that can in-

sufflate an air flow proportional to pair throughthe controlling restriction, whereaspairis smallenough for the air to behave nearly as anincompressible liquid (Moore, 1972). Largerpair values, however, can also be used, but aircompressibility is also a relevant nonlinearmodulating component for the air flow throughthe instrument restrictions. Another air flow-related response time component is the flow-meter manometer, in which response rapidityisinversely related to its internal dead air volumechange caused by the liquid column movementduring a measurement. To achieve responseof a few seconds, the flowmeter used in thiswiltmeter was set to work between zero and

90 mLmin1

and in this range, the observedinternal air volume change in a flowmetermanometer U tube was%0.4 mL. Additionally,ethanol was used as the manometric fluid

because it is less dense and has a much lowersurface tension than water, which also helps inattaining a fast and accurate response.

The 6-kPa working air pressure (pair) usedis small-inlet air pressure considering that acommon battery-driven microcompressor typ-ically develops air pressures above 10 kPa,andas a consequence, 6 kPa was easily adjustedin an escape valve [Fig. 1 (10)]. After definingpair, the instrument air restrictions (Fig. 1)were estimated as follows: the flowmeter re-

striction R2 [Fig. 1 (3)] is a plastic capillarytube cut to allow an air flow of 90 mLmin1

with a 200 mm of pressure difference at itsreading in an alcohol manometric column. Theair inlet restrictionR1 [Fig. 1 (11)], on the otherhand, is set to develop a 200-mm reading atthe flowmeter, when was fed by an adjusted6.0-kPa air pressure source in the openedwiltmeter. Both R1 and R2 are prepared bycutting pieces of a plastic capillary tube withan internal diameter of %110 mm. The airrestrictions R1 andR2 work in series with theleaf/flattening plate air restriction RL F3(Fig. 3).

Consequently, as the wiltmeter-applied pres-sure increases, RL also increases, because it isindicated by the air flow reduction (Fig. 2A).When RL becomes much greater than R1 plusR2, then the applied air pressure (pair) is alldissipated at the restriction RL.

Instrument features. This instrument issuitable to measure near laminar vegetablesamples such as leaves or segmented organswith thickness ranging from 0.050 mm to2 mm. For measurement, these laminar struc-

tures are hydraulic-compressed with aid ofa membrane [Fig. 1 (6)], which should beelastic and thin to uniformly compress theorgan against theflattening plate,across a freecircular area, with a diameter of 15 mm. A50-mm flexible polybutene rubber membrane,which is also robust durable, is suitable to

perform this function. It is also important to point out that this membrane can inflateexcessively causing rupture if the wiltmeteris not properly closed with the bolt/screw[Fig. 1 (7)] before compressing the leaf. Theuse of thicker and more robust membranescan cause uneven compression for veinedleaves, whereas the use of thinner and more

flexible 20-mm latex membranes, which canapply a more uniform compression, even forsmall veins, are fragile and short-lived. Forspecific fine experimental work, however,such thin membranes can be installed, be-cause they are fixed and sealed between afixing plate and the instrument base.

The flattening plate (Fig. 1) with a finelyengraved surface net made of 10- to 30-mmdeep obtuse indentations is another essentialinstrument feature. The engraved surface en-ables the measurement of smooth surfacedsam-

ples, because it helps in assuring that the airpassages are only blocked after the compressedleaf surface is molded to the flattening plate

shape. This engraved network also helps in al-lowing measurement of small samples with aminimum length of 15 mm and a minimalwidth of%6 mm. During a measurement, theadaxial leaf side, which is usually the mostregular one, is the side that should preferably

be placed in contact with the flattening plate,considering that large veins and other leafirregularities should not be placed directly un-der the central flattening plate mechanism area.

Instrument gauging. In the wiltmeter in-strument comparison against the cell turgor

pressure probe (Fig. 4), it can be observed

Fig. 3. Restrictions that modulate the air flow driven by a controlled inlet air pressure source (6.0 kPa) inthe wiltmeter instrument. R1 = inlet air flow restriction; R2 = flow meter restriction; RL = variable leaf/flattening plate interface restriction.



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Fig. 4. Relationship between leaf flattening pressure, measured with the wiltmeter instrument, and cell turgor pressure, measured with the pressure probe. (A)Lettuce, (B) chicory, and (C) kale.

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Proof Onlythat readings by these two methods wereproportional with slopes of 1.16 for lettuce,1.13 for kale, and 1.04 for chicory. Thesevalues are consistent with the fact that theflattening method conceptually should gen-erate pressure readings smaller than the cellturgor pressure measured with the pressure

probe, except for limiting tissues in whichthe cell wall thickness and the intercellularair volumes could be considered vanishinglysmall (Calbo and Nery, 2001). The percent-

age of intercellular air volume (v/v) in theleaf is expressive and for most crops and it isin the range between 20% and 30% (Spector,1956). Estimates of the cell wall thicknesscan be assessed in terms of percentage ofapoplastic water volume, which forBrassicaleaves is on the order of 10% of the leafvolume (Husted and Schjoerring, 1995).Consequently, despite the relevant apoplas-mic and intercellular air volumes of theleaves, the wiltmeter produces leaf pressureestimates that are very close to the leaf cellturgor pressure as measured by the pressure

probe method, which is an indication thatthis instrument can be used for quantitativestudies devoted to postharvest and other fieldexperiments where portability, rapidity, androbustness are crucial requirements.

Measuring leaf turgidity with the portableflattening procedures herein described repre-sents progress over current simple subjectivesensory procedures that are used to evaluatewater status postharvest, irrigation scheduling,and in some plant physiology applications.The closeness of the wiltmeter instrumentreadings to the leaf turgor pressure measuredwith the pressure probe makes this flatteningmethod a practical field substitute for the

pressure probe, which can be easily used in

the field for most of the commercially relevantcrops during quantitative evaluations of wilt-ing, freshness, and turgor pressure needed foragronomic and ecological applications.

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A Leaf Lamina Compression Method for Estimating Turgor Pressure