An equation is developed that relates apparent photosynthesis
to CO? light compensation values, resistance to
CO? diffusion in air, and the apparent mesophyll resistance
to CO? transport. The equation also yields values
for total photosynthesis and light respiration from measurements
of apparent photosynthesis. A simple method
for measuring leaf CO? exchange rates in the field with
a hand-operated syringe is described. Results obtained
with this device and data published in recent literature
are used with the new equation to show that photosynthesis
may be limited more by mesophyll resistance than
by photorespiration

Cary, J.W.

An equation is developed that relates apparent photosynthesis
to CO₂ light compensation values, resistance to
CO₂ diffusion in air, and the apparent mesophyll resistance
to CO₂ transport. The equation also yields values
for total photosynthesis and light respiration from measurements
of apparent photosynthesis. A simple method
for measuring leaf CO₂ exchange rates in the field with
a hand-operated syringe is described. Results obtained
with this device and data published in recent literature
are used with the new equation to show that photosynthesis
may be limited more by mesophyll resistance than
by photorespiration

USDA - ARS - NWISRL

10fl ut•O-.. • .1:4	 'byReprinted from CROP SCIENCEVol. 17, May-June 1977, p. 453-456Relations between CO, Exchange Rate, 0::), Compensation, and MesophyllResistance from a Simple Field Method'J- W.ABSTRACTAn equation is developed that relates apparent photo-synthesis to CO, light compensation values, resistance toCO, diffusion in air, and the apparent mesophyll resist-ance to CO, transport. Theequation also yields valuesfor total photosynthesis and liOit respiration from mea-surements of apparent photosynthesis. A simple methodfor measuring leaf CO, exchange rates in the field witha hand-operated syringe is described. Results obtainedwith this device and data published in recent literatureare used with the new equation to show that photosyn-thesis may be limited more by mesophyll resistance thanby photorespiration.Addilionai index words: Stomatal resistance, Photo-respiration, Gross photosynthesis, Chloroplast CO, concen-trations.THE growth of any plant ultimately depends onits carbon balance. Leaf absorption of CO 2 isan important component of this balance and has beendescribed by relations of the typeCa - Cr _ ca - ci _	 - CrGER - 	r, +	 rmwhere CER is the leaf CO 2 exchange rate, nmoi-s- Lcm- 2, and Ca the concentration of CO 2 in ,,L1 - liter- 1 ,in the air around the leaf with C i and Ce the respectiveconcentrations in the substomatal cavities and at thechloroplast surfaces. The gas diffusion resistances toCO 2 transport, s•cm- 1 , are ra , indicating the viscousflow air boundary, and ra, indicating the leaf sur-face including both the stomata and the cuticular layer.The symbol r represents the sum of ra and r a, whilerm is the total app-LLrent mesophyll CO2 transfer resis-tance. The constant, 7 _.=-4.44k makes the dimensionsconsistent (assuming standard pressure and 24.4 litersof gas/mole of CO 2 at 20 C). The variables CER, Ca,r, and I-, can be measured, but values for C i , Ca , andrm must be obtained indirectly.Values for rm have been calculated from equation1 by estimating a value for Ce and measuring CER, r,and C a . Other methods of obtaining rm follow fromassuming C, constant, so thatAC JEICF.R	 ACERwhich require two simultaneous measurements of CERand r under different levels of light intensity, CO2or Cl, concentrations, or water stress. Lake (6) hasdiscussed the problems associated with these variousmethods of calculating mesophyll resistance.' Contribution from the Western Region, ARS/USDA, Univ.of Idaho College of Agriculture Research and Extension Center,Kimberly. cooperating. Received 12 June 1970."Soil scientist, Snake River Conservation Research Center,Kimberly, Ill 13334l-CaryThe method reported here enables one to calculateTM from a single set of measurements of CER, r a, ,and where 1" is the CO2 compensation point inlight. These measurements can be made quickly onplant leaves in the field with simple equipment.MATERIALS AND METHODSTheory. The concentration of CO, in the substornatal cavitiesdepends on the resistances to CO, diffusion through the airboundary layer, the leaf surface, and the mesophyll tissue. TheCO, compensation point in light and the concentration of CO,at the chloroplasts will also affect C. As r O. C, —0 C., andas r becomes large, C, —0 1'. so the relation between r and C 1must be a curve with the general shape shown in Fig. 1. Thereare a number of functions one might choose to represent thiscurve, one of the simplest beingC, = a exp (— r/p)	 b.	 (31The boundary conditions require that a =_- C, — 1' and b =The term 11 represents the sink strength of the mesophyll tissuefor CO. and so is a function of	 and C,. The relationship be-tween /3 and r,, will be investigated using experimental valuesof ra.300250200t".;	 150U100500r(s CM -I )Fig. L The concentration of CO, in the substomatal cavities asa function of r and 0, as given by equation 3 with C.330 a l -z-' and r	 SO ar 1-1.[ I[2]453CROP SCIENCE, VOL. 17, M.kY-JUNE 1977Hypodermic NeedleThin ClearPlastic Coverer	 Flexible TubingThinMetal Ri-15-1Air inlet Port---,..Arm Fastenedto Syringe4—Spring ClipSupport Ring---/ 	Leafc,- CER '[9 ](1.5 I	 - CER	 - 1.51Fig. 2. A cross-sectional diagram of the apparatus used to measure net CO, uptake by leaves in the field. The cover is 9.8 cm indiameter and 2 cm high with r", = 4 at 500 cm 8 -min-1.Combining equations 1 and 3 leads to—r	CER•r-y	 (C. —	 (1 — expwhich solved for g gives,prir	-I'"ri-(11Once p has been evaluated from equation 5 using a singleset of experimental measurements, equation 4 may be used toestimate 1lw total or true photosynthetic rate by taking r 0.The (fillet titer between this value and the measured CER is anestimate of the light respiration rate.It is logical to suppose that values of Cr in the immediateneighborhood of the chioroplasts will ordinarily fall betweenF and zero. depending on the mesophyll resistance and the con-centration of CO.-. in the substomata! cavities. As a first approxi-mation using logic similar to that forming equation 3, one maywrite	C, i f ex !, —	 (C, 10-8)1	 [ 6 1where l0- 2 is a somewhat arbitrary constant used to weight theeffect of C, on C,. values.	 Using this relation in equation 1 gives(,-I ., \1 5 - 10,1C , Di 211I N.1	 -	 Rsubject to the assumption that transport between the stomata!c:avities and the chlo •oplasts of the carbon to be used in photo-synthesis is directly proportional	 to the difference between 	 C,and C.,. Values of rm calculated	 from equation 7 will fall be-ween those calculated from equation 1, using the limits C, = 0or C, T. Generally. changing from one of these limitingvalues to the other leads to a change in the calculated values ofr,, that is less than the normal variation in experimental mea-surf:MI:Ms of r under field conditions.Mra,oering CFR, r, and T. A number of methods for measuringCER hate been proposed (8). In this case, the flux of CO,into 1he leaf was measured in the field with the simple appara-tus shuts 	 in Fig. 2.	 The chamber	 was first clipped	 on	 theleaf and flushed li emptying the syringe, immediately followedhr Brass ing 500 m1 of air through	 the inlet port across	 theleal sin fare and hack	 into ihe syringe during a	 period.• W1 a Mile practice and 1 he use of a stopwatch, the operatorran make	 Ow lion rate	 !wall • enlist:nil. The air sample WaSIhrn	 :Disk! ed lime	 the sylringe into an evacuated teflon	 bag1 iirniii4h	 suplutil pot!. Liken into	 the lahoratrity. and	 its CO,toticturi;iiimi measured with an infrared :waiver. Teflon bagsmay be purchased through most specialty gas supply catalogsand are sufficiently impermeable to C.0, to permit gas samplestorage for several hours.The rate of CO„ exchange by the leaf in the chamber isCER' = 1.4 x 10-a	 (C, — C)	 [8]where q is the air flow in cin a •mirri, A is the area coveredon the leaf in cm', C is the concentration in pi-liter- 1 of theCO, sample drawn off from the leaf, and 1.4 x 10-8 is a con-stant making the dimensions consistent (assuming 24.4 litersof gas/mole of CO,, and including a factor of 2 because theexchange is measured on only one side of the leaf). The CO,uptake by the leaf is less in the chamber than under naturalfield conditions because the CO, concentration in the chamberis lower and the air boundry layer resistance higher. Theseeffects can he accounted for by solving equation I simultane-ously for r.	 rm with both the natural and chamber conditions,givingThe prime marks indicate conditions in the chamber, 0.5 is theaverage boundary layer resistance in the field (11), C'. is takenas 0.5 (C. C), and C, has been dropped as it is much lessthan C. and C'.. There is, of course, some experimental varia-tion associated with this simple procedure for measuring CO,.exchange rates in the field. in general, values of CER meas-ured over consecutive 1 min time periods on the same sugar-beet (Beta vulgaris L.) or corn (Zen mays L.) leaf agreedwithin	 0,13 iiniol•s- l •cm-' unless the light changed rapidlyor the wind was strong and gusty.Leaf resistance to water vapor transfer was measured witha commercially available diffusion porometer'. These valueswere converted to r., rounded to the next higher whole num-ber and then taken as r. This was justified because of thenormally low houndaty layer resistance in the field (11), andthe variation of one or more s•citt-' often encountered in ran-dom measurements of r. on different parts of the same leaf un-der field conditions.The CO, light compensation values were measured by bring-ing leaf samples with about 75 cm' of surface area into the lab-" Lamba Instrument Cofporation, Lincoln, Nebr. Trade namesand company names are included for the benefit of the readerand (10 not imply any endorsement or preferential treatmentilie product listed by the USDA.[41[51[7]0205	 10	 15r or rm (s . crnlr(, 7 I )25	 50	 75	 10032CARY: MEASURING CO, EXCHANGE RATES 	 455Table 1. Examples of measured and calculated gas exchange and transport parameters using data obtained in this study from re-cent literature.PlantTotalCER	 photosynthesismeasured	 eq. 4, r = fl measured eq. 5 measuredrMeq. 7 Source''cm -2-nmol's .Cm -ISSugarbeet (8, • a r ulpris L.} 1.64	 L96 52 6.6 1 7.30.79	 1.01 70 13.4 16.50.98	 1.20 60 9.5 4 10.21.70	 2.08 501 5.0 5.62.15	 2.65 50 + 3.3 2 3.2 121Corn (le. may, L.} 3.28	 3.34 12 2.3 3 1.2 t0.82	 0.88 16 12.3 7 9.9 t2.40	 2.46 101 2.7 4.5 1.2 1513.66	 3.79 10+ 1.7 3 0.7 151Bean (Phu w,,/	 •rdigar t r [J.) 1.14	 1.39 601 7.8 3 8.60.63	 0.76 60t. 13.2 7 14.0 t i lSunflower (Heliant hal	 unnateN L.) 2.37	 2.78 40t 3.4 1 3.5 (3}Sorghum (.Sorghum rulgare L.} 2.52	 2.59 10.t. 3.5 3 2.4 (412.84	 2.90 10± 3.5 2 2.7 (41t Observations from this study. t Estimated value.oratory and placing them in glass jars submerged in a waterbath at 20 C. The samples were illuminated with 40-watt in-candescent bulbs outside the jar, giving an average photosyn-thetic photon flux density of 42.5 nE•cm- 2 •s-1 on the leafsurface in the wavelength range of 400 to 700 nm. The jarwas flushed with air low in CO, (bubbled through a KOHsolution), and then equilibrated for 15 to 20 min before measur-ing the CO, level, which was taken as I'. The leaf surfacetemperature in the jar was about 25 C. When necessary, thebath temperature can be increased so that the leaf in the jaris about the same temperature as in the field. The light inten-sity can also be reduced to match that of shaded leaves in thefield that arc not light saturated.While the leaves of the plants studied here were largeenough to accommodate the 75 cm = cover shown in Fig. 2, thesame technique could he used on smaller leaves by completelyenclosing them and measuring their surface area. A betterapproach for smaller leaves might be to reduce the cover sizeto fit the leaf of interest and use a smaller syringe. Thiswould yield a smaller air sample that may require a gas chro-niatograph for analysis, but there is an advantage in coveringonly one side of the leaf because perturbations of its normalenvironment are less. In any case, the distance between theleaf and the top of the cover should be small to reduce theboundary layer resistance. This resistance can he measured forany shaped cover. Desiccant traps on the chamber's inlet andoutlet may be used to get the steady state rate of evaporationfrom a wet filter paper at a known temperature and givenair flow under the cover as[156[ 	 101where Q is the evaporation rate in mg•s- 1 •cm-2, C. is the con-centration of water vapor at the wet surface in mg•cm- 3 (knownfrom the temperature of the filter paper and the vapor pressureof water at that temperature), C o is the concentration of waterVapor leaving the chamber ing•cm-a 0.8 is a weighting factorwith water-free air entering the chamber. and 1.56 accountsfor she difference between the diffusion rates of CO, and H 2Oin air. Oiher simple methods are also available for estimatingr',	 (8, 11).RESULTS AND DISCUSSIONSome representative values of measured and calculat-ed photosynthetic parameters are presented in Table1. The inesophyll resistance to CO 2 transport is adynamic leaf property. Its normal range of values de-pends on plant species, environmental c•onditions. andprobably other factors that are not vet recognited.Indeed. rM and r are probabiv linked to some degreeits most plants (7).Fig. 3. The affects on CER caused by varying individually thevalues of r, r,,, or r given the conditions that r - 3, r N -1'	 60, and C. = 330.The values of 13 and rM in Table 1 are. for practicalpurposes, equal within the limits of random and ex-perimental errors of field measurements. Because themodel for rM suggested by equation 1 is such a grossoversimplification of CO 2 internal transport pathwa y s •and because of the uncertaint y of C 4 . Vititteti (7. 11).it would probabl y be better to think of rm as a pa ; . ; 1 -muter cha •act•rifing ihe int•rual sink strength oraffinit y of the leaf mesoph)11 for CO, This givesequation 1 the formC. - 0.8 Co456	 CROP SCIENCE, VOL. 17, MAY-JUNE 1977CER ca -r r	 r	 	 - exp -).	 [1 1 ]It is interesting to compare this form to the assimila-tion equation proposed by van Bavel (10) because ofthe relationship that is implied between F, TM, andphoton flux density. It appears that changes in CERresulting from changes in light intensity may largelybe affected through the mesophyll sink strength forCO2, i.e., through g (rm).Equation 11 has been used to draw the curves inFig. 3 illustrating the relative effects of F, r, and rMon limiting CER. Reducing the mesophyll resistancewould be an effective way to increase CO2 assimila-tion. Reducing the CO 2 light compensation pointwould have a lesser effect on (X) 2 exchange, as shownby the slope of the curves in Fig. 3 and by the smalldifferences between CER and total photosynthesis inTable I. These differences are, in fact, estimates ofthe light respiration rates. The low mesophyll resis-tances for corn and sorghum in Table 1 suggest, inconjunction with the curves in Fig. 3, that their po-tential for rapid growth results from small rm valueseven more than from their low photorespiration rates.ACKNOWLEDGMENTThe author sincerely appreciates the stimulating discussionswith Dr. Gale Kleinkopf. University of Idaho, and Dr. SamBrewster, Ricks College.REFERENCES1. Fraser, D. E. and R. G. S. Bidwell. 1974. Photosynthesisand photorespiration during ontogeny of the bean plant.Can. J. Bat. 52:2561-2570.2. Hall, A. E. and R. S. Loomis. 1972. Photosynthesis andrespiration by healthy and beet yellows virus-infected sugar-beets (Beta vulgaris L.). Crop. Sci. 12:566-572,3. Holmgren, P., P. G. Jarvis, and M. S. Jarvis. 1965. Resis-stances to carbon dioxide and water vapor transfer in leavesof different plant species. Physiol. Plant. 18:557473.4. Kanetnasu, E. T. and C. K. Hiebach. 1975. Net carbondioxide exchange of wheat, sorghum, and soybean. Can. J.Sot. 53:382-389.5. Koch, D. W. and G. 0. Estes. 1975. Influence of potassiumstress on growth, stomata' behavior, and CO„ asstmiliationin corn. Crop Sci, 15:697-699.6. Lake, J. V. 1967. Respiration of leaves during photosynthe-sis. II. Effects on the estimation of mesophyll resistance.Aust. J. Biol. Sci. 20:495-499.7. Peisker, M. and P. Apel. 1975. Influence of oxygen onphotosynthesis and photorespiration in leaves of Triiicumaestivum L. Photosynthetica 9:16-23.8. Sestakz, Z., J. Catsky, and P. G. Jarvis, 1971. Plant photo-synthetic production: Manual of methods. Junk, Hague,818 p.9. Terry, N. and R. P. Huston. 1975. Effects of calcium onthe photosynthesis of intact leaves and isolated chloroplastsof sugarbeets. Plant Physiol. 55:923-927.10. van Bavel, C. H. M. 1975. A behavioral equation for leafcarbon dioxide assimilation and a test of its validity. Pho-tosynthetica 9:165-176.11. Zelitch, I, 1971. Photosynthesis, photorespiration, and plantproductivity, 347 p. Academic Press, New York.

Relations Between CO2 Exchange Rate, CO2 Compensation, and Mesophyll Resistance from a Simple Field Method

Northwest Irrigation & Soils Research Laboratory Publications

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Relations Between CO2 Exchange Rate, CO2 Compensation, and Mesophyll Resistance from a Simple Field Method

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