periments in which we have confirmed the observa-tion by Galston and Baker (2) that elongation in thedark of green pea stem segments is much more de-pendent upon sucrose than is the elongation of etio-lated stem segments. Furthermore, since we have de-tected no promotion by cobalt of the growth of thegreen segments, the condition affected by cobalt ap-parently is not limiting to elongation. In one experi-ment, the elongation of such stem segments (origi-nally 5.3 nun long) was L52 mm on 10 mg/l IAA, and4.02 mm on IAA plus 2% sucrose. The elongation oneither solution was not increased by inclusion of CoC12in the concentration range from 4 to 20 x 10-5 M.
SUMMARYSucrose in some experiments slightly promoted and
in others slightly inhibited the elongation of sub-apicalsegments from etiolated pea stems. The sugar, how-ever, markedly increased the fresh weights of the seg-ments. Cobaltous salts, when added to test solutionscontaining only indole-3-acetic acid (IAA), slightlypromoted elongation. When sugar and cobalt wereadded together to the IAA solution, great increases ofelongation occurred. The evidence reveals a distinc-tion between the roles of water uptake and cell wallincrease in the enlargement of plant cells.
It is suggested that sucrose increases cell volumeand that cobalt promotes the ability of the cell wallsto increase in surface area.
This work was supported in part by a grant-in-aid from the American Cancer Society upon recom-
mendation of the Committee on Growth of the Na-tional Research Council and in part by the ResearchCommittee of the Graduate School on funds from theWisconsin Alumni Research Foundation.
LITERATURE CITED
1. CHRISTIANSEN, G. S. and THIMANN, K. V. The me-tabolism of stem tissue during growth and itsinhibition. I. Carbohydrates. Arch. Biochem. 26:230-247. 1950.
2. GALSTON, A. W. and BAKER, R. S. Studies on thephysiology of light action. IV. Light enhance-ment of auxin-induced growth in green peas.Plant Physiol. 26: 311-317. 1951.
3. GALSTON, A. W. and HAND, M. E. Studies on thephysiology of light action. I. Auxin and the lightinhibition of growth. Amer. Jour. Bot. 36: 85-94.1949.
4. KENT, MARTHA and GORTNER, W. A. Effect of pre-illumination on the response of split pea stems togrowth substances. Bot. Gaz. 112: 307-311. 1951.
5. MILLER, C. 0. Promoting effect of cobaltous andnickelous ions on expansion of etiolated bean leafdisks. Arch. Biochem. and Biophys. 32: 216-218.1951.
6. MILLER, C. 0. Relationship of the cobalt and lighteffects on expansion of etiolated bean leaf disks.Plant Physiol. 27: 408412. 1952.
7. SCHNEIDER, C. L. The interdependence of auxin andsugar for growth. Amer. Jour. Bot. 25: 258-270.1938.
8. THIMANN, K. V. Studies on the physiology of cellenlargement. Growth (Supplement) 15: 5-22. 1951.
APPLICABILITY OF POISEUILLE'S LAW TO EXUDATION OFOLEORESIN FROM WOUNDS ON SLASH PINE1, 2
C. S. SCHOPMEYER, FRANCOIS MERGEN AND THOMAS C. EVANSSOUTHEASTERN FOREST EXPERIMENT STATION. LAKE CITY, FLORIDA
Yields of oleoresin from wounds on slash pine(Pinus elliottii Engelm. var. elliottii) and of longleafpine (P. palustris Mill.) are known to vary greatlyamong trees which are morphologically similar, asshown by Cary (3) and Wyman (16). An estimated0.5% of the population consists of individuals capableof yielding two to three times more oleoresin than themean yield for the population, according to Downs(5). Inheritance of capacity to produce high yieldsof oleoresin has been demonstrated in longleaf pine byMergen (10), and breeding studies have been startedat this station by Dorman (4) and Downs (5) todetermine the possibilities of breeding strains of slashand longleaf pine having higher yielding capacitiesthan the average of the existing population. High
1 Received May 11, 1953.2 A preliminary report on the results of this work was
presented at the annual meeting of the Plant PhysiologySection, Association of Southern Agricultural Workers,February 4-6, 1952, in Atlanta, Georgia.
oleoresin yielding ability is probably attributable toseveral factors which, if measurable, could be used forselecting individual trees for crossing. The identifica-tion of such measurable factors is the purpose of thisstudy.Many characteristics of pine trees have been meas-
ured or suggested for possible correlation with thecapacity of the tree to yield oleoresin. These charac-teristics are listed in the following classification.
A. Morphological factors1. crown size (16)2. diameter of the bole (16)3. radial growth rate of the bole (16)
B. Anatomical factors1. size of tracheids (11)2. number of longitudinal resin ducts (8, 11)3. diameter of longitudinal resin ducts (11)
C. Physical and chemical factors1. turpentine content of oleoresin (2, 13)
2. osmotic pressure of epithelial cells sur-rounding resin ducts (11)
3. rate of absorption of water by epithelialcells (8, 11)
4. frictional resistance to flow on walls ofducts (8)
5. viscosity of oleoresin (8, 13)6. pressure on oleoresin in ducts (1, 8, 14)
Correlation between yields of oleoresin and themorphological factors listed above have been observedby Wyman (16), but the degree of correlation is notgreat. Unpublished data from a recent study at thisstation show that diameter and radial growth rateduring several years preceding oleoresin extraction ac-counted for about 40% of the variation in oleoresinyields from samples of slash and longleaf pine. Hencethese morphologieal criteria are not very reliable forselecting trees having high productive capacity foroleoresin.For the anatomical and physico-chemical factors,
quantitative data are not sufficient to determine thedegree of correlation with oleoresin yielding capacityof individual trees. All of these factors, however,have some conceivable effect on oleoresin yields.A logical selection of factors which may provide a
reliable index of yielding capacity is possible by set-ting up two hypotheses: 1. The flow rate of oleoresinfrom a wound during a short time interval may becontrolled by the same factors that determine the rateof flow of a liquid through a capillary tube in vitro.2. The flow rate of oleoresin from a tree during a shorttime interval may be correlated with the yielding ca-pacity of the tree. If these two hypotheses can beproved, the purpose of the study will be accomplished.The flow of a viscous liquid through capillary tubes
is quantitatively defined by the equation for Poi-seuille's Law as modified by Stokes (15) and Hagen-bach (7), which is cited by Gortner (6). For truesolutions, this equation is
V(t r4P8(1) ~~~~t8L77
where V = volume of flowt = time of flowr = radius of capillary
Po = hydrostatic pressurecapillary
L = length of capillaryv = viscosity of solution
on liquid entering the
Modifications of this equation are necessary forapplying it to exudation of oleoresin. A pressure P,was used which differs from P0 above in that P was
measured at the point of discharge from the capillaryresin ducts. Since P accounts for the reduction in Pocaused by the length L of the capillary, L can beomitted from the equation. This omission may causea change in the constant, 8. Only relative values forthe predicted flow rate are needed for accomplishingthe purpose of this experiment, so a constant K can
be substituted for 7r/8. The interconnected system oflongitudinal and radial resin ducts has multiple dis-
charge orifices on any wound surface, hence a factorN for number of exposed radial resin ducts per unitarea of tangential wound surface is added to the equa-tion. The transverse sections of the radial resin ductsobserved in this study were more elliptical than circu-lar, hence instead of r, the major and minor semi-axesof an ellipse, a and b are used. With these modifica-tions the equation becomes
N(2) V KP:(ab)2 Y KPN(ab)2
t o Ywhere Y is the yield of oleoresin during the first 24hours after the wounding and (ab)2 is the mean of themeasured ducts. The applicability of this modifiedequation was tested.
EXPERIMENTAL PROCEDUREThe 12 slash pine trees used in this test were grow-
ing on a low site with the water table at the surfaceof the ground at the time of the experiment. The siteis in the Olustee Experimental Forest in BakerCounty, Florida. The trees were marked at randomfor the test with the restriction that they be at least25 cm in diameter at breast height. The range indiameter was from 28 to 40 cm. Duplicate sets ofobservations were made on each of the 12 trees.FLOW RATE OF OLEORESIN: The flow of oleoresin
was measured on August 10, 1951. A steel punch wasused to make wounds 37 mm in diameter extendingthrough the bark to the xylem. Wounds were sprayedwith a 50% solution of sulfuric acid by weight to openthe radial resin ducts which are normally closed at thesurface of the xylem. Oleoresin was collected in flat-bottomed glass test tubes suspended on circular plasticfittings, as shown in figure 1 B. The diameter of thesefittings is the same as that of a steel punch used inmaking the wounds. Hence when the fitting is in-serted into the hole through the bark, a tight fit isobtained.Measurements of flow were started after a few
drops of oleoresin had drained into the test tube so asto allow time for the level of liquid in the plasticfitting to come to equilibrium. Yields were measuredgravimetrically to ± 0.01 gim. Although in the equa-tion tested, the yield unit is volumetric, substitutionof a gravimetric unit introduces a negligible error be-cause the specific gravity of oleoresin is only slightlygreater than 1.00. Yields were measured for about 24hours and the time of flow for each sample recordedto the nearest minute. Flow rates were then com-puted as gm/hr.
VISCOSITY: A major difficulty in determining vis-cosity of oleoresin is to get samples before the resinacids crystallize out of solution. To overcome thisdifficulty, samples were collected in a closed systemwhich minimized loss of the solvent (turpentine), andthe viscosity determinations were made on the day ofcollection. In this experiment, samples were obtainedat the beginning of the 24-hour period of flow meas-urements, from the same wounds on which flow rateswere determined. Samples weighing about 2 gm were
collected in small viscometer tubes (fig. 1 A) each ofwhich had been previously calibrated against a stand-ard. Collection time for viscosity samples was be-tween one and two hours for most trees, but on one,20 hours were required.
Viscosity was determined by the bubble rise methodwhich involves the time required for an air bubbleto rise through the liquid in a vertically mounted tube.The small tubes used in this test were calibratedagainst a standard Gardner viscometer tube in whichthe time of bubble rise is said to equal the viscosityin Stokes. Calibration of the standard tube for accu-racy of absolute units is not necessary for relativevalues and was not done. All samples were run in awater bath at 300 ± 0.02°C. The time required forbubble rise was repeated on each sample until threesuccessive observations were obtained which checkedwithin 2%.PRESSURE: Closed-end manometers were used for
measuring pressure of gum at the point of dischargefrom the resin ducts. The manometers were made
FIG. 1. (A) Installation on tree for measuring exuda-tion pressure of oleoresin with a closed-end manometerclamped against a cambium-depth wound (above) andthe receptacle for collecting samples of oleoresin for vis-cosity determinations. (B) Viscometer tubes shown in(A) are replaced with larger vials in which oleoresinyields are collected and weighed.
from heavy-walled capillary tubing with an inside di-ameter of about 2 mm and a length of about 250 mm.The exact length of the manometers was measured
after sealing one end and before making the 900 bendnear the open end. A millimeter scale cut from graphpaper was glued to the outside of the tube. Tubeswere dried at 110°C for 2 hours to remove moisturefrom the bore. The bores were kept dry in a desicca-tor until installed on the tree. This precaution was
taken to delay the occurrence of crystallization of theoleoresin which takes place rapidly in the presence ofwater.With a uniform bore within each manometer, pres-
sure in atmospheres on a liquid entering the open endof the manometer is the total length divided by thelength of the unfilled portion of the tube. The accu-
racy of this ratio was confirmed on two of the ma-
nometers using a deadweight gauge tester. The ratiois accurate only with a rising pressure. The film ofliquid remaining on the wall of the tube causes error
in the ratio when pressure is decreasing.For setting up the manometers, a smooth flat sur-
face about 10 cm in diameter was prepared on the
bark of the tree with the center at the desired pointof attachment. Selection of a point of attachment inthe center of a large bark plate was essential for agood seal. A circular wound 10 mm in diameter wasmade in the center of the prepared surface andsprayed with a 50% solution of sulfuric acid. Theopen end of the manometer was pushed through anumber 10 rubber stopper so that about 2 mm of thelength extended through the stopper. The open endof the manometer was placed in the wound and heldthere by pressure obtained with a system of cables,clamps, and stretchers as shown in figure 1.Although this method of measuring pressure of ex-
udingf oleoresin involves very little injury to the tree,it was not satisfactory because as soon as appreciablepressure was built up in the system, loss of pressureoccurred by penetration of oleoresin into the exposedxylem and bark. The phenomenon probably pre-vented the establishment of pressure in the manome-ter in equilibrium with that in the resin ducts.
In use, the manometers were set up during theafternoon of the day preceding the flow measurementsso as to show, as nearly as possible, the equilibriumpressure at the time of making the wounds for flowmeasurements.Maximum resin pressure in the diurnal cycle oc-
curred shortly after sunrise (between 5:30 and 6:30A.M.). This maximum pressure, although includingmuch experimental error, was used in the equation tofind out if it contributed anything at all to the flowrate. Subsequent pressure readings were made, butbecause of the loss of oleoresin from the system bypenetration of oleoresin into the xylem and throughexternal leaks which developed in a few of the sys-tems, these readings were useless.RESIN DUCT MEASUREMENTS: The material on
which radial resin duct measurements wvere to bemade, was collected in the field with the aid of a bowsarw, chisel and mallet. Two parallel saw cuts 2.5 cmapart were made first. The first cut was just abovethe lowest point of the punch wound. A block ofwood measuring 2.5 x 3.2 x 2.5 cm was then taken outwith the aid of the chisel and mallet. Immediatelyafter collection, the block was dropped into a bottlecontaining a formalin-acetic alcid-alcohol-water (13-7-40-40) fixing and killing fluid. All samples were leftin this reagent at least 24 hours.
Blocks suitable for a sliding microtome were saw-edoff and trimmed out with a knife. Tangential sec-tions, 20,u thick were cut with a sliding microtomeequipped with an automatic feed. The wood sampleand the knife wvere kept moist with 50% alcohol bymeans of a camelshair brush. About three times asmany sections as were actually needed were cut, sothat the best ones could be picked out for studyingafter staining and dehydration were completed.The freshly cut material was transferred to a dish
containing acetone for 5 minutes to dissolve the oleo-resin. The sections were first stained in Ehrlich'shaematoxylin and later counterstained with safraninusing methods described by Johansen (9). The stainedsections were trimmed off carefully with a microtome
knife so that their respective sides were parallel.Resin duct counts were made under 100 x magnifica-tion. A complete count was made of the radial ductswithin each section, starting at the lower left cornerand moving across to the right using the diameter ofthe field of vision as the width of each strip. The area
of each section was determined from measurementsmade with the mechanical stage.When examined, the transverse sections through
radial resin ducts were more elliptical than circular.Hence the major axis (2a) and the minor axis (2b)were measured including the thickness of the epithelialcells. In a tangential section from each wound, 8ducts were measured. Flow through any one duct isassumed to be proportional to (ab)2. Hence thisvalue was computed for each measured duct in thesection before computing the mean used in the equa-
tion. Expressed mathematically, this mean is8
(ab)2 = (ab)2
8
for each of the two wounds on each of the 12 trees.Where possible the axes of every fifth duct along
each strip were measured but where the fifth duct hadruptured epithelial cells as a result of sectioning, thenearest duct to it was measured. Measurements were
made under a magnification of 440 x with an ocularmicrometer in which each division was equal to 3 ,u atthe magnification used.
ANALYSES AND DISCUSSION OF RESULTSThe individual measurements obtained in this study
are listed in table I. For regression computations, thesums of the two values on each tree were used.The first step was to determine the significance of
each variable in equation (2). For convenience, thisequation was re-written
Y = KXlX2X3X4where X1 = 1/n, X2 = N, X3 = (ab)2, and X4 =P.Then the assumption was made that each variablehas an unknown power so the equation was rewrittenagain.
Tests of significance on each of the coefficients showedthat b1 and b2 are significant at the 1% level and thatb3 is significant at the 5% level. The coefficient b4 issignificant at the 25% level.Hence the observed values of N, (ab)2, and 1/4j are
each correlated with flow rate. Pressure measure-ments contributed nothing to the precision of the re-
gression because of the inadequate techniques used.Although accurate measurements of exudation pres-
sure of oleoresin were not obtained, the existence of
BLE IINDIVIDUAL MEASUREMENTS OF THE VARIABLES
TREE AND VISCOSITY NUMBER _ OBSERVED
WOUND FLOW OF OF DUCTS * (ab)2 EXUDATIONBATE OLEORESIN PER a 2b107 PRESSRDDESIGNATION (n1) (N) (P)
such pressure was demonstrated. Hence it is proba-bly a determining factor in rate of flow of oleoresin.To determine how much of the variation in flow
rate (in absolute units rather than logarithms) wasaccounted for by the three significant factors, the fol-lowing regression equation was set up with the pres-sure factor omitted:
Y =a + bX whereX= N(ab) 2
One solution of this equation is Y= - 0.367 + 2.864XThis solution is based on the assumption that the vari-ance of Y is proportional to X2.The deviations of the observed values from the line
representing this equation are shown in figure 2. Thecoefficient of determination is 0.83, which means that
the regression accounts for 83% of the variation of Yin the sample of 12 trees. A logical deduction is thatsome of the remaining 17% of the variance in the
3.5
> 3.0 _
2.5
2.0o
> 1.0w /O.5 -
0 .2 .4 .6 .8 1.0 1.2
X a "(b)2
FIG. 2. Deviations from the regression equation
Y' = -0.367 + 2.864 N (ab)2
sample trees could probably have been accounted forwith more precise measurements of pressure.Although the viscosity of gum and the size and
number of radial resin ducts are correlated with flowrate during the first 24 hours after wounding, theproduct of these independent variables can be an in-dex of oleoresin yielding capacity only if flow rate or
total yield during the first 24 hours after wounding is
correlated with the yielding capacity. The true yield-ing capacity of a tree is an abstract concept which, atthe present time, cannot be quantitatively defined.The actual yield of oleoresin obtained by working a
tree for one season with a specified technique is thebest available quantitative index of the yieldingcapacity.In 1952, individual yield records were made on 25
slash pine trees worked with a standard commercialchipping method. For the wound made on June 26 on
these trees the yield during the first 24 hours was de-termined. The coefficient for the correlation betweenthis one-day yield and the total yield for the season
is 0.440, which is significant at the 5% level. Hencethe product of the three independent variables isprobably correlated with the current annual yield ofindividual trees.
If the first correlation holds for a stratified sampleof high-yielding trees, the probability of getting prog-eny with oleoresin-yielding capacity greater than thatof either parent possibly could be increased by cross-ing a tree rating high on one factor with another treerating high with respect to another factor. If allthree factors are independently inherited, successiveselections and crosses may result in trees rating highon all three factors. Such trees may have an oleo-resin-yielding capacity several times greater than thatof the existing population.
SUMMARYThe variables in a modification of the equation for
Poiseuille's Law for flow of liquids through capillarieswere measured in the resin duct system of 12 slashpine trees (Pinu.s elliottii Engelm. var. elliottii). Themodified equation is given.
Regression analysis showed that the product ofthree of the independent variables, i.e., the reciprocalof the.viscosity of oleoresin, number and size of resinducts, accounted for 83% of the variation in flow ratebetween trees. Pressure could not be measured accu-rately because of the loss of pressure by penetrationof oleoresin into the xylem behind the enclosed woundto which manometers were attached. Hence the im-provement in fit of the regression line from the inclu-sion of pressure values in the product of the inde-pendent variables was not significant.Flow rate of oleoresin during the first 24 hours after
wounding was correlated with annual yield of oleoresinwhich is the only available index of yielding capacity.The three factors, N, (ab)2, and -, therefore may be
useful criteria for selection of parent trees to be usedin producing progeny having a high oleoresin yieldingcapacity.
LITERATURE CITEDARBuzov, A. E. The process of flow of oleoresinfrom some species of the conifer family. Bull.Inst. Pin No. 37, pp. 137-139. Chem. Abstr. 21:3754. 1927.
?. BLACK, A. P. and THRONSON, S. M. Oleoresin fromindividual trees of slash and longleaf pines. Ind.and Eng. Chem. 26(1): 66-69. 1934.
3. CARY, AUSTIN. Studies on flow of gum in relationto profit in the naval stores industry. NavalStores Review 43(17): 6, 8, 17. 1933.
4. DORMAN, KEITH W. Better pines for turpentine.American Forests 53(11): 498-500. 1947.
5. DowNs, ALBERT A. Developing better pines for gumproduction. Southern Lumberman 179(2249):233-236. 1949.
6. GORTNER, R. A. Outlines of Biochemistry, pp. 45-55.John Wiley & Sons, Inc., New York. 1938.
7. HAGENBACH, E. tVber die Bestimmung der Ziihigheiteiner Fliussigkeit durch den Ausfluss aus R6hren.Prog. Ann. Phys. Chem. 109: 385-426. 1860.
8. IVANOV, L. A. Scientific principles underlying the
technique of streaking pines. Federal Sci. Inst.for Research in Forest Management. Contribu-tions in Forest Research 1: 1-55. 1930. Transl.from the Russian by L. J. Pessin.
9. JOHANSEN, D. A. Plant Microtechnique. McGraw-Hill. 1940.
10. MERGEN, FRANgoIs. Gum yields in longleaf pine areinherited. Research Notes No. 29. Jan., 1953.Southeastern Forest Experiment Station, Ashe-ville, North Carolina.
11. MUNCH, ERNST. Naturwissenschaftliche Grundlagender Kiefernharznutzung. Arbeiten aus der Bio-logischen Reichsanstalt fur Land- und Forstwirt-schaft. 10(1): 1-140. 1919-1921.
12. NIKOLAEV, N. F. and SINELOBOV, M. A. The influ-
ence of chemicals on gum yields. Forest ChemicalIndustry No. 6: 4-9. 1936. Translated from theRussian by L. J. Pessin.
13. RUNCKEL, W. J. and KNAPP, I. E. Viscosity of pinegum. Ind. and Eng. Chem., Ind. Ed. 38: 555-556.May, 1946.
14. SOLODKI, TH. and VASSKOUSKAYA, T. Experimentsin tapping. Forest Chemical Industry 2(8): 36-43. 1933.
15. STOKES, G. G. On the theories of the internal fric-tion of fluids in motion and of the equilibrium andmotion of elastic solids. Trans. Camb. Phil. Soc.8 (Pt. III): 287-319. 1847.
THE EFFECTS OF THREE LEVELS OF MAGNESIUM ON THE NUTRIENT-ELEMENTCOMPOSITION OF TWO INBRED LINES OF CORN AND ON THEIR
SUSCEPTIBILITY TO HELMINTHOSPORIUM MAYDIS 1, 2
GEORGE A. TAYLORDEPARTMENT OF HORTICULTURE, PENNSYLVANIA STATE COLLEGE, STATE COLLEGE, PENNSYLVANIA
This study was undertaken to ascertain the effectsof different levels of magnesium on the nutrient-ele-ment balance of two inbred lines of corn and its rela-tionship to the incidence of leaf blight caused byHelminthosporium maydis. The influence of nutritionon the severity of plant diseases has received con-siderable attention (3, 7, 15,16, 17, 18, 20, 21, 22, 23).Wingard (25) gives a general review of the literaturepertaining to this subject. In general, the literaturesupports the view that high concentrations of nitrogenpredispose plants to disease, while high potassiumtends to retard disease development. Many conflict-ing cases, however, may be found. Some of the dis-crepancies may be caused by inadequate knowledgeconcerningf the nutritional status of the plants understudy, as reflected by leaf composition, while othersmay be caused by differences in the nutriti6nal re-quirements of various pathogens.
Investigations dealing with the effect of magnesiumon the susceptibility of plants to disease are limitedto a few papers. Bledsoe et al. (3) reported a rela-tionship between the magnesium content of leaf tissueand the incidence of peanut leafspot. They foundthat plants grown at low magnesium levels had lessmagnesium in the leaf tissue and developed aboutfifty times more leaf spots than did plants grown athigh magnesium levels. A fung,us, Mycosphaerellaarachidicola W. A. Jenk, was isolated from the leafspots, leading them to believe it was responsible forthe disease.
Differences between inbred lines of corn with respectto leaf-blight susceptibility are well established (19,24). Little is known, however, concerning differences
1 Received November 17, 1952.2Authorized for publication November 12, 1952, as
paper No. 1768 in the Journal Series of the PennsylvaniaAgricultural Experiment Station.
in their nutrient requirements. Using solution culturetechniques, Sayre (14) found that growth curves ofvarious inbred lines differed although grown at thesame magnesium level. It has been noted from fieldobservations that some inbred lines exhibited visualdeficiency symptoms, while others growing nearby didnot (10).Problems involving magnesium deficiency in crop
plants can be expected to become more numerous inthe future (1). Since little is known regarding theeffect of magnesium on disease susceptibility and thenutrition of different inbred lines of corn, it is im-portant that further information be obtained.
MATERIALS AND METHODSTwo inbred lines of corn, Illinois Hy and Ohio 41,
were grown in a greenhouse in nutrient solutions con-taining three levels of magnesium, while all other ele-ments were kept as constant as possible. A prelimi-nary experiment was carried out to determine thelowest and the highest concentrations which did notproduce deficiency or toxicity symptoms respectively.To determine these levels both inbred lines were grownin solutions to which the following amounts of mag-nesium were added: 0, 3, 5, 10, 25, 45, 60, 75, 100, 250and 500 ppm. The sole purpose of the preliminaryexperiment was to study the response of both inbredsto a wide range of magnesium levels and to select thelevels most appropriate for further experiments.
This information was used in setting up the mainexperiment. Three levels of magnesium were chosen:3 ppm (low), 75 ppm (medium), and 300 ppm (high).The experiment consisted of four replications; mag-nesium treatments and inbred lines were randomizedwithin each replication. After the plants had devel-oped under the differential magnesium treatments forabout 12 weeks, they were inoculated with cultures
