Soybean Leaflets Research Articles

Heliotropic Leaf Movement Response to H + /ATPase Activation, H + /ATPase Inhibition, and K + Channel Inhibition in Vivo

The pulvinus, located at the base of soybean leaflets, is both the light perception and motor organ for heliotropic leaf movements. Our objective was to investigate the role of plasma membrane H+/ATPase and TEA-sensitive K+ channels in mediating pulvinar response to light. The plasma membrane H+/ATPase activator, fusicoccin, plasma membrane H+/ATPase inhibitors, vanadate and erythrosin-B, and the K+ channel blocker TEA were introduced to the intact pulvinus through the transpiration stream. The pulvinus was illuminated by a vertical light beam of 1,400 μmol m-2 s-1 to stimulate leaf movement. Leaf orientation was measured every 5 min for 60 min of illumination. All compounds tested inhibited pulvinar bending, but concentration and uptake time required for inhibition varied: 12.5 μM fusicoccin reduced leaf movement after 3 hr uptake, 2 mM vanadate reduced leaf movement after 6 hr uptake, 100 μM erythrosin-B reduced leaf movement after 3 hr uptake, and 15 mM TEA reduced leaf movement after 6 hr uptake. In all cases final leaf angle was reduced by higher concentrations and/or increased time for uptake of the chemical into the pulvinus. Results support the hypothesis that the proximal mechanism of heliotropic movement is similar to that of nyctinastic movements.

HELIOTROPIC LEAF MOVEMENT RESPONSE TO H+/ATPase activation, H+/ATPase inhibition, AND K+ CHANNEL INHIBITION IN VIVO

The pulvinus, located at the base of soybean leaflets, is both the light perception and motor organ for heliotropic leaf movements. Our objective was to investigate the role of plasma membrane H+/ATPase and TEA‐sensitive K+ channels in mediating pulvinar response to light. The plasma membrane H+/ATPase activator, fusicoccin, plasma membrane H+/ATPase inhibitors, vanadate and erythrosin‐B, and the K+ channel blocker TEA were introduced to the intact pulvinus through the transpiration stream. The pulvinus was illuminated by a vertical light beam of 1,400 μmol m‐2 s‐1 to stimulate leaf movement. Leaf orientation was measured every 5 min for 60 min of illumination. All compounds tested inhibited pulvinar bending, but concentration and uptake time required for inhibition varied: 12.5 μM fusicoccin reduced leaf movement after 3 hr uptake, 2 mM vanadate reduced leaf movement after 6 hr uptake, 100 μM erythrosin‐B reduced leaf movement after 3 hr uptake, and 15 mM TEA reduced leaf movement after 6 hr uptake. In all cases final leaf angle was reduced by higher concentrations and/or increased time for uptake of the chemical into the pulvinus. Results support the hypothesis that the proximal mechanism of heliotropic movement is similar to that of nyctinastic movements.

An in vivo Acetolactate Synthase Assay

A method was developed and tested for in vivo assay of acetolactate synthase (ALS). The method used foliar application of 1,1-cyclopropanedicarboxylic acid (CPCA) to inhibit ketol-acid reductoisomerase, the enzyme immediately following ALS in biosynthesis of branched-chain amino acids, thereby causing accumulation of acetolactate. Since the amount of acetolactate accumulation is a function of carbon flux through ALS, quantification of acetolactate accumulation determined ALS activity. Accumulation of acetolactate in soybean leaves resulted from CPCA rates as low as 15 g/ha and occurred within 1.5 h. Accumulation rates in soybean leaflets declined with leaf age from 84 μg/h/g tissue at 3 d to 17 μg/h/g tissue at 7 d. Foliar application of CPCA also caused acetolactate accumulation in corn, grain sorghum, velvetleaf, common cocklebur, and smooth pigweed. The ability of the in vivo assay to quantify the reduction in ALS activity following applications of ALS-inhibiting herbicides was validated by comparing ALS activity following thifensulfuron application to ‘Williams 82’ soybean, which has a sulfonylurea-sensitive ALS, and ‘Asgrow 3200’ soybean, which has a sulfonylurea-insensitive ALS. Thifensulfuron reduced ALS activity in Williams 82 soybean to 0, 0.8, 3.3, and 15.6% of the CPCA control at 6, 12, 24, and 48 HAT, but ALS activity in Asgrow 3200 soybean was reduced only to 34, 40, 57, and 88% of the CPCA control.

Photosynthetic acclimation and photosynthate partitioning in soybean leaves in response to carbon dioxide enrichment

Photosynthetic rates and photosynthate partitioning were studied in three-week-old soybean [Glycine max (L.) Merr. cv. Williams] plants exposed to either ambient (35 Pa) or elevated (70 Pa) CO2 in controlled environment chambers. Ambient CO2-grown plants also were given a single 24 h treatment with 70 Pa CO2 1 d prior to sampling. Photosynthetic rates of ambient CO2-grown plants initially increased 36% when the measurement CO2 was doubled from 35 to 70 Pa. Photosynthetic rates of the third trifoliolate leaf, both after 1 and 21 d of elevated CO2 treatment, were 30 to 45% below those of ambient CO2-grown plants when measured at 35 Pa CO2. These reduced photosynthetic rates were not due to increased stomatal resistance and were observed for 2 to 8 h after plants given 1 d of CO2 enrichment were returned to ambient CO2. Initial and total ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities, percent activation, Rubisco protein, soluble protein and leaf chlorophyll content were similar in all CO2 treatments. Quantum yields of photosynthesis, determined at limiting irradiances and at 35 Pa CO2, were 0.049±0.003 and 0.038±0.005 mol CO2 fixed per mol quanta for ambient and elevated CO2-grown plants, respectively (p<0.05). Leaf starch and sucrose levels were greater in plants grown at 70 than at 35 Pa CO2. Starch accumulation rates during the day were greater in ambient CO2-grown plants than in plants exposed to elevated CO2 for either 1 or 21 d. However, the percentage of C partitioned to starch relative to total C fixed was unaffected by 1 d of CO2 enrichment. The above results showed that both photosynthetic and starch accumulation rates of soybean leaflets measured at 35 Pa CO2 were temporarily reduced after 1 and 21 d of CO2 enrichment. The biochemical mechanism affecting these responses was not identified.

Selection of soybean plant leaves which yield mesophyll cell isolates with maximal rates of CO2 and NO inf2 sup? photoassimilation

A problem often encountered when assaying mesophyll cell isolates prepared from mature soybean leaves, was that of poor reproducibility in rates of net (14)CO2 photoassimilation and NO2 (-) photoreduction. It was known that soybean source leaves repeatedly displayed their most active net CO2 photoassimilation in the period from attainment of maximal leaf area to approximately two to five days subsequent to that point. Advantage was taken of the fact that when soybean leaflets of each leaf reach their maximal area they also have reached their maximal leaf length from base to tip. This facilitates a more rapid determination of the point in time in which leaflet areas had reached Amax. Soybean plants (Glycine max cv. Williams) were propagated in the growth chamber with a 12 h light-12 h dark cycle, 25δC, 65% RH, and 700 microeinsteins per meter squared per second. At 24 d post-emergence, the third leaf (numbered acropetally from the unifoliates) of each plant had just attained maximum leaflet areas (≈110 cm(2)) and lengths (≈13 cm). For this study, leaf mesophyll cells were enzymatically isolated, using commercially prepared pectinase, from leaflet sets of leaves selected from each of the second, third, and fourth leaf positions. Maximal rates of net (14)CO2 photoassimilation (with 5 mM HCO3 (-)) for the second, third and fourth leaf (leaflet) isolates were, respectively, 27.0, 57.0, and 41.7 μmol (14)CO2 assimilated per milligram chlorophyll per hour; simultaneously maximal rates of NO inf2 (sup-) photoreduction (1 mM NO inf2 (sup-) ) were, respectively, 4.4, 8.1, and 0.0 μmol NO inf2 (sup-) reduced per milligram chlorophyll per hour. These studies made it clear that in order repeatedly to attain reproducible maximal rates of leaf cell isolate net (14)CO2 photoassimilation and NO inf2 (sup-) photoreduction, it always was necessary to select the newest, fully expanded leaves (e.g. leaf number 3) for cell isolation. Leaves from several plants only were pooled if they were excised from identically the same node on each of the plants.

Feeding Activity and Mortality of Permethrin-Resistant and Susceptible Soybean Looper (Lepidoptera: Noctuidae) Larvae on Soybean Foliage Treated with Insecticide

Third and fifth instar soybean loopers, Pseudoplusia includens (Walker), from three field strains resistant to permethrin and a susceptible laboratory strain were exposed individually to untreated soybean leaflets or leaflets treated with field rates of permethrin, thiodicarb, and Bacillus thuringiensis Berliner (var. kurstaki ). Leaf area was measured before a larva was placed on a leaflet. Leaf area and mortality were measured 24, 48, and 72 h after a larva was exposed to treated and untreated foliage. Mortality of permethrin-resistant larvae on foliage treated with permethrin was significantly lower than that observed for susceptible larvae. Thiodicarb and B. thuringiensis were equally effective against permethrin-resistant and susceptible larvae after 72 h, but mortality was delayed with resistant larvae. All insecticide treatments significantly reduced leaf area consumption by both permethrin-resistant and susceptible larvae regardless of instar, compared with consumption when they fed on untreated foliage. However, permethrin-resistant third and fifth instars consumed significantly more foliage treated with permethrin compared with any other larvae on treated foliage in most experiments. Reductions in consumption by resistant larvae on foliage treated with permethrin ranged from 42 to 68% and 55 to 76% for third and fifth instars, respectively, compared with consumption on untreated foliage.

氷核活性細菌Ｐｓｅｕｄｏｍｏｎａｓ ｓｙｒｉｎｇａｅの氷核活性に及ぼす培養温度の影響について

The study was conducted to clarify the effects of cultural temperature on the activities of INA (ice nucleation-active) bacteria Pseudomonas syringae.INA bacteria were cuttured at 5 levels of temperature (0, 10, 20, 25 and 30°C) for 1 to 5 days of cultural periods respectively. The soybean leaflets contacted with the suspension of these INA bacteria cultured under above cultural conditions. The ice nucleation activities were evaluated through the freezing temperatures of water droplets suspended with INA bacteria and of soybean leaflets spread with INA bacterial suspension.There were no large differences between the freezing temperatures of water droplets suspended with INA bacteria cultured under the temperature condition of 20°C and below. However, the freezing temperatures of droplets suspended with INA bacteria cultured at 25 and 30°C decreased with the cultural duration.On the other hand, the freezing temperatures of soybean leaflets also dependent on cultural temperature, although they were generally lower than those of water droplets. The ice nucleation activities of the INA bacteria cultured under 25 and 30°C were almost nil.

Variations in Serologically Detectable Antigen of Soybean Dwarf Virus in Soybean Leaflets as a Function of Time After Inoculation and Plant Age

Two enzyme-linked immunosorbent assay (ELISA) systems were developed for detection of soybean dwarf virus (SDV) and compared for sensitivity. With direct double-antibody sandwich (DAS) ELISA, dilution of conjugated immunoglobulins with uninfected tissue extracts prepared with phosphate buffered saline (PBS) (1:5, w/v) improved absorbance compared with conjugates prepared with PBS alone. The apparent amplification of the positive signal occurred without changing nonspecific (background) absorbance. Indirect antigen-coated ELISA detected the dwarfing strain (SDV-D) but was much less sensitive than DAS-ELISA (...)

Effects of CO2 Concentration on Rubisco Activity, Amount, and Photosynthesis in Soybean Leaves

Growth at an elevated CO(2) concentration resulted in an enhanced capacity for soybean (Glycine max L. Merr. cv Bragg) leaflet photosynthesis. Plants were grown from seed in outdoor controlled-environment chambers under natural solar irradiance. Photosynthetic rates, measured during the seed filling stage, were up to 150% greater with leaflets grown at 660 compared to 330 microliters of CO(2) per liter when measured across a range of intercellular CO(2) concentrations and irradiance. Soybean plants grown at elevated CO(2) concentrations had heavier pod weights per plant, 44% heavier with 660 compared to 330 microliters of CO(2) per liter grown plants, and also greater specific leaf weights. Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activity showed no response (mean activity of 96 micromoles of CO(2) per square meter per second expressed on a leaflet area basis) to short-term ( approximately 1 hour) exposures to a range of CO(2) concentrations (110-880 microliters per liter), nor was a response of activity (mean activity of 1.01 micromoles of CO(2) per minute per milligram of protein) to growth CO(2) concentration (160-990 microliters per liter) observed. The amount of rubisco protein was constant, as growth CO(2) concentration was varied, and averaged 55% of the total leaflet soluble protein. Although CO(2) is required for activation of rubisco, results indicated that within the range of CO(2) concentrations used (110-990 microliters per liter), rubisco activity in soybean leaflets, in the light, was not regulated by CO(2).

Automated Microanalysis for Reducing Sugars, Sucrose, and Starch in Single Soybean Leaflets

Abstract An automated enzymatic hydrolysis procedure was developed to analyze aqueous leaf extracts for soluble carbohydrates in the presence of chlorophyll. Following an initial estimate of free reducing substances in the extracts from individual soybean {Glycine max [L.] Merr. 'Braxton') leaflets, separate portions were incubated with invertase or amyloglucosidase. The increase in ferricyanide reduction after incubation with the enzyme was assumed to represent the concentration of sucrose or starch, respectively, in the original extract. Use of an autoanalyzer operating in a double-beam mode compensated for interference from chlorophyll and other endogenous pigments. Extraction efficiency was checked by successive boiling-water extractions, and enzymatic incubation conditions were verified by a series of internal calibration standards. After 3 successive boilingwater extractions of § min each, more than 95% of the soluble sucrose was removed from the leaflets. Although the conditions of these procedures were insufficient for quantitative removal of all soluble starch present in the soybean leaflet, they represent a compromise that yields an aqueous extract which is amenable to enzymatic assay while avoiding excessive degradation of labile carbohydrates during the extraction process. This method allowed for a rapid comparison between drought-stressed and non-stressed soybean leaflets, which were collected during field experiments and stored in a laboratory freezer for later analysis of osmotic potential and soluble carbohydrates, e.g., glucose, fructose, sucrose, and starch.

Effect of planting date and growth stage on secondary and micronutrient content of soybean tissue

Abstract Field experiments were conducted to determine the effect of planting date and cultivar on B, Zn, Mn, Fe, Mg, and Ca content of soybean leaflets, petioles, and stems at beginning bloom (R1), beginning seed (R5) and physiological maturity (R7) growth stages. The concentrations of all the nutrients analyzed were influenced by planting date and cultivar in at least one year. The concentration ranges were: B, 16–167 ppm; Zn, 14–98 ppm; Fe, trace‐575 ppm; Mn, 12–122 ppm; Mg, 0.18–0.89%, and Ca, 0.47–3.02%. As the plants developed, [Fe] generally decreased, [Mn] increased, and [Zn] showed no consistent response. Changes in B, Mg, and Ca concentrations with plant development varied with the plant part. Leaflet [Ca] and [B], and petiole [Ca] and [Mg] increased during seedfill. Petiole and stem [B], and stem [Mg] and [Ca] decreased during seedfill.

Nitric Oxide and Nitrous Oxide Production by Soybean and Winged Bean during the in Vivo Nitrate Reductase Assay

This study was conducted to determine by gas chromatography (GC) and mass spectrometry (MS) the identity and the quantity of volatile N products produced during the helium-purged in vivo NR assay of soybean (Glycine max [L.] Merr. cv Williams) and winged bean (Psophocarpus tetragonolobus [L.] DC. cv Lunita) leaflets. Gaseous material for identification and quantitation was collected by cryogenic trapping of volatile compounds carried in the He-purge gas stream. As opposed to an earlier report, acetaldehyde oxime production was not detected by our GC method, and acetaldehyde oxime was shown to be much more soluble in water than the compound(s) evolved from soybean leaflets. Nitric oxide (NO) and nitrous oxide (N(2)O) were identified by GC and GC/MS as the main N products formed. NO and N(2)O produced from soybean leaflets were both labeled with (15)N when (15)N-nitrate was used in the assay medium, demonstrating that both were produced from nitrate during nitrate reduction. Other compounds co-trapped with NO and N(2)O were identified as air (N(2), O(2)), CO(2), methanol, acetaldehyde, and ethanol. Leaves of winged bean, subjected to the purged in vivo NR assay, evolved greater quantities of NO and N(2)O (13.9 and 0.37 micromole per gram fresh weight per 30 minutes, respectively) than did the soybean cv Williams (1.67 and 0.09 micromole per gram fresh weight per 30 minutes, respectively). In both species NO production was dominant. In contrast, with similar assays, NO and N(2)O were not evolved from leaves of the nr(1) soybean mutant which lacks the constitutive NR enzymes. In addition to soybean cv Williams, six other Glycine sp. examined evolved significant quantities of NO((x)) (NO and NO(2)). Other species including Neonotonia wightii (Arn.) Lackey comb. nov., Pueraria montana (Lour.) Merr., and Pueraria thunbergiana Benth. evolved lower levels of NO((x)).

Susceptibility of the Velvetbean Caterpillar and Soybean Looper (Lepidoptera: Noctuidae) to Nomuraea rileyi: Effects of Pathotype, Dosage, Temperature, and Host Age

Comparisons among four Nomuraea rileyi isolates tested against Anticarsia gemmaatalis and Pseudoplusia includens demonstrated distinct differences in both the susceptibility of the two noctuid hosts and the virulence of strains. Host susceptibility to N. rileyi infection was directly influenced by larval age. Maturation of host larvae resulted in increased Lc50 values and in the mean days to death of the N. rileyi strains. The in vivo developmental rate of N. rileyi was affected by the ambient temperature. Temperatures below the calculated optima (26°C) for N. rileyi decreased the developmental rate of the infection process but did not alter the percent N. rileyi-induced mortality of treated A. gemmatalis larvae. A. gemmatalis, with a developmental optimum of 31°C, became increasingly resistant to N. rileyi infection when reared at temperatures above 26°C. The assay and food substrate influenced both the development of the host and the relative virulence of N. rileyi to A. gemmatalis larvae. Comparisons of concentration and time responses demonstrated that larvae assayed on the soybean leaflets were more resistant to N. rileyi infection than larvae assayed on agar substrate and fed artificial diet. TheA. gemmatalis feeding on mature soybean foliage (pod set stage) appeared to be more resistant to N. rileyi infections than did the larvae feeding on prebloom and bloom soybean stages.

Oxygen and Carbon Dioxide Effects on the Pool Size of Some Photosynthetic and Photorespiratory Intermediates in Soybean (Glycine max [L.] Merr.)

The levels of ribulose 1,5-bisphosphate (RuBP), 3-phosphoglyceric acid (PGA), glycolate, glycine, and serine were measured in soybean leaflets during photosynthesis in atmospheres ranging from 1 to 60% O(2) and from 0 to 500 microliters per liter CO(2.)The RuBP level remained constant as CO(2) concentration was decreased in atmospheres containing 20 or 60% O(2), but increased as CO(2) concentration was decreased in atmospheres containing 1% O(2.) PGA levels decreased at CO(2) concentrations near or below the CO(2) compensation point under all O(2) concentrations. The glycolate pool at 300 microliters per liter CO(2) increased slightly with increasing O(2) concentration, but remained nearly constant at very low CO(2). The serine pool showed no measurable change over the range of CO(2) or O(2) concentrations tested. The glycine pool did not change significantly with varying CO(2) concentration but increased linearly with increasing O(2) concentration.Measured RuBP levels indicate an RuBP concentration less than the estimated concentration of RuBP carboxylase/oxygenase active sites. The constant RuBP pool size in 20% O(2), however, indicates that RuBP level does not limit photosynthesis or photorespiration any more at 50 microliters per liter CO(2) than at 450 microliters per liter.

Nozzle Size-Pressure and Concentration Combinations for Heliothis zea Control with an Aqueous Suspension of Polyvinyl Alcohol and Baculovirus heliothis12

Droplet sizes produced by a TX-1 nozzle operated at 552 kPa produced higher mortality of bollworm larvae, Heliothis zea (Boddie), for a given technical deposit than 2 larger size nozzles operated at lower pressures when an aqueous suspension of polyvinyl alcohol (PVA), UV protectant (Shade), and Baculovirus heliothis was applied to soybean leaflets in laboratory tests. For this formulation-TX-l nozzle combination, virus concentrations of 9.63 × 108 and 19.3 × 108 polyhedral inclusion bodies (PIB)/liter suggested biological superiority to lower concentrations. The concentration of 9.63 × 108 PIB/liter possibly would be preferred when considering plant coverage under field situations. The mortality-deposit response for the PYA and Shade treatment at 9.67 × 108 PIB/liter was similar to that for the water-surfactant formulation at 2.42 × 108 PIB/liter.

Effects of Artificial and Insect Defoliation on Soybean Net Photosynthesis1

Greenhouse and field trials were conducted during 1974–75 to determine the effects of artificial and insect defoliation on soybean net photosynthesis. Net carbon exchange rates were measured by using an excised-leaflet technique. Cork-borer, paper-punch, and along-the-midrib leaflet bisections adequately simulated defoliation of soybean leaflets by Plathypena scabra (F.) and Cynthia cardui (L.). Across-the-midrib leaflet bisections increased net photosynthesis and, thus, did not adequately simulate defoliation by these insects. Net carbon exchange rate was significantly reduced 12 h postdefoliation but was not significantly different from the check 24 h postdefoliation.

The Soybean Looper: Pupal Weight, Development Time, and Consumption of Soybean Foliage

Pseudoplusia includens (Walker) reared singly on soybean leaflets at 29.4 @+ 1.5@*C consumed 81.96 cm of leaflet area. Approximately 97% of the consumption occurred during the last 3 stadia. Development time from eclosion to pupation was 13.74 days. Mean pupal weights for larvae reared in the laboratory on leaves or artificial diet were less than those of field collected larvae.

Orientation and Distribution of Leaves Within Soybean Canopies1

Abstract‘Chippewa 64’ and ‘Hark’ soybean [Glycine max (L.) Merr.] varieties were studied at Lamberton and St. Paul, Minn. The average leaf sizes, numbers, and total leaf area index were determined during the growing season. The distribution of leaf area with height was obtained in the field and in a controlled environment chamber. The chamber‐grown plants exhibited a distribution similar to the area distribution of a cylindrical cross‐section. The field‐grown soybeans had most of the leaf area near the upper portions of the plant canopy and did not have a cylindrical distribution. These variations in leaf area distributions are largely explained by differences in the light intensities at the lower canopy levels with the different growing conditions.Measurements of the azimuth directions of the soybean leaflets revealed that soybean leaves do not show any preferred direction. Leaf inclination distributions showed that soybean canopies were planophile through most of the growing season, except in the early and lategrowth stages, when the leaves were almost evenly distributed among all inclination classes. The Hark variety had smaller and slightly more vertically oriented leaves than the Chippewa 64 variety, which would allow greater penetration of light to the lower canopy levels.

Ovipositional Response of Plathypena scabra (Lepidoptera: Noctuidae) to Selected Surfaces1

The ovipositional response of Plathypena scabra (F.), the green cloverworm, was determined by counting eggs laid on natural and artificial surfaces presented to moths. Natural surfaces included leaves of alfalfa, clover, and soybean. Artificial surfaces included rough yellow blotter paper, various treatments to rough yellow blotter paper (amyl acetate, ink containing triacetin, water), and 4 blotter-paper colors (red, yellow, blue, green). Leaf pubescence had an important influence on oviposition. Significantly more eggs were laid on lower surfaces of alfalfa and clover leaflets, where there was a significantly higher leaf-hair density. Pubescent soybean leaflets showed no significant difference between eggs laid on upper and lower surface, but over 20 times more eggs were laid on pubescent than on glabrous soybean. The pubescent leaf surface was simulated by roughening blotter paper with a wire brush. More than 10 times more eggs were laid on roughened than on nonroughened paper. Amyl acetate-treated surfaces seemed to have little effect, as did blotter-paper color. Ink- and water-treated blotter paper showed ovipositional abnegation. Of the ovipositional factors studied, surface texture had the greatest stimulating effect. The relationship between leaf-hair density, a measure of texture, seemed curvilinear, but further studies would be required to define this relationship more precisely.

Beta Gauging Leaf Water Status: Influence of Changing Leaf Characteristics

Since the intvroduction (3) o,f the technique of beta gauging as a method for continuous and nondestructive mon!itoriing the water status of leaves, severcal reipo,rts (1-4) have appeared verifying the technique and its appl,ication in physio;logical research. Impliciit in this applicatiioni o?f beta gauging is the assutmption that, except for diurinal variations in water content, the physical characteristics of the leaf at the time it is removed for calibralbio,n are not sign1ifican-tly differenit fronm those that prevai,l while leiaf vater content is being mon,i(tored. In other wordis, the fuincitional reliati,on between beta-ray absorption by the leaf and some measutre of leaf water stress estiabl'iished wihleni the leaf is calibrated s'houlld not be dcifferent from thait whiich prevailed while the leaf was attached to the plant. If the atitached leaf is monitioreTl for onily severall hours there is liittle probability that eirrors in prediction wou,ld arise fr,o!m c'hianges in leaf comiposition, and it is reasonable to ass,ume that variatiolns in beta-ray attenualtion are soilely the result of vari,ations in mass of water per uniit area of leaf surface. At the time of its proposal, the belta gauge technique was envisioned as one thatt woulld enable monitoring oif leaf-water status for a period of at least 24 hours or perhiaps for several days. In thle course of calibrating, that is, determining tihe regression cutrve relating relative water contenit, R, (5) to count rate for a large nutmiber of soybean leaflets we observed that under conditions of constaant source-gaulge geometry, the curves folr a group oif youtng, developing leaflets were significantly differeniit from those fo,r older, inore mature leaflets. Youtng leaflets have characteristically less dry matter and less water per unit area at saturation contenit (100 % R) and shrink more with lo,ss of water thfan matuire, ful,ly devel,oped leaflets. We hypothesized thait because there is a rapid, progres).aive change in the physical characteristics of youing developing leaflets they shoutld not be monitored fo!r mo,re than a few hours befoire removing the leaflet for calibration. But mature leaflets with either constant or onily slowily changing physical characteristics would provide sites for