10
The Use of the 11 C Technique to Measure Plant Responses to Herbivorous Soil Nematodes Author(s): D. W. Freckman, K. R. Barker, D. C. Coleman, M. Acra, M. I. Dyer, B. R. Strain and S. J. McNaughton Source: Functional Ecology, Vol. 5, No. 6 (1991), pp. 810-818 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/2389545 . Accessed: 02/10/2013 19:08 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Functional Ecology. http://www.jstor.org This content downloaded from 128.153.5.49 on Wed, 2 Oct 2013 19:08:31 PM All use subject to JSTOR Terms and Conditions

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The Use of the 11C Technique to Measure Plant Responses to Herbivorous Soil NematodesAuthor(s): D. W. Freckman, K. R. Barker, D. C. Coleman, M. Acra, M. I. Dyer, B. R. Strain andS. J. McNaughtonSource: Functional Ecology, Vol. 5, No. 6 (1991), pp. 810-818Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2389545 .

Accessed: 02/10/2013 19:08

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

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Page 2: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

Functional Ecology 1991, 5, 810-818

810

TECHNICAL REPORT

The use of the 11C technique to measure plant responses to herbivorous soil nematodes

D. W. FRECKMAN*, K. R. BARKERt, D. C. COLEMANt, M. ACRA?, M. I. DYERt, B. R. STRAIN? and S. J. McNAUGHTON? *Department of Nematology, University of California, Riverside, California 92521, JDepartment of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27695, *Department of Entomology and Institute of Ecology, University of Georgia, Athens, Georgia 30602, ?Department of Botany, Duke University, Durham, North Carolina 27706 and /Biological Laboratories, Syracuse University, Syracuse, New York 13210, USA

Abstract. An advantage of the short half-life (20_3 min) "C isotope method is the simultaneous measurement of several crucial plant processes without hindering or destroying whole-plant function. The "C technique was used to examine the effects of the herbivorous soil nematode, Meloidogyne javanica (Treub) Chitwood, on carbon assimilation and redistribution in tomato (Lycopersicum esculentum L.) and in two eco- types [grazing adapted (GA) and non-grazing adapted (NGA)], of an African C4 grass, Panicum coloratum L. Preliminary tests, based on nematode eggs and galls, showed that M. javanica repro- duced differentially on both P. coloratum ecotypes and suppressed GA shoot growth (weight), but had little effect on fresh root weight.

In tomatoes, control plants had a higher pool of labile C in the roots than the nematode-infected plants. With P. coloratum, there were no differ- ences in "C parameters between ecotypes. Plants with herbivorous nematodes had a lower stem sink activity, root activity and root transport speed than control plants. The technique proved valuable for measuring instantaneous changes in photo- synthate accumulation and movement induced by herbivorous soil nematodes. Key-words: Carbon allocation, phytophages, root herbi- vory, soil nematodes

Introduction

Understanding the effects of herbivores such as

phytophagous nematodes on plant-carbon allo- cation and flux to metabolic, structural and storage components is central to both ecological and agricultural science. Several techniques have been employed to quantify plant responses to these parasites including total biomass, changes in root/ shoot ratios, galling and changes in reproductive structures (Barker, 1985). Molecular and bio- chemical research have increased our knowledge of carbon utilization in cells (Hyman, 1990). Quan- titative models have been developed to explain the effects of soil phytophagous nematodes on net photosynthesis and plant growth (Barker & Noe, 1987; Noling, 1987). Infra-red gas analysis has been used for determining the instantaneous effect of environmental changes on photosynthetic carbon gain by continuous, non-destructive measures of CO2 exchange (Wallace, 1974).

The transport of carbon is measured usually by time-course studies of changes in pool size or movement of R-emitting, radioactive 14C. This technique requires harvesting plants destructively and measuring changes between significant time intervals (McClure, 1977; Bird & Loveys, 1980). The "C method instantaneously measures changes in plant carbon pools on a live, real-time basis (Strain et al., 1983; Coleman et al., 1989; Wang et al., 1989; Dyer et al., 1991). As a tracer, 1"C has the advantage of having sufficient energy (15 GBq mol') to allow detection in the decaBec- querel range through several centimetres of plant tissue in vivo (Fares et al., 1978; Strain et al., 1983). The short half-life (203 min) of the isotope permits simultaneous measures of transfer rates of labile sugar pools and starch sinks throughout the leaf, stem and roots (Table 1, Fig. 1) without hindering whole-plant function.

Our recent work has described the effects of above-ground herbivores, grasshoppers (Dyer et al., 1991) and plant symbionts, VA mycorrhizas, (Wang et al., 1989) on carbon allocation in two ecotypes of an African grass. The purpose of this study was to:

1 Examine the responses of plants to herbivorous nematodes using the 1"C technique. 2 Determine the usefulness of the technique for

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Page 3: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

811 110C2 Source

'IC technique Leaf

and Fixed "CO2 (LTR) 0 herbivorous nematodes StarC< Labile

sink (LSS) sugar poolX(LSP)

1C Export to stem Stem ? (ER)

Starch Labile sink SS) sugar

Pool (SSP)

1C Export to root

Root (RTS)

Labile Labile sugar sugar

pool (RTA) pool (RTA)

Starch Sarc sink (RTA) sink (RTA)

Fig. 1. A flow diagram (after Dyer et al., 1991) showing the three-part model used as the basis for assessing carbon dynamics in plants. The 11C activity in labile sugar pools and starch sinks of the leaves, stems and roots was computed from the analytical model of Mag- nusson, Goeschl & Fares (1986). Nos. 1-6 show locations of paired scintillation detectors (see also Table 1 for key to each variable and its unit measure and Fig. 3). For the short-term 11C analyses after carbon is fixed, labile sugar components are either converted to starch or translo- cated into phloem tissue in the stem and root.

assessing differential effects of herbivorous nema- todes on carbon accumulation and movement in selected plants.

Materials and methods

Two-week-old tomato (Lycopersicum -esculentum L.) seedlings were used for the first 1"C experi- ments. Seedlings were grown at 260C in a fumi- gated loam-sand (v:v 1:1) soil in pots in greenhouses at North Carolina State University. Nutrients (1/4 strength Hoagland's and Peters' nutrient solution) were applied in excess daily to prevent accumulation of salts.

Two populations (ecotypes) of the African savannah C4 grass Panicum coloratum L. (McNaughton, 1983) were used in the second experiments (Wang et al., 1989). The two Panicum ecotypes, termed grazing adapted (GA) and non- grazing adapted (NGA) (McNaughton, 1983; Dyer et al., 1991) have been maintained for the last 10 years under greenhouse conditions at Syracuse University imimcking the Serengeti environment.

In July 1986, 12 plants of each population were transported to the Duke University Phytotron, Durham, North Carolina. The plants were grown in a controlled environmental chamber with a 12:12 photoperiod, a light intensity of 600 ,mol m-2 s-' at leaf level and a 30 0C day/15 0C night temperature schedule. The light levels were consistent with levels in the 1"C testing facility (Wang et al., 1989). Plants were watered to field capacity twice daily, once with one-half strength Hoagland's nutrient solution and once with deionized water and were transplanted to new containers every 2 months.

These plants had been isolated without their indigenous nematode herbivores (Martin, 1955, 1961), which are quarantined from the USA. To select a USA nematode population that repro- duced on both grass ecotypes, eggs of five popula- tions of Meloidogyne arenaria (Neal) Chitwood and three populations of M. javanica (Treub) Chitwood were inoculated onto individual plants of the two ecotypes. Eight replicates were used for each plant. Eight weeks later, the M. javanica population from Georgia reproduced most rapidly on both ecotypes, as measured by eggs and gall development. This nematode was then increased on tomato and on both ecotypes of P. coloratum. Based on a greenhouse experiment using four inoculum levels (Fig. 2), there were no significant differences in final nematode population densities or root weight in the GA vs NGA plants. Inoculum density affected final population densities and root weights. These data indicated that both eco- types were tolerant to M. javanica. An inoculum level of 200000 eggs per plant was chosen for subsequent experiments.

For the 11C experiments, plants were carefully matched for uniform size. The roots were separated equally into each side of a 'split-root' system, which was 'sandwiched' between two pieces of plexiglass (Fig. 3). The 'soil' medium was as described previously for pots. Each half-root section was contained by plastic wrap with a few small holes at the base to allow for drainage. All plexiglass containers were covered with alumi- nium foil to prevent light from reaching soil and roots. There were two treatments, nematode treated (nematodes on one side of the split-root) and control (no nematodes on either side of the split-root). All nematode inoculations were made on tomato and P. coloratum plants 14 days after transplanting. Nematode-treated plants were inoculated with NaOCl-extracted eggs (Barker, 1985) by pipette into a plastic centrifuge tube with a terminal orifice. The centrifuge tube was placed into the soil onL one side of the split-root system

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Page 4: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

812 (a) D. W. 12000 - Grazing Adapted

Freckman et o 10000 T 0 Non-grazing Adapted

al. 3c00-c 1

LL 6000-

'~4000-

2000 -

0 0 1000 10000 100000

Inoculum Level

(b) 700 -

600-

500 -

400-

0 o 300-

.~200-

LI. 1 00

0 0 1000 10000 100000

Inoculum Level

Fig. 2. (a) Numbers of eggs g-1 fresh root weight resulting from four initial population levels (0, 1000, 10000 and 100000 eggs plant-1) of the herbivorous nematide., Meloidogyne javanica on two Panicum coloratum eco- types, grazing adapted (GA) and non-grazing adapted (NGA). (b) Effect of initial population levels of M. javanica on fresh root weight of the two P. coloratum ecotypes. The nematodes were inoculated as eggs and plants grown for 110 days. Data are means of eight replicates. Vertical bars = 1 SE.

(Fig. 3). For control plants, water only was inoculated into one side of a split-root system. The age of the plants from transplanting to 1"C testing ranged from 40 to 120 days old depending on the experiments outlined below.

After about 30 days, paired plants, control (no nematodes on either side of the split-root) and nematode-treated (nematodes on one side of the split-root) plants, were transferred from the growth facilities to the 1"C apparatus in a con- trolled environmental chamber for acclimation 24h prior to testing (Strain et al., 1983). A small water-cooled plastic cuvette (6 cm2) was clamped on the third leAf of each plant. Six pairs of scintillation detectors were placed along each plant, four on tillers and two on roots in the split-root container (Fig. 3). For both grasses and tomato, the source of the 1"CO2 was the third leaf on the right side of the plant, detector no. 1 (Fig. 3). Detector no. 6 was positioned on the split-root on the same side of the plant as detector no. 1.

The next day, initial steady state CO2 exchange

rates at 335 p-p-m. CO2 were obtained for each plant at 2 7 0C using a Horiba PIR 2000 Infra Red Gas Analyzer (IRGA). The "C-labelled CO2 was pro- duced at the Duke University Medical School Cyclotron and transported to and used in the Duke Phytotron. Initial steady state C fixation and trans- location rates were established with the introduc- tion of "C-labelled CO2 [see Magnuson et al. (1982) and Strain et a]. (198.3) for general 1"C design and details; Wang et a]. (1989) for specific details about P. coloratum experiments]. After equilibration (c 120 min) measures of parameters (Table 1) were taken continuously for up to 2 h. Carbon movement through the plants was recorded with a minute-by-minute integration time.

Tomato experiments

Two experiments involved testing of control and nematode-infected tomato plants. In one test with

11C Real-Time Carbon Flow Detection

1C02

Cuvette

\ /0~~2 /-Panicum coloratum

Location of detectors (six pairs total)

Acrylic plastic 'sandwich' with split-root design

--Centrifuge tube for nematode inoculation

Fig. 3. Diagram of the ̀ C experiment (modified from Dyer et al, 1991). 11C-tagged CO2 is delivered to a temperature-regulated cuvette connected to an Infra Red Gas Analyzer (IRGA). Six pairs of scintillation detectors are located along the tagged leaf, the stem and at two locations in a split-root container [see Wang et a]. (1989) for additional details]. Two of these modules, each containing carefully matched plants of either tomato or the ecotypes of P. coloratum were prepared for each experiment.

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Page 5: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

813 Table 1. Variables used in 11C experimental approach to examine responses of plants to soil herbivorous nematodes 11C technique under laboratory conditions. See model in Fig. 1 for additional details. and

Variable Abbreviation Units herbivorous -

nematodes Leaf Carbon exchange rate (net photosynthesis) CER ,imol m2 s- Stomatal conductance (water gradient) SC mmol m-2 s- Total rate 11C fixed [detectors 1+(2-3)] LTR Bq m-2 S- Leaf starch sink (11C at detector 1) LSS Bq m-2 S- Export rate (11C at detectors 2-3) ER Bq m-2 S- Leaf sugar pool (11C within cuvette) LSP Bq m-2 S-

Stem Stem starch sink (steady state stem - SSP) SSS Bq cm-' axis cm-2

leaf Stem sugar pool (11C at detectors 3-4) SSP Bq cm-1 axis cm-2

leaf Leaf to stem, average transport speed ('1C at detectors 2-4) ATS cm min-

Root and root transport Stem to root - transport speed-5 (11C from 4 to 5) RTS-5 cm min- Stem to root - transport speed-6 (11C from 4 to 6) RTS-6 cm min- Root total activity-5 (11C at detector 5) RTA-5 Bq cm-1 s-1 Root total activity-6 (11C at- detector 6) RTA-6 Bq cm-1 s-1

the nematode-infected plants, the nematodes weie inoculated on the split-root side (detector no. 5) opposite the source of 11CO2. In the second, the nematodes were inoculated on the same (detector no. 6) split-root side as the source of 11CO2. The plants were 42 days old from transplanting to the 1"C testing, and the nematodes were inoculated 14 days following transplanting, as described pre- viously.

P. coloratum experiments

A series of three tests were conducted with P. coloratum.

Series no. 1. The first series on Panicum was with the NGA ecotype, ? nematodes as treatments, with two replicates each. These plants were 100 days old from transplanting to the 1"C testing. Nema- todes were inoculated on the same side (no. 6) as the 1"CO2 source.

Series no. 2. The second series of plants was with the GA and NGA ecotypes, approximately 40 days old. In this experiment whole-plant effects were considered. Analysis of variance for the four replicates of the control and nematode treatments did not compare the - split-roots of each plant individually.

Series no. 3. The final test was run on the GA ecotype only with five replicates of the ? nema-

tode treatment. The plants were 120 days old. Nematodes were inoculated at the split-root (detector no. 5) opposite the source of 11CO2. After this 1"C test was completed, the plants were harvested immediately and fresh shoot and root weight from each side of the split-root chambers determined.

Results

Tomato experiments

In the test (Fig. 4a and b) with nematodes inoculated on the split-root side (detector no. 5) opposite the source of 1"CO2, the photosynthate activity was greatly reduced in the leaf (detectors no. 2 and 3). At the roots of control plants (Fig. 4a), most of the 1"C moved to the split-root on the same side (detector no. 6) as the source (detector no. 1). There was only a small amount of 1"C at detector no. 5, the split-root side opposite the source of 1"CO2. Fig. 4b illustrates the activity of the isotope in a nematode-infected plant. There was more carbon activity (Fig. 4b) in the nematode side of the split-root (detector no. 5) than the non-nematode side (detector no. 6), and considerably more than in the control-plant roots (Fig. 4a) opposite (detector no. 5) the 1"CO2 source.

Results of tests of control tomato plants and plants with nematodes inoculated to roots on the same side (detector no. 6) as the 1"CO2 source are shown in Table 2. Only the tomato root total

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Page 6: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

814 (a) SD BK

D. W. 2000- CR SR

P70 1 B T

Freckman et 40

al. 2 -0

1500 - source

Bq2 1000

4~~~~~~~~~~~~~

500- 61

SD BK

- W ~~~~~~~~~~~~~~~CA SR

2000 000R

Time ~ ~ ~ ~ ~ ~~~P0 R TTs

40

1500 --11co02

1000 2 3

4

2 5 ~~~~~~~~~~~~~~~~0 6

500- 5 ~~~-Nematode side

C 240 300 ~~~~~~~~~~~~~~~~~0 I 0 30 60 90 120 150 180 21020 27030 330 1

Time (mns)

Fig. 4. Graph showing minute by minute movement of "C at detectors 1-6 [see Coleman et al. (1989) and Dyer et al. (1991) for further explanation]. Detector no. 1 is the source of 11CO2 which is supplied to the leaf at time zero and stopped when the plant reaches steady state at all scintillation counters about 90-120 min later. Results are shown from the control (no nematodes) plant (a), and the nematode-infected plant (b). Note the increased activity (Bq) at the nematode-inoculated split-root (detector no. 5) (b). The data are corrected for background, detector sensitivity, specific activity and the mean steady state transit time from detector no. 1 to each detector location.

activity (RTA) was significantly affected by nema- tode parasitism. The control-plant root total activity at detector no. 5 was greater than detector no. 5 on the non-inoculated side of the nematode plant. The root activity (RTA) at detector no. 6 was lower than that of detector no. 5 for both control and nematode plants. The nematode-inoculated

side of the split-root had less carbon activity than the control.

P. coloratum experiments

Series no. 1. Paired t-test analyses indicated no significant differences between the 1O0-day-old'

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Page 7: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

815 Table 2. Comparison of variables measured using the "C technique on control and nematode-inoculated tomatoes for llC technique preliminary experiments. The nematodes were inoculated on roots (detector no. 6) on the same side as the 11CO2 source and (n = 4). See Table 1 for description of abbreviations and units. herbivorous Means t-test (one-tail) nematodes

Control + SE Nematode + SE Paired t Probability Leaf CER 13-3 ? 0 13-9 ? 0-8 NS SC 516-0 ? 0 445-5 ? 0 7 NS LTR 14-5 ? 0-4 15-1 ? 2-4 NS LSS 7-2 ?+0-1 8-1 ? 1-5 NS ER 7-3?+05 6-9?+0-9 NS LSP 28263 ? 3415 24322 ? 5973 NS

Stem SSP 95+?1-4 9-1+?0-2 ATS 1-9 ? 0-02 2-0 ? 0-2 SSS 1-0 ? 04 0-2 0-2 4-1 0-076

Root and root transport RTS-5 1-4 ? 0-1 1-6 0-3 RTS-6 1-5 ? 0-5 1-4 0-4 RTA-5 4-5 ? 0-4 1-9 ? 0-5 53-0 0-006 RTA-6 1-0 ? 0*5 0-6 ? 0-6 3*9 0-079

NS indicates not significant.

Table 3. (a) ANOVA comparisons of carbon allocation and photosynthate transport on two ecotypes of 40-day-old Panicum coloratum, grazing adapted (GA) or non-grazing adapted (NGA), as determined by the "C technique. Plants were inoculated with nematodes or nematode-free (control). Nematodes were inoculated on the same side (detector no. 6) as the 11C02 source (detector no. 1). Data are means of four replicates (n = 8). Refer to Table 1 for units of variables .measured.

Treatment Ecotype

Variables F PR > F Control Nematodes GA NGA

Leaf CER 4-2 0095 15-1 12-6 LTR 7-7 0 039 4-85 5-45 ER 4-3 0-092 2-98 3-59 LSP 9-9 0-025 7540 8944

Root RTS-4-6 5-0 0-075 3-63 1-32 RTA-6 5-2 0-072 5-04 0-78

Table 3. (b) ANOVA comparisons for nematodes on young (40-day-old) and old (100-day-old) and both ecotypes of P. coloratum plants tested with "C. Nematodes were inoculated on the same side (detector no. 6) as the 11CO2 source (detector no. 1). Data are means of six replicates (n = 12).

Treatment Age

Variables F PR > F Control Nematodes Young Old

Leaf ER 8-4 0-03 0-52 0 23 3 28 4-28

Root RTS-4-6 7-5 0-41 2-69 1.11 RTA-6 11-8 0-02 7-33 1-34 RTA-6 4-7 0-81 2-91 718

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Page 8: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

816 Table 4. Carbon allocation and photosynthate transport due to grazing by nematodes on the grazing-adapted (GA) D. W. ecotype of Panicum coloratum as determined by the "C technique. Nematodes were inoculated on the opposite side Freckman et (detector no. 5) of the "CO2 source (detector no. 1). Data are means of five replicates (n = 10). All variables except shoot

al. weight (SW) and root:shoot weight (RSW) are divided by root:shoot weights. Refer to Table 1 for units of variables measured.

Treatment

Variables F PR > F Control Nematodes

SW 74 0-027 28 2 18-2 RSW 12-0 0009 4-8 10-6

Leaf CER 9-7 0 014 2-96 1-38 SC 15-5 0-006 23 6 10-9 LTR 20-8 0-0002 0-73 0-33 LSS 14-9 0 005 0 22 0-10 ER 22-0 0-002 0-52 0 23

Stem ATS 8-3 0-021 0-25 0-11

Root RTS-4-5 14-1 0-006 0-16 0-05 RTA-5 14-2 0-006 0 53 0-17

nematode-inoculated and control plants. Trans- formation of the data to log(x+ 1) showed the root total activity at detector no. 6 (the nematode-ino- culated side) to be significantly greater on the nematode-infected plant (F = 16-8, PR > F= 0.055) (mean = 6 1), than the control plant (mean = 3.7).

Series no. 2. Results indicated six significant (P < 0.10) variables (Table 3a). The only difference between the 40-day-old ecotypes was a higher CER on the GA ecotype, consistent with previous studies (Wang et al., 1989). Transport differences between ecotypes noted earlier (Wang et al., 1989; Dyer et al., 1991) were not as obvious as the present experiments. Plants with nematodes (Table 3a) had a greater rate of 1"C fixed in the leaf (LTR), export rate and leaf sugar pool (LSP) of photosynthate, but stem-to-root transport speed and the root total activity pool (RTA-6) at detector no. 6 were greater on the control plants. Although the stem starch sink activity (SSS) was not signifi- cant (PR > F = 0-194), the mean of the nematode- plant starch sink activity (7-8) was almost twice that of the control (4.0).

An analysis of variance on all plants from series nos. 1 and 2 (Table 3b) indicated that leaf export rate and root total activity at detector no. 6 were greater for the older 100-day-old vs 40-day-old plants. Stem to root transport speed from detectors nos. 4-6, and the root total activity at no. 6 were

higher for control plants. Other parameters were notstatistically significant.

Series no. 3. Major differences were evident in plant biomass between nematode-infected and control plants (Table 4). Shoot weight was greater on control plants (F = 7 4, PR > F = 0 027, mean = 28-2g fresh weight) compared to the nematode- inoculated plants (18.2g). Because of the differ- ences in root and shoot weights, and the corres- ponding effect on all plant variables measured by the 1"C technique, the variables were divided by root:shoot biomass to standardize and minimize variability between plants.

Carbon allocation and transport were suppressed in the presence of nematodes in GA ecotypes (Table 4). The control plants had signifi- cantly (P = 0-006) greater 11C stored in the leaf/unit area (leaf starch sink), a greater rate of photo- synthate exported and a larger export pool of photosynthate. Root total activity levels were greater on the control plant than at the nematode- infected detector no. 5 site. The greater amount of the root total activity (RTA) on the control plant, compared to the nematode-infected side of a split-root, was consistent with all but the first experiments on the older, NGA ecotypes.

Discussion Greenhouse experiments with varying inoculum levels of nematodes showed limited differences in

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Page 9: The Use of the11C Technique to Measure Plant Responses to Herbivorous Soil Nematodes

817 nematode reproduction on the two P. coloratum 11C technique ecotypes. Galls produced were small, but egg and masses were present. The mild root gall and foliar herbivorous symptoms observed on the native plants indicate nematodes that P. coloratum was highly tolerant to M.

javanica in contrast to the responses on the cultivated crop, tomato. This differential response is an example of native plants having tolerance to environmental stresses and coincidentally, nema- tode parasites, as suggested by Wallace (1987b). He noted native plants might have a general tolerance to nematodes, which is lost through agricultural cultivation (1987a).

This study and others have shown decreased host-photosynthetic rates with nematode herbi- vory, although the exact mechanism remains unclear. Our results with tomato are similar to those of Loveys & Bird (1973) who noted a decreased net photosynthetic rate in Meloidogyne- infected plants. This decrease was maintained for the duration of the plant's growth. Wallace (1974) observed a decreased photosynthetic rate and incorporation of 14C02 into Meloidogyne-infected tomato roots compared to a nematode-free control. He suggested that the effect of metabolic siiiks created by these parasites did not have a major effect on photosynthesis and shoot growth and that the inhibition of upward translocation of water was a greater factor in suppressing shoot growth.

We had expected that the nematode-infected plants would have greater rate of transport and a higher sink activity, root total activity and root transport speed than the control plant. Giant cells induced by Meloidogyne spp. generally act as sinks sequestering plant host photosynthate (McClure, 1977). Infection by Meloidogyne spp. results in disrupted vascular tissue which presu- mably affects water and _ photosynthate trans- location (Wallace, 1987a). The opposite response, however, occurred in our experiments. The plants infected with nematodes generally had small root galls, lower root activity and movement of carbon than did the control plant. These results apparen- tly vary depending on plant age, nematode density and reproduction time of the nematodes. We observed initial effects of nematode parasitism which over time contribute to reduced root bio- mass and diminished plant function.

Wallace (1987a) noted the limited research on effects of nematode herbivores on photosynthesis. Our results, combined with studies on plant growth and development (Potvin, Goeschl & Strain, 1984; Potvin, Strain & Goeschl, 1985; Welker et a]., 1985) as affected by mycorrhizas

(Wang et al., 1989) or grasshoppers (Dyer et al., 1991) have identified a carbon-isotope technique useful for determining plant responses to soil herbivores. The technique has the sensitivity to measure several plant processes simultaneously without hindering plant function. However, its availability is limited because the 1"C isotope, with its short half-life, requires immediate accessi- bility to a cyclotron.

Acknowledgments

This project was supported by National Science Foundation Grant BSR-86-00605 to D.C. Coleman, M.I. Dyer, D.W. Freckman and S.J. McNaughton. We thank Drs J. Coeschl, G. Wang and S.R. Koenning for their assistance during this project.

References

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Received 20 August 1990; revised 20 February 1991; accepted 10 March 1991

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