PLANTÐINSECT INTERACTIONS Wheat Stem Sawfly, cinctus Norton, Impact on Wheat Primary Metabolism: An Ecophysiological Approach

1 TULIO B. MACEDO, ROBERT K. D. PETERSON, DAVID K. WEAVER, AND WENDELL L. MORRILL

Department of Entomology, 333 Leon Johnson Hall, Montana State University, Bozeman, MT 59717Ð3020

Environ. Entomol. 34(3): 719Ð726 (2005) ABSTRACT The impact of wheat stem sawßy, Norton (: ), on the photosynthetic capacity and primary metabolism of wheat, Triticum aestivum L., was evaluated in three different environments: environmental growth chamber, greenhouse, and Þeld. C. cinctus elicited different photosynthetic responses in different environments. Wheat gas exchange param-

eters, such as photosynthesis, stomatal conductance, intercellular CO2, and transpiration in the growth chamber environment were negatively affected by C. cinctus feeding. Conversely, the same gas exchange responses were not observed under greenhouse and Þeld conditions. This study shows the

important role of environmental variables, such as ambient CO2 concentrations and light intensity, on plant responses to herbivores.

KEY WORDS Triticum aestivum, herbivory, photosynthesis, eco-physiology, injury guilds

INJURY BY MANY HERBIVOROUS species has long (as traditionally had been done). Furthermore, Peter- been known to have a negative impact on plant pho- son and Higley (2001) argued that similarities in plant tosynthesis and yield in both agricultural and natural response to speciÞc injury types (also known as injury systems (Welter 1989, Peterson and Higley 1993, guilds) can be used effectively to address many basic Delaney and Macedo 2001, Peterson 2001). Charac- and applied research questions. terizing primary metabolic mechanisms, such as gas The most complete studies of arthropod injury and exchange, underlying plant responses to arthropod plant gas exchange responses involve the leaf-mass injury is needed to adequately explain Þtness and yield consumption (defoliation) injury guild (Peterson loss and develop general models of plant response 2001, Peterson et al. 2004). Other injury guilds, such as (Peterson 2001, Peterson and Higley 2001). Plant gas assimilate sapping, mesophyll feeding, and leaf mining, exchange processes, such as photosynthesis, water- have been studied much less. Virtually nothing is vapor transfer, and respiration, represent a subset of a known about plant physiological responses to the plantÕs primary metabolic processes. Consequently, stem-boring injury guild. understanding how arthropod injury inßuences these Heichel and Turner (1973) and Madden (1977) parameters is important because these are the primary suggested that stem boring impaired photosynthetic processes determining plant growth, development, capacity primarily because of injury to vascular tissue. and, ultimately, Þtness (Peterson and Higley 1993). Godfrey et al. (1991) observed reductions in photo- Furthermore, these processes respond very rapidly to synthesis of corn injured by European corn borer, external factors, so their measurement provides an Ostrinia nubilalis (Hu¨ bner). In particular, they noted immediate indication of plant stress (Peterson and 10Ð20% photosynthetic rate reductions associated Higley 1996, Peterson et al. 1998, Macedo et al. 2003). with larval stem boring and concluded that the re- Unfortunately, the physiological mechanisms un- ductions may have been related to alterations of derlying plant responses to arthropod injury are still source-sink relationships. Rubia et al. (1996) observed poorly understood (Peterson and Higley 1993, 2001). compensatory mechanisms leading to an increase in Progress in understanding plant physiological re- photosynthesis of borer-injured rice tillers. sponses to arthropod injury resulted primarily from a Plant primary metabolic responses to the wheat consideration of injury types. Boote (1981), Pedigo et stem sawßy, Cephus cinctus Norton, a larval stem al. (1986), and Higley et al. (1993) emphasized the use borer, have not been examined. C. cinctus is consid- of categorizing plant biotic stressors based on injury ered the most important pest of dryland wheat type and response rather than on the taxonomic clas- in the northern Great Plains. In Montana alone, losses siÞcation of stressors or physical appearance of injury by C. cinctus are approximately U.S. $25 million per year. To date, most of the research surrounding C. cinctus is 1 Corresponding author, e-mail: [email protected]. directed at aspects of detection and management.

0046-225X/05/0719Ð0726$04.00/0 ᭧ 2005 Entomological Society of America 720 ENVIRONMENTAL ENTOMOLOGY Vol. 34, no. 3

However, understanding photosynthetic responses of replicates. The treatment design consisted of two wheat to C. cinctus also is of considerable practical and treatments: noninfested wheat (control) and C. cinc- theoretical interest. tusÐinfested wheat (infested). Experimental units In Montana, C. cinctus adults emerge from late May consisted of individual pots, each containing four to late June, but emergence may continue until the wheat plants. The plants and C. cinctus were main- third week of July (Criddle 1915, 1923, Wallace and tained in environmental growth chambers (Conviron) McNeal 1966, Weiss et al. 1990, Weiss and Morrill 1992, at 21 Ϯ 1ЊC, photoperiod of 14:10 (L:D) h, with a light Morrill and Kushnak 1996), which is synchronized intensity of 63 ␮mol photons/m2/s at plant level and with host plant developmental stage that is suitable for 40Ð50% RH for the duration of the study. Supplemen- C. cinctus oviposition (developmental stage 32Ð69) tal light consisted of ßuorescent lamps installed at a 3 (Zadoks et al. 1974). Generally, the uppermost inter- Cool White:1 Gro-Lux ratio (F96T12/CW/1500, nodes of an elongating stem are preferred for ovipo- F72T12/CW/1500, F96T12/GRO/VHO, and F72T12/ sition (Holmes and Peterson 1960). Each female typ- GRO/VHO; Sylvania Lighting Center, Dancers, MA). ically lays only one egg per stem (Ainslie 1920), but Gas exchange parameters, such as photosynthesis egg laying by more than one female per stem is com- (Photo), transpiration (E), and stomatal conductance

mon. (gs) rates were recorded from one leaf, ßag-leaf, on Larval development may include four or Þve instars each plant using a portable photosynthesis system (Ainslie 1920), and at the onset of plant maturation, (model LI-6400; Licor, Lincoln, NE) at 1,200 ␮mol 2 ␮ larvae chew a notch around the inside perimeter of the photons/m /s light intensity, 400 mol/mol CO2 con- stem near the soil level. Infested stems usually lodge, centration, and a constant ßow of 500 ␮mol/s. A and the broken stems are highly visible to farmers smaller subset of leaves (i.e., four leaves/treatment)

during harvest (Ainslie 1920). Neonate larvae initially was used to determine CO2 response curve (A-Ci) and feed on parenchyma tissue near the oviposition site light response curve. A-Ci response curves were per- (Holmes 1954). Developing larvae disperse through- formed at constant light intensity (1,500 ␮mol pho- 2 out the stems, and stem nodes are injured during the tons/m /s) and varying the intercellular CO2 (Ci) movement. Disruption of vascular tissues that are con- concentration at 50, 100, 200, 300, 400, 600, 800, and stricted at the nodes can result in an accumulation of 1,000 ␮mol/mol. material that appears as darkened areas below nodes Light-response curves or photosynthetic photon (Morrill et al. 1992b). Stems are Þlled with “frass,” ßux density (PPFD) response curves were performed consisting of fecal material and chewed plant tissue. at constant intercellular CO2 (Ci) concentration (400 ␮ ␮ Estimation of the reduction of grain resulting from mol/mol CO2) and constant ßow (500 mol/s) and larval feeding is complicated because ovipositing fe- varying light intensity at 2,000, 1,500, 1,000, 500, 200, males prefer the largest stems that have the potential 100, 50, 20, and 0 ␮mol photons/m2/s. Plant gas ex- to produce comparatively more grain (Morrill et al. change parameters were measured weekly, 17, 24, 31, 1992a). However, estimates of loss range up to 25% 38, and 45 d after removal of the plants from the cages. (Morrill et al. 1992a). Responses of A to PPFD and responses of assimilation

The objective of this study was to characterize the (A) to internal CO2 concentration (Ci) were analyzed impact of injury by the stem borer, C. cinctus, on for each treatment by nonlinear regression (Layne primary metabolism of wheat. To do this, we con- and Flore 1995). The best-Þt curve was evaluated by 2 ⌫ ducted studies during 2002, 2003, and 2004 in three analysis of r . The CO2 compensation point ( ) was different environment settings: growth chamber, extrapolated as the Ci at which assimilation was greenhouse, and Þeld. 0.0 ␮mol/m2/s. Estimated carboxylation efÞciency (k) was calculated as the slope in the linear portion of the A-C response curve. Ribulose-1,5-bisphosphate Materials and Methods i (RuBP2) regeneration rate limitations were expressed Study 1. In 2002 (4 June to 19 July), we conducted by a reduced assimilation at saturating CO2 concen- a growth chamber experiment at the Plant Growth trations (Farquhar and Sharkey 1982). Carboxylation

Center, Montana State University-Bozeman. Spring efÞciencies from assimilation versus Ci were deter- wheat, ÔMcNealÕ, was grown in 10.15-cm pots in a mined through linear regression analysis. Homogene- mixture of Sunshine soil mix and sand mix (1:1 ratio) ity of the independent regression was evaluated by in a environmental growth chamber (Conviron, Win- StudentÕs t-test (Steele and Torrie 1980). nipeg, Canada) under a photoperiod of 14:10 (L:D) h. Study 2. In the summers of 2003 and 2004, we con- Plants were watered regularly and fertilized twice per ducted a total of three greenhouse experiments at the week with a 100-ppm mix (Peters 20Ð20-20 General, Plant Growth Center, Montana State University-Boz- Scotts, Marysville, OH). To infest plants, four large eman. Spring wheat, ÔMcNealÕ, was grown, watered, Plexiglas cages (95 by 46 by 46 cm), each containing and fertilized following procedures described in study four pots of wheat at developmental stage 32 (Zadoks 1. To infest plants in the Þrst (13 May to 23 June 2003) et al. 1974), were infested with 20 male and 20 female and second (16 May to 20 June 2003) experiments, C. cinctus adults. were allowed to mate and four large cages (95 by 46 by 46 cm), each containing oviposit freely for a period of 7 d, after which plants four pots of wheat at developmental stage 32 (Zadoks were removed from the cages and arranged in a ran- et al. 1974), were infested with 20 male and 20 fe- domized complete block design (RCBD) with four male C. cinctus adults. Insects were allowed to mate June 2005 MACEDO ET AL.: WHEAT PHOTOSYNTHETIC RESPONSE TO C. cinctus INJURY 721

␮ and oviposit freely for a period of 7 d, after which tion rate (E) were measured at 400 mol/mol CO2 plants were removed from the cages and arranged in concentration and 1,200 ␮mol photons/m2/s light in- a RCBD with four replicates. The treatment design tensity. consisted of two treatments: noninfested wheat (con- To measure chlorophyll a ßuorescence, a leaf trol) and C. cinctusÐinfested wheat (infested). Exper- chamber ßuorometer (model LI-6400Ð40; Li-Cor) imental units consisted of individual pots, each con- was used. We performed a kinetic test with the pri- taining four wheat plants. The plants and sawßy were mary objective of determining the photochemical ef- maintained in a greenhouse bay at 21 Ϯ 1ЊC, photo- Þciency of photosystem II (PSII) in determining all period of 14:10 (L:D) h, and 40Ð50% RH for the du- ßuorescence parameters, such as the nonvariable

ration of the study. The light intensity in the green- ßuorescence (Fo), the overall photochemical quan- house at plant level was Ϸ334 ␮mol photons/m2/s tum yield (Y), the apparent photosynthetic electron during the experiment. Light supplement consisted of transfer rate (ETR), and the quenching coefÞcientsÑ GE Multi-Vapor lamps (MVR1000/C/U; GE Lighting, nonphotochemical quenching (qN), and photochem- General Electric Co., Cleveland, OH). Gas exchange ical quenching (qP). Chlorophyll a kinetics were mea- ␮ ␮ measurements were recorded 14, 21, 29, and 35 d after sured at 400 mol/mol CO2 concentration, 1,200 mol plant removal from the cages in the Þrst experiment photons/m2/s light intensity, with measuring intensity and 14, 21, 28, and 35 d after plant removal form the 1 Int, measuring modulation 0.25 kHz, measuring Þlter cages in the second experiment, and consisted of 1 Hz, measuring gain 10 Gn, ßash duration 0.8 s, ßash the same measurements and settings described in the intensity 7 Int, ßash modulation 20 kHz, and ßash Þlter study 1. To infest plants in the third experiment 50 Hz settings. Such measurements will indicate the (30 July to 21 September 2004), we used the same overall photochemical efÞciency of PSII. cages as in the previous experiments, but there were All measurements were taken from the ßag leaves only two plants per cage at developmental stage 39 on the primary stem and a tiller (i.e., two measure- (Zadoks et al. 1974). Twenty-two females and four ments/plant) during wheat reproductive and matu- males were allowed to mate and oviposit freely for a ration stages, comprised of growth stages 49Ð87 period of 7 d. After the infestation period, plants were (Zadoks et al. 1974). removed from the cages and arranged in a completely Data for studies 1 and 2 were analyzed using re- randomized design (CRD) with six replicates. Gas peated measures (PROC MIXED; SAS Institute 2001). exchange measurements were recorded 17 and 22 d For study 3, data were analyzed asa2by2factorial after the infestation period. using PROC MIXED procedure of the SAS program Study 3. The study was conducted during June and (SAS Institute 2001). Means were separated by t-test July 2004 in a Þeld located in Gallatin County, MT. (␣ ϭ 0.05). Spring wheat, ÔMcNealÕ, was grown in 0.30-m-wide row at Ϸ494,000 plants/ha in a Broko silt loam (0Ð4% Results slopes) according to the soil survey of Gallatin County Area, MT (USDA-NRCS). The Þeld was maintained Study 1. We measured a total of 23 plants (7 infested with standard agronomic practices for western Mon- and 16 control) starting 17 d after infestation. Only 7 tana. An individual plot was three rows by 15 m in of 16 plants were positively infested by C. cinctus. We length. The distance between plots was 152 m. The observed a signiÞcant reduction of photosynthetic ca- study had Þve replications, consisting of two infesta- pacity of wheat plants as they senesced (Photo: F ϭ ϭ Ͻ ϭ ϭ ϭ tion treatments (infested and noninfested). Experi- 76.9, df 4,29, P 0.0001; gs: F 4.52, df 4,29, P ϭ ϭ ϭ ϭ mental plots were naturally infested by C. cinctus that 0.0058; Ci: F 6.88, df 4,29, P 0.0005; E: F 12.1, had emerged in a fallow wheat Þeld located near the df ϭ 4,29, P Ͻ 0.0001). No signiÞcant interactions were plots and had immigrated into the crop during the observed between time and treatment (uninfested wheat stem elongation period (JuneÐJuly). and infested). A signiÞcantly lower net photosyn- We determined C. cinctus presence/absence in thetic rate was observed for infested plants at each wheat plants (infested or noninfested), levels of in- measuring event on days 17, 24, 31, 38, and 45 (Fig.

festation (no. of infested stems/no. total of stems), 1A). The linear portion of the A-Ci response curve did insect developmental stages (egg, larva), and levels of not show any signiÞcant difference in CO2 assimila- injury (no. of damaged nodes) by randomly harvest- tion. However, a signiÞcant reduction in CO2 assim- ing 25 plants/plot (n ϭ 75) and splitting all the stems ilation was observed at the saturation point in infested weekly using a X-ACTO knife (Hunt, Statesville, NC) plants (Fig. 2). No signiÞcant difference was observed during the total period of the experiment (17 June to in the light-response curve at any point of the curve 30 July). (Fig. 3). Measures of the impact of C. cinctus injury on wheat Study 2. We measured Ϸ60 ßag leaves weekly dur- primary metabolism were obtained by determinations ing the period of 27 May to 20 June and 30 May to of impairment on photosynthetic capacity after we 23 June 2003 and 42 ßag leaves during the period of observed larval feeding in the plant material. Photo- 22 August to 3 September 2004. Because we observed synthetic parameters were measured with a portable a signiÞcant difference in the variance among runs, we photosynthesis system (model LI-6400; Licor). Pho- analyzed the data separately. We did not observe

tosynthetic rate (Photo), stomatal conductance (gs), signiÞcant differences for any photosynthetic param- intercellular CO2 concentration (Ci), and transpira- eter between infested and uninfested plants measured 722 ENVIRONMENTAL ENTOMOLOGY Vol. 34, no. 3

Fig. 2. A-Ci response curves to C. cinctus injury (means Ϯ SEM).

ϭ ϭ Ͻ ϭ 0.0107; gs: F 84.85, df 3,21, P 0.0001; Ci: F 20.61, df ϭ 3,21, P Ͻ 0.0001; E: F ϭ 11.86, df ϭ 3,21, P Ͻ 0.0001). No signiÞcant interactions between time and treatment (infested and uninfested) were observed.

Results from A-Ci and light-response curves were sim- ilar to those observed in study 1 (Fig. 2). Similar re- sults were obtained from experiment 3 (2004). Brießy, C. cinctus did not elicit signiÞcant changes in any of the photosynthetic parameters measured, such as Photo,

gs,Ci, and E. We observed that photosynthetic values were lower than those observed in the two 2003 stud- ies, which could be attributed to the more advanced developmental stage of the plants when they were Þrst infested. However, the general pattern was consistent with previous greenhouse studies in which a signiÞ- cant reduction in these parameters was observed as plants senesced. Study 3. We destructively sampled Ϸ225 plants within a period of three sampling events (i.e., 75 plants/event). A total of 304 stems were evaluated, and Ϸ60% were infested. We Þrst observed eggs in the experimental plots on 17 June, and 67% of the eggs were located in tillers.

Fig. 1. Impact of C. cinctus on wheat photosynthetic rates measured over time (means Ϯ SEM). (A) Wheat photo- synthetic rates under environmental growth chamber mea- sured at 17, 25, 31, 38, and 45 d after C. cinctus infestation. (B) Wheat photosynthetic rates under greenhouse environment measured at 14, 21, 28, and 35 d after C. cinctus infestation. (C) Wheat photosynthetic rates under Þeld environment measured at 13, 22, 29, 36, and 43 d after C. cinctus infestation. in all three greenhouse experiments. Conversely, sig- niÞcant reductions in photosynthetic capacity were observed as plants senesced in both 2003 experiments (experiment 1ÑPhoto: F ϭ 167.09, df ϭ 3,21, P Ͻ ϭ ϭ Ͻ ϭ 0.0001; gs: F 107.07, df 3,21, P 0.0001; Ci: F 24.97, df ϭ 3,21, P Ͻ 0.0001; E: F ϭ 52.27, df ϭ 3,21, P Ͻ Fig. 3. Light response curve to C. cinctus injury (means Ϯ 0.0001; experiment 2ÑPhoto: F ϭ 3.36, df ϭ 3,21, P ϭ SEM). June 2005 MACEDO ET AL.: WHEAT PHOTOSYNTHETIC RESPONSE TO C. cinctus INJURY 723

␮ ؎ Table 1. Study 3 summary of mean SEM values for photosynthetic capacity ( mol CO2/m/s) measured at days 13, 22, 29, 36, and 43 after C. cinctus infestation

Days after infestation 13 22 29 36 43 Main Stem Control 30.8 Ϯ 1.2a 26.4 Ϯ 1.7b 23.8 Ϯ 1.9b 18.8 Ϯ 3.7c 15.6 Ϯ 1.5c Infested 31.7 Ϯ 1.6a 24.2 Ϯ 1.4b 25.1 Ϯ 2.3b 17.0 Ϯ 2.4c 17.8 Ϯ 1.8c Secondary stem Control 26.1 Ϯ 0.2a 20.8 Ϯ 1.5b 24.0 Ϯ 2.7c 16.1 Ϯ 1.6d 15.1 Ϯ 1.6d Infested 28.2 Ϯ 1.7a 20.6 Ϯ 1.5b 21.0 Ϯ 3.0c 17.6 Ϯ 1.9d 11.2 Ϯ 1.6d

Means Ϯ SEM followed by same letters are not signiÞcantly different at ␣ ϭ 0.05.

Primary metabolism measurements were Þrst taken to C. cinctus injury. Additionally, the responses are after we observed signiÞcant C. cinctus feeding on the revealing as we consider if there are generalized phys- parenchyma tissue. Frass resulting from larval feeding iological response modalities for the stem-boring in- was observed on 30 June, 6 d after the Þrst larva was jury guild. found. Severe node injury (at least two of the three Our results were inconsistent across different en- nodes damaged) was observed 10 d after we began vironments. Wheat leaves did not respond in a similar taking physiological measurements. We did not ob- manner among different environments, suggesting serve signiÞcant effects of C. cinctus on the overall that environmental conditions are important determi- photosynthetic capacity measured in the period of the nants for plant response to insect injuries. Wheat ϭ ϭ ϭ ϭ study (Photo: F 0.18, df 1,38, P 0.6749; gs: F plants injured by C. cinctus under environmental ϭ ϭ ϭ ϭ ϭ 0.18, df 1,38, P 0.6696; Ci: F 0.03; df 1,38, P growth chamber conditions clearly had their photo- 0.8585; E: F ϭ 0.30, df ϭ 1,38, P ϭ 0.8780). Conversely, synthetic capacity impaired (Fig. 1A). Conversely, no we observed a signiÞcant reduction in wheatÕs pho- signiÞcant reductions in photosynthetic capacity were tosynthetic capacity as plants senesced (Photo: F ϭ observed in either greenhouse or Þeld environments ϭ Ͻ ϭ ϭ Ͻ 50.72, df 4,38, P 0.0001; gs: F 58.01, df 4,38, P (Fig. 1B and C). These results support the hypothesis ϭ ϭ ϭ ϭ 0.0001; Ci: F 5.75; df 4,38, P 0.001; E: F 53.15, of Pedigo et al. (1986), which stated that environmen- df ϭ 4,38, P Ͻ 0.0001; Fig. 1C). We observed a reduc- tal effects represent one of Þve key factors that de- tion on the photosynthetic capacity of Ϸ50% from termine how plants respond to arthropod herbivory. growth stage 49Ð87 (Zadoks et al. 1974). We also The environmental conditions inside the environ- observed a signiÞcant difference between main stem mental growth chamber used in this study were char- and tiller photosynthetic capacity (F ϭ 50.72, df ϭ acterized by low-quality (intensity) light (Ϸ63 ␮mol 1,38, P ϭ 0.0001). In general, tillers had a 14% lower photons/m/s), concurrent with elevated atmospheric Ϸ ␮ photosynthetic capacity. Tillers also showed a re- CO2 concentrations ( 467 mol CO2/mol). Both of duced photosynthetic capacity as the plant ap- these abiotic factors are well documented to inßuence proached growth stage 87 (F ϭ 2.90, df ϭ 4,38, P ϭ the overall photosynthetic capacity (Perchorowicz et 0.0344). A summary of wheat photosynthetic capacity al. 1981, Paul and Foyer 2001, Bidart-Bouzat et al. 2004, results is presented in Table 1. Melkonian et al. 2004, van Dongen et al. 2004, Van Cephus cinctus did not elicit signiÞcant impairment Heerden et al. 2004). Although light drives photosyn- of chlorophyll a ßuorescence on wheat ßag leaves. thesis, it can also cause severe damage to the photo- None of the parameters measured with the ßuores- synthetic apparatus (Baroli and Melis 1998). cence kinetic test was signiÞcantly affected by insect Recent studies by Johnson and Barber (2003) ϭ ϭ ϭ ϭ feeding (Fo: F 5.41, df 1,5, P 0.0676; Fm: F 2.35, showed that low-light irradiance can have a similar ϭ ϭ ϭ ϭ ϭ df 1,5, P 0.1861; Fs: F 1.24; df 1,5, P 0.3277; impact on photosynthetic quantum yield. Light also ϭ ϭ ϭ ϭ ϭ Fv/Fm: F 1.11, df 1,5, P 0.3406; FoÕ: F 2.17, df seems to play an important role on the biochemical ϭ ϭ ϭ ϭ 1,5, P 0.2003; FmÕ: F 2.19, df 1,5, P 0.1991; regulation of CO2 Þxation. Perchorowicz et al. (1981) ϭ ϭ ϭ ϭ ϭ FvÕ/FmÕ: F 1.28, df 1,5, P 0.3214; qP: F 0.91, df showed that activation of RuBP2 carboxylase (3-phos- 1,5, P ϭ 0.3400; qN: F ϭ 1.11, df ϭ 1,5, P ϭ 0.3412; NPQ: pho-D-glycerate carboxylase [dimerizing]) is depen- F ϭ 0.00, df ϭ 1,5, P ϭ 0.9964; ETR: F ϭ 0.21, df ϭ 1,5, dent on light intensity. They observed higher photo- P ϭ 0.6672). We also did not observe signiÞcant impact synthesis rates in leaves under higher irradiances, of tissue maturation on any of the ßuorescence pa- concurrent with increased inactivation of RuBP2 car- rameters. boxylase. In addition, light serves to coordinate assim- ilation of inorganic nitrogen with available carbon backbones produced during photosynthesis. Light Discussion also can affect plant gene expression by acting through Despite the lack of signiÞcant effects of injury on phytochrome activation or through changes in levels wheat primary metabolic processes, with the excep- of carbon metabolites (Oliveira et al. 2001). tion of study 1 (i.e., growth chamber study), the re- CO2 seems to be the most important limiting factor sponses observed in these studies are informative as for the photosynthetic capacity of plants. Bryant et al. we characterize how wheat responds physiologically (1998) showed that plants exposed to elevated CO2 724 ENVIRONMENTAL ENTOMOLOGY Vol. 34, no. 3 concentrations for periods during their development tiles than uninfested; however, a few volatile com- showed signiÞcant reductions in gas exchange. In- pounds seem to be less abundant for infested (D.K.W. deed, short-term photosynthetic responses of C3 and W.L.M., unpublished data). Elevated levels of plants species to elevated CO2 are usually modiÞed by ripening compounds appear for infested plants 1Ð2 wk some degree of longer-term acclimation of the pho- earlier than for control plants, suggesting premature tosynthetic apparatus (Oechel and Strain 1995). For senescence (D.K.W. and W.L.M., unpublished data). example, increases in global CO2 concentration may Another series of greenhouse experiments evaluating alter plant compensation through increased biomass varietal variation in wheat volatile production show accumulation and photosynthetic rates (Bazzaz 1990, that nutrient limitation and reduced light intensity Hughes and Bazzaz 1997). However, it is important to both decrease volatile production (D.K.W. and point out that potential interactions with both abiotic W.L.M., unpublished data). and biotic limiting factors may result in over expres- In addition to yield loss because of lodging and sion of damage by elevated CO2 concentrations. breakage of stems, wheat stem sawßy injury also re- Considering the limiting factors discussed above sults in physiological yield loss (Morrill et al. 1992a). and the results observed in this study, we suggest that Physiological yield loss, of course, implies impairment plants under abiotically stressful environments, such of physiological processes. However, based on this as growth chamber environments, have their meta- study, more research is needed before the physiolog- bolic homeostasis altered. In such conditions, wheat ical mechanism or group of mechanisms underlying plants are not able to tolerate C. cinctus injury. Indeed, yield loss are more thoroughly understood. a signiÞcant reduction in CO2 assimilation was ob- Pedigo et al. (1986) suggested that responses of served in the A-Ci response curve at the CO2 satura- plants to arthropod injury are dependent on Þve fac- tion point, showing a limitation on assimilation may be tors: (1) time of injury, (2) plant part injured, (3) caused by RuBP2 carboxylase/oxygenase (Rubisco) injury type, (4) intensity of injury, and (5) environ- activity inhibition. We compared the photosynthetic mental effects. Results from this study indicate that capacity of uninjured plants across the tested envi- physiological responses of wheat to C. cinctus stem- ronments and found that plants under environmental boring injury may be highly dependent on environ- Ϸ chamber exhibited 24% lower gas exchange in rela- mental interactions. This is not unexpected. Indeed, tion to the rates observed in the greenhouse and Þeld the ability of abiotic stressors to interact with biotic environments. Conversely, plants in better environ- stressors is well known (Haile 2001, Peterson and mental conditions, such as in a greenhouse environ- Higley 2001). ment, seem to be able to tolerate C. cinctus injury. In The interactions among arthropod injury, plant re- our greenhouse environments, light quality and CO2 sponses, and environmental factors necessitates that levels were relatively better than in the growth cham- characterizations of physiological responses also con- Ϸ ␮ 2 ␮ bers ( 334 mol photons/m /s and 382 mol CO2/ tain detailed descriptions of abiotic conditions. This is mol, respectively). Additionally, other potential lim- salient not only for characterizing a single host plant iting factors such as, nutrients, water, and temperature and herbivore species, but also for developing gener- were better for wheat primary metabolism. The Þeld alized models of response to an injury guild. environment during this study also was better, with ␮ This idea of injury guilds based on injury develop- ideal light quality, atmospheric CO2 levels (376 mol ment (i.e., symptoms) to physiological response has CO2/mol), nutrients, and temperatures for wheat de- been purposed for defoliators. Higley et al. (1993) velopment. divided the defoliation injury type into the following Even though the primary metabolism parameters three injury guilds: leaf-mass reduction, leaf photo- measured in this study are very sensitive to changes synthetic rate reduction, and leaf senescence alter- induced by plant stressors, we observed statistically ation. Based on our Þndings in this study and previous signiÞcant gas-exchange reductions associated with studies, it seems likely that there is no generalized C. cinctus injury only in the growth chamber study. model of plant physiological response to stem boring. Injury to wheat by C. cinctus is characterized by its This is not surprising because stem boring currently subtlety. The injury is visually asymptomatic even “ ” refers to a type of injury based on physical appearance. though the larva often feeds inside the length of the Therefore, it seems likely that the stem-boring injury entire stem, including the nodes. A more sensitive guild also will need to be divided into additional guilds measure of effects on primary physiology is chloro- based on physiological response. phyll ßuorescence. However, we did not observe any differences in ßuorescence. Therefore, the injury seems to be not only visually, but also physiologically, asymptomatic under certain environmental condi- Acknowledgments tions. Injury by C. cinctus has been detected using other We thank M. Flikkema for allowing us repeated access to his wheat Þeld. We also thank P. Macedo for assisting with the physiological parameters. Studies evaluating plant Þeld work, along with staff members of the wheat stem sawßy volatiles produced by infested and uninfested wheat project at Montana State University. Funding for this project plants in the greenhouse show relatively subtle was provided by USDA, CREES, Special Research Grant changes in the amounts of certain compounds pro- entitled, “Novel Semiochemical-and Pathogen-Based Man- duced. In general, infested plants produce more vola- agement Strategies for the Wheat Stem Sawßy,” the Montana June 2005 MACEDO ET AL.: WHEAT PHOTOSYNTHETIC RESPONSE TO C. cinctus INJURY 725

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