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Morphogenic light reflected to developing cotton leaves affects insect-attracting terpene concentrations.

COTTON LEAVES ACCUMULATE VOLATILE TERPENES in mesophyllar glands or subdermal trichomes that attract both insect pests and their natural enemies, arthropods that prey on them. Because of the extensive hectarage of cotton worldwide, and the wide range of insect pests associated with it, the volatile constituents of these glands have been the subject of numerous studies. For example, blends of cotton terpenes containing compounds such as [alpha]-pinene, [beta]-pinene, and caryophyllene have been shown to be attractive to pests such as boll weevils (Anthonomus grandis grandis Boheman) (Thompson et al., 1971; Hedin, 1976; Chang et al., 1986), tobacco budworm (Heliothis virescens Fabricus) (Chang et al., 1988; Rostelien et al., 2000), and cotton bollworm (Helicoverpa armigera Hubner) (Jallow et al., 1999), as well as insect parasites and predators such as Campoletis sonorensis (Cameron) (Elzen et al., 1983) and Cotesia marginiventris (Cresson) (Turlings et al., 1990). The odor profile emitted by growing cotton plants is highly characteristic and serves as an efficient means of location for insects (Chang et al., 1986).

The emission of terpenes by plants is affected by environmental factors such as temperature (Staudt et al., 1997), season (Hedin, 1976), and time of day (Loughrin et al., 1994, 1995, 1997). Light quantity and its spectral distribution are both affected by factors such as time of day and season. Closeness of neighboring plants, plant residues, and variously colored mulches on the soil surface also affect PPF and morphogenic light that is received by developing plants (Kasperbauer 1971, 1987; Ballare et al., 1990). For example, nearby growing plants and some colors of mulch can affect the amounts of FR reflected to developing plants. In nature, a high FR/R ratio acts through the natural growth regulating system within a plant to influence allocation and use of photosynthate to favor survival among competing plants. The regulatory system also influences the chemical composition of the developing plant.

Color of light, as a direct visual cue for insects, has been the subject of many studies (Antignus, 2000). Some colors attract certain insects and others repel. Mulch color, for example, has been shown to affect insect populations in crops such as pepper (Capsicum annuum L.), tomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativus L.), and watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] (Lobenstein et al., 1975; Cohen, 1982; Csizinski et al., 1995; Farias-Larios and Orozco-Santos, 1997a, 1997b). The effect of color of light on mesophyllar glands and the synthesis and emission of volatile insect attractants from leaves, however, is not understood.

Recently we found that fragrant compounds including volatile terpenoids emitted by basil (Ocimum basilicum L.) leaves could be influenced by altering the color of light reflected from mulches to the developing plants (Loughrin and Kasperbauer, 2000). Like cotton leaves, basil leaves accumulate volatile terpenoids in specialized glands. The objective of the present study was to determine whether color of light reflected to developing cotton leaves could alter the concentrations of terpenes that are known to serve as insect attractants.

MATERIALS AND METHODS

Plant Material and Growing Conditions

Cotton plants were grown in drip-irrigated plots of Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudults) at the Coastal Plains Soil, Water, and Plant Research Center near Florence, SC. A randomized complete block design with four replicate plots was used each year of the 2-yr study. Raised beds were prepared, irrigation tubes were placed, and the 90-cm wide by 15-cm high raised-bed plots were covered with standard black plastic. In 2000, each plastic-covered plot was divided into three 6-m long subplots, two of which were painted with green or white exterior enamel. The other was covered with a red plastic (SRM-Red, Ken-Bar Agricultural Plastics, Reading, MA) that was held in place by taping it over the black plastic. In 2001, each plot was divided into four 6-m long subplots, three of which were painted with green, white, or yellow exterior enamel. The fourth plot was covered with the red plastic as described above. In 2000, seeds of the cultivar McNair 235 were sown, while in 2001, seeds of McNair 235 and 'SC-1' were sown. In both years, 5-cm-diam. holes were cut in the plastic at within-row distances of 0.6 m. Four seeds were sown in each hole. When the seedlings were in the cotyledon stage, all but one per hole were removed by cutting below the cotyledons. This allowed us to avoid disrupting the roots of the remaining seedling in each hole.

Reflected Light Measurements

Light measurements were taken at solar noon [+ or -] 30 min on a cloudless day. The spectral distribution and quantity of upwardly reflected light was measured about 15 cm above the colored surfaces with a LI-COR LI-1800 spectroradiometer (LI-COR Inc., Lincoln, NE) equipped with a remote hemispherical light collector on a 1.5-m fiber optic probe. The spectroradiometer recorded measurements at 5-nm intervals between 400 and 800 nm. A reference spectrum was obtained by measuring incoming sunlight at the same wavelengths. The reflected light values were then calculated as percentages of incoming sunlight at each measured wavelength. We expressed values for R at 645 [+ or -] 5 nm because that is the approximate action peak for the red absorbing form of phytochrome ([P.sub.R]) in green plants due to competitive absorption by chlorophyll at 660 nm (Kasperbauer et al., 1964), which is the absorption peak for [P.sub.R] in vitro (Butler et al., 1964). Our values for FR were expressed at 735 [+ or -] 5 nm, which is the absorption peak for the far-red absorbing form of phytochrome ([P.sub.FR]) in vitro, and FR' was expressed at 755 [+ or -] 5 nm, which is the beginning of the far-red reflection plateau (percentagewise) from green leaves (Kasperbauer, 1987). The FR' measurement is important morphologically because prolonged exposure (as occurs in the field) to FR at 735 nm results in a R response due to the overlapping of [P.sub.R] absorption into that wave band (Kasperbauer et al., 1963). At 755 nm, the [P.sub.R] absorption is extremely low, resulting in a FR response due to prolonged reflection of the 755- to 770-nm waveband from nearby plants as observed in nature (Kasperbauer, 1987). The FR/R and FR'/R ratios in upwardly reflected light were expressed relative to the ratios in incoming sunlight.

The values for light reflected from the red, green, yellow, and white surfaces were similar to those previously measured from the same colors (Loughrin and Kasperbauer, 2000). The white surface reflected approximately 40% of the BL and PPF that impinged on it, and FR/R and FR'/R ratios were similar to those of incoming sunlight. Yellow also reflected much PPF, but about half as much BL as was reflected by white. Red and green surfaces reflected only about 5% of the BL, relatively low PPF, and higher FR/R and FR'/R ratios than were present in sunlight at the same time and place.

Terpene Analysis

Since volatile emission from leaves is known to vary diurnally (Loughrin et al., 1994, 1997), all leaf samples were collected at 0900 h [+ or -] 20 min to eliminate potential variation from this factor. All leaves were collected within 25 to 30 cm of the soil surface. In 2000, leaves of the cultivar McNair 235 were collected from plants before flowering, on sunny vs. overcast days occurring within 1 wk of each other. In 2001, leaves were collected from McNair 235 and SC-1. Fully expanded leaves were collected from plants of each cultivar at preflowering and again at early boll set. In both 2000 and 2001, leaves were kept cool and in darkness in insulated boxes until sampled.

Each sample consisted of 10 leaf disks that were punched out of two leaves with a No. 10 cork borer (total area per sample was 20 [cm.sup.2]). Care was taken to avoid major leaf veins. The disks were weighed and placed in 8-mL glass vials with Teflon-lined caps. Five milliliters of high purity pentane (amyl hydride) (J.T. Baker, Phillipsburg, NJ) was added to each vial and the samples were stored in darkness at -65[degrees]C until analyzed.

Samples were allowed to reach room temperature before analysis. The samples were vortexed and passed through 3-mL solid phase extraction cartridges containing 1000 mg of silica gel absorbent (Supelco Inc., Bellefonte, PA) to remove oxygenated compounds from the extracts. The nonretained portions of the extracts were used for gas chromatographic analyses as described below.

In 2000, 10 [micro]g of cumene in pentane was added to each vial as an internal standard. Samples were concentrated to approximately 1 mL under a stream of N before analysis. In 2001, the samples were used directly for analyses after removal of the oxygenated compounds. Compounds were quantified by injections of external standards of [alpha]-pinene for monoterpenes and caryophyllene for sesquiterpenes.

Samples were analyzed by injection into a Varian model 3800 gas chromatograph (GC, Varian Associates, Walnut Creek, CA) equipped with a 60-m by 0.32-mm SPB-5 column with a film thickness of 1.0 [micro]m (Supelco, Inc.). Two-microliter injections were made in splitless mode for 1 min with an injector temperature of 220[degrees]C, column initial temperature of 50[degrees]C for 4 min, and column oven programming at 2[degrees]C [min.sup.-1] to 100[degrees]C. The column oven was then programmed at 5[degrees]C [min.sup.-1] to 230[degrees]C. Other GC operating conditions included flame ionization detector at 260[degrees]C, column helium linear flow rate of 17 cm [s.sup.-1], injector split ratio of 75:1, and helium make-up gas flow rate of 25 mL [min.sup.-1].

Compound Identification

Gas chromatography and mass spectroscopy were performed on a GC equipped with a 30-m by 0.25-mm HP-5 column (Hewlett-Packard, Palo Alto, CA) interfaced to a Hewlett-Packard Model 1800 mass selective detector. Ten-milliliter samples from both cultivars were concentrated to about 1 mL at 50[degrees]C with a micro Kudema-Danish concentrator (Supelco Inc.). One-microliter aliquots were injected onto the GC in splitless mode for 1 min and the mass ion detector used a scanning range of 40 to 450 amu. Operating conditions for the GC included injector temperature of 220[degrees]C and column oven at 40[degrees]C for 1 min then programmed at 3[degrees]C [min.sup.-1] to 180[degrees]C. Compound identifications were performed by computer database searches and retention time matches of the natural compounds with those of authentic samples of compounds on the SPB-5 column. Authentic samples of compounds were obtained from commercial sources.

Statistical Analysis

In both years, terpene concentrations were expressed relative to leaf weight (ng [mg.sup.-1] fresh weight) and leaf area (ng [cm.sup.-2]) and subjected to analysis of variance with PROC MIXED using the SAS system for Windows (SAS Institute, 1996). Means and standard errors of the mean were calculated by PROC MEANS.

RESULTS AND DISCUSSION

Leaf Morphology

Altering the color of light reflected to developing cotton leaves affected their morphology. Leaf disks were heavier over white mulch than over red or green, and those grown over yellow were intermediate (Fig. 1). Therefore, increasing PPF and BL reflected to leaves resulted in heavier leaves while increasing the ratio of FR to R resulted in lighter leaves. These results are consistent with those of an earlier study of influence of reflected color on leaf morphology and concentration of photosynthetic pigments in cotton seedlings (Bradburne et al., 1989).

[FIGURE 1 OMITTED]

Terpene Concentrations

We identified six terpene hydrocarbons from the leaf extracts in 2000, and nine in 2001. The identity of these compounds was confirmed by comparison of fragmentation patterns with samples of authentic compounds and retention time matches on the SPB-5 column. However, an authentic sample of the sesquiterpene [delta]-guaiene was not available to us and was tentatively identified on the basis of its fragmentation pattern and reports of its occurrence in cotton (Minyard et al., 1966).

2000

The levels of [alpha]-pinene, [beta]-pinene, and myrcene from leaves of McNair 235 were affected (P [less than or equal to] 0.1) by color of light received by the leaves during development (Table 1). In contrast, we found no significant effect of sunny vs. overcast weather on the accumulation of volatile terpenes. This is consistent with accumulation in peppermint (Mentha x piperita L.) leaves during the vegetative growth stage, in which the synthesis of monoterpenoids in leaves occurred during a relatively short period corresponding with rapid leaf expansion (Gershenzon et al., 2000). Apparently no significant losses due to catabolism or volatilization occurred in the accumulated pool thereafter. It appears that terpene accumulation is similarly regulated in cotton; therefore, changes in weather occurring within a relatively short time span would not be expected to affect the level of stored terpenes.

Table 2 presents data on the concentrations of terpenes (ng [mg.sup.-1]) from McNair 235 collected during sunny and overcast days. Although terpene concentrations were variable, greater concentrations of these compounds were found in leaves that had developed over red and green reflectors than those that had developed over white. The same trends were observed when terpene concentrations were expressed in terms of leaf area (ng [cm.sup.-2], data not shown).

2001

Differences in the amounts of terpene hydrocarbons accumulated in leaves were found on the basis of cultivar, developmental stage (prebloom vs. early boll set), and color of soil cover. Table 3 presents analysis of variance of the effects of these factors, and their interactions, on the amounts of volatile terpenes per weight of leaf. Color of soil cover was found to be a significant factor (P [less than or equal to] 0.05) affecting the amount of the monoterpenes [alpha]-pinene, [beta]-pinene, myrcene, [gamma]-terpinene, and the sesquiterpenes caryophyllene and [alpha]-humulene as well as total terpenes. Developmental stage of the plants was a significant factor affecting the amounts of 2-carene, [gamma]-terpinene, and [delta]-guiaene. Significant differences between cultivars were seen in the amounts of [gamma]-terpinene, caryophyllene, [delta]-guaiene, and total terpenes. No interactions were found between color of light reflected to the developing leaves and cotton cultivar nor color of light and developmental stage of the plant.

When analyzing each cultivar separately, color of light significantly (P [less than or equal to] 0.05) affected [alpha]-pinene, [beta]-pinene, [gamma]-terpinene, caryophyllene, and [alpha]-humulene, as well as total terpenes in McNair 235 (Table 4). Developmental stage was a significant factor affecting [alpha]-pinene, [beta]-pinene, 2-carene, myrcene, [gamma]-terpinene, caryophyllene, [alpha]-humulene, [delta]-guaiene, and total terpenes. Significant interactions between color of light and developmental stage were seen for [alpha]-pinene, [beta]-pinene, caryophyllene, and total terpenes. Color of light reflected to developing leaves significantly affected [alpha]-pinene, [beta]-pinene, myrcene, [alpha]-humulene, and total terpenes in SC-1. Also in SC-1, developmental stage significantly affected [alpha]-pinene, [beta]-pinene, myrcene, caryophyllene, [alpha]-humulene, and total terpenes. However, there were no significant interactions between color of reflected light and developmental stage for SC-1.

While amounts of foliar terpenes increased from pre-bloom to early boll set for the cultivar McNair 235 and decreased for the cultivar SC-1, some trends were apparent (Tables 5, 6). For McNair 235, the highest levels of total terpenes were obtained from leaves which were grown over the red reflective surfaces at both developmental stages. For SC-1, leaves which had been grown over green surfaces contained the highest levels of total terpenes, followed by leaves which had been grown over red. The same pattern occurred at both growth stages. This is consistent because both the red and green soil covers reflected a relatively low PPF and a high FR/R ratio as compared with the white and yellow surfaces.

The trend observed in 2001 was consistent with that observed in 2000 (see Table 2). That is, cotton leaves that developed over soil covers that reflected a FR/R ratio higher than that of incoming sunlight contained the highest levels of terpenes. It appears, therefore, that increasing the FR/R ratio reflected to developing cotton leaves can increase the amount of volatile terpenoids accumulated in leaves despite their having less weight per unit area. This, in turn, could increase the emission of volatile insect attractants from cotton. It should be noted, however, that in the present study the red and green surfaces reflected a FR/R ratio higher than was present in sunlight without decreasing incoming PPF. Whether a high FR/R ratio at reduced PPF, as occurs when the FR/R ratio is altered by nearness of other plants, would result in increased accumulation of terpenes is unclear and beyond the scope of the present study.

Conversely, increased PPF as was reflected from the yellow and white soil covers was less effective in inducing the accumulation of leaf terpenes than was a high FR/R ratio. Thus, within the range of natural outdoor light intensities used in the present study, altering R and FR light reflected to developing leaves was more effective in inducing terpene accumulation than was altering PPF reflected to the leaves.

In summary, phytophagous insects locate hosts by a combination of visual and olfactory cues. Many studies have shown that colored soil covers and traps can affect insect populations in crops, presumably by acting as a direct visual cue (Antignus, 2000). Results from the present work show that reflected color may also affect insect orientation to plants by affecting plant metabolism and boosting available olfactory cues. Our work clearly shows that altering morphogenic light reflected to developing cotton leaves can modify the concentration of volatile insect attractants. This finding has potential application in any cultural practice or integrated pest management program that can influence morphogenic light received by growing cotton leaves.

Abbreviations: BL, blue; FR, far-red; GC, gas chromatograph; PPF, photosynthetic photon flux; [P.sub.FR], far-red absorbing form of phytochrome; [P.sub.R], red absorbing form of phytochrome; R, red.
Table 1. Analysis of variance comparing the concentrations of
volatile terpenes isolated from McNair 235 leaves grown over
red, green, or white soil covers and sampled during sunny vs.
overcast weather in 2000.

                                     Weather (W)
Compound              Color (C)      ([dagger])    C X W

[alpha]-Pinene     0.0965 ([double     0.8160      0.6366
                          dagger])
[beta]-Pinene           0.0619         0.9184      0.4310
Myrcene                 0.0696         0.7213      0.9527
Limonene                0.5150         0.5645      0.2104
Caryophyllene           0.6445         0.8302      0.8897
[alpha]-Humulene        0.6222         0.7682      0.8534
  Total                 0.1747         0.9952      0.7529

([dagger]) Sunny vs. overcast.

([double dagger]) Values are probabilities from an analysis of
variance with 10 observations. Terpene content was analyzed
on a fresh-weight basis.

Table 2. Concentrations of terpenes from cotton cultivar McNair
235 grown in 2000 over red, green, or white soil covers and
sampled during sunny vs. overcast weather.

                               Soil cover

Compound                 Red               Green

                             ng [mg.sup.-1]

                            Sunny weather

[alpha]-Pinene     281 [+ or -] 59    329 [+ or -] 109
                       ([dagger])
[beta]-Pinene       61 [+ or -] 9      81 [+ or -] 31
Myrcene            125 [+ or -] 31    201 [+ or -] 108
Limonene            23 [+ or -] 4      28 [+ or -] 11
Caryophyllene      119 [+ or -] 31    142 [+ or -] 40
[alpha]-Humulene    35 [+ or -] 8      47 [+ or -] 15
  Total            644 [+ or -] 139   828 [+ or -] 310

                           Overcast weather

[alpha]-Pinene     410 [+ or -] 96    336 [+ or -] 119
[beta]-Pinene       84 [+ or -] 18     74 [+ or -] 19
Myrcene            152 [+ or -] 37    167 [+ or -] 59
Limonene            30 [+ or -] 7      25 [+ or -] 6
Caryophyllene      156 [+ or -] 38    184 [+ or -] 87
[alpha]-Humulene    48 [+ or -] 11     55 [+ or -] 24
  Total            879 [+ or -] 196   978 [+ or -] 373

                      Soil cover

Compound                White

                    ng [mg.sup.-1]

[alpha]-Pinene     211 [+ or -] 50

[beta]-Pinene       52 [+ or -] 10
Myrcene             89 [+ or -] 36
Limonene            27 [+ or -] 5
Caryophyllene      109 [+ or -] 29
[alpha]-Humulene    33 [+ or -] 8
  Total            522 [+ or -] 133

[alpha]-Pinene     171 [+ or -] 53
[beta]-Pinene       38 [+ or -] 8
Myrcene             67 [+ or -] 18
Limonene            15 [+ or -] 3
Caryophyllene      118 [+ or -] 35
[alpha]-Humulene    37 [+ or -] 10
  Total            445 [+ or -] 124

([dagger]) Values are the mean of 10 observations
[+ or -] standard error of the mean.

Table 3. Analysis of variance comparing the concentrations of
volatile terpenes isolated from leaves of two cotton cultivars
grown in 2001 over four colors of soil covers and sampled at
two developmental stages, expressed on the basis of leaf weight.

                                      Stage (S)
Compound               Color (C)      ([dagger])   Cultivar (V)

[alpha]-Pinene      0.0002 ([double     0.4928        0.1806
                           dagger])
[beta]-Pinene           0.0001          0.8610        0.2517
2-Carene                0.1107          0.0001        0.0846
Myrcene                 0.0028          0.9227        0.1261
Limonene                0.2214          0.2739        0.7814
[gamma]-Terpinene       0.0421          0.0093        0.0030
Caryophyllene           0.0013          0.1377        0.0432
[alpha]-Humulene        0.0016          0.3184        0.1263
[delta]-Guaiene         0.4150          0.0028        0.0317
  Total terpenes        0.0106          0.0840        0.0315

Compound            C X V    C X S

[alpha]-Pinene      0.1404   0.4369

[beta]-Pinene       0.0779   0.3114
2-Carene            0.3205   0.2332
Myrcene             0.0852   0.8547
Limonene            0.1033   0.9308
[gamma]-Terpinene   0.0824   0.3487
Caryophyllene       0.1699   0.0512
[alpha]-Humulene    0.4379   0.4431
[delta]-Guaiene     0.1399   0.4347
  Total terpenes    0.2340   0.2866

([dagger]) Prebloom vs. boll set.

([double dagger]) Values are probabilities from an analysis of
variance with eight observations. Terpene content was analyzed
on a fresh-weight basis.

Table 4. Analysis of variance comparing the concentrations of
volatile terpenes isolated from leaves of two cotton cultivars
grown in 2001 over four colors of soil covers and sampled at
two developmental stages, expressed on the basis of leaf weight.

                                      Stage (S)
Compound            Color (C)         ([dagger])   C X S

                             'McNair 235'

[alpha]-Pinene      0.0008 ([double     0.0001     0.0185
                           dagger])
[beta]-Pinene           0.0001          0.0001     0.0102
2-Carene                0.1173          0.0001     0.4487
Myrcene                 0.0773          0.0001     0.4179
Limonene                0.1983          0.6295     0.6609
[gamma]-Terpinene       0.0344          0.0225     0.1971
Caryophyllene           0.0115          0.0009     0.0374
[alpha]-Humulene        0.0127          0.0023     0.0659
[delta]-Guaiene         0.3286          0.0168     0.5064
  Total                 0.0249          0.0001     0.0476

                                'SC-1'

[alpha]-Pinene          0.0025          0.0001     0.2916
[beta]-Pinene           0.0014          0.0001     0.3902
2-Carene                0.2384          0.2691     0.3029
Myrcene                 0.0002          0.0001     0.1663
Limonene                0.1366          0.0540     0.8814
[gamma]-Terpinene       0.4303          0.1923     0.6329
Caryophyllene           0.1428          0.0217     0.1072
[alpha]-humulene        0.0139          0.0011     0.2190
[delta]-guaiene         0.1645          0.0728     0.6056
  Total                 0.0084          0.0001     0.4526

([dagger]) Prebloom vs. boll set.

([double dagger]) Values are probabilities from an analysis
of variance with eight observations. Terpene content was
analyzed on a fresh-weight basis.

Table 5. Concentrations of terpenes from cotton cultivar McNair
grown over four colors of soil covers and sampled at two
developmental stages in 2001.

                                      Soil cover

Compound                      Red                    Green

                                    ng [mg.sup.-1]

                                   Prebloom stage

[alpha]-Pinene      180 [+ or -] 13 ([dagger])   80 [+ or -] 26
[beta]-Pinene        38 [+ or -] 4               19 [+ or -] 7
2-Carene             43 [+ or -] 6               37 [+ or -] 10
Myrcene              42 [+ or -] 4               32 [+ or -] 12
Limonene             44 [+ or -] 5               35 [+ or -] 6
[gamma]-Terpinene    32 [+ or -] 3               39 [+ or -] 15
Caryophyllene        58 [+ or -] 5               35 [+ or -] 12
[alpha]-Humulene     30 [+ or -] 4               12 [+ or -] 3
[delta]-Guaiene      29 [+ or -] 3               45 [+ or -] 10
  Total             734 [+ or -] 28             420 [+ or -] 112

                                Early boll set stage

[alpha]-Pinene      708 [+ or -] 198            335 [+ or -] 50
[beta]-Pinene       128 [+ or -] 31              61 [+ or -] 7
2-Carene             32 [+ or -] 8               18 [+ or -] 7
Myrcene             111 [+ or -] 30              76 [+ or -] 16
Limonene             54 [+ or -] 7               40 [+ or -] 6
[gamma]-Terpinene    56 [+ or -] 15              60 [+ or -] 8
Caryophyllene       183 [+ or -] 65             128 [+ or -] 21
[alpha]-Humulene     48 [+ or -] 18              34 [+ or -] 6
  Total            1660 [+ or -] 417            967 [+ or -] 102

                              Soil cover

Compound               Yellow             White

                            ng [mg.sup.-1]

[alpha]-Pinene      76 [+ or -] 7     76 [+ or -] 33
[beta]-Pinene       16 [+ or -] 2     17 [+ or -] 2
2-Carene            38 [+ or -] 5     39 [+ or -] 6
Myrcene             22 [+ or -] 4     32 [+ or -] 9
Limonene            38 [+ or -] 4     38 [+ or -] 4
[gamma]-Terpinene   24 [+ or -] 3     24 [+ or -] 3
Caryophyllene       23 [+ or -] 3     23 [+ or -] 3
[alpha]-Humulene     8 [+ or -] 2      5 [+ or -] 1
[delta]-Guaiene     66 [+ or -] 13    49 [+ or -] 8
  Total            447 [+ or -] 54   381 [+ or -] 34

[alpha]-Pinene     160 [+ or -] 18   103 [+ or -] 15
[beta]-Pinene       25 [+ or -] 4     23 [+ or -] 3
2-Carene             7 [+ or -] 5     12 [+ or -] 6
Myrcene             60 [+ or -] 10    30 [+ or -] 6
Limonene            36 [+ or -] 5     38 [+ or -] 5
[gamma]-Terpinene   23 [+ or -] 4     27 [+ or -] 3
Caryophyllene       37 [+ or -] 4    252 [+ or -] 85
[alpha]-Humulene     9 [+ or -] 2     61 [+ or -] 22
  Total            512 [+ or -] 29   819 [+ or -] 157

([dagger]) Values are the mean of eight observations
[+ or -] standard error of the mean.

Table 6. Concentrations of terpenes from cotton cultivar SC-1
grown over four colors of soil covers and sampled at two
developmental stages in 2001.

                                        Soil cover

Compound                      Green                   Yellow

                                      ng [mg.sup.-1]

                                       Prebloom stage

[alpha]-Pinene      383 [+ or -] 90 ([dagger])    374 [+ or -] 48
[beta]-Pinene        74 [+ or -] 16                75 [+ or -] 9
2-Carene             38 [+ or -] 7                 43 [+ or -] 10
Myrcene              53 [+ or -] 11               131 [+ or -] 26
Limonene             45 [+ or -] 9                 54 [+ or -] 8
[gamma]-Terpinene    25 [+ or -] 6                 32 [+ or -] 4
Caryophyllene        89 [+ or -] 24                44 [+ or -] 16
[alpha]-Humulene     28 [+ or -] 6                 26 [+ or -] 7
[delta]-Guaiene      50 [+ or -] 5                 60 [+ or -] 8
  Total terpenes    923 [+ or -] 180             1050 [+ or -] 104

                                  Early boll set stage

[alpha]-Pinene       84 [+ or -] 18               130 [+ or -] 19
[beta]-Pinene        18 [+ or -] 4                 26 [+ or -] 5
Myrcene              12 [+ or -] 2                 36 [+ or -] 7
Limonene             32 [+ or -] 5                 41 [+ or -] 5
[gamma]-Terpinene    25 [+ or -] 4                 27 [+ or -] 4
Caryophyllene        20 [+ or -] 5                 38 [+ or -] 8
[alpha]-Humulene      6 [+ or -] 2                  9 [+ or -] 2
[delta]-Guaiene      62 [+ or -] 3                 70 [+ or -] 11
  Total terpenes    401 [+ or -] 32               523 [+ or -] 57

                                Soil cover

Compound                 White

                              ng [mg.sup.-1]

[alpha]-Pinene      276 [+ or -] 60     80 [+ or -] 26
[beta]-Pinene        55 [+ or -] 9      21 [+ or -] 3
2-Carene             32 [+ or -] 7      17 [+ or -] 6
Myrcene              68 [+ or -] 15     12 [+ or -] 2
Limonene             48 [+ or -] 5      34 [+ or -] 7
[gamma]-Terpinene    25 [+ or -] 4      15 [+ or -] 4
Caryophyllene        55 [+ or -] 15     14 [+ or -] 2
[alpha]-Humulene     18 [+ or -] 5       5 [+ or -] 1
[delta]-Guaiene      58 [+ or -] 10     41 [+ or -] 6
  Total terpenes    770 [+ or -] 120   337 [+ or -] 36

[alpha]-Pinene       44 [+ or -] 7      18 [+ or -] 3
[beta]-Pinene         7 [+ or -] 2       3 [+ or -] 1
Myrcene              15 [+ or -] 9       3 [+ or -] 1
Limonene             39 [+ or -] 6      30 [+ or -] 5
[gamma]-Terpinene    27 [+ or -] 5      22 [+ or -] 2
Caryophyllene        14 [+ or -] 2      10 [+ or -] 1
[alpha]-Humulene      8 [+ or -] 3       2 [+ or -] 1
[delta]-Guaiene      57 [+ or -] 8      36 [+ or -] 7
  Total terpenes    333 [+ or -] 34    217 [+ or -] 22

([dagger]) Values are the mean of eight observations [+ or -]
standard error of the mean.


ACKNOWLEDGMENTS

We thank Thomas R. Hamilton-Kemp and David Hildebrand for assistance with mass spectrometric identification of compounds. We also thank Leo McLaurin and Victor Rogers for technical assistance. Mention of a trademark or product anywhere in this paper is to describe experimental procedures and does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products or vendors that may also be suitable.

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Michael J. Kasperbauer * and John H. Loughrin

USDA/ARS, Coastal Plains Soil, Water, and Plant Research Center, 2611 West Lucas St., Florence, SC 29501-1242. Received 16 Jan. 2003. * Corresponding Author (michaeljkasper@aol.com).
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