Diel patterns of volatile organic compounds released by maize plants: The chemical environment of the Ostrinia nubilalis moth

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VOCs collection

Maize field VOCs were collected by placing SPME fibers in open-air condition in the middle of the maize field. The fiber was attached at the top part of a maize stem. The forest VOCs were collected by placing SPME fibers in open-air conditions on the lower branches of trees at 1.50 m above the ground. The sampling was conducted at least 5 meters inside of the forest.

SPME fiber

SPME fibers DVB/CARBOXEN/PDMS 50/30 $m (Supelco) were conditioned by heating in the gas chromatograph injector at 250 °C for 5·min with helium as carrier flow. Cleaned fibers were then wrapped in aluminum foil and stored in individual screw-capped Pyrex glass tubes until use.

Chemical analysis

After volatile collection, SPME fibers were desorbed in a Varian 3400 GC injector held at 250°C. The GC was coupled to a MS detector Varian QIMS. Compound separation was carried out using a Rxi-5ms column (Restek, France) 30 m % 0.32 mm i.d., film thickness 1.0 $m). The column was programmed from 50 °C for after 3 minutes at 8 °C/min to 300 °C. Helium was used as carrier gas. Mass spectra were obtained in electron impact mode (70 eV) with the ion source at 230 °C.
Compounds eluted from SPME collections were identified according to their mass spectra and retention index (RI). RIs were computed using C10 to C24 n-alkanes, eluted under the same conditions as the samples. Every compound spectrum and RI was compared with the RI and spectrum of standard and NIST 1998 library as reference using deconvolution software AMDIS32. The calibration curves of green leaf volatiles (GLV), homoterpenes (HT), monoterpenes (MT) and sesquiterpenes (SQT) were obtained by injection of standards: !-linalool, !-pinene, !-humulene, methyl salicylate (MeSA), cis-3-hexenol (Sigma-Aldrich), « -farnesene (Chemtech), ocimene (Fluka), cis-3-hexenyl acetate (Lancaster) and (E)-4,8-dimethyl-1,3,7- nonatriene (DMNT) (gift from Jarmo K. 50! Holopainen (University of Kuopio, Finland). Each compound was injected at least three times at given concentrations: 5, 10 25, 50 ng/μl.

Maize headspace VOC

From maize headspace VOC collections, 21 components were detected and identified based on comparing the RI and mass spectra to authentic samples or appropriate data bases (Table 1a). Nineteen components out of 21 had previously been identified. Two VOCs were newly identified as maize VOCs: p-cymene, and a compound tentatively identified as selina-3,7 (11) diene (SQT). The maize scent was found to be a mixture of three GLVs, six MTs, two homoterpenes (HTs), and 12 SQTs (Table 1a). The amounts of each component varied considerably throughout the 24-h cycle. The relative ratios of MT and SQT changed from day to night (Fig. 2). The peak SQT emission occurred during the day. In contrast, the peak MT emission was observed at night and dawn. HT was always present in low quantities, and the quantity did not change substantially over time. The diel GLV composition was characterized by the absence of Z3-6:Ac during the day and at dawn, but the relative amount of Z3-6:OH increased at dusk. MeSA (an induced GLV) and !-copaene, were the two main compounds in maize headspace collections; they accounted for half of all the VOCs detected (Table 1a). The relative amounts of MeSA and !-copaene varied over time. The diel variations in these two VOCs were clearly separate at dusk and dawn. The ratio of MeSA to !-copaene in the headspace varied from 0.39 at dusk to 2.32 at dawn. The ratio between day and night was less impressive; it ranged from 1 during the day to 0.68 at night (Fig. 3). During the day, the emission rates of !-copaene and MeSA were nearly 67! equal. At dusk, however, individual maize plants emitted about 2.5 times more !- copaene than MeSA. At night, they tended to be emitted at similar levels At dawn, the emission rates were the reverse of those at dusk, and maize plants emitted about 2.3 times more MeSA than !-copaene.

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Maize field atmosphere VOCs 

In the maize field atmosphere, a total of 13 VOCs were detected and identified (Table 1b). The VOCs profile was dominated by MeSA and a complex of p-cymene with limonene. There was also a constant low level of dimethyl nonatriene (DMNT). The  ratio of MeSA to the complex of p-cymene-limonene did not change with diel periods; !-pinene, 3 carene, linalool, !-copaene, « -farnesene, and trans-nerolidol were detected in random amounts in the atmosphere without a clear diel pattern. None of the 13 VOCs detected in the maize field significantly changed in amount over time. Furthermore, when the 13 VOCs were grouped into chemical classes, no significant diel variation was observed for the relative amounts (Kruskal Wallis test, p!0.05 for all cases).

Table of contents :

List of tables
List of figures
List of original publications
Introduction
Study objectives
Insect-plant relationship
Chapter 1: Study subjects
European corn borer
Chemical communication in European corn borer
Taxonomy of European corn borer pherotypes
European corn borer in agriculture
European corn borer host plants
Maize
Mugwort
Hop
Chapter 2: Volatile organic compounds
Volatile organic compounds in plants
Green leaf volatiles
Terpenes
Monoterpenes
Sesquiterpenes
Homoterpenes
Inducible volatile organic compounds
Olfactory environment
Paper I Chemical landscape of maize field for host-seeking moth
Paper II Diel patterns of volatile organic compounds released by maize plants: The chemical
environment of the Ostrinia nubilalis moth
Chapter 3: Host plant specialization
Host plant choice and recognition
European corn borer host plants
Paper III Volatile organic compounds and host-plant specialization in European corn borer E an Z pheromone races
Chapter 4: Assortative mating
Male scent organs
Male pheromones
Courtship behaviour
European corn borer courtship behaviour
Speciation
Sympatric speciation
Paper IV Male hairpencils and assortative mating in European corn borer pherotypes
Discussion and perspectives
References

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