Elsevier

Phytochemistry Letters

Volume 7, February 2014, Pages 225-230
Phytochemistry Letters

Ex situ volatile survey of ground almond and pistachio hulls for emission of spiroketals: Analysis of hull fatty acid composition, water content, and water activity

https://doi.org/10.1016/j.phytol.2013.03.004Get rights and content

Highlights

  • Analysis of ground almond hull and shell emissions detected the spiroketals conophthorin and chalcogran.

  • Conophthorin is an important semiochemical of the insect pest navel orangeworm.

  • The spiroketals appear to be produced by damaged hull tissue.

  • The fatty acid composition, water activity, and water content of developing almond and pistachio hulls were determined.

Abstract

The spiroketal conophthorin has recently been implicated as an important semiochemical of the navel orangeworm moth (Amyelois transitella), a major insect pest to California tree nuts. Additionally, new evidence demonstrates that fungal spores in the presence of linoleic acid produce conophthorin. Numerous investigations have analyzed the volatile emissions of almonds and pistachios under varying conditions, yet there are few reports of conophthorin as a volatile component. Previous studies by our laboratories have suggested almond hulls may be a source of conophthorin production. Accordingly, the volatile emissions of ex situ almond and pistachio ground hulls were surveyed at several developmental stages. Each ground sample was analyzed at various intervals to determine if conophthorin was produced. The almond and pistachio samples were presumed to have a natural fungal bouquet present. Additionally, the fatty acid composition, water content, and water activity of the hulls were analyzed for each sample. Conophthorin and the structurally similar compound chalcogran were detected from almond hulls and shells, but not from the pistachio samples. The almond and pistachio hulls were investigated for four fatty acid components – palmitic, oleic, linoleic, and linolenic. The fatty acid composition of almond hulls varied greatly throughout the growing season, whereas the composition of pistachio hulls remained relatively constant. Both water content and activity were constant in early stages of almond growth then dropped in the later stages of hull split. Spiroketal emission along with other associated volatiles is discussed. This is the first report of the fatty acid composition, water content, and water activity of developing almond and pistachio hulls.

Introduction

The structurally simple spiroketal conophthorin (7-methyl-1,6-dioxaspiro[4.5]decane, 1 in Fig. 1) has recently been reported as an active component in a blend of host plant volatiles that attract both male and female navel orangeworm (Amyelois transitella) moths (Beck et al., 2012a). A. transitella is a major insect pest that inflicts significant economic loss to California almonds, pistachios, and walnuts (Campbell et al., 2003). Moreover, A. transitella larvae are purported to vector toxigenic aspergilli to almonds (Palumbo et al., 2008). Yet, conophthorin and the isomeric chalcogran (2-ethyl-1,6-dioxaspiro[4.4]nonane, 2 in Fig. 1) have long histories as semiochemicals of scolytid beetles with insect and plant origins (Francke and Kitching, 2001). Compared to the history of these spiroketals and scolytid beetles, reports of conophthorin (1) and chalcogran (2) from almonds are relatively new. As an example, recent investigations reported compound 1 from almonds at hull split (Beck et al., 2012a) and both 1 and 2 from mechanically damaged almonds (Beck et al., 2008). More recent was a report of 1 and 2 from various fungal spores on fatty acids common to almond and pistachio (Beck et al., 2012b).

What was interesting regarding the recent detection of 1 and 2 from almonds (all components present – hull, shell, and kernel) was their lack of detection in a number of investigations that reported on the volatile emissions of almonds or pistachios under varying conditions. For instance, spiroketal 1 or 2 was not detected from the following: the vacuum steam volatiles of almond hulls (Buttery et al., 1980); in situ intact and undamaged almonds (Beck et al., 2009); ambient almond orchard volatiles (Beck et al., 2011a); intact ex situ Pistacia spp. (Roitman et al., 2011); and, almond kernels naturally contaminated with orchard fungi (Beck et al., 2011b).

The present investigation was initiated to help delineate the true origin of conophthorin from almonds, and to determine if pistachios were a possible source. This is important for a number of reasons: the inconsistent detection of spiroketals in the aforementioned studies; the rich history of spiroketals and scolytid beetles; the recent assessment that conophthorin (1) is a semiochemical of A. transitella; and, the recent study that demonstrated spores produce both spiroketals (Beck et al., 2012b). Despite the mounting evidence, the question remains – is conophthorin produced by fungi, the host plant, or both? The fungal spore study demonstrated that spores were capable of producing both conophthorin (1) and chalcogran (2), but did not decisively eliminate the plant as an active participant – not just a carbon source for the spores.

Anecdotal evidence from previous volatile emission studies and preliminary exploratory experiments suggested we turn our attention to the hulls of almonds and pistachios as a possible source of compounds 1 and/or 2. Thus, the objectives of this study were to: (1) monitor the volatiles emitted from ground almond and pistachio components, primarily the hulls, to determine a condition for consistent spiroketal production; (2) determine the corresponding fatty acid profiles of the hulls at progressive stages of hull development; and, (3) determine the water content and water activity of the hulls at the varying stages of hull development.

Section snippets

Results and discussion

The objectives of determining the relative fatty acid composition, water content, and water activity of the hulls were successfully met. Furthermore, both spiroketals were observed from ground almond hulls and shells at varying times and amounts (Table 1), yet no spiroketals were observed from the ground almond kernel treatments or any of the pistachio material. The objective of determining specific conditions for consistent spiroketal production was more elusive; however, based on our results

Almond and pistachio collections

In the 2011 growing season, 10 undamaged Nonpareil almonds from three different trees were removed and placed in a 1-quart wide-mouth Mason jar (Ace Hardware, El Cerrito, CA) with a modified lid containing a 1 cm hole and lined with Teflon. Jars were shipped overnight to the laboratory. For pistachios, ca. 40 undamaged Kerman pistachios were removed from three different trees and placed in similar jars. Both almonds and pistachios were collected from commercial orchards located in the southern

Acknowledgements

The authors thank Nausheena Baig, Divya Donthi (USDA-ARS), and Johnny Magana (Paramount Farming) for their valuable contributions. Research was conducted under USDA-ARS CRIS Project 5325-42000-037-00D, TFCA 5325-42000-037-05 with the Almond Board of California, TFCA 5325-42000-037-07 with the California Pistachio Research Board, and portions of RCA 5325-42000-037-13 with the California Department of Food and Agriculture.

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