Foliar herbivory increases sucrose concentration in bracteal extrafloral nectar of cotton

Abstract

Cultivated cotton, such as Gossypium hirsutum L., produces extrafloral (EF) nectar on leaves (foliar) and reproductive structures (bracteal) as an indirect anti-herbivore defense. In exchange for this carbohydrate-rich substance, predatory insects such as ants protect the plant against herbivorous insects. Some EF nectar-bearing plants respond to herbivory by increasing EF nectar production. For instance, herbivore-free G. hirsutum produces more bracteal than foliar EF nectar, but increases its foliar EF nectar production in response to herbivory. This study is the first to test for systemically induced changes to the carbohydrate composition of bracteal EF nectar in response to foliar herbivory on G. hirsutum. We found that foliar herbivory significantly increased the sucrose content of bracteal EF nectar while glucose and fructose remained unchanged. Sucrose content is known to influence ant foraging behavior and previous studies of an herbivore-induced increase to EF nectar caloric content found that it led to increased ant activity on the plant. As a follow-up to our finding, ant recruitment to mock EF nectar solutions that varied in sucrose content was tested in the field. The ants did not exhibit any preference for either solution, potentially because sucrose is a minor carbohydrate component in G. hirsutum EF nectar: total sugar content was not significantly affected by the increase in sucrose. Nonetheless, our findings raise new questions about cotton’s inducible EF nectar responses to herbivory. Further research is needed to determine whether an herbivore-induced increase in sucrose content is typical of Gossypium spp., and whether it constitutes a corollary of systemic sucrose induction, or a potentially adaptive mechanism which enhances ant attraction to the plant

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“…Samples were then transferred to two ml screw top vials containing 150 μL glass inserts (Agilent Technologies, Santa Clara, CA, USA) and run on the Agilent 1200 binary LC gradient system using the Hi-Plex Ca (Duo) 7.7 × 50 mm column and 8 μL guard column (Agilent Technologies, Santa Clara, CA, USA). The column was eluted with 100% HPLC water at a flow rate of 4.0 mL/min at 80 °C [ 51 ]. The amino acid composition of SA honeydew was analyzed using an Agilent 1200 HPLC connected to a diode array detector (Agilent Technologies, Santa Clara, CA, USA).…”