Department of Entomology, Texas A&M University, College Station, Texas, United States of America.
Southern Plains Agricultural Research Center, USDA-ARS, College Station, Texas, United States of America.
PLoS One. 2021 Oct 29;16(10):e0258836. doi: 10.1371/journal.pone.0258836. eCollection 2021.
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.
栽培棉，如陆地棉（Gossypium hirsutum L.），在叶片（叶状）和生殖结构（苞片）上产生额外的花（EF）蜜，作为一种间接的抗食草动物防御。作为这种富含碳水化合物物质的交换，捕食性昆虫，如蚂蚁，保护植物免受草食性昆虫的侵害。一些产生 EF 蜜的植物通过增加 EF 蜜的产生来响应食草动物的侵害。例如，无食草动物的陆地棉产生的苞片 EF 蜜比叶状 EF 蜜多，但在受到食草动物侵害时，它会增加叶状 EF 蜜的产生。本研究首次测试了对陆地棉叶片食草动物侵害后苞片 EF 蜜的碳水化合物组成的系统诱导变化。我们发现，叶片食草动物显著增加了苞片 EF 蜜中的蔗糖含量，而葡萄糖和果糖保持不变。蔗糖含量已知会影响蚂蚁觅食行为，先前对 EF 蜜热量含量的食草动物诱导增加的研究发现，这会导致蚂蚁在植物上的活动增加。作为我们发现的后续研究，在野外测试了模拟 EF 蜜溶液中蔗糖含量变化对蚂蚁招募的影响。蚂蚁对两种溶液都没有表现出任何偏好，这可能是因为蔗糖是陆地棉 EF 蜜中一种次要的碳水化合物成分：总糖含量没有因蔗糖的增加而显著受到影响。尽管如此，我们的研究结果提出了关于棉花对食草动物侵害的诱导 EF 蜜反应的新问题。需要进一步的研究来确定蔗糖含量的食草动物诱导增加是否是棉属植物的典型特征，以及它是否构成系统蔗糖诱导的必然结果，或者是一种潜在的适应性机制，增强了蚂蚁对植物的吸引力。
