Floral nectar is the primary reward directly consumed by floral visitors and its chemical composition affects their behaviour and fidelity. In turn, floral visitors are expected to alter floral nectar composition directly or indirectly through the introduction of external contaminants, such as pollen grains and microorganisms. To understand the effect of exogenous factors on nectar chemistry, we investigated the modifications of nectar sugar and amino acid composition in relation to floral visitors in the perennial herb Gentiana lutea. We quantified nectar contamination by floral visitors through visitor exclusion experiments in the field, estimating pollen and yeast concentration in visited flowers and in uncontaminated nectar at two different flower stages (flower bud and older open flowers). We then assessed changes in the composition and concentration of sugars and both protein and non-protein amino acid in the nectar. We found clear differences in the chemical composition of flower nectar in relation to flower stage and floral visitation. Nectar sugar and amino acid concentrations were significantly higher in older flowers than in flower buds. In addition, nectar sugar concentration was significantly higher in older flowers where visitors were excluded than in visited flowers. On the contrary, amino acid concentration was comparable between visited and non-visited older flowers. We also found that pollen contamination enriched the amino acid profile of the host nectar both quantitatively and qualitatively. Finally, nectar exposed to floral visitors showed a higher number of yeast cells than unvisited flowers although too low to cause the observed changes in the glucose:fructose ratio, suggesting the presence of fructose-related bacteria. Our results show that the chemical composition of floral nectar is altered by several endogenous and exogenous factors in a complex process of ecological relations, and suggest that the modulation of floral visitation through nectar is not solely related to the plant itself.

中文翻译：

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花蜜化学不仅仅是植物的事情：花粉访客影响花蜜糖和氨基酸组成

花蜜是花卉游客直接消费的主要奖励，其化学成分会影响他们的行为和忠诚度。反过来，花粉游客预计会通过引入外部污染物（如花粉粒和微生物）直接或间接改变花蜜成分。为了了解外源因素对花蜜化学的影响，我们研究了与多年生草本植物Gentiana lutea中花蜜相关的花蜜糖和氨基酸组成的变化. 我们通过实地的访客排除实验量化了花卉访客的花蜜污染，估计了在两个不同花期（花蕾和较老的开放花）访问的花朵和未受污染的花蜜中的花粉和酵母浓度。然后，我们评估了花蜜中糖类以及蛋白质和非蛋白质氨基酸的组成和浓度的变化。我们发现花蜜的化学成分与花期和花的访问量存在明显差异。老花中的花蜜糖和氨基酸浓度显着高于花蕾。此外，在排除访客的老花中，花蜜糖浓度显着高于参观花。相反，访问和未访问的老花之间的氨基酸浓度相当。我们还发现花粉污染在数量和质量上都丰富了宿主花蜜的氨基酸谱。最后，暴露于花卉访客的花蜜显示出比未访问的花朵更多的酵母细胞，尽管太低而不会导致观察到的葡萄糖：果糖比率的变化，这表明存在与果糖相关的细菌。我们的研究结果表明，花蜜的化学成分在生态关系的复杂过程中受到多种内源和外源因素的影响，并表明通过花蜜对花卉访问的调节不仅仅与植物本身有关。我们还发现花粉污染在数量和质量上都丰富了宿主花蜜的氨基酸谱。最后，暴露于花卉访客的花蜜显示出比未访问的花朵更多的酵母细胞，尽管太低而不会导致观察到的葡萄糖：果糖比率的变化，这表明存在与果糖相关的细菌。我们的研究结果表明，花蜜的化学成分在生态关系的复杂过程中受到多种内源和外源因素的影响，并表明通过花蜜对花卉访问的调节不仅仅与植物本身有关。我们还发现花粉污染在数量和质量上都丰富了宿主花蜜的氨基酸谱。最后，暴露于花卉访客的花蜜显示出比未访问的花朵更多的酵母细胞，尽管太低而不会导致观察到的葡萄糖：果糖比率的变化，这表明存在与果糖相关的细菌。我们的研究结果表明，花蜜的化学成分在生态关系的复杂过程中受到多种内源和外源因素的影响，并表明通过花蜜对花卉访问的调节不仅仅与植物本身有关。表明存在果糖相关细菌。我们的研究结果表明，花蜜的化学成分在生态关系的复杂过程中受到多种内源和外源因素的影响，并表明通过花蜜对花卉访问的调节不仅仅与植物本身有关。表明存在果糖相关细菌。我们的研究结果表明，花蜜的化学成分在生态关系的复杂过程中受到多种内源和外源因素的影响，并表明通过花蜜对花卉访问的调节不仅仅与植物本身有关。