Nectar peroxide: assessing variation among plant species, microbial tolerance, and effects on microbial community assembly.

Nectar contains antimicrobial constituents including hydrogen peroxide, yet it is unclear how widespread nectar hydrogen peroxide might be among plant species or how effective it is against common nectar microbes. Here, we surveyed 45 flowering plant species across 23 families and reviewed the literature to assess the field-realistic range of nectar hydrogen peroxide (Aim 1). We experimentally explored whether plant defense hormones increase nectar hydrogen peroxide (Aim 2). Further, we tested the hypotheses that variation in microbial tolerance to peroxide is predicted by the microbe isolation environment (Aim 3); increasing hydrogen peroxide in flowers alters microbial abundance and community assembly (Aim 4), and that the microbial community context affects microbial tolerance to peroxide (Aim 5). Peroxide in sampled plants ranged from undetectable to c3000 μM, with 50% of species containing less than 100 μM. Plant defensive hormones did not affect hydrogen peroxide in floral nectar, but enzymatically upregulated hydrogen peroxide significantly reduced microbial growth. Together, our results suggest that nectar peroxide is a common but not pervasive antimicrobial defense among nectar-producing plants. Microbes vary in tolerance and detoxification ability, and co-growth can facilitate the survival and growth of less tolerant species, suggesting a key role for community dynamics in the microbial colonization of nectar.