Foliar uptake of nitrogen from ant faecal droplets: An overlooked service to ant-plants

1. Nutrient supplies to plants from ants are well known from specialised myrmecophytic symbioses and from plants growing in soil close to ant nests. However, above-ground nutrient pathways may play a largely unrecognised role also in less specialised ant-plant interactions—the numerous facultative relationships, where ants forage on plants. 2. In a laboratory experiment, weaver ants (Oecophylla smaragdina) were confined to the canopies of coffee (Coffea arabica) seedlings, excluding any ant-to-plant transfer of nutrients via the soil strata. When ants were fed ¹⁵N-labelled glycine and subsequently deposited faecal droplets on the seedlings, coffee leaves showed increased levels of ¹⁵N and total nitrogen compared to control plants without ants. This was evident for both exposed leaves and leaves covered in plastic bags (i.e. not directly exposed to ants). Thus, nitrogen from ant excretions was absorbed through the coffee leaves, and subsequently, translocated within the plants and possibly leading to the observed higher shoot/root (wet weight) ratios observed on ant-plants compared to controls. 3. Synthesis. These results reveal an undescribed foliar uptake of ant-provided nutrients. If this is a general mechanism, a vast amount of ant-plant interactions involves fertilisation. Foliar fertilisation by ants may be an important steady benefit to plants, which has driven the evolution of ant-plant mutualisms in parallel with the well-recognised but probably more fluctuating benefit from herbivore protection. Given the world-wide abundance of plant canopies foraged by ants, this nutrient pathway may be of high ecological significance.