Sugar-mediated physical constraints drive the evolution of pollination drops into nectar

It is now well established that the chemical composition of ovular secretions in gymnosperms and angiosperms plays a key role in pollination biology, and the evolutionary history of this chemical profile has attracted considerable attention. In this work, for the first time, we focus on the physical properties of ovular secretions and their potential central role in the evolutionary success of angiosperms. Through wettability measurements on model systems, artificial secretions deposited on female cones of Taxus baccata, chosen as a representative gymnosperm species, we demonstrate that at the cone apex, characterized by 3D confocal scanning profilometry, a highly hydrophobic interaction arises. This interaction depends on both the nanostructure of the cone surface and the specific sugar composition typical of the pollination drop, enabling the secretion to maintain an almost perfect droplet shape and thereby maximizing the interception of airborne pollen grains. Additional observations performed by optical microscopy on the same solutions revealed that the chemical composition of the pollination drop, that is typically dominated by glucose and fructose and low in sucrose, ensures a high degree of pollen stabilization at the droplet surface, unlike solutions having higher concentration of sucrose, such as those typical of angiosperm nectar, which appear highly unstable. Overall the results point out that the sugar profile of the pollination drop is optimal for maximizing airborne pollen capture and to interact with pollen grains once this has landed on the drop. On the other hand this was most probably a constraint that reduced the possibility of chemical adaptation of pollination drop to interact with new pollinating agents, i.e. insects.