Question

Yes, weird plant scientist asking weird plant questions...

Background: I am looking at how the twigs of trees absorb water, a poorly understood means by which trees alleviate water stress. Of 4 fluorescent tracers in an aqueous solution, experimentally applied to twig surfaces, 2 enter the twigs and 2 do not. The 2 that are absorbed have a hydrogen bond donor count that is 6-8X greater than the 2 that don't. Of all the chemical properties, this is by far the most different between the pairs. Oddly, it is the largest and smallest molecules that are absorbed, so I am thinking it is not molecular weight. All 4 compounds have a formal charge of zero.

The solutions were applied directly to the intact surface of detached (but live) twigs, on the twig epidermis (not leaves), during the day. Transport was detected throughout, both symplastically (inside the cells so a membrane was crossed) and apoplastically (outside the membrane, so a layer of suberin was crossed).

The apoplastic tracers were: Sulforhodamine B sodium salt (not absorbed) and Calcofluor white (absorbed)

The symplastic tracers were: HPTS (not absorbed) and 5(6)-Carboxyfluorescein (absorbed)

Question: What does it mean? Can an H-bond donor count positively influence absorption, particularly across suberin/other waxes or membranes (the 2 possible entry points)? Or could the smaller HBD:HBA ratio impact absorption? How?

Answer 1

The higher hydrogen bond donor count of the absorbed fluorescent tracers suggests that hydrogen bonding capacity influences the absorption by plant twigs. This can affect transport through hydrophilic and hydrophobic barriers in plants by enhancing solubility and interaction with water potential in the plant. A smaller HBD:HBA ratio may reduce this effect and limit absorption.

Step-by-step explanation:

The observation that the two absorbed fluorescent tracers have a significantly higher hydrogen bond donor (HBD) count suggests that the ability to form hydrogen bonds can influence the absorption of molecules by twigs in plants.

The high HBD count of the absorbed molecules may play a crucial role in their interaction with the water potential within the plant's tissues.

Water molecules in plant tissues readily form hydrogen bonds with solutes, which can affect the passage of these molecules across various barriers such as the suberin layer in the apoplast or cellular membranes encountered during symplastic transport.

These findings could indicate that the higher HBD count provides multiple points of attachment for the water molecules, facilitating the traversal of these tracer molecules through both hydrophilic and hydrophobic barriers.

This would be consistent with the known role of hydrogen bonding in increasing solute potential and the permeability of the cell wall to water and water-soluble molecules through adhesion and cohesion mechanisms.

Consequently, a smaller HBD to hydrogen bond acceptor (HBA) ratio may inherently impact absorption due to a lower capacity for hydrogen bonding, potentially leading to reduced solubility or permeability through plant tissues.