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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?

Thanks for any insight! :)

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    $\begingroup$ Unfortunately I cannot answer your question, but I think it would be helpful if you could indicate the nature of the fluorescent probes you are using in the experiments. $\endgroup$
    – PAEP
    Commented Sep 11, 2023 at 17:44
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    $\begingroup$ Well, if someone had started the biology class with that, I would have been very curious to know more. Anyway, to the point, and without knowing anything about how twigs gobble up water, I could imagine any kind of hydrogen bonding will positively enhance permeability. But I guess that's hardly helpful here. Still think it's an interesting question. +1 $\endgroup$
    – Martin - マーチン ♦
    Commented Sep 11, 2023 at 23:53

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All else being constant, more H bond donors/acceptors will correlate with lower hydrophobicity. Greater hydrophobicity is expected to lower aqueous solubility. This is a well known principle within drug design as it influences the compound's ADME properties (adsorption, distribution, metabolism and excretion). See for instance Lipinski's rule of 5 or RO5. Note that in the case of RO5 too many HA or HD implies poor ADME due to low lipophilicity and therefore poor membrane permeability. Too high lipophilicity (indicated by the octanol/water partition coefficient) also correlates with poor ADME. The "sweet spot" is somewhere in between.

Similar concepts have been developed for plants, see Ref. 1. You don't explain exactly how the solutions were applied (nighttime or daytime, for instance), or where the compounds were detected, and there may be differences in the transport of the compounds in different parts of the plant (within a leaf as opposed to in sap transported by conducting cells within the phloem). It is expected that transport of the larger compound by diffusion alone is slower (see the Stokes-Einstein equation for instance). Diffusion across a membrane without participation of a transporter would by the same principle also be expected to occur more slowly for larger or heavier compounds (for organic compounds molecular weight is a reasonable proxy of size). In the case of the RO5, some violations occur due to active translocation.

References

  1. Tomer Malchi, Sara Eyal, Henryk Czosnek, Moshe Shenker & Benny Chefetz (2022) Plant pharmacology: Insights into in-planta kinetic and dynamic processes of xenobiotics, Critical Reviews in Environmental Science and Technology, 52:19, 3525-3546, DOI: 10.1080/10643389.2021.1946360
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