We are planning an experiment with pine and oak seedlings, for which we would like to reduce transpiration flow, but keeping as much as possible plant functionality. Our first thought was ABA, however as a plant hormone this would also induce other stress responses.
Dry down experient is an option. You must be sure that roots explore all the soil before you begin the dry down (covering pots to avoid looses of water by evaporation). So you will be sure that plants will regulate water content in the plant with stomatal conduntance and not by avoidance mechanisms with root behaviour.
Do you want to close stomata or reduce transpiration? If you only want to reduce transpiration, you can place your seedlings in a high humidity growth chamber.
Dear Hsien and María. Thank you for your quick answers. We are interested in looking at how drought affects the residence time of water in the xylem, and the effect on water loss and its isotope composition through evaporation in the stem. Thus, what we want is to reduce leaf transpiration, but keeping high VPD to allow stem evaporation, as happens under water stress, and also avoiding changes in the soil water content, which would confound our isotope measurements.
Hi Juan.
I do not know what is "the best" solution, but you may want to consider spraying pinolene, a polymer of beta-pinene. It forms a thin layer on leaves, blocking transpiration and overall gas-exchange. It has been used in viticulture to provoke a light carbon starving stress, and reduce fruit set. Again, you have to say and decide whether you want just a stomata closure or a reduction in transpiration. Actually, if you spray the whole tree, stem transpiration would be reduced as well by pinolene. It is just an idea, cheaper than ABA (or fusicoccin...).
Best,
Andrea
Have a look at this paper: http://repository.ksu.edu.sa/jspui/bitstream/123456789/136/1/Effects%20of%20Kaolin%20and%20Pinolene%20Film-forming%20Polymers%20on%20Water%20Relation%20and%20Photosynthetic%20Rate%20of%20Tuberose%20(Polianthes%20tuberosa%20L.).pdf?origin=publication_detail
As leaf water potential drops, stomata tend to close. Again, this effect may be direct or indirect. If water potential is very low, the guard cells themselves, and the surrounding subsidiary cells, may lose turgor, causing stomatal closure directly.Much more often, the response to leaf water potential is indirect. As leaf water potential drops, the content of ABA in the leaves increases. This ABA appears to sensitize the stomata to other signals that would normally cause closing, and so the average stomatal aperture is decreased. It appears that as the soil dries, the roots "sense" the lower soil water potential, and synthesize ABA. This ABA is transported to the leaves in the transpiration stream (i.e., via the xylem), and produces the same effect on stomata as leafsource ABA. This mechanism may allow plants to anticipate drought conditions and respond accordingly, even before the leaf water potential has been affected.
I don't know if I understood very well, but Dr. Pitacco's answer seem to be an option to consider. With some care, perhaps you can protect the stem from the spray. Stomata may be the most sensitive part of the plant, and with many redundant control mechanisms. You sure cannot mess with them without collateral effects. As Dr. Pitacco suggested, even a thin polymer film will cause a response elsewhere (reduced fructification in Vitis). Good luck.
I think that spraying seedlings with yeast solution will decrease the rate of transpiration by inducing stomatal closure as well as protect seedlings infection.
I've published many years ago that leaf spraying with ABA promote shoot growth in Ilex paraguariensis by alleviating water stress at midday.
Please, see the attached paper.
ABA promoted stomatal closure that was positively correlated with an increase of the RWC in leaves. This situation was reflected in higher leaf expansion implying augmentation of expansion growth; that is, the new leaves and sprouts of the ABA-treated plants grew more than those of untreated control plants. These results are in agreement with the concept that expansion growth is very sensitive to water deficit, especially in leaf tissues (Granier and Tardieu 1999). In fact, the role of ABA in the
adaptation of a variety of plant species under extreme conditions of water deficit has been well established (Zeevaart and Creelman 1988). ABA-deficient mutants showed higher damage at leaf level under conditions of water stress, in correlation
with their ABA contents (Fambrini et al. 1994).