Help! Help! The Sky is Falling!!!

You all remember the story of Chicken Little, right? Chicken Little thought she’d been hit on the head by a piece of the sky and ran around alarming the rest of the barnyard animals, who assumed Chicken Little knew what she was talking about. Had they not all been eaten by Foxy Loxy, I’m sure they would have felt foolish discovering that an acorn, not a piece of the sky, had bopped Chicken Little on the noggin.

On to today’s science rant.

Ann McCormick, one of my GWA (Garden Writers Association) colleagues, alerted me to an online story from Live Science entitled “Water Drops Magnify Sunlight and Burn Leaves.” A quick look at the internet shows that this report has gone viral, with similar headlines from other websites including the venerable Scientific American (“A study in the journal New Phytologist confirms the gardener’s belief that droplets of water resting on some types of leaves can focus sunlight until the plant’s surface actually burns”). It gets more and more ridiculous by the day (“Sun shining?  Then don’t water your plants” courtesy of the Daily Mail in London; “Water droplets can form forest fires” from Calcutta). And so on.

I tracked down the original article in New Phytologist, entitled “Optics of sunlit water drops on leaves: conditions under which sunburn is possible.”  You should take a look at it, if only to become completely intimidated by the physics and computer modeling it contains. I’ll be honest – I didn’t even try to understand this portion but focused on the plant science.

The authors had three actual experiments in addition to the optical modeling.  The first experiment involved placing glass spheres on detached leaves (Acer platanoides – Norway maple) and exposing them to sunlight. Yup, glass spheres caused leaf burn on sunny days – no big surprise there. The second experiment substituted water droplets for the glass spheres and tested Ginkgo biloba as well as maple leaves. Not surprisingly (to me anyway) there was no damage to leaves of either species. The third experiment repeated the second, but tested the leaves of the aquatic fern Salvinia natans and voila! Leaf damage!

I have a lot of issues with this paper and maybe we’ll have to extend blog coverage for a day or two to keep today’s discussion as short as possible. Let me point out just two of the experimental problems

The leaves for experiments 1 and 2 were detached from the plant prior to treatment. How a detached leaf resembles anything in a natural situation is beyond me. Furthermore, these leaves were laid out, covered with glass spheres or water droplets, and left in the full sun for as long as nine hours. (Even so, the leaves covered with water droplets didn’t burn! You go, detached leaves!)

The Salvinia experiments were conducted on leaves scooped out of a pond, placed in two containers, sprinkled with water, and left in the sun for two hours. Then, in the authors’ words, “the experiment was concluded by cutting and scanning several Salvinia leaves – still holding water drops – in the laboratory in order to document their sunburn.” We are not told (a) how many leaves were scanned, (b) how the leaves were chosen, since they didn’t scan them all, and (c) how the reported damage was proven to be from water droplets. >Worse, there are no statistical data. We are simply asked to believe their report in the absence of any evidence except a handful of photographs. (As an aside, I really would like to know how they were able to cut and transport leaves without the water droplets moving!)

The lack of scientific rigor in this article is disappointing, especially in a peer-reviewed journal. Is there any plant scientist would consider detached leaves to be a model for those on an intact plant? Is there any gardener who would consider an aquatic fern comparable to trees and shrubs? Would any species – including aquatic ferns – easily burned by the combination of water and sunlight survive in the real world for long? I don’t think so – hence my myth posting on this very topic several years ago.

Let’s review: leaves of one species of aquatic fern was damaged by something – possibly sunlight – but without enough data presented to really evaluate the claim. None of the tree leaves tested were affected, even though they were detached from the plant and could not benefit from transpirational cooling. Yet the alarm has gone out! Don’t water your garden plants when it’s sunny, or the leaves will burn!!!

Be sure to take everything you read with a grain of salt – or an acorn.

(I will continue discussion of this article further if there is enough interest – that means you need to post a comment!)

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

25 thoughts on “Help! Help! The Sky is Falling!!!”

  1. I read the abstract of this report which offered no hard data that would entice me to even want to read the full study. If this were a medical study, I’d pass it by without another glance. That a story is published by a website or periodical with the word Science in it’s title doesn’t mean it’s backed up by good science. Too bad more readers neglect to get this. Besides, how many times has rain or dew covered leaves, yet when the sun comes out immediately afterwards there is no leaf damage. Common sense!

  2. I’m speechless. Almost.
    Why does the press publicize this crap? If NPR picks this up, I’m going ballistic.

    I do know that water droplets can cause spots on fuzzy Gloxinia leaves (for various reasons, including water that is too cold, botrytis, scald), thus many growers use subirrigation on glox and african violets (while petting them with lotioned hands ;). But this is an aesthetic consideration…I truly doubt they will spontaneously combust. Sheesh.

  3. Keep writing this stuff, Linda! It’s so refreshing to read the voice of reason when I open The Garden Professors, and just great to learn something new with every post.

  4. The paper is a baffling interpretation of some highly questionable data, if you can call it data. If I was to submit that as an undergraduat I’d be torn to shreds by the lecturer. Yet it’s published in a bloody journal! Ha!
    I read the ” leaves for experiments 1 and 2 were detached from the plant prior to treatment” and laughed out loud. It’s like cutting off someone’s hand, dunking it in LN2, then asking them how it feels.
    There are two who should be the focus of this ire, though. The first of course is the author and journal, the second is the media for reporting such nonsense.

  5. I’ll say it before, I’ll say it again: Never have I jumped out of the pool on a sunny day and burst into flames. Never.

  6. (LOL) Dave, the inevitable question – are you pubescent or glabrous? The authors would say that makes all the difference!

  7. I was hoping you’d done a little test on this myth. Since I just found your blog maybe you have and I didn’t get there yet. BTW- great blog!
    Me thinks you should test this out …on canna ‘bengal tiger’ in full sun. I believe it has burned for me. Perhaps my testing was flawed. PLease give it whirl. thx

  8. Karen, check out my web page under “Horticultural Myths” and look way back to August 2002 – that’s when I first wrote about this myth. Nothing has changed since then, other than there are some newer articles that continue to disprove the myth. If you’re interested in running your own test on this – you’ll need several plants to test – about 20 would be best, all in the same general location and ideally in separate containers. Half of these you would sprinkle with water on a sunny day, the others you would not. You would need to select your test subjects randomly, by numbering them and then pulling numbers out of a hat (or something similar). This is a very simplistic (and incomplete) explanation on how to set up an experiment.

  9. Linda,

    Yes, yes, continue the discussion please. This myth has ballfed me for several years. When investigating an apparent scorch on my Echinacea (I’m a grower) several other professional growers suggested overhead irrigation on hot sunny days may be the problem. Clearly the “full sun” plants of the world have experienced rain followed by sun without leaf burn, but the myth pesists. BTW, I never solved the Echinacea foliage burn issue – I’ll send pics if you’re interested. Now I’m off to read your earlier post. More! More!

  10. OK! On Wednesday (my appointed blog day) I’ll post further commentary on this article. Let me know if there’s anything specific you’d like me to include.

  11. Linda. I am the second author on the paper you are ranting about. I truly admit, I am not a plant scientist. However, I think you are barking up the wrong tree. In the original manuscript, the emphasis was more on the results, which showed no burn damage to the leaves due to water drops. The reviewers demanded the title to be changed to reflect the case of hairy leaves, which did suffer burn damage. It is not our fault that news sites highlight that case, and maybe misrepresent the gist of the paper. Second, your criticism about not performing the experiments in nature is splitting hairs. Science is hierarchic, you start from the simplest possible setup/model, extract what you can, and then move on to a more complex model/experiment. That is how physicist approach a problem, and this has worked just fine for centuries (see Consider a Spherical Cow: A Course in Environmental Problem Solving by John Harte). By your argument, a problem should not be studied until you are able to make to most complex and realistic experiment. Our study is not the most definitive answer to the problem at hand, but it is one step. Your biggest bone with us seems to be that we did not cite your previous blogs/articles in our paper. That might be a valid criticism, but it was not intentional. Finally, I would not be proud of the fact that I did not even TRY to understand the physics and computer modeling presented in the paper I am ranting about; as you so proudly stated in the third paragraph of your blog entry.

  12. Akos, thank you for your comments. However, let me respond: First, your paper presents no statistically valid data for us to interpret regarding leaf burn of Salvinia leaves. I think you need to read that section of my critique more carefully. Second, there has been a great deal of peer-reviewed plant science on sources of leaf burn (you cited some of these in the article). The problem has been studied, and the results clearly refute the premise that uncontaminated water burns leaves. Third, it’s immaterial to me whether or not you use my online information – it’s intended for a nonacademic audience. What does concern me is that you did not fully review the peer literature on the topic. Finally, I’m a plant scientist, not a physicist. I actually do have a Master’s degree in photobiology, but I am not qualified to review whether or not your optical modeling experiments are valid. It’s a statement of fact – nothing more. Why do you think I was “proud” of this?

  13. Linda. Thank you for your reply. Here are my thoughts on your blog entry. 1. It might be true we did not fully review the literature. Point taken. Nobody prevents you from submitting a letter/comment on our paper to New Phytologist, and go through the peer-review process yourself. In fact, I urge you to do that, which would give us the chance to respond to your criticism in an official forum. 2. It is beyond our control how tabloids, such as the Daily Mail of London, misrepresent our results. This newspaper is the British equivalent of The National Enquirer or the New York Post. Would it be fair to hold you responsible if one of your research papers were misrepresented by such news outlets? Citing the Daily Mail in your blog as if it represented our views amounts to the same spin you are rightly criticizing. It frankly is dishonest. 3. As I explained before, the gist of our paper was that water drops do not burn leaves. In fact, our original title explicitly stated the refutation of this premise/myth. 4. The experiments might not have been numerous enough to reach statistical significance; however, the optical modeling, I believe, is sound and represents new results/insights. 5. In other blogs where you rant about this paper, you called it “junk science”. I maintain, such unconstructive language is rather bold coming from a person who openly admits her inability to follow the physics/ray tracing modeling in our work. 6. I understand a nonacademic audience requires simpler language; however, it does not give you license for engaging in spin on par with Daily Mail. 7. You criticize scientific rigor in our work, but then you rant on a patently non-peer reviewed blog, which is fully under your control. This is hardly the right avenue to advance knowledge. In sum, I might find some of your criticism valid, but I also find your tactics and language rather unconstructive.

  14. Akos, I actually find debating this topic on an open blog a better forum than going through the journal. Had your article not received vast public coverage, I would agree with you. The proverbial cat is, however, out of the bag. People are hearing of your results – misrepresented or otherwise – and believing that they should not water their plants when the sun is out. This becomes my problem, because I’m involved in outreach education and this is bad advice. I now have to refute this information and the best way to do that, for my audience, is in a public rather than academic forum. I hope you would see the validity of that. Here are some further points: (1) Your results were reported/misinterpreted by Scientific American online – which many people respect as a valid scientific resource. (2) I have no beef with your optical modeling experiments, and they may be perfectly valid. Is there a reason you chose not to publish them in a more subject-appropriate journal, rather than a plant science journal? (3) If you carefully read my comment about “junk science”, it was in direct reference to your claim that you reviewed the “forestry literature” when in fact you looked at 3 web sites with little, if any, scientific value. That is a blatant misreprentation and what I refered to as “junk science.” (4) I did not refer to your optical modeling as “junk science” as I have already admitted I am not an expert in that field. I imagine you would be insulted if I pretended to be. Consider how I feel, as an applied plant physiololgist, when I read an article in a plant science journal that misrepresents the science, employs questionable methologies, and lacks statistical relevance.

  15. Linda. As I stated before, it is beyond our control how Scientific American, or any other news outlet for that matter, (mis)represents our results. You should direct your ire toward them, and only criticize us for what we have actually stated in the paper but you disagree with. I see your point of reaching the broadest possible audience, but the particular way you did it falsely suggested we warned people not to water their plants when the sun is out. If anything, our results refute this myth. Why do you consider New Phytologist a non-subject
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    -oriented journal for an article discussing the issue of leaf damage due to focusing of sunlight by water drops? Just like you, we aim to reach the broadest possible audience, and submitting the paper to Applied Optics, for example, would have limited its impact. The very fact that, according to your own words, our “article received vast public coverage” confirms the choice of NP. Well, you definitely did not bend over backwards to clarify which parts you considered junk and which you might accept. The strong criticism in your last sentence would, IMHO, merit a peer-reviewed comment in NP.

  16. Actually, I’m with Akos on this. Call me old school but if you have a problem with someone’s science, you take it up in the venue where it’s published. It’s one thing to comment on the interpretation or mis-interpretation of a paper in the popular press. But when we start getting into point by point criticisms of a scientific paper and impugning the integrity of a journal in which none us could ever get published, I think we’re stepping over the line.

  17. Just thought I’d jump in with a quick comment — Linda is out giving talks today — Akos, I’m another “garden professor” who posts on this blog. I really am enjoying following the back and forth between the two of you and I think that you both have raised excellent points. Akos, I strongly support your right to publish wherever you want/can. Addit
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    ionally, I don’t have a problem with you adding the data that you collected on leaves (Data is, after all, meant to be shared), though I must agree with Linda that this was, by far, the weakest part of your paper. Look, you chose to target plant researchers as your audience by selecting to publish in the journal that you did — but the plant science in this paper was amazingly poor — as it stands I think that the data which you collected from these leaves is insufficient to reach any sort of a conclusion — it’s hard for me to believe that you didn’t expect to get backlash that you did. Once again, I don’t think that you were wrong for including this data — but my goodness — just a quick conference with a plant researcher should have provided some much, much better methodology. I also agree that it’s not your fault that someone spun your paper inappropriately — I’ve had it happen to me (though on a smaller stage) and I empathize. That said, as “Garden Professors”, we reserve the right to get mad about poor science — we also reserve the right to do it on a public stage. Perhaps Linda will send a comment to NP, but you can bet that response won’t receive 1/4 the attention that her response here did.

  18. Dear Garden Professor Chalker-Scott,

    I am the senior and corresponding author of the paper entitled

    Adam Egri, Akos Horvath, Gyorgy Kriska, Gabor Horvath (2010) Optics of sunlit water drops on leaves: Conditions under which sunburn is possible. New Phytologist, Vol.185(4), doi:10.1111/j.1469-8137.2009.03150.x.

    I began to read your blog etitled “Help! Help! The Sky is Falling!!!”

    https://sharepoint.cahnrs.wsu.edu/blogs/urbanhort/archive/2010/01/20/help-help-the-sky-is-falling.aspx

    with interest, but soon I had the feeling that you must have seriously misunderstood something, and your superficial comments were merely your first impressions elicited by the poor, ambiguous summaries appearing on the internet, rather than sound reflections on our theoretical, computational and experimental studies. I would like to ask you to reply to my comments below in your capacity as a professor, rather than an emotional, common blogger.

    I understand your frustration: You are a professor of plant science, who until now seemed to understand, in depth, all aspects of the photobiology and biooptics of water-droplet-covered plant leaves, and now came four “outsiders”, two biophysicists, one biologists, and one meteorologist, who dared to perform three basic experiments and do computer modelling on your turf, in order to clarify the biooptics/photobiology of leaves after rain/watering, and who partly refuted, but also partly confirmed a long-lasting myth, the well-known gardener’s belief. Our results were published in a highly-ranked international peer-reviewed plant journal, one of our photos appeared on the cover of this journal, and our paper elicited a huge media echo. Unfortunately, we did not cite your earlier works on this topic. I admit, I was unaware of your peer-reviewed scientific publications on this subject (if any), and surely do not know all the relevant papers. However, in the continuation of our present paper we will certainly refer to your experimental results, if you kindly forward copies of your relevant publications to us.

    Let me now respond to some of your specific points.

    YOU WROTE: Ann McCormick, one of my GWA (Garden Writers Association) colleagues, alerted me to an online story from Live Science entitled “Water Drops Magnify Sunlight and Burn Leaves.”

    ANSWER: This bombastic title from Live Science is obviously misleading, for our major finding is just the contrary: We showed experimentally, that smooth (hairless) leaves cannot suffer sunburn due to adhered water droplets, thereby mostly refuting the common myth. However, we also sophisticated further by the experimental fact that only hairy leaves can suffer sunburn in certain light environments. Note that the misleading title originated not from us, but from the journalist of Live Science, who did not communicate with us prior to publishing her/his piece of work.

    YOU WROTE: A quick look at the internet shows that this report has gone viral, with similar headlines from other websites including the venerable Scientific American (“A study in the journal New Phytologist confirms the gardener’s belief that droplets of water resting on some types of leaves can focus sunlight until the plant’s surface actually burns”).

    ANSWER: As mentioned above, our paper confirmed this belief only partly, for hairy enough leaves, but this general belief was actually refuted for smooth leaves.

    YOU WROTE: It gets more and more ridiculous by the day (“Sun shining? Then don’t water your plants” courtesy of the Daily Mail in London; “Water droplets can form forest fires” from Calcutta). And so on.

    ANSWER: All these interpretations of our results are indeed ridiculous. In the future, please debate such bombastic headlines with the non-professional journalists who wrote them, rather than with us.

    YOU WROTE: I tracked down the original article in New Phytologist, entitled “Optics of sunlit water drops on leaves: conditions under which sunburn is possible.” You should take a look at it, if only to become completely intimidated by the physics and computer modeling it contains. I’ll be honest – I didn’t even try to understand this portion but focused on the plant science.

    ANSWER: Did you really get the WHOLE article including its online SUPPLEMENT? Our paper will be print-published only in the next issue of New Phytologist. Or did you have access to the full online material of our proof-corrected paper? This is a crucial question, because if you could not get the full paper and its supplement, you should be only half-informed about our methods, details and results, like the poor journalists mentioned above.

    On the other hand, as my brother, Akos Horvath pointed out, in your situation I would not be too proud of the lack of basic knowledge in physics, optics, especially, if I were a researcher dealing with PHOTObiology; the photo-half of which is (radiation) physics. Lacking possession of basic optics, you should be more careful when debating this topic with actual biophysicists (AE, GH) and an atmospheric scientist (AH).

    Our computational study was ESSENTIAL to understand the fine optical details of non-spherical water droplets adhered to smooth leaves, or held by wax-hairs above the leaf epidermis. Note that half of our new results concerned optical physics, because famous earlier studies (from Newton and Descartes through Rayleigh, Mie, Airy, to Nussenzweig, Können, Lee, etc.) did not deal with this optical problem, but concentrated only on light collecting by spherical or semi-spherical water droplets, such as, the well-known rainbow phenomenon. Consequently, without TRYing to understand the basic physics/optics of the phenomenon investigated by us, you seriously undermine the credibility of your criticism. In my opinion, neglecting the (bio)physics, (bio)optics of sunlit water droplets resulted in your misunderstanding the gist of our paper. The possible sunburn of leaves by water droplets is a typical interdisciplinary, complex problem: partly physics, partly plant biology. It simply cannot be dealt with using an incomplete approach, that is, “focusing on the plant science” only.

    YOU WROTE: The authors had three actual experiments in addition to the optical modeling. The first experiment involved placing glass spheres on detached leaves (Acer platanoides – Norway maple) and exposing them to sunlight. Yup, glass spheres caused leaf burn on sunny days – no big surprise there.

    ANSWER: Do you know of anybody (e.g., researcher, scientist, teacher, gardener, amateur), who already conducted this simple experiment with plant leaves and glass spheres of various diameters? I was really surprised, that such sunlit glass spheres can so seriously burn the leaves at any time of day, independently of their diameter. This experiment should/could be a basic experiment in text-books of plant biology and optics, but as far as I know, it is not. Why was the mentioned result not a big surprise to you? How did you expect, for example, that the capability of glass spheres to cause sunburn in leaves is independent of their radius? How could you expect that any sunlit glass sphere can burn the underlying leaf independently of the solar elevation angle? Having a hunch is not a substitute for conducting actual experiments!

    YOU WROTE: The second experiment substituted water droplets for the glass spheres and tested Ginkgo biloba as well as maple leaves. Not surprisingly (to me anyway) there was no damage to leaves of either species.

    ANSWER: How did you expect that a spheroid water droplet adhered to a smooth, water-repelling ginkgo leaf cannot burn the leaf? According to our computer modelling, at a solar elevation of about 23 degrees the focal region of a spheroid droplet falls onto the leaf epidermis outside the contact area (thus, not subject to water cooling), and the intensity of sunlight is enhanced by a factor of 251 compared to a flat drop (see online supplement of our paper). How did you know that a 251-fold intensification of sunlight cannot burn a detached leaf? Are you suggesting that you know the species-specific intensity thresholds, which can cause burn damage in leaf tissues? By the way, do you know of anybody, who performed this very simple experiment with different plant leaves and water drops of various sizes prior to us? I was rather surprised, that the spheroid water droplets adhered to ginkgo leaves did not cause sunburn, in spite of the fact that their shape was quite similar to the glass spheres used in our 1st experiment. This experiment, too, could be a nice basic experiment in any text-book of plant biology and optics. However, it is not!

    YOU WROTE: The third experiment repeated the second, but tested the leaves of the aquatic fern Salvinia natans and voila! Leaf damage!

    ANSWER: How did you expect that spheroid water droplets on extremely water-repellent (superhydrophobic) ginkgo leaves cannot cause sunburn, but the same droplets on the similarly superhydrophobic leaves of floating fern can? What is the fundamental difference between these two similar situations? The answer is clearly spelled out in our paper! But without our results could you have definitely answered this question? I can only suggest authors of future text-books in optics and plant science to include this simple experiment, the result of which is surprising, but convincing.

    YOU WROTE: I have a lot of issues with this paper and maybe we’ll have to extend blog coverage for a day or two to keep today’s discussion as short as possible. Let me point out just two of the experimental problems:
    1) The leaves for experiments 1 and 2 were detached from the plant prior to treatment. How a detached leaf resembles anything in a natural situation is beyond me. Furthermore, these leaves were laid out, covered with glass spheres or water droplets, and left in the full sun for as long as nine hours. (Even so, the leaves covered with water droplets didn’t burn! You go, detached leaves!)

    ANSWER: If you had read our paper carefully, you would know that we applied sunlight exposures of several different time spans. It was only the longest exposure that lasted 9 hours, the other ones were much shorter: 1 and 3 hours. The exposure of water-droplet-covered leaves lasted until the droplets evaporated within 1-2.5 h, depending on the time of day (see Table 1 of our paper). Hence, not all exposures were 9 h long. During these exposures the intensity of sunlight changed drastically, because the solar elevation changed too, from low values in morning to high values at noon. Thus, we could study the phenomenon under all relevant illumination conditions. Hence, this issue is not a methodological error, but this was intended to cover the range of natural light conditions.

    Furthermore, if our detached leaves could not suffer sunburn at any solar elevation, independently of the shape of the water droplets, it can surely be stated that intact leaves do not suffer sunburn either. Why? Because in case of intact leaves the plant can benefit from transpirational cooling, which was lacking for detached leaves. This is the trivial plant biological reason why it was enough to use detached leaves in our 1st and 2nd experiments. Would you accept this? The referees of our paper certainly did.

    YOU WROTE: 2) The Salvinia experiments were conducted on leaves scooped out of a pond, placed in two containers, sprinkled with water, and left in the sun for two hours. Then, in the authors’ words, “the experiment was concluded by cutting and scanning several Salvinia leaves – still holding water drops – in the laboratory in order to document their sunburn.” We are not told (a) how many leaves were scanned,

    ANSWER: Although we stated that there were 120 Salvinia leaves involved in our 3rd experiment, we, indeed, did not give the number of leaves scanned. In our Fig. 3D-I we show only 6 Salvinia leaves with brown sunburnt patches; however, we observed more numerous sunburnt leaves, but scanned only some of them (a few dozen). The actual number, however, is rather irrelevant. Based on our results and photos, everybody can easily imagine that among Salvinia leaves oriented randomly in all possible directions and covered by numerous (5-20) water drops of varying sizes (0.5-8 mm in diameter), several will suffer burn damage similar to the ones shown in Fig. 3D-I.

    YOU WROTE: (b) how the leaves were chosen, since they didn’t scan them all,

    ANSWER: Prior to our experiment, we chose 120 intact, green Salvinia leaves, the surface of which displayed no brown spot visible either by the naked eye, or with a common magnifier lens. After the 2-hour exposure we randomly selected some leaves, on which at least one water droplet remained. All of them suffered sunburn in the form of clearly visible brown patches, which, of course were apparent in the scanned pictures as well. Does this satisfactorily answer your question?

    YOU WROTE: and (c) how the reported damage was proven to be from water droplets. Worse, there are no statistical data.

    ANSWER: As mentioned above, prior to exposure the leaves were intact, homogenous green, without any brown spot, while after the exposure there were several brown patches on them. From this everybody can draw the trivial conclusion that the brown patches were caused by the intense sunlight focused by the water droplets. This conclusion was furt
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    her confirmed by the fact that the shape of these brown patches was similar to that of the arched sunburnt patches observed on the leaves covered by glass spheres (Fig. 1 of our paper). Do you have an alternative and physically/biologically plausible explanation for the observed brown patches?

    Because a considerable portion of the treated Salvinia leaves suffered such sunburn damage, we deemed it unnecessary to perform any statistical acrobatics to convince ourselves and the referees that Salvinia leaves covered by water droplets CAN indeed suffer sunburn after exposure. Quantifying what percentage of Salvinia leaves might suffer damage was beyond the scope of the current paper.

    YOU WROTE: We are simply asked to believe their report in the absence of any evidence except a handful of photographs.

    ANSWER: The photos in Fig. 3D-I alone should be enough evidence to prove that hairy Salvinia leaves can suffer sunburn under the conditions described in our paper. I suggest you to replicate this simple experiment, if you are not fully convinced by our results. An essential characteristic of good science is reproducibility. This is also the case for our paper; therefore, we would welcome your own experiments.

    YOU WROTE: (As an aside, I really would like to know how they were able to cut and transport leaves without the water droplets moving!)

    ANSWER: In our paper we wrote:

    “Floating fern (Salvinia natans) with a large number (120) of leaves were put into two small water-filled containers (Fig. 3A), which were then exposed to direct
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    sunlight for two hours from 13:00 to 15:00 h local solar time (UTC + 2 h). Prior to exposure numerous (5-20) water drops of varying sizes (0.5-8 mm in diameter) were dripped/sprayed onto the hairy Salvinia leaves (Fig. 3B,C) with an eye-dropper/water-sprayer. Throughout the experiment the positions and orientations of the Salvinia leaves floating on the water surface did not change. After the two-hour exposure smaller water droplets completely evaporated, while larger drops did not.”

    On the basis of this description nobody needs an enormous fantasy to imagine the situation and solve this “enigma”: We first scooped the Salvinia plants out of an inside pool of the botanic garden, placed them in two containers, and only after this did we sprinkled them with water, and left them in the sun for two hours. Hence, the leaves were not cut, but small Salvinia plants floating on the water surface were transported to the open-air containers, where their leaves were sprayed with water. Thus, the water droplets did not move. Does this answer your concerns?

    YOU WROTE: The lack of scientific rigor in this article is disappointing, especially in a peer-reviewed journal. Is there any plant scientist would consider detached leaves to be a model for those on an intact plant?

    ANSWER: I honestly say that the lack of scientific correctness in your blog has disappointed us.

    Is there any scientist who would question the following trivial fact? If a detached leaf (without transpirational cooling) of a given plant species does not suffer sunburn due to adhered water droplets, the same leaf should not suffer sunburn even in its intact stage, when transpirational cooling is a further mitigating factor. Consequently, it was enough and pertinent to conduct our 2nd experiment with detached leaves. This made the logistics of our experiment considerably simpler, because it would have been very difficult, if not impossible, to perform an experiment with intact leaves, which could not have been oriented horizontally, and irradiated by direct sunlight at any time of the day. In sum, our detached leaves were not a model for intact leaves, but rather a limiting case most susceptible to sunburn; the lack of burn damage has obvious implications for the less-then-ideal case of intact leaves under natural lighting conditions.

    YOU WROTE: Is there any gardener who would consider an aquatic fern comparable to trees and shrubs?

    ANSWER: Why should anybody consider an aquatic fern comparable to trees and shrubs? The leaves of floating fern were an ideal model for hairy plant leaves (of trees, or bush, or other plant forms), by which we could demonstrate that such leaves can indeed suffer sunburn due to hair-held water droplets.

    YOU WROTE: Would any species – including aquatic ferns – easily burned by the combination of water and sunlight survive in the real world for long?

    ANSWER: In our paper we refuted the gardener’s misbelief, but showed that the leaves of floating fern, for example, can indeed suffer water-droplet-induced sunburn. However, such sun damage is highly localized (see Fig. 3D-I of our paper), thus, not extensive enough to endanger the survival of the whole plant itself.

    YOU WROTE: I don’t think so – hence my myth posting on this very topic several years ago.

    ANSWER: Unfortunately, I am not familiar with your peer-reviewed scientific publications on this topic. Could you please send me copies? If they are scientifically sound, correct and valuable, I will certainly cite them in my next publication on this subject.

    YOU WROTE: Let’s review: leaves of one species of aquatic fern was damaged by something – possibly sunlight – but without enough data presented to really evaluate the claim. None of the tree leaves tested were affected, even though they were detached from the plant and could not benefit from transpirational cooling. Yet the alarm has gone out! Don’t water your garden plants when it’s sunny, or the leaves will burn!!!

    ANSWER: You simply err again! Let us review:

    (i) In our 3rd experiment numerous hairy leaves of floating fern (Salvinia natans) were damaged not by “something”, but evidently by the intense sunlight focused by those spheroid water droplets, which were held by the wax hairs at an appropriate height above the leaf so that their focal region fell onto the leaf epidermis. In this experiment about 120 Salvinia leaves were involved, and at least several dozen of them suffered sunburn. In our paper we could, of course, present only the photos of a few (six) sunburnt Salvinia leaves (Fig. 3D-I). Hence, we had enough data to prove our statement, that is, hairy Salvinia leaves CAN suffer sunburn due to water droplets held in focus above the leaf epidermis.

    (ii) Indeed, none of the tree leaves tested (Ginkgo biloba and Acer platanoides) were sunburnt by any water droplet, independent of the solar elevation angle and the drop shape, even though they were detached from the plant and could not benefit from transpirational cooling, BUT could benefit from the cooling of water in the circular contact area. This experiment alone refuted the gardener’s belief mentioned above. This exactly is one of the key take-home messages of our paper. Not this, but the other key point (that is, hairy leaves can suffer sunburn due to sunlit hair-held water droplets) was, unfortunately, misinterpreted by many web-journalists.

    (iii) The criticized alarm – Don’t water your garden plants when it’s sunny, or the leaves will burn!!! – has been sent out by non-professional journalists, rather than by us in our paper.

    I suggest you, as a garden professor, to repeat our experiments before speculating on or criticizing our results without any supporting evidence.

    YOU WROTE: Be sure to take everything you read with a grain of salt – or an acorn.

    ANSWER: We could not agree more. However, we would add, be sure to inform yourself from the original, pure source, rather than from second- or third-hand misinterpretations by journalists in cyber space. Be sure to take everything you read in bombastic-titled summaries written by non-professional journalists with a grain of salt.

    Finally, I do not want to turn this debate into a “pissing contest”, pardon my French, sprinkled with ad hominem attacks. I hope our debate can be conducted in a civilized, constructive manner, so we can both benefit from it.

    Sincerely yours: Gabor Horvath (corresponding and senior author)

    Gabor Horvath (habil., Ph.D., C.Sc., D.Sc.)
    Environmental Optics Laboratory,
    Department of Biological Physics,
    Physical Institute, Eotvos University,
    H-1117 Budapest, Pazmany setany 1, Hungary
    e-mail: gh@arago.elte.hu
    http://gh@arago.elte.hu

  19. I can remember as a child when my dad taught me to never water plants after 8 am in the summer time because the sun could scorch the leaves through water droplets. It’s one of my first memories working in the garden as a youngster 🙂

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