Written by Matt DiLeo
This is why farmers like hybrid seed. The parents on the left and right are inbred lines that have been self-pollinated for many, many years.* The two rows of much bigger plants in the middle are simply their hybrid offspring – they grow faster, produce higher yields and are tougher in the face of unfriendly environments.
Hybrid vigor (aka heterosis) is the tendency of hybrid offspring to be more vigorous than either their inbred parents or open-pollinated (OP) ancestors. The right combination of inbred lines can produce hybrid seed that produces twice the yield of naturally-crossing OP varieties. Plant breeders spend a lot of time trying to produce better and better inbred lines that have both excellent agronomic characteristics on their own and “combine” well to produce extra-vigorous hybrids. Unfortunately, about half of this extra vigor is lost in each generation that’s descended from the hybrid individual. The scientific reasons for this haven’t been completed settled but the economic implications are clear – the extra yield that comes from F1 hybrid seed more than covers the cost of buying new seed every year.
|Northern flint, Modern, Southern dent|
Prior to the 1920s, farmers grew only OP varieties. Although there are now over 100 races within the species Zea mays, Amerindians were primarily growing two when the Europeans showed up: Southern Dents and Northern Flints.** Southern dent varieties produce single stubby, white cobs on moderately tall and thick single stalks. Cobs are slow to mature and kernels are skinny, soft and develop deep dents when mature. Northern flint varieties produce multiple long, skinny, reddish cobs per stalk and plants produce multiple short, skinny stalks (tillers) per plant. Cobs mature quickly and kernels are squat and flinty when mature. European pioneers mixed up these two genotypes as they settled the Plains, creating Corn Belt Dents, and the now-familiar corn phenotype that we recognize in nearly all modern hybrid maize.
Many now-famous OP varieties were developed in the 1800s to early 1900s by American farmers: Lancaster Sure Crop, Reid Yellow Dent, Midland and Jarvis. OP variety development could be a very personal affair at this time. Reid and his dad selected their self-titled variety to have easily-plucked ears in order to spare the wrist of the aspiring artist son. Despite the care of many individual farmers, the national average yield of corn stayed very low from 1870-1920 (~ 27 bushels/acre).***
I think I’ve seen some variation of this graph in about half of the maize research presentations I’ve ever sat through. It displays the amazing increase in U.S. average corn yields since the appearance of professional plant breeders. I’ve been told that about 60% of this increase in yield is due to genetics (primarily the introduction of hybrid seed) and about 40% is due to agronomy (improved fertilization, tillage, herbicides and denser planting).
Modern corn is incredibly optimized to its agricultural environment. I’d be surprised if any crop has been so massively and unrecognizable modified from its wild relative into a creature so suited to our needs. It’s become beneficially oblivious to the shade cast by its neighbors, holds its leaves straight at a high angle from a single stem to share light with its neighbors, and even twists while it grows to take advantage of sunlight between rows!
Since then, generations of Midwestern teenagers have earned their summer beer money de-tasseling corn. Corn is wind-pollinated, dropping pollen from its male tassels onto its (and other plants’) female ear silks. If you want to produce large amounts of hybrid seed, the way to do it is plant a row of “male” plants for every couple rows of “female” plants. Female plants are simply plants that have had their tassels chopped off (hence the need for cheap teenage/immigrant labor). Seed harvested from these de-tasseled plants are only cross-pollinated, and therefore reliably hybrid.
In the 1970s, some hybrid seed producers began using de-tasseling tractor attachements to save money. An even better solution was to plant “female” lines that were genetically incapable of producing pollen, thereby completely avoiding the whole de-tasseling process. Cytoplasmic male sterility (CMS) mutations were commonly used from the 1960s through the 1980s, with infamous consequences in 1970, when a fungus discovered an unknown chink in the most popular CMS genotype’s armor. Currently, the major seed companies are developing ingenious selection and sorting systems that allow plants with pollen-killing transgenes (nuclear male sterility) to produce hybrid (but non-transgenic) offspring.
Genetic male sterility is really key for hybrid corn seed producers because it saves a ton of money and oil and spares the inevitable worker injuries that occur when you work in a hot, rocky field all day, squinting into the sun while you make hundreds of knife cuts above your head. Transgenic male sterility also avoids the monoculture issue of CMS, since it works in all genetic backgrounds. Most importantly, transgenic male sterility will allow us to get the great benefits of heterosis from crops that aren’t considerate enough to keep their pollen and seeds in separate flowers (rice, wheat, oilseed rape, etc.)
I began composing this post back in July as I worked my
way through our tall, sticky corn fields. June bugs dripped drunkenly from emerging tassels under a darkening sky as I pawed through stiff leaves and Johnson grass. My co-workers stuck with hand shears but I quickly switched to yanking the tassels straight up out of their rosette whenever they had emerged enough to wrap my fingers around them. There was something very satisfying about hearing that wet POP! as you ripped the manhood out of yet another corn plant that stood between you and the end of the work day. Our main field would be carefully hand-crossed to make specific combinations of different genotypes, but these two side fields were just meant to produce enough hybrid seed so that the next year enough could be planted to feed to animals. 4 rows de-tasseled for ever row left intact (and unharvested). The early season weather had given us a great head start. We were pollinating well before the “go to hell date” and had high hopes for our season. Late summer smut or hail, or an early frost could still destroy it all, but so far so good.
As the welcome, cool rainshower draped over us, the irony was not lost on me of the song that had been stuck in my head all day
“Rain makes corn…”
* the inbred lines are B73 and MO17, which are somewhat obsolete in real world ag but are established lab rats (B73 got its genome sequenced)
** Much of the information and pictures from this post were found in plant breeding lessons in the UNL’s excellent Plant and Soil Sciences eLibrary
*** Today, it’s becoming common to get 300 bushels/acre with cutting edge varieties grown in the best environments. Selecting seed from the best plant in your field is actually a very inefficient way to improve your germplasm. I’ll explain in a later post… (and the Corn Shows didn’t help…)
Written by Guest Expert
Matt DiLeo has a PhD in Plant Pathology from UC, Davis. During his postdoctoral research at Boyce Thompson Institute, he researched unintentional effects of genetic engineering. Matt builds R&D teams and biotech platforms: genome editing, gene discovery, microbials, and controlled environment agriculture.
Thanks for this article! It is clearer and more up-to-date than my own effort a little while ago to explain some of this. I was addressing the all-too-common misconception that farmers COULD NOT keep and plant the seed of (non-GMO) hybrid crops, either because they were not ALLOWED to, or because the seed could not grow.
Also, the historical yield graph you showed is not just one that is copied over from place to place, but is one representative of very similar graphs produced all over the world where the results of breeders’ efforts and improved technology could reach.
My knowledge of corn breeding (and of other modern breeding techniques) is, sadly, out-of-date. I am sure there are lots of interesting discoveries and techniques that I would like to learn of. Obviously, I would be grateful if you could publish some info here (but I wouldn’t expect a thorough and complete exposition). Some good references would be nice. Since people are so prone to getting all Dunning-Kruger-ish when they don’t know much, I like to see sites that prove that those directly involved in a field DO have more to consider than someone on the outside would ever guess.
I hope that you will post more on the topic of Maize breeding (and maybe how the principles apply to other species).
As I understand it, another big problem for seed-saving in countries like the US, besides yield, is that the descendants of hybrids are not UNIFORM. The heterozygosity of the hybrid is reassorted, and the traits are not uniformly distributed. It’s no good for mechanized agriculture to have variation in height, size, maturation rate and composition. Though in successive generations, the polymorphism decreases as traits are randomly lost or selected-out by the seed-saver.) Because of the decreased productivity and uniformity that results from planting seed of saved hybrids, almost all farmers everywhere choose to plant fresh hybrid seed each year: the cost is minuscule compared to the gain in productivity, and it is usually impractical, even on huge farms, to maintain a private breeding program.
I would particularly like to hear more about what has happened due to/since the 1970 Southern Corn Leaf Blight/Texas “T-Sterile” maternal genome. All the even moderately detailed histories I can find have a definite –ahem- UN-scientific –ahem- bent. http://www.sciencemag.org/cgi/content/short/173/3991/67
I, myself, grow land-race OP corn, but I am not burdened with high pressure for productivity; I grow boutique sweet corn. Since, as you know, the properties of a FRUIT are determined only by the female parent (with few exceptions, including passionfruit), we can have a tree called “Gala” or “MacIntosh” and not worry about where the pollen comes from. In corn, it’s different: we eat the seeds, and the pollen parent definitely DOES affect the properties. When my white-corn “Country Gentleman” http://www.territorialseed.com/product/584/191 is pollinated by “Black Aztec,” http://www.abundantlifeseeds.com/product/27/15 I get modest-sized lovely ears of white sweet corn with lavender dots. But if I plant those seeds, I get vigorous productive non-tasty ugly corn, so I don’t bother.
Until recently I was under the misconception that descendants of hybrids should yield at least what the inbred parents did, although less than the hybrid – however when you see the resulting field (a colleague provided the photo) of what happens when you plant a selfed hybrid (at least an elite commercial hybrid, not sure how much variation you’d see for different hybrids) it becomes pretty apparent that in terms of mechanized ag the resulting field is likely to be horrible – plants ranged in height from maybe 2′ to maybe 12′ (with the look of light stressed greenhouse grown plants, which are odd looking beasts) and a range of stresses across the field – the small plants were heavily shaded by the tall plants and so their individual yield would be awful, the small plants in turn basically acted like weeds, and a good number of the tallest plants were likely, according to my colleague, to be of no use whatsoever as they’d lodge the second a mild breeze blew through – worlds away from the relatively uniform height and performance of a field of hybrid corn.
One possible addition to the male sterility aspect of things (and here I bang my corporate drum) – Monsanto is currently in late stages of development of the RHS (roundup hybridization system) for corn – which entails engineering corn which is essentially RR in all tissues bar the tassles, no more lengthy detassling of corn – just aerially apply roundup around tassling and you do the job in one quick pass – I’m thinking the utility of this system may be limited for the first 20 or so years (or however long the patent has left) but it seems a pretty cool way to get around the hard work of detassling a field of any appreciable size.
On leaf angle – I havent seen it portrayed as a ‘sharing with neighbors’ thing before – I was under the impression that leaf angle was primarily a way to maximize capture of incoming solar radiation – a flat surface will get hit with so much radiation that most is wasted, a surface which is highly angled to the incoming radiation spreads this over a greater area and loses less – this becomes important when dealing with the massive amounts of solar radiation you deal with in the midwest summer – although I suppose as this talk was by a crop physiologist who refuses to deal in anything less than field level science the concept of sharing wasn’t really a great consideration (if I remember right the speaker was M Tollenaar – who is about as passionate as you can be about corn without it crossing the bounds of decency)
Thanks for all the interesting info! I’ll definitely add more info on breeding (with references) as I have time to put it together. I’m a plant pathologist by grad school so I’m still working on learning the details of breeding.
Orchidgrowinman – do you have a description of your work anywhere? do you recommend any good references on OP maize varieties? I’ve been very curious about it recently..
Ewan – I agree with your points on leaf angle, I was more referring to the idea that we’ve selected, in many cases, for plants with a small area footprints to allow more plants to be packed into one field. And the RR detasseling is brilliant! I’m routinely amazed how such simple, obvious ideas (in hindsight) are missed by so many. Great point on uniformity too – that’s no doubt a bigger problem than yield if it routinely occurs with anywhere near the severity you describe.
I’ll look into both your comments more when I have time but wanted to give a quick reply before I’m whisked away again…
Uh… Does Frank N. Foode™ know of your thoughts re: “There was something very satisfying about hearing that wet POP! as you ripped the manhood out of yet another corn plant that stood between you and the end of the work day.” ? LOL
Thanks for the post:)
From your description of that clever technique for breeding I make these conclusions. Correct me if I’m wrong:
RR trait involves an alternate, “invulnerable” version of Shikimate phosphatase. If blocking this enzyme in part of the plant stops the growth of that part, then that part has only the original, vulnerable version active. So 1) this corn has its original version AND the RR version, and 2) you have figured-out a tissue-specific way to silence the RR version. Interesting.
If blocking the enzyme in one part of a plant stops the growth of that part, then there must not be any systemic redistribution of (aromatic) amino acids: they are manufactured and consumed locally. Now, supposedly, plants, before dropping their naturally-senescent leaves, mobilise and retrieve nutrients from those leaves, including metal ions, phosphate and nitrogen. Is the nitrogen retrieved as amino acids or something simpler? Would inducing senescence in part of a corn plant (and thereby inducing a flow of amino acids) render it insensitive to the type of treatment you disclose? ‘Time to go back to the library!
I’m not really an expert on OP varieties, though I have practical interests about them and self-identify as an environmentalist concerned about the preservation of genetic diversity, but not a woo-soaked one. I don’t think anyone has a right to force a farmer to be part of a “land-race preserve” and endure poverty in the interest of perpetuating local varieties. I am rich, and choose to grow less-productive-than-is-available varieties for purely aesthetic reasons (and a little filthy lucre from my fellow first-worlders willing to pay exorbitant prices for produce).
In my case, one thing I am a supplier of CANNOT (so far) be produced by planting one kind of seed, the corn I mentioned. The only way to get it is to plant two varieties, detassling one (which is the keyword I found this article by). I don’t know if this phenomenon even has a NAME: “Heterozygote Advantage” doesn’t really cut it, nor is it the same as pink petunias (Rr: F1 you buy is all pink, but F2 is 1/4 white, 1/4 red, 2/4 pink: a GREAT Mendelian field-experiment for kids).
Some species do not experience (much) heterosis (inbreeders like peas, tomatoes, wheat), so old traditional varieties have a much better chance of competing with hybrids than they do in outbreeders like maize. That’s not to say, though, that substantially improved strains can’t exist. In fact, OP strains are not uniform across their areas of cultivation; they experience introgression and selection differently in every field, and are quite plastic within the bounds of their descriptions (“Bob’s Yellow Pole Bean” might be a little different field-to-field, but it had better be vining and yellow!)
I have access to lots of books that touch on these topics, from Vavilov to Deppe, but many I cannot recommend because of what I find a toxic anti-scientific bias. Off the top of my head, (and the library database), I can recommend these accessible and informative ones:
Shattering: Food, Politics, and the Loss of Genetic Diversity Fowler, Cary and Mooney, Pat
Seeds Spades Hearths & Herds: The Domestication of Animals and Foodstuffs Sauer, Carl O.
Seeds of the Earth, A Private or Public Resource Mooney, Pat Roy
Origin, Variation Immunity and Breeding of Cultivated Plants, The Vavilov, N.I., Translated by Chester, K. Starr, Ph.D.
Nutrient Use in Crop Production Zdenko Rengel, PhD, Editor
Lost Crops of the Incas Vietmeyer, Noel D., Editor
Gardener’s Guide to Plant Conservation, The Marshall, Nina T.
Enduring Seeds: Native American Agriculture and Wild Plant Conservation Nabhan, Gary Paul
Development of Garden Flowers, The Gorer, Richard
Breed Your Own Vegetable Varieties: The Gardener’s & Farmer’s Guide to Plant Breeding & Seed Saving Carol Deppe
Wow: that looks messy.
One more thing: both “Country Gentleman” and “Black Aztec” maize vars figure prominently in the historical annals of breeding: I think I remember that CG is ancestral to a lot of elite strains and was one of the first used to produce commercial F2 seed.
OGM – alas I’m too lazy to see what is in the public domain and therefore have to shut my mouth about how it is done! (it is very cool, and I’d have to guess there’s a patent out there, but without knowledge of what I can say I’m just repeating stuff that’s publicly available (from a 2 min search on google – which I did to check I could even mention it at all)) Which illustrates how frustrating it can be to do corporate science….
Now this I know a little about, and most of the knowledge is in the literature! Generally senescing leaves will break down all (well most) of their proteins into amino acids – I believe a fair amount of ammonium gets generated in the proteolytic process which has to be reassimilated by cytosolic GS1 (GS2 (and GOGAT!) do all the work of primary N assimilation – in the chloroplast, which I think means this pathway would deal with a bunch of photorespiratory NH4 also, which probably explains the localization to the chloroplast in the first place) – Asparagine, glutamine and glutamate form the main transport amino acids (asparagine is a better transporter of nitrogen in terms of the C:N balance – glutamate is converted to asparagine in the mitochondrion of the phloem to take advantage of this, and glutamine by asparagine synthetase in the cytoplasm (I presume of phloem companion cells) – http://aob.oxfordjournals.org/content/early/2010/03/18/aob.mcq028.full.pdf
details this a bit better than I just did (used it to check I didn’t get anything too wrong) afaik AAs are the best way the plant has to transport nitrogen around – ammonium would be too toxic and nitrate too energy inefficient to convert back to AAs (nitrate takes a silly amount of ATP or reducing equivalents to convert to AAs in the first place – almost as bad as fixing nitrogen)
So based on the literature – inducing sensescence wouldn’t have an impact because you’d have to convert your AAs from rather simple forms up to aromatics and would run into the same problem of broken EPSP.
I dimly recalled that nitrogen was moved-around as “amino acids,” but your explanation makes perfect sense: I do not recall knowing, or asking, WHICH AAs. As to the nitrate-energy thing, why then is NO3 preferred to NH4 in fertilizer? I suppose the energy hit is modest enough that other factors mask it in RW applications. I wonder if it would be interesting to try substituting asparagine for NH4NO3 in Knudson’s? http://www.sigmaaldrich.com/catalog/ProductDetail.do?D7=0&N5=SEARCH_CONCAT_PNO%7CBRAND_KEY&N4=K4003%7CSIGMA&N25=0&QS=ON&F=SPEC
The article is great: bedtime reading!
The reason I brought up senescence was wondering if damaged/stressed/diseased plants might resist being sterilized.
On another topic, I wonder if you know anything about silicon metabolism? I’m casually interested.
On preferred N types for application – I’m not totally sure on the NH4/NO3 preference – if I remember my soil chemistry right, and I rarely do, ammonium will become pretty tightly bound to soil particles (which are generally negatively charged) and be rendered pretty immobile while nitrate stays pretty mobile in the soil – generally in the field soil nitrate concentrations will be an order of magnitude (or more) greater than ammonium (at least in corn fields).
Corn is built to use nitrate rather than ammonium – rice on the other hand is the other way around (as evidenced by the comparitive size of the ammonium transporter family in each species) – although you categorically do see improved growth in hydroponically grown corn if you utilize ammonium nitrate rather than straight up nitrate as an N source (even in equimolar quantities) – I’m going to check with our physiologist tomorrow on why preferential nitrate use though (I’m thinking mineralization plays a role, but can’t remember off hand – this seems to make sense to me (guessing mineralization doesn’t occur as fast in a paddy field as compared to a drier field environment)) – I can’t quite remember the numbers now but it’s something like 20 ATP/reducing doodad equivalents to get NO3 –> NO2 –> NH4 and only somewhat more to fix nitrogen.
On whether or not AAs (particularly asparagine) could be used in fertilizer – I’m guessing expense wise this would be prohbitive, although if possible would probably make the plant grow even better again than using ammonium – although it might cause some weirdness with root growth etc (and I have a feeling something as bioavailable as asparagine or other AAs would end up being a feast for bacteria and fungi rather than for your plants) – root growth can be pretty dependant on NO3 (another reason perhaps for using NO3) with NRT1.1 (a nitrate transporter/nitrate signalling molecule) playing a pretty significant role in soil exploration and root expansion etc (google search Tsay et al for oodles of NRT awesomeness)
The reason I brought up types of AAs in transport is that Glu Gln and Asn are all downstream of the aromatics and therefore I wouldn’t expect that senescing tissues would impact the effects of a glyphosate treatment (although now I’m intruiged to see how the levels of minor AAs change in phloem during senescence) also, and I may be mistaken here, I’d expect most of the remobilized reserves to start heading either to the developing ear or ear leaf around the V10-VT stage – although this is idle speculation rather than grounded in any actual science I’ve seen – AA starvation in the tassels doesn’t seem, thinking about it, to have been a pressure that’d have come up much so I wouldnt expect to find strategies to overcome it – whereas getting as many reserves to the ear as possible is one of the main goals of breeding corn.
Not the first thing – I only know enough N metabolism to fake it (and get silly excited about it), and tend to get baffled by C metabolism enough that I try to stick to just N & C without worrying too much about other elements!
It looks like my last message got ate….
At the moment I’m interested in in-vitro applications: maybe AA (glycine, aparagine?) instead of NH4NO3 would advance the techniques. It’s a competitive field, and right now Taiwan Sugar Corp is on top with high-quality inexpensive product dominating markets everywhere:
Intact plants generally don’t absorb big molecules like sucrose or urea (or DO they?), but orchids are a special case due to adaptations to their symbiotic lifestyle (even the “seedlings”: they get all their energy and nutrients externally: no storage, no endosperm, no embryo, no starch).
I’ve been brainstorming with some colleagues to see if we can develop a high-throughput, affordable + useful field phenotyping platform (UAV robots perhaps;). Do you have an idea of the extent to which big companies like Monsanto find themselves needing to screen very large numbers of plants in the field for things more complex than grain yield (e.g. growth rate, transpiration, leaf area/angle/necrosis, etc. etc.)? I’d appreciate any input you have, or contact with any field scientists who have to do large scale field phenotyping. mvdileo at gmail dotcom. Thanks!
I’ve been brainstorming with some colleagues to see if we can develop a high-throughput, affordable + useful field phenotyping platform (UAV robots perhaps;). Do you have an idea of the extent to which big companies like Monsanto find themselves needing to screen very large numbers of plants in the field for things more complex than grain yield (e.g. growth rate, transpiration, leaf area/angle/necrosis, etc. etc.)? I’d appreciate any input you have, or contact with any field scientists who have to do large scale field phenotyping. mvdileo at g mail dotcom. Thanks!
OGM – im not sure what the uptake capacity of maize in particular is in terms of large molecules – I know there is some scope (a visiting professor whose name escapes me was very excited about the exudation and re-uptake of carbon by corn roots (I’m thinking glucose or fructose are more likely candidates here though) but imagine it would be limited simply by what the plants have been bred to deal with, and have dealt with historically – soil AA levels, and soil carbon levels are low enough generally for uptake to probably not play a big role, and for the plant not to be equipped to take advantage should all C and N be provided this way – it’d be interesting to see how corn grown on AAs as an N source fared compared to grown on ammonium nitrate for instance (my bet would be to use glutamine or glutamate as they’re the next in line after ammonium in terms of nitrogen assimilation)
a somewhat better answer on why nitrate (or ammonium nitrate) rather than straight up ammonium – first ammonium nitrate gives more N per molecule than most ammonium compounds (sulfate etc) second is the charge/pH consideration – if you preferentially accumulate ammonium you’ll get all positive and whatnot, if you take up a nitrate with each ammonium you’ll be all balanced and cool… (how scientific was that?) – plus ammonium buildup can be toxic whereas in times of plenty a plant can simply stick nitrate into the vacuole (nitrite is toxic also afaik)
However how other plants do things (orchids etc) may be completely different – so all this needs to be taken with a pinch of salt (I’d suggest Hoaglands salt, rather than Knudsons)
Matt – I’ll talk with our physiology folk – any high throughput affordable + useful field phenotyping platform would be of great interest (there are some tools available, but nothing, as far as I am aware, that is earth shatteringly awesome – we still do a lot of phenotyping the old fashioned way)- I can’t go in to the things we actually do in the field (yay corporate science) nor I believe could any of our field scientists – however I think that any of our field physiologists would probably be interested in techs developed outside of Monsanto which could be used in house.
Mugineic acid? I have always wanted to learn more about siderophores and the like, ever since I had a nosocomial infection…. (Pseudomonas is a fascinating organism, and I would have kept some as a pet, except that it kept trying to EAT me.) I think there are also phosphate-getting exudates. If you’re going to get me a Christmas present, may I suggest http://www.amazon.com/Rhizosphere-Biochemistry-Substances-Soil-Plant-Environment/dp/0849338557/ref=dp_ob_title_bk
Orchids in particular are reliant on (a special type of) arbuscular mycorrhizae to bring them nutrients in nature: most species have low root surface-area (no root hairs). The seedling (“protocorm”) is a “cheater” in that it has nothing to offer the fungus (but pheromones) and functions as a parasite (myco-heterotroph). Certain species continue this lifestyle into adulthood, like Corralorhiza and Kato Kaelin. Heuristically, it was discovered that orchids in general have the ability to take-up complex nutrients, and thus the commercial in-vitro breeding and production industry was made possible with plants that normally take 7+ years to come to maturity under natural conditions.
I have exceeded my knowledge here, but I assume that maize would be facultatively mycorrhizal, probably under nutrient deficiency. ‘Know anything about that? It might be a yield factor in marginal agriculture. (Coming full-circle, as it were.)
BTW, how does hoadland’s taste? I like my Knudson’s hot, and with bananas….
_HoaGland’s_ obviously. And _Corallorhiza_.
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