Animals can survive without eating if nutrition is injected directly into their blood.
Can the equivalent be done with plants? By injecting carbohydrate rich nutrient solution, either without photosynthesis or in addition to photosynthesis.
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3 Answers
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To answer if the equivalent could be done with plants, we have to understand the histological and anatomical differences between plants and animals.
Just as spraying a nutritive solution over an animal (like you and me) doesn't work, spraying a nutritive solution over the leaves doesn't work as well: the nutritive molecules (let's use glucose in our hypothetical example) don't penetrate the leaf's epidermis. The same happens if you put the nutritive solution in the soil: there are several membrane layers between the external medium and the phloem, and glucose will never reach its destination.
Now, let's try to compare with a IV injection of nutrients, which we can do in an animal. The plant's structure that (kind of) can be compared to blood vessels is the phloem (actually, xylem and phloem, but in our case, dealing with nutrients, we're gonna talk only about the phloem). To inject the nutrients we have to reach the phloem, because the periderm in plants with secondary growth ("cork" in the image) is very impermeable:
Now, even if we reach the phloem, using methods like this...
...we still have a problem: in an animal like you and me, a substance injected in a given vein will quickly spread to all the systemic blood vessels. But the same doesn't happen with the phloem. The injected substance can go up or down (depending on the pressure flow), but it doesn't spread to all of the plant's tissues, unless we perforate the phloem in a lot of places.
Thus, so far, this method is used to deliver chemicals (like pesticides) into the tree. Notice that, just like a person in a hospital, the bag with the solution has to he higher than the perforation:
For a chemical treatment like this, the amounts of the substance are small. But for "feeding" a plant using this technique, the amounts of necessary nutrients are way higher...
Finally, answering your question: is it hypothetically possible to feed a plant in the dark with a nutritive solution? yes. Is it practical, or doable? very hardly.
Source: Utah State University
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$\begingroup$ Thank you, I did suspect it was not very easy or practical. The reason I asked is because I am trying it imagine some kind of high-tech agriculture, beyond aeroponic vertical farming, that uses carbohydrates as its energy source rather than light. Imagine for example if a pollinated tomato flower could be 'plumbed' into some kind of artificial machine that appears to the developing tomato like the tomato plant. I know it sounds overly complicated, but maybe one day such a system could be automated. $\endgroup$ Jul 15, 2016 at 9:02
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$\begingroup$ If that's what you are imagining, there is something way more practical, efficient and actually doable: search "plant tissue culture" or "tissue culture growth". $\endgroup$– user24284Jul 15, 2016 at 9:07
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$\begingroup$ @Nick Collier: Where would you get the carbohydrates from in the long run? $\endgroup$ Jul 15, 2016 at 9:51
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$\begingroup$ Good question @AlexDeLarge, it would be pointless to get them from other plants. Maybe some kind artificial photosynthesis if it were much more efficient than natural photosynthesis. Or maybe there is a way to turn electrical energy in to carbohydrates. I am speculating about the future here, I know none of this is practical now. $\endgroup$ Jul 15, 2016 at 10:07
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$\begingroup$ @Nick Collier: Yes, it's interesting yet speculative. You could also ask people in Worldbuilding.SE and see if they have further ideas as that question may also be addressable from a more science-creative perspective. $\endgroup$ Jul 15, 2016 at 10:35
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Survive without eating: no
Survive without photosynthesis: 1. Yes, and 2. Yes-if considering only "nutritional" aspect of photosynthesis
- Some parasitic plants lack photosynthesis (see the beautiful ghost plant)
- Plants also grow during the night, and plants are able of respiration, which can exceed photosynthesis - and trees do so excessively when they are young and growing (learnt some time ago during an undergraduate lecture on botany)
Survive by adding carbohydrate rich nutrient solution: not sure
Survive without photosynthesis at a philosophical scale: probably not as most molecules of sugars, which we have on earth, had required photosynthesis for their generation
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$\begingroup$ Very good answer! But as far as I know cellular respiration always exceeds photosynthesis. Photosynthesis, even though it does make some energy available for the plant directly, is only the process that provides glucose for respiration. The major ATP synthesis in plants also comes from cellular respiration. The difference is the carbon source. In that way, photosynthesis is analogous to animal eating as it provides the carbon source. However, the energy conversion of the carbon source then is very similar. $\endgroup$ Jul 15, 2016 at 8:16
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$\begingroup$ tsttst Yes, I am trying to imagine a way to produce food without photosynthesis on a philosophical scale. You have highlighted the other side to the problem, namely production of sugars without using plants in the first place. The reason I ask is because I am wondering if humans can survive on this planet in future without destroying most of its natural habitat for agriculture. $\endgroup$ Jul 15, 2016 at 9:13
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$\begingroup$ Hi Nick. Photosynthesis will have been required for carbon fixation and thus indirectly most present carbohydrates. Note that already now, most of the energy in ordinary plant-derived food comes indirectly from fertilizer, and not photosynthesis. (with some components of fertilizer coming indirectly from fossil oil....) . You might infer an unacceptable answer by reading the first chapter of Harari's "A Brief History of Mankind", which discusses the time before agriculture. $\endgroup$– tsttstJul 16, 2016 at 1:17
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No, not in the long run at least. (Except for parasitic plants as I have just learned!) Plants rely on photosynthesis to make up glucose but that is just the tip of the iceberg, there are tons of cascade reactions (also genes regulation) going on linked to the photosynthesis. Read this just to get an idea. So, photosynthesis is so deeply linked to the metabolism of plants that replacing it with some nutrient-rich buffer will not be enough. There are organisms that are facultative phototroph they do photosynthesis but they can live also without, they are not classified as plants, though. One example of such an organism is Chlamydomonas reinhardtii.
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1$\begingroup$ This is not true for all plants, as mentioned in another answer. There are parasitic plants that are incapable of photosynthesis. $\endgroup$ Jul 15, 2016 at 9:52
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$\begingroup$ @alec_dijinn: Thanks for editing (withdrawed the -1). As the exception of parasitic plants is there, I would not be so sure about the generality of your answer for all non-parasitc plants, though. Still a god thing to point out! $\endgroup$ Jul 15, 2016 at 16:19