The dominant thing that sugar is used for in plants is it’s structural potential, such as the polysaccharide cellulose which makes plant cell walls. It’s secondary function is energy storage. Perhaps surprisingly, it’s primary function is not energy as it has multiple forms of energy that precursor it’s production of sugar, such as ATP. Likewise, human use of plants is for these two reasons, but perhaps sugar for energy is primary to it’s harvest for fiber.
Carbohydrates are essentially carbon atoms and water molecules, hence “carbo” and “hydrate”. In terms of sugar and it’s aggregates, carbohydrates can exist as monosaccharides, oligosaccharides, and polysaccharides (“single-sugar”, “few-sugar”, and “many-sugar”, respectively). The aggregation occurs as monosaccharides link together via glycosidic bonds (named after glucose [a type of saccharide]). Glycosidic bonds are not exclusively sugar-sugar bonds but rather the way in which sugars bond to other things. In the case of plants and their carbohydrate fibers, glycosidic bonding links the monosaccharides they produce into polysaccharides, which are deposited into cell walls and other structural regions.
Note: wood is lignin and a hydrocarbon-based polymer, not a carbohydrate polymer, but all plant cell walls are made of cellulose. Wood and bark are lignin deposits.
So, to focus on cellulose, it is a long chain of hundreds or more glucose molecules covalently bonded via glycosidic bonds into an insoluble and high tensile strength fiber. In this sense, and a broad sense, plants are made of sugar. A lot of the photosynthetic work they do to make sugar is used to make cellulose for their cells and then the left over is kept in the cells for later energy use and cell growth.
Notable: The hydrocarbon sugar structure of plants is sourced primarily from the carbon oxides in the atmosphere and water. Plant protein’s nitrogen sources come indirectly from the atmosphere via soil bacteria that can make bioavailable amines from otherwise inert N₂; a small but significant source of N also comes from the release of biomass from rocks. Overall, the mineral and micronutrients come from both the dirt and soil, as well as wind-borne dust deposits. Point is: plant structure is mostly water and air with a little dirt and germ poop; all assembled with solar energy.
The Broad Spectrum Life 
I like this! I’m curious. What inspires you, besides being so knowledgeable, to write these posts? I’m always impressed with your expertise in a wide range of subjects, not to mention your poetic self. Do you just wake up in the morning and say… “I’m going to talk about plants today”? Or are you working on stuff that is related to what you write?
Just curious and I know you’re incredibly intelligent, that is obvious but I’m not your only fan. Inquiring minds want to know, I’m sure! LOL!
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BTW… I think I feel like a plant today. My structural potential… very sugary hopefully fibery. LOL! I love that – “structural potential”.
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Fiber and sugar help the Sunday go down, I reckon. I write in a somewhat random manner, going on the whims of concepts that pop-up in the ol’ TomBot’s head. Usually, I have a stack of partial drafts and once in a while I’ll complete one, that was the case here (I found a few sentences I wrote in October this morning when I was perusing around my laptop files and decided to develop it into a mini-article/lesson). Plus, I try to keep up with the “Broad” part of my blog so if I think of a topic that is different from recent things I’m more likely to work on it.
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I like your answer! You get an A+. ;-
Happy Sunday to you!
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I have sort of a question Tom. I know with grasses the amount of sugar varies depending on the time of day. Fructan levels are high in the morning and on overcast, cooler days. That is when horses are most likely to founder from high sugar content. Do you think it matters what time of day we pick our vegetables? Is there any difference in nutritional content or is my question overthinking this?
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I think you answered your own question! It absolutely matters because all plants regulate their functions based on light, hence where the resources will be focused. Often, plants focus on reproductive functions at night, so anything that carries seed will probably be juiciest and tastiest right at dawn.
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Thank you. That makes sense but I was curious if it was enough to worry about. I guess even a little advantage helps when it comes to food these days considering soil depletion and limited choices. Thank you. Tom has great readers!!
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I think in terms of root and fruit, they are packed and store sugar in a way that won’t fluctuate much. For roots, they are crammed with sugar and only have it withdrawn when photosynthesis decreases or stress increases, which would be more of a seasonal variation rather than a time of the day variation. Likewise for fruit, they are packed with sugar to a point where they are genetically set, giving the fruit the sugar they are encoded to without a return pathway (at least for any fruit I can think of); so again, it’s season timing rather than time-of-day timing. You are probably alright to extend your grass-sugar logic to leafy vegetable harvesting, harvesting when the season is right AND the time of day is right. However, grasses more distantly related to most leafy veggies we grow (like kale, spinach, basil, etc.) so their characteristics may not be applicable to things other than grasses. And overall, I bet the sugar content variation is small enough you may not have to worry about harvesting at a certain time, but I would take the horses advice still and harvest when they do.
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Interesting and informative! A meaningful post! Thanks so much! 👍
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I learned something new reading this.
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