What is the source of energy for plants in the seed and seedling stages, prior to photosynthesis? The seeds have lipid, but also substantial carbohydrate. Do they use carbohydrate during germination? Also, general sources say the fatty acids are converted to glucose (using the glyoxylate cycle) to provide an energy source. But that would also require energy; why not just oxidize the fatty acid? The sequence of utilization must be known. My own area is mammailan metaboism, but I just finished reading "The terroir of whiskey" and became interested in how and when grants utilize their energy sources.
Seed germination is a complex process, which involves several signals and is influenced by both intrinsic and extrinsic factors. The processes involved in seed germination can be categorized into three prominent stages.
Phase I, rapid imbibition of water by the dry seed;
Phase II, metabolism reactivation, including mobilization of food reserves and protein synthesis; and
Phase III, radicle protrusion.
However, before germination initiation in seeds, the photosynthetic phase of the embryo starts early upon embryogenesis, at the globular stage when plastids differentiate into mature chloroplasts and chlorophyll accumulates. Overall, embryonic chloroplasts resemble mature chloroplasts in leaves. Still, they contain less grana with fewer stacks than leaf chloroplasts, possibly due to exposure to far-red enriched light within the fruit. As the embryo begins desiccating, the chloroplasts de-differentiate into non-photosynthetic plastids, called eoplasts, and the embryo loses its green color, becoming white.
The developing embryo is surrounded by green, photosynthetically active maternal tissues (silique, ovule), which filter the incoming light toward the embryo and render it less suitable for photosynthesis. It has been suggested, in rapeseed (Brassica napus), maize (Zea mays), and pea (Pisum sativum), that embryonic photosynthesis could provide oxygen (O2) and ATP to cells located in the inner parts of the embryo where it was shown that the concentration of O2 is limited. Thus, despite its low efficiency, embryonic photosynthesis could favor the developing embryo by supplying necessary oxygen.
Thanks for that information; it is interesting that the chloroplast development is so early. My question is, prior to the ability to use photosynthesis -- in the early seed or just germination stages -- how does this organism utilize two potential energy sources, starch, and lipid. I was assuming that it only used lipid, but that was because germination triggers the release of enzymes to break down the starch, but I have no references for this. Also, I understand that, with the glycoxylate cycle, it is possible for the lipid to be converted to glucose, but then why should the seed do that? It doesn't need to transport it anywhere. Perhaps a full Krebs cycle is not developed?
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