Re: How do reserve organs, in plants, work?

That is a long answer. I'm afraid I don't have room enough to explain here, but here we go:

As in animals, the idea is to store something when it is abundant to use when it is scarce. Since it is (somehow) programmed in the life cycle of the plant, there has to be some ecological pressure on the population in order to stablish this characteristic on the plants. There are other possible reasons of need for storage, but that would require a very long discussion.

Depending on the morphological and anatomical characteristics of the plant, different organs can be used for storage. Some plants store nutrients in the roots, others store them in the stem or even leaves. A typical place of storage is the seed. Actually, the seed is on of the critical stages of the live cycle of a plant because at this stage, the plant is not able to make photosynthesis and doesn't have roots developed to extract from the soil the nutrients it needs. That is why seeds usually have reserves of nitrogen, carbon, RNA, phosphate and other ions.

What is stored also varies a lot. Some plants store water (like cacti), some store nitrogen in the form of aminoacids (on the seeds and on the sap of some tropical plants) and others store phosphate (in the form of Phytin (mio-inositol hexaphosphate)) and other ions associated (calcium, magnesium...). Lipids are also stored in the seeds of olive, peanuts, sunflower, etc. One of the most usual storage compound is carbohydrate. Some plants store fructans in the vacuole of the root cells (have you ever heard of "Heliantus tuberosus") and some seeds store cell wall polysaccharides (like Mannan and xyloglucans). Starch is the universal storage molecule of carbohydrates that is used in many parts of the plant: on the leaves (when the plant produces more carbohydrate than it is capable of using) and on the seeds (corn, beans).

As you can see, the variety of storage compounds is as large as the places where it can be stored. If you have any specific question about what I said, mail me and I will be very pleased to give you more information and references. This is actually a very complicated subject due to the variety of possibilities. Hope I answered what you wanted to know.