A friend of mine was sitting at the dinner table recently, discussing science and plants (how nerdy can you get?) with his progeny and he was struck with the question: how do plants rid themselves of waste? Humans breathe, sweat and excrete as means of eliminating molecular waste; how do plants do the same?
Plants, like all organisms, must excrete the waste products of critical metabolic activities before they reach life threatening levels. Excretion generally refers to a waste separating and discarding process, but should not be confused with secretion, which also involves material movement, wherein the material is usually produced by specialized cells for a functional purpose.
Plants produce two types of waste: metabolic and non-metabolic. Non-metabolic waste is not actually produced via the plant cell, but comes in and leaves. As in other organisms,there are three types of metabolic waste in plants: gaseous, liquid and solid.
The biggest waste products for plants are oxygen (gas) and water (liquid). We do not normally think of these as waste products, but these two by-products of photosynthesis diffuse through leaf stomata (Yes, plants can sweat too). Incidentally, plants release carbon dioxide, just as animals do, in the absence of light. Solid wastes might include: resins, tannins, alkaloids, salt crystals, saps and latexes (i.e. milkweed and dandelions). However, it should be stated that what science thought of as plant “waste” fifty years ago is being revised as new roles for these different compounds are suggested. Many secondary metabolites are critical for plant defense, eliminating the competition, etc. Humans are rely on them for medicinal and culinary uses as well.
Humans eliminate waste via the kidneys, lungs and sweat. Plants do not possess organs like humans have to eliminate waste. Plants utilize leaves, stems, roots, xylem, fruit and flowers and aging tissue to compartmentalize/store or eliminate wastes. One key structure in the plant cell to waste storage is the central vacuole. In some cases, waste products are stored in the central vacuole of aging plant tissues, which then in turn fall from the plant. Some plants excrete wastes into the surrounding soil, while others store by products in wood. Basically, plants are like chemical hoarders. They stack and pile refuse wherever they can until trash pick up day.
Josh Steffen, Horticulture and Facilities Manager