Location: Outside U.S.
Date: Summer 2011
How does a plant manage without an excretory system ? Some plants are known to live for a hundred or more years. During their life sapn, they, doubtless, intake a lot of unwanted stuff ? How do they get rid of it ? Not possibly through transpiration, as unwanted salts cannot be excreted that way. Please explain
Plants take up a variety of unusable compounds including toxins. These are sequestered within plant tissues.
Anthony R. Brach, PhD
Missouri Botanical Garden / Harvard University Herbaria
If you think of plants (autotrophs) the same way you think of humans and animals (heterotrophs), your question is very appropriate. If all your energy and nutrients came from eating other plants and animals, you would need to take the good nutrients with the bad toxins and find a way to remove the toxins. Plants produce all of their chemical energy from light and can produce all of their nutrients from inorganic minerals and carbon dioxide. So the level of toxins taken up by the plant will be much less than for animals. Nevertheless, toxins are taken up by plants (herbicides are great examples of how we use the property to human advantage) and they have enzymes that can alter some toxins so that they can be excreted through the roots or by deposition at the leaf margins with removal by the next rainfall. Often times toxins are sequestered in the vacuoles of plant cells, which are often considered to be less biologically active than the rest of the cell.
The toxins can be lost if they remain in parts of the plant that are discarded annually such as leaves or bark.
As autotrophs, plants don’t need elaborate excretion pathways. The major “waste” products are oxygen and CO2. Technically, the term waste is a misnomer since both are required to fuel respiration and photosynthesis. As gases, they are also readily released.
However, there are many other compounds which are deleterious to plant survival. In these cases, plants have evolved coping mechanisms. Most are reactive, involving storage or removal. Others are preventative, involving interception and refusal at the point of entry.
Plants can eliminate excess salts through a process known as guttation. This occurs through specialized structures known as hydathodes. On some plants you can actually see a fine precipitate of salt on the leaf fringes.
Alternatively, plant cells possess storage sacs called vacuoles. This organelle effectively compartmentalizes (cordons off) waste and prevents interaction with the remainder of the cell. Wastes are shuttled here for storage. In this case, the plant does not actively expunge wastes, but keeps them housed onsite, not unlike a spent fuel rod pool at a nuclear facility. Similarly, wastes also can be incorporated into less sensitive structural areas such as cell walls, epidermal cells, and trichomes. Trees are also reported to use their heartwood to store waste products.
Heavy metals may be complexed with organic molecules to render them inactive and then stored in a vacuole. Some plants have a tendency to hyperaccumulate heavy metals in their tissues. This makes them useful for phyto (meaning plant-based) remediation of brownfields. The issue is that they essentially become biohazards, which requires harvest and proper disposal.
Noted British biologist Brian Ford has suggested that dropped leaves are “excretophores”, providing a medium for plants to eliminate stored wastes.
For a time, it was thought that wastes were converted into secondary metabolites. These are compounds that are produced as offshoots of a primary metabolic pathway. Instead of taking the “normal” route, raw materials are redirected and alternatively processed to make advantageous compounds. For instance: bitterants and/or natural pesticides that make plants unpalatable to herbivores. Current evidence indicates that wastes are not repurposed in this fashion.
Dr. Tim Durham
Instructor, Office of Curriculum and Instruction
Department of Biological Sciences
Florida Gulf Coast University
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Update: June 2012