|MadSci Network: Botany|
Acids react with molecules and change them. Sulfuric acid with a couple of loosly bound hydrogens is very good at reacting with other molecules.
For seed germination to occur, lots of things have to go right and many environmental conditions need to be met. Some plant species are much more finicky than others. I do not know what the germination requirements are for cress, but I do know that there are very many species of cress. In any case, I am getting off track.
Most of the stuff inside the seed coat of a cress seed is endosperm. It is full of compounds that will help the plant grow and be the those cotyledon leaves that you first see after germination. Much of the material here can be lost and the plant survive. What the acid is having an effect on is the very tiny and very specialized part, the embryo, where cells are already formed and where most of the initial meristem cells are. These meristem cells are the ones that do all the dividing. If cell division does not happen, the plant does not grow and eventually it will die. The acidity is having an effect on those cells. Cells are kept separate from each other by membranes, these membranes keep areas isolated from each other so that reactions in one area don't interfere with reactions in another. (I'm being a little simplistic here but I don't want to delve into plant anatomy and physiology as it will take hours.) Water and some other things such as hydrogen pass freely though the membranes, but most things are only let in through expenditures of energy (ATP, NADP, etc.) or as a result of a pH gradient. The hydrogens in an acid (like sulfuric) disrupt the pH gradient. The cell must work overtime to deal with the additional hydrogens and processes such as photosynthesis and respiration are disrupted, which at first slows growth and may eventually result cell death.
Once the hydrogens are gone from the sulfuric acid, the sulfate that is left will bind with minerals and nutrients in the seed and remove them from use by the plant. There are many other things that go on as a result of the presence of sulfuric acid in the plant, but the above are some of the most obvious and important. In addition, there are many more things that go on in the soil due to increased acidity. Most of these things are not good for plants. Although some plants, such as sheep sorral and pine trees do better with a more acid soil. Still, there are limits and at some point the pH can become too low for even these plants.
David Hershey adds the following:
In the soil solution, sulfuric acid completely dissociates into hydrogen ions and sulfate ions. The sulfate is not harmful to the plant in the low concentrations found in acid rain. Sulfur is actually beneficial to the plant because it is required by the plant and is absorbed from the soil by the roots as sulfate.
The hydrogen ions or low pH of acid rain can harm plants in several ways. At low pH, potentially toxic metals in the soil, such as zinc, copper and aluminum, become more soluble and are more readily absorbed by the plant. The plant can thus be injured by metal toxicity. Hydrogen ions can also displace calcium ions from plant membranes and damage them.
Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.
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