JARS v63n1 - The Word: Metabolism

The Word: Metabolism
Bruce Palmer
Cutten, California

In the Northern Hemisphere, the winter season is back. Our rhododendrons still hold their leaves, but most of the other trees and shrubs, including deciduous azaleas, are bare. Things are going on in the cells, though. The collective term for those activities in all organisms is metabolism , the word for this issue of the Journal. The word derives from the Greek meta , beyond and ballein , to throw. A good definition for metabolism is the sum total of all chemical processes occurring within a living cell. It's appropriate at this season because we have done or soon will do our winter fertilizing to induce outstanding flowers in our rhodies come spring. Why do we need to fertilize when plants are autotrophic (Greek autos , self, and trophos , feeder)? As we know, they trap light energy in carbohydrate molecules by the process of photosynthesis (Greek photos , light, syn, together and titheni , to place). Animals could not exist without the molecules plants produce. We fertilize plants because not all of the elements necessary for cells are trapped by photosynthesis. All organisms use the same basic elements. The most central ones can be remembered by the mnemonic device CHNOPS. It's easy to remember because it sounds like the German word for strong liquor. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are found in all cells, along with smaller quantities of several other elements, including potassium, iron, magnesium, calcium and a number of others. Plants extract carbon dioxide from the air and water from the soil and air to produce carbohydrates and give off oxygen as a waste product. To do so they use a molecule called chlorophyll (Greek, kloros , green and phyllos , leaf). Chlorophyll has an atom of magnesium at its center that acts to assist photosynthesis. Adding Epsom salts, magnesium sulfate, to our gardens often helps plants take up sufficient quantities of magnesium.

Both plants and animals use the byproducts of photosynthesis and other molecules to carry out respiration, the central metabolic process in all cells. Respiration yields the energy needed for all other processes. In the broadest sense, respiration results in the production of proteins. Proteins are important because all major molecules in cells are proteins or, as with carbohydrates and fats, are assembled by enzymes which themselves are proteins. Plants often cannot get enough of some elements to produce leaves, growth and flowers that are satisfactory to us, so, as with people, we feed them supplements. We call these supplements fertilizers and we look at the contents to determine what fertilizer to use. The contents are listed on the container as something like 10-10-10 or some other set of three numbers. The first number tells us what percentage of usable nitrogen is in the mix. All proteins contain nitrogen and here's where we run into the first reason for fertilizing our garden plants. Nearly 80 percent of the gas in the atmosphere is nitrogen. You'd think that it would be easy for plants to extract it, but it's not. Nitrogen by itself is inert; it doesn't combine easily with other atoms. That's why nitrogen floats around alone in the atmosphere. With some exceptions, plants can't use it unless it is attached to oxygen as a nitrate. The process of combining nitrogen with oxygen takes place mainly in the soil and in the roots of some plants and is carried out mostly by certain bacteria called nitrogen-fixing bacteria. Higher plants depend on the nitrates produced this way to make their proteins. When we want "better" plants we often add fertilizers containing significant amounts of fixed nitrogen. The second number on the container indicates how much phosphorus is in the mix. Energy is required to keep all of the cell processes going; it is held in molecules containing phosphorus. Plants require a lot of energy to make flowers; thus we use fertilizers containing high phosphorus numbers when we want to encourage flower production. The third number on the container tells how much potassium the mix contains. Potassium is extremely important in small quantities in plant cells because it helps a number of other elements get into cells. Sulfur is an essential part of many proteins including some enzymes. Because of that we often find by reading the fine print that the potassium is represented as potassium sulfate or some of the nitrogen is ammonium sulfate.

It's not terribly difficult to discover if a plant needs fertilizer or has had too much. Just like people, if a plant looks pale and unhealthy it probably needs some help. Plants with an overall yellowish look to the leaves are likely to be deficient in nitrogen. That's the most common nutrient deficiency. A general fertilizer with a high first number (nitrogen) is usually just the ticket; the plant will respond by greening up nicely. Sometimes after we have "fed" a plant some or most of the leaves, especially new ones, will develop brown tips and edges a few weeks later. We have given the plant so much fertilizer that it has been transported to the tips of the leaves in excessive quantities and the cells there have been killed with kindness. It may be that a plant needs fertilizer, but discovering what is deficient in a plant is not always straightforward. In the photograph accompanying this article an older leaf of Rhododendron 'Fire Rim' is shown with yellow areas called chlorosis (Greek, kloros , green and - sis , disease or condition) between the veins but with dark green with dark green veins. This pattern, sometimes called a "Christmas tree" effect, is probably magnesium deficiency. The cells closest to the veins are gobbling up all the available magnesium before it can get out to the other cells. It could be iron deficiency except that the younger leaves, not shown in this photo, don't show the same problem. It's difficult to be certain which problem it is, but giving the plant some trace minerals that include chelated iron (iron attached to an organic molecule) and/or Epsom salts should help.

To carry out the processes of metabolism our rhododendrons and all other organisms need to take in quantities of specific nutrients. The overall requirements and processes are essentially the same, although some of the specific atomic and quantitative requirements vary slightly from one organism to another. In the case of our rhodies, to get the best performance we need to give them an extra boost from time to time. Except in the areas where there is little to no frost we avoid nitrogen-rich fertilizers in fall. In winter we give them a boost of phosphorus (the second number) to encourage great blooming in spring. After blooming they need a general fertilizer (all three numbers in roughly equal quantities) to help their growth spurt in spring and early summer. In the temperate, mostly frost-free U. S. West Coastal region where I live we give them a fertilizer boost in early fall or late summer, often with slightly higher nitrogen (the first number). It has to be early fall, though, or we may lose nitrogen induced new growth during the winter, as happens in colder climates. We remember the times by fertilizing near Valentine's Day, Mother's or Father's Day and Labor Day. If you haven't done your winter fertilizing yet, it's probably time. Put on your heavy clothes and get out there. Your efforts will be well-rewarded when your plants bloom.

Bruce Palmer is a member of the Eureka Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii System for twenty-five years.