Unit Seven. Plant Life
34.7. Growth and Nutrition
Just as human beings need certain nutrients, such as carbohydrates, amino acids, and vitamins, to survive, plants also need various nutrients to grow and remain healthy. Lack of an important nutrient may slow a plant’s growth or make the plant more susceptible to disease or even death.
Plants require a number of nutrients. Some of these are macronutrients, which plants need in relatively large amounts, and others are micronutrients, which are required in trace amounts. These nutrients are called essential because the plant cannot manufacture them; they have to be brought into the plant. For example, a plant can manufacture amino acids used to build proteins, but it can’t manufacture the carbon or nitrogen atoms that make up the amino acids; therefore, these are essential nutrients. There are nine macronutrients: carbon, hydrogen, and oxygen—the three elements found in all organic compounds—as well as nitrogen (essential for amino acids), potassium, calcium, phosphorus, magnesium (the center of the chlorophyll molecule), and sulfur. Each of these nutrients approaches, or as in the case with carbon, may greatly exceed, 1% of the dry weight of a healthy plant.
Macronutrients are involved in plant metabolism in many ways. Nitrogen (N), acquired from the soil with the help of nitrogen-fixing bacteria, is an essential part of proteins and nucleic acids. Potassium (K) ions regulate the turgor pressure (the pressure within a cell that results from water moving into the cell) of guard cells and therefore the rate at which the plant loses water and takes in carbon dioxide. Calcium (Ca) is an essential component of the middle lamellae, the structural elements laid down between plant cell walls, and it also helps to maintain the physical integrity of membranes. Magnesium (Mg) is a part of the chlorophyll molecule. The presence of phosphorus (P) in many key biological molecules such as nucleic acids and ATP has been explored in detail in earlier chapters. Sulfur (S) is a key component of an amino acid (cysteine), essential in building proteins.
The seven micronutrient elements—iron, chlorine, copper, manganese, zinc, molybdenum, and boron—constitute from less than one to several hundred parts per million in most plants. While the macronutrients were generally discovered in the last century, most micronutrients have been detected much more recently when technology developed that allowed investigators to identify and work with very small quantities.
Identifying Essential Nutrients
Nutritional requirements are assessed in hydroponic cultures as shown in figure 34.9; a plant’s roots are suspended in aerated water containing nutrients. The solutions contain all the necessary nutrients in the correct proportions but with certain known or suspected nutrients left out. The plants are then allowed to grow and are studied for the presence of abnormal symptoms that might indicate a need for the missing element. To give an idea of how small the quantities of micronutrients may be, the standard dose of molybdenum added to seriously deficient soils in Australia amounts to about 34 grams (about one handful) per hectare, once every 10 years! Most plants grow satisfactorily in hydroponic culture, and the method, although expensive, is occasionally practical for commercial purposes. Analytical chemistry has made it much easier to take plant material and test for levels of different molecules. One application has been the investigation of elevated levels of carbon (a result of global warming) on plant growth. With increasing levels of CO2, the leaves of some plants increase in size, but the amount of nitrogen decreases relative to carbon. This decreases the nutritional value of the leaves to herbivores.
Figure 34.9. Identifying nutritional requirements of plants.
A seedling is first grown in a complete nutrient solution. The seedling is then transplanted to a solution that lacks one suspected essential nutrient. The growth of the seedling is then studied for the presence of abnormal symptoms, such as discolored leaves and stunted growth. If the seedling's growth is normal, the nutrient that was left out may not be essential; if the seedling's growth is abnormal, the lacking nutrient is essential for growth.
Key Learning Outcome 34.7. All plants require significant amounts of nine macronutrients to survive. They also require trace amounts of seven other elements.