   Basic Plant Processes - October 6, 1999 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County In our never-ending quest to better understand our gardens and environment, we need basic knowledge of the processes that occur on the cellular and molecular levels. My motives are not to cram science down your throat, but to inspire interest and increase your scientific knowledge of plant processes. Greater understanding of botany will make you a better gardener. So, here is a Plant Science 101 lesson. All living organisms are formed from minute units called cells. All cells are surrounded by a semipermeable membrane and contain genetic information (DNA) to create other cells. The word "semipermeable" means that the cell can allow some substances to get through the membrane while others cannot. The genetic information is the "blueprint" for the organism. This basic definition best describes bacterial cells. Plant cells are more complex having cell walls, specialized internal subunits called organelles, an internal structural network of micro fibers that form a cytoskeleton, and a membrane bound nucleus containing the DNA. Plants are distinguished from other organisms such as animals and fungi by several characteristics: cell walls made from cellulose; embryos are protected by the parent plant tissue; specialized organelles called chloroplasts (the site where photosynthesis occurs) that contain chlorophyll; excess sugars are stored as starch; and sexual reproduction that alternates between cells that have one copy of genetic information (pollen and eggs) and cells having two copies (what we call the plant). The cell wall provides a barrier for the cell membrane to press against and gives the plant a rigid structure. Cellulose forms the cell wall and is a valuable human commodity: fiber for paper and wood for structures and furniture among other products. The unique cross-linkages that form cellulose are difficult for most organisms to digest. Humans need to eat cellulose but cannot digest it. Dieticians call it fiber. Termites and some fungi have no problems digesting it though. Parent tissues protect the embryos found in seeds. There are many different types of seeds that use a variety of strategies to survive environmental challenges. Fire, soil chemicals, animal digestive systems, daylight hours, and specific temperature ranges can act independently of together to influence when a seed can germinate. Organelles are surrounded by membranes and have many different functions. Chloroplasts are unique to plants and they harvest light energy to be stored in chemical compounds such as sugars, fats, and proteins. There are many other organelle types that: metabolize sugars (mitochondria), form proteins (ribosomes), recycle cellular junk (lysosomes), create complex molecules (golgi complex), and storage containers (central vacuoles). Organelles compartmentalize the cell and allow different metabolic processes to occur simultaneously in the privacy of their own membrane. Plant store excess sugar as starch. We recognize starch as a major component of foods such as potatoes and cereal grains. To the plant, these are stored food reserves to be used later. Potatoes are tubers that will resprout the following growing season. Cereal grains use starch as energy to grow after germinating. Starch is also stored in woody plant stems and roots as energy reserves. Trees and shrubs create sugar in the leaves all during the growing season via photosynthesis. The unused portion is transported downward through the phloem and stored in the trunk and roots. When growth resumes in the spring, the starch is converted back to sugar and is used as energy to resume active growth. Plant reproduction is somewhat similar to human reproduction. Pollen is transported to the stigma where it germinates and grows a pollen tube toward the ovule. Both pollen and ovule have only one set of chromosomes. At fertilization, the pollen tube deposits it=s set of chromosomes into the ovule. At this point, the fertilized ovule is called a zygote which matures into an embryo. The mother plant forms the seed around the embryo along with a food reserve that will keep the embryo alive until germination. This is a lot of information on basic plant function, but it gives backyard gardeners some background information in plant science. It may even inspire some of you to study up and learn more on your own. The University of Arizona Cooperative Extension has publications and information on plant science and gardening. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |