| abscisic acid | A growth-inhibiting hormone. |
| abscission | The dropping of leaves, flowers, or fruit by a plant. This can result from natural growth processes (e.g., fruit ripening) or from external factors such as temperature or chemicals. |
| abscission layer | Specialized cells, usually at the base of a leaf stalk or fruit stem, that trigger both the separation of the leaf or fruit and the development of scar tissue to protect the plant. |
| absorption | The intake of water and other materials through root or leaf cells. |
| accumulated heat units | The number of hours in a growing season. Usually calculated at temperatures above 50°F, but can be calculated at other temperatures, depending on the crop. A day’s heat units (above 50°F) are calculated as:
Daily values are then totaled for the season, with values less than zero ignored (but not deducted from the total). |
| acid soil | Soil with a pH below 7 on a pH scale of 0 to 14. The lower the pH, the more acid the soil. See pH. |
| actinomorphic flower | A flower possessing radial symmetry. Any cut through the center divides the flower into two equal parts. |
| active ingredient | The chemical in a pesticide formulation that actually kills the target pest. |
| additive | A substance that, when added to a pesticide, reduces the surface tension between two unlike materials (e.g., spray droplets and a plant surface), thus improving adherence. Also called an adjuvant or surfactant. |
| adjuvant | See additive. |
| adventitious | Growth not ordinarily expected, usually the result of stress or injury. A plant’s normal growth comes from meristematic tissue, but adventitious growth comes from nonmeristematic tissue. |
| adventitious bud | A bud in an unusual place on a plant, often on an internode. This may be the result of an injury. Suckers and water sprouts usually grow from adventitious buds. |
| adventitious root | A root in an unusual place, often where a branch contacts soil or damp material. A plant cannot be reproduced from cuttings or layering unless adventitious roots develop. |
| aeration | Mechanically loosening or puncturing soil to increase permeability to water and air. |
| aerial root | A root emerging above the soil level. |
| aerobic | Active in the presence of free oxygen. |
| after-ripening | The seed maturation process that must be completed before germination can occur. |
| aggregate fruit | A group of small fruits derived from several ovaries within a single flower. |
| aggregation | The process by which individual particles of sand, silt and clay cluster and bind together to form soil peds. |
| alkaline soil | Soil with a pH above 7 on a pH scale of 0 to 14. The higher the reading, the more alkaline the soil. See pH. |
| alkaloid | A nitrogen-containing compound frequently used as a chemical defense by plants. |
| allelopathy | The excretion by some plants of compounds from their leaves and/or roots that inhibit the growth of other plants. |
| ammonium (NH4+) | A plant-available form of nitrogen contained in many fertilizers and generated in the soil by the breakdown of organic matter. See nitrogen cycle. |
| anaerobic | Active in the absence of free oxygen. |
| angiosperm | A member of a class of plants characterized by the formation of flowers and seeds in fruits. |
| anion | A negatively charged ion. Plant nutrient examples include nitrate (NO3–), phosphate (H2PO4–), and sulfate (SO42-). See cation. |
| annual | A plant that completes its life cycle in one growing season. |
| annual ring | A cylinder of secondary xylem added to the wood in a single growing season. |
| anther | The pollen-bearing part of a flower’s male sexual organ. The filament supports the anther; together they are referred to as the stamen. |
| anvil pruner | A pruning tool that cuts a branch between one sharpened blade and a flat, anvil-shaped piece of metal. These have a tendency to crush rather than make a smooth cut. |
| apex | The tip of a stem or root. |
| apical bud | A bud at the tip of a stem. |
| apical dominance | The inhibition of lateral bud growth by the presence of the hormone auxin in a plant’s terminal bud. Removing the growing tip removes auxin and promotes lateral bud break and subsequent branching, usually directly below the cut. |
| apical meristem | A region of actively dividing cells at the tip of a growing stem or root. |
| arboretum | An area devoted to specimen plantings of trees and shrubs. |
| asexual reproduction | See vegetative propagation. |
| aspect | Direction of exposure to sunlight. |
| assimilation | The building of cell matter from inorganic and organic materials (carbohydrates and sugars). |
| attractant | A material that lures pests. |
| autotrophic nutrition | A form of nutrition in which complex food molecules are produced by photosynthesis from carbon dioxide, water, and minerals. |
| auxin | One of the best known and most important plant hormones. Most abundantly produced in a plant’s actively growing tips. Generally, stimulates growth by cell division in the tip region and by cell elongation lower down the shoot. Growth of lateral buds is strongly inhibited by the normal concentration of auxin in the growing tip. |
| available water supply | Soil water that is available for plant uptake. Excludes water bound tightly to soil particles. |
| axil | The upper angle formed by a leaf’s stalk (petiole) and the internodes above it on a stem. |
| axillary bud | A bud that forms on an axil. |
| axillary bud primordium | An immature axillary bud. |
