Plant Science Terms - A

Abiotic disease - condition caused by nonliving, nonparasitic, or noninfectious agents

Abscisic acid (ABA) - considered a “stress hormone.” It is a signaling molecule that induces stomatal closure under drought or extreme temperature stress conditions

Abscission - dropping of leaves, flowers, or fruit by a plant. 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

Abscission zone - layer of cells at the base of a leaf petiole, flower or fruit stalk where the organ separates from the plant

Absorption - intake of water and other materials through root or leaf cells

Acuminate - prolonged apex tapering to a long, narrow point 

Accumulated heat units - number of heat units in a growing season.  Calculated at temperatures above 50°F, but can be calculated at other temperatures, depending on the crop. A day’s heat units are calculated as: Max temp (°F) + Min temp (°F) divided by 2 - 50°F

Acid soil - soil with pH below 7 on a pH scale of 1 to 14. The lower the pH, the more acid the soil

Actinomycete - bacterium of an order of typically nonmotile filamentous form. They include the economically important streptomycetes, and were formerly regarded as fungi

Active ingredient - chemical in a pesticide formulation that actually kills the target pest

Acute - ending in an acute angle, with a sharp point

Additive - 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

Adhesion - force of attraction that causes two different substances to join

Adventitious - growth arising in an unexpected location or not ordinarily expected, usually the result of stress or injury. Normal growth in plants comes from meristematic tissue, but adventitious growth starts from non-meristematic tissue

Adventitious bud - bud that develops in locations where buds usually do not occur. An example would be buds found on root pieces used for propagation; roots do not have buds

Adventitious root - root that forms at any place on the plant other than the primary root system

Adjuvant - See additive

Aeration or aerification - practice involving removal of cores or turf plugs and soil with the purpose of reducing compaction and improving air flow

Aerial root - unusual type of root that develops on stems above ground

Aerobic - active in the presence of free oxygen

After-ripening - seed maturation process that must be completed before germination can occur

Aggregate - soil aggregates are groups of soil particles that bind to each other more strongly than to adjacent particles. The space between the aggregates provide pore space for retention and exchange of air and water

Aggregation - process by which individual particles of sand, silt, and clay cluster and bind together to form soil peds

Agriculture - science or practice of farming

Agronomy - science of land cultivation, soil management, and crop production

Air drainage - downward flow of cold air through the soil caused by gravity. Plant crops that are sensitive to late spring and early fall frost on slopes, preferably south-facing, so that cold air will drain downhill and settle in the low spots below the planted area. This because cold air is heavier than warm air, it flows downhill and often fills hollows which become frost pockets

Air layering - propagation method. A cut is made three-quartets of the way through the stem at an angle of 45 degrees. A toothpick is inserted to keep the cut from sealing itself closed. The cut is wrapped in sphagnum moss that has been soaked in water and wrung out. This is then wrapped in plastic and tied at the top and bottom with ties

Alkaline soil. Soil with pH above 7 on a pH scale of 1 to 14. The higher the reading, the more alkaline the soil. (See also pH.)

Allée - walkway lined with trees or tall shrubs

Allelopathy - excretion by some plants’ leaves and roots of compounds that inhibit the growth of other plants

Alternate leaf arrangement - leaves are attached at one leaf per node on different sides along the stem

Alternate - plant parts arranged singly on the main stem or axis

Ammonium - plant-available form of nitrogen contained in many fertilizers and generated in the soil by the breakdown of organic matter. (See also Nitrogen cycle.)

Anaerobic - active in the absence of free oxygen

Analogous - landscaping, use of adjacent colors on the color wheel such as blue, violet, and red

Anatomy - study of plant structure

Angiosperm - flowering plants. Plants that have a highly evolved reproductive system. Seeds enclosed in an ovary such as a fruit, grain, or pod

Anion - negatively charged ion, for example, chloride

Anion exchange - anion exchange capacity (AEC). The sum total of exchangeable anions that a soil can absorb expressed in meq/100g (milliequivalents per 100 grams) soil

Annuals - plants that complete their life cycles in within a single growing season which one year or less.  The plants started from seed and grow, mature, flower, produce seed, and die in the same growing season

Anoplura - major order of insects that have two pairs of wings, or are wingless, and piercing-sucking mouthparts (sucking lice)

Anther - pollen-bearing part of the flower and also the flower’s male sexual organ (on the upper part of the stamen) in which pollen is produced. The filament supports the anther; together they are referred to as the stamen

Anthracnose - type of leaf or fruit spot plant disease characterized by black or brown dead areas on leaves, stems, or fruits and caused by a fungus

Anvil pruner - pruning tool that cuts a branch between one sharpened blade and a flat, anvil-shaped piece of metal. Has a tendency to crush rather than make a smooth cut

Apical - at the tip

Apical dominance - 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

Apex - tip of a stem or root

Apical meristem - area of the plant shoot and root tips where cells actively divide to provide more cells that will expand and develop into the tissues and organs of the plant. Also called shoot meristem

Arboretum - an area devoted to specimen plantings of trees and shrubs

Arms - for grape plants, all main branches two years and older

Asexual propagation. Reproduction of a plant using its own vegetative parts. (See also Vegetative propagation.)

Asexual reproduction - duplicating a plant from any cell, tissue or organ of the plant

Assimilation- building of cell matter from inorganic (minerals) and organic (carbohydrates and sugars) materials

Aspect - direction of exposure to sunlight

Attenuate - prolonged base with thin tissue extending along part of the petiole 

ATP (adenosine triphosphate) - molecule that is used in a number of metabolic reactions in plant cells to carry out cellular work

Attractant - material that lures pests

Auricle - claw-like, or ear-like, appendage projecting from the collar of the leaf

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 - upper angle formed between the a leaf stalk (petiole) and the internodes above it on the stem 

Axillary buds - buds that are borne laterally on the stem in the axils of the leaves. This undeveloped (embryonic) shoot lies at the junction of the stem and petiole of a plant. As the apical meristem grows and forms leaves, it leaves behind a region of meristematic cells at the node between the stem and the leaf, an undeveloped shoot or flower at the node. Also called the lateral bud


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