BISC 2224 Final Exam Key Terms

Photosynthetically Active Radiation (PAR)

The specific spectrum of light that plants are capable of perceiving and utilizing for photosynthesis.

Sun Loving Plants

Plants that evolved to grow in high light conditions; while they may survive in shade, they typically exhibit light-seeking behaviors.

Etiolation

A classic light-seeking behavior characterized by 'leggy', tall, and stretched growth with excessive internodal stretching, pale coloration, floppiness, and physical weakness.

Heliotropism

A light-seeking behavior in which plants physically bend, stretch, and turn their organs toward a light source.

Phytochrome

A plant photoreceptor that detects shifts in the red to far-red light ($R:Fr$) ratio.

$R:Fr$ (Red Light : Far Red Light)

A ratio describing the relative proportion of red light to far-red light in a plant's ambient light environment.

Pfr

The physiologically active conformation of phytochrome that inhibits internodal stretching and stimulates leaf and chloroplast development.

Pr

The physiologically inactive conformation of phytochrome.

Shade Loving Plants

Plants that evolved under continuous canopy shade, typically possessing broad, thin leaves to harvest sunlight like solar panels; they display lower sensitivity to $R:Fr$ and lack drought-resistance adaptations.

Abscisic Acid (ABA)

A plant hormone synthesized by root tips upon sensing dry soil, serving as an internal chemical signal that triggers stomatal closure to conserve water.

Temporary Wilt (Sun Wilt)

A transient wilt triggered by high transpiration rates exceeding uptake on hot, dry days rather than a lack of soil moisture, recovering overnight once stomata close.

Permanent Wilt

Wilt caused by deficient soil moisture; recovery begins upon irrigation unless prolonged desiccation causes irreversible tissue damage.

Drought Avoidance

An evolutionary strategy to evade drought through mechanisms such as germination inhibitors, geophytic dormancy, succulent water storage, or deep taproots.

Drought Tolerance

An evolved physiological capacity allowing plant cells to withstand severe water deficits, with some species surviving up to a $70\%$ loss of tissue water weight without adverse effects.

C-4 Photosynthesis

An adapted photosynthetic pathway found in prairie grasses and crops like corn, where an additional cellular reaction fixes $CO_2$ into a 4-carbon chain for Calvin cycle use during stomatal closure.

CAM (Crassulacean Acid Metabolism) Photosynthesis

An adaptation found in succulents and orchids where stomata open at night to fix atmospheric $CO_2$ into malic acid in vacuoles, which is broken down during the day for the Calvin cycle.

Aerenchyma

A specialized plant tissue containing continuous, gas-filled channels connecting shoots to roots, allowing atmospheric oxygen delivery to roots in waterlogged, low-oxygen substrates.

Forcing

A horticultural technique manipulating controlled environmental cues to induce a plant into specific seasonal behaviors, such as out-of-season flowering.

Photoperiodism

The physiological response of an organism to the relative lengths of light and dark intervals in a 24-hour cycle.

Night Break Response

The disruption of a plant's dark-period tracking by brief exposure to light during the night, preventing dark-dependent responses such as bract formation in poinsettias.

Critical Day Length (CDL)

The specific light duration within a 24-hour cycle required to initiate flowering in long-day plants or inhibit flowering in short-day plants.

Critical Night Length (CNL)

The precise period of continuous darkness required within a 24-hour cycle to inhibit flowering in long-day plants or stimulate flowering in short-day plants.

Short Day Plant (SDP)

Plants that flower only when day length is shorter than the critical day length (and the continuous dark period exceeds the critical night length).

Long Day Plant (LDP)

Plants that flower only when day length exceeds their critical day length (and the night period is shorter than the critical night length).

Florigen (FT Protein)

A long-range mobile signaling protein produced in leaves that travels through the phloem to the shoot apex to stimulate floral initiation.

Leaf Senescence

The programmed degradation and remobilization of leaf nutrients and compounds to the root system in autumn, initiated by the breakdown of chlorophyll.

Foliar Fruit Flag

Early-season leaf senescence where foliage turns bright red against an otherwise green canopy to visually attract frugivorous birds to distribute seeds.

Vernalization

The acquisition of the competence to flower or germinate triggered by prolonged exposure to chilling temperatures, with visible developmental responses occurring once warmth returns.

Cold Hardiness

The physiological ability of a plant to endure low and freezing temperatures without sustaining cellular damage or injury.

USDA Hardiness Zones

A system dividing geographic regions into 13 zones based on extreme minimum winter temperatures to classify plant species hardiness.

DIF

The mathematical difference between day and night temperatures; using negative DIF (-DIF), where nights are warmer than days, suppresses stem elongation to produce compact, bushy plants.

Hardening Off

The process of gradually acclimating greenhouse-grown plants to outdoor fluctuations in temperature, wind, and light to minimize transplant shock.

False Spring

A transient warm spell during late winter or early spring that triggers premature de-acclimation or bud break, followed by damaging below-freezing temperatures.

Glazing

The structural framing members of a greenhouse, commonly manufactured from non-rusting aluminum.

Cladding

The exterior translucent covering installed on greenhouse glazing, selected primarily based on total light transmissibility.

Greenhouse Cladding Materials Comparison

A comparative overview of the major cladding materials: Glass (durable 50+ year lifespan, high cost), Film Plastics (3-5 year lifespan, low cost, prone to fog/drip), and Polycarbonate Sheets (5-10 year lifespan, easy to cut, moderate cost).

Headhouse

A climate-controlled service structure attached to a greenhouse, used for storage and staging to buffer interior greenhouse microclimates from outside air.

Horizontal Air Flow (HAF) Fans

Greenhouse circulation fans that intake air through sidewall openings and move it horizontally across the plant canopy.

Vertical Air Flow (VAF) Fans

Circulation fans that draw air downward from upper roof ridge vents to generate vertical air patterns.

Thermal Curtain

A retractable fabric screen deployed horizontally below greenhouse ridges on cold nights to trap heat and reduce winter heating expenses.

Evaporative Cooler (Swamp Cooler)

A low-energy greenhouse cooling system passing intake air across wet corrugated cellulose pads, dropping sensible heat via moisture evaporation.

High Pressure Sodium (HPS) Lighting

Supplemental discharge lamps that have low upfront equipment costs but generate large quantities of waste heat and consume high amounts of electrical energy.

Light-Emitting Diode (LED) Lighting

Highly customizable, energy-efficient horticultural lighting that produces low radiant heat but requires higher initial capital investment.

Hydroponics

A soilless cultivation technique wherein plant root systems are directly suspended in an aerated water and nutrient solution.

Disease Triangle

A conceptual paradigm stating that infectious plant disease requires the concurrent presence of three factors: a susceptible host, a virulent pathogen, and favorable environmental conditions.

Hyphae

Microscopic filamentous tubular structures through which fungal pathogens invade plant tissues and secrete digestive enzymes.

Fruiting Body

The specialized, visible reproductive structure produced by a fungal organism containing spores, widely used for taxonomic identification.

Vector

An organism (typically a pest insect) that transfers pathogenic bacteria, viruses, or fungi from infected plants to uninfected hosts during feeding.

Horticultural Soaps and Oils

Topical contact sprays composed of refined fatty acids or oils that physically seal and block insect spiracles, causing death via asphyxiation.

Biological Controls

The intentional management of agricultural pests using natural predators, parasitoids, or microbial formulations such as Bacillus thuringiensis.

Systemic Pesticide

A chemical agent absorbed by plant organs and translocated systemically throughout vascular tissue, rendering entire plant structures toxic to pests.

Integrated Pest Management (IPM)

A pest management approach developed to reduce chemical dependency, comprising four core phases: setting action thresholds, pest monitoring and identification, prevention, and control.

Action Threshold

The predefined pest population density or level of damage at which control treatments must be implemented to prevent unacceptable loss.

Mode of Action (MOA)

The specific physiological and biochemical mechanism by which an active pesticide ingredient disrupts or terminates a target pest organism.

Re-Entry Interval (REI)

The minimum period of time that must pass after a pesticide application before workers can enter the treated area without personal protective equipment (PPE).

Gibberellins

A class of endogenous plant growth hormones primarily responsible for promoting internodal stem elongation and lateral branch formation.

Auxins

A class of plant hormones produced in apical buds that travel downward through stems, driving cell elongation, adventitious rooting, and apical dominance.

Cytokinins

A class of plant hormones synthesized largely in root systems that primarily stimulate active cell division (cytokinesis).

Phototropins

Specialized photoreceptors that detect directional blue light and mediate phototropic curvature by translocating auxins away from illuminated tissues to shaded sides.

Apical Dominance

The physiological mechanism where the primary terminal bud suppresses the outgrowth of lateral axillary buds via the basipetal stream of auxin.

Plant Growth Regulators (PGRs)

Foliar chemical treatments applied to modulate vegetative architecture and size without delaying bloom time or requiring structural pruning.

Root:Shoot Ratio

The equilibrium maintained by a plant between underground root biomass and aboveground shoot biomass.

Notching

Removing a small wedge of tissue directly above a dormant lateral bud to cut off descending auxin flow, thereby stimulating its emergence and growth.

Nicking

Removing a small wedge of tissue directly below a dormant lateral bud to disrupt the ascent of water and nutrients from roots, inhibiting that bud's development.

Cordon

A training technique where fruit trees are pruned to a single main stem trained at an angle to simplify spraying and improve light penetration across all fruits.

Espalier

A formal horticultural training system where lateral branches are trained along a flat, two-dimensional plane against walls or wire trellises.

Coppice

The traditional practice of cutting woody trunks back to one foot or less above soil level to generate a dense flush of rapidly growing new canes.

Pollard

A pruning system where upper secondary branches are cut back repeatedly to a structural head roughly 8 feet above ground level to keep regrowth and flowering near eye level.