Definitions of plant and light science terms for growing indoors under lights

My new book on Growing Plants Under Lights will be released in spring 2018. To accompany the book, here is a Glossary of Terms related to plant and light science that you'll use when growing indoors under lights. Use these terms to help you better understand processes such as photosynthesis, photoperiodism, plant growth, propagation, and light science.


Absolute long-day plants. (qualitative long-day plants, obligate long-day plants). The plants whose flowering absolutely depends on a long-day (short-night) condition. They flower only under long-day conditions, and never flower under unsuitable photoperiodic cycle conditions. Antonym is “facultative long-day plants”.

Absolute response. (qualitative response, obligate response). The plants whose flowering absolutely depends on the photoperiodic cycle condition or low temperature. They never flower under unsuitable photoperiodic cycles or temperature conditions.

Absolute short-day plants. (qualitative short-day plants, obligate short-day plants). The plants whose flowering absolutely depends on short-day (long-night) conditions. They flower only under short-day conditions, and never flower under unsuitable photoperiodic cycle conditions.

Aeroponic. Soilless growing by continuously, or intermittently, misting suspended roots with water.

Auto-flowering.  Entering the flowering stage based on plant age rather than season or daylength.

Auxins. Plant hormones found in shoots and roots

Ballast. A resister that limits the electrical current to a light bulb (lamp).

Biochrome. Biological pigment in plants.

Bolting. The process by which rosette plants elongate their stems. It occurs when flowering is induced in long-day plants or cold-requiring plants. However, bolting is not the same as flowering. Sometimes, the stem would elongate but flower buds would not be formed. However, the term is often used synonymously with flowering.

Canopy. The top of a grouping of plants.

Carotenoid. Plant pigments usually red, orange or yellow in color.

CFM. Cubit Foot per Minute.

Chitting. Encouraging seeds to sprout prior to planting, ending dormancy early.

Chlorophyll. Pigment that turns leaves green and allows light absorption.

Chloroplast. Organelle in a plant that conducts photosynthesis.

Coir. Coconut fiber, used in potting media and hydroponics.

Cold-requiring plants. The plants which are induced to flower when exposed to low temperatures.

Come true. Seeds of a given plant species or variety will produce the same variety as the parent plant. Seeds of open-pollinated plants come true.

Cotyledon-seed coat. A hard seed coat which requires swelling.

Critical dark period. (critical nightlength, critical night) The length of night or dark period in a 24-hour cycle required to induce flowering of short-day plants, or inhibit flowering of long-day plants. When the length of night is longer than the critical dark period, short-day plants are induced to flower and long-day plants do not flower. In a natural setting, critical dark period is 24 hours minus critical daylength. Since the factor regulating photoperiodic flowering is not daylength but nightlength, the critical dark period rather than critical daylength should be referred to. “Nightlength” is also spelled “night length”.

Critical day. The same as “critical daylength”.

Critical daylength (critical day, critical photoperiod). The length of daylight within a 24-hour cycle (balanced with critical dark period) required to induce flowering of long-day plants or to inhibit flowering of short-day plants. When the daylength is longer than the critical daylength, long-day plants are induced to flower and short-day plants do not flower.  

Critical night. The same as “critical dark period”.

Critical nightlength. The same as “critical dark period”.

Critical photoperiod. The same as “critical daylength”.

Cryptochromes. Blue light photoreceptors.

Cultivar. A plant variety resulting from an intentional hybridization of two related plants, or intentional selection of a naturally occurring variety, for cultivation in the marketplace. Seeds from cultivars to not come true, but cultivars may be replicated via vegetative cuttings or division.

Cytokinins. Phytohormones that promote lateral growth

Daylength. The same as “photoperiod”. Also spelled “day length”.

Day-neutral plants. The plants whose flowering does not depend on night length. The flowering of day-neutral plants is triggered when plants reach a certain age.

Deciduous.  Plants that lose their leaves when the season changes.

Devernalization. The reversal of the effect of an inductive cold temperature in vernalization by exposure to warm temperatures.

DIF. The numeric value between daytime and nighttime temperatures, which you calculate by subtracting the nighttime temperature from the daytime temperature.

DLI Daily Light Integral. Total quantity of light delivered over an entire day, with the time period measured in hours.

Dormancy. Temporary inactivity at a metabolic level.

Facultative long-day plants (quantitative long-day plants). The plants whose flowering is promoted by a long-day (short-night) photoperiod. They can flower even under alternate photoperiods, although flowering is delayed. Antonym is “absolute long-day plants”.

Facultative response (quantitative response). The response which is promoted by a suitable photoperiod. The response could occur even under an inappropriate condition, although it is delayed. Antonym is “absolute response”.

Facultative short-day plants (quantitative short-day plants). The plants whose flowering is promoted by a short-day (long-night) photoperiod. They can flower even under inappropriate photoperiods, although flowering is delayed. Antonym is “absolute short-day plants”.

Fermenting. Process that converts sugar to acids, gases, or alcohol. Process used to prepare wet seeds.

Fluorescent. Lamp type that creates light by excitation of mercury vapor.

Foliar feeding. Spray feeding plants by applying a liquid solution to the leaves instead of the roots.

Footcandle. A measurement unit of light comparable to shady, part sun, or sunny outdoor conditions.

Fruit set. The process in which a flower makes the transition to a developing fruit.

Germinate. To sprout, begin growing.

Gibberellic acid. Plant growth regulator promoting elongation of cells and root tissue.

Go to seed. When a plant bolts, flowers, and develops mature seed.

Greensprouting. Encouraging seeds to sprout prior to planting, ending dormancy early.

Herbaceous. Plants with non-woody stems.

Hybrid. A cross-breed of two plants that is not necessarily reproducible

Hydrophilic. Water absorbent.

Hydroponic. Growing plants in water.

Incandescent. Lamp type that creates light by heating a filament.

Indole butyric acid (IBA). A hormone used to produce clones via cuttings and for rooting

Infrared. Light with a longer wavelength than the visible spectrum---felt as heat.

Intermediate-day plants. The plants which flower when the dark length is between certain durations. An example is Salsola komarovii, which flowers when nightlength is around 12 hours. Flowering is delayed under both longer and shorter nightlengths. This type of plant is quite rare. The intermediate-day plant is a modification of a short-day plant.

Internode Length. The stalk of a plant as measured between nodes

Kelvin. Unit of visual light temperature measurement (cool to warm spectrum).

Long-day plants. The plants that flower under dark period shorter than the critical night length (light period longer than the critical daylength).

Long-short-day plants. The plants that flower when placed under long-day (short-night) photoperiods and then under short-day (long-night) photoperiods.

Lumen. A unit of light measurement.

LPW. Lumens per watt.

Lux. A measurement unit of light.

Media. The substrate material in which a plant grows, i.e. potting soil or coir.

Moss. A small, flowerless plant used as a soil-additive for moisture retention.

Microgreens. Juvenile edible plants---young seedlings and leaves of many different crops.

Micromole. One millionth of a mole or 602 quadrillion.

Mitochondria. Organelle in cytoplasm that functions in energy production.

Mole. A mole is a term used in science and is equal to something called “Avogadro’s number”, which is 602,214,150,000,000,000,000,000.

N-P-K.  A ratio representation of nitrogen, phosphorus, and potassium in fertilizers.

Obligatory long-day plants. The same as “absolute long-day plants”.

Obligatory response. The same as “absolute response”.

Obligatory short-day plants. The same as “absolute short-day plants”.

Oedema. Leaking of fluids into plant tissue due to swelling of cells. Can be caused by high-humidity or certain light spectrums.

Open-Pollinated. An open-pollinated plant variety can self-pollinate, or be pollinated by another specimen of the same variety and produce essentially the same variety from seed.

Organelle. Cellular structure with a specialized function.

Ornamental. A plant whose purpose is decorative; blooming and foliage plants.

Peat. Accumulation of partially decayed vegetation or organic matter.

Perennials. Plants that grow and bloom in the same location year-after-year. Winter-hardy.

Perlite. Volcanic mineral used as a soil additive.

Petiole.  The thin stalk that attaches a leaf to a stem.

PFR. The phytochrome molecule that absorbs and initiates biological response to Infrared light.

Photomorphogenesis. The influence of light on plant development.

Photons. The name physicists give to light particles.

Photoperiod. The period of time during which natural or artificial light is available to promote photosynthesis in plant life each day.

Photoperiod (daylength). The length of day (light period) in the daily cycle.

Photoperiodic cycle. The cycling of day and night (light and darkness).

Photoperiodic flowering. The flowering which occurs in response to the photoperiodic cycle.

Photoperiodism. The nature in which a plant responds to photoperiodic cycle, such as flowering.

Photosynthesis. The process by which plants make food from light energy plus using chlorophyll.

Photosynthetically Active Radiation (PAR). The amount of light in the visible spectrum available for photosynthesis.

Pollination. Transfer of pollen grains from a flower’s anthers to the same or another flower’s stigmas.

PPF = Photosynthetic Photon Flux. Measurement of PAR that is produced by a light source each second.

PPFD = Photosynthetic Photon Flux Density. Measurement of the amount of PAR light produced by a light source that actually reaches a plant surface at a specific distance from the light source.

PR. The phytochrome molecule that absorbs and response to red light.

Propagation. Growing new plants from seeds, or taking vegetative cuttings from a plant to make new clones.

Qualitative long-day plants. The same as “absolute long-day plants”.

Qualitative response. The same as “absolute response”.

Qualitative short-day plants. The same as “absolute short-day plants”.

Quanta. Quantity of light photons.

Quantitative long-day plants. The same as “facultative long-day plants”.

Quantitative response. The same as “facultative response”.

Quantitative short-day plants. The same as “facultative short-day plants”.

Respiration. The release of stored energy by plants (burning fuel).

Rootstock. The underground part of a plant. Roots and growth crown.

Rosette. A form of plant whose stem elongation is restricted in the vegetative growth phase. Many leaves are attached to a short stem, and therefore the plant has the appearance of a rose flower.

Scarification. Literally scarring the outer hard coat of a larger seed by scratching or breaking to allow water in, to break dormancy and initiate germination.

Seed-eye. A bud that grows into a new plant.

Seedling. Very young plant, as grown from a seed.

Shoot. New plant growth consisting of an underdeveloped stem.

Short-long-day plants. The plants that flower when placed under short-day (long-night) photoperiods and then under long-day (short-night) photoperiods.

Snips. Small hand sheers for pruning smaller parts of plants, or harvesting herbs, taking cuttings.

Stomata. Pores in plant leaves that open and close, allowing for the exchange of water and gasses.

Stratification. Simulating winter conditions for stored seeds to break dormancy.

Substrate. The surface on or in which plants live or grow which supports their root system.

Succulent. A plant with specialized cells to store water for a later time; a desert plant

Thermoperiod. Period of daytime and nighttime a plant experiences a given temperature.

Transpiration. Water and gas exchange into and out of a plant –water traveling through the plant from roots to stomata.

Turgidity/Turgor pressure. The pressure of the cell contents against the plant cell wall (resulting in an upright plant).

Ultraviolet Radiation (UV). Light with a shorter wavelength than the visible spectrum.

Vapor Pressure Deficit. (VPD) Moisture amount in the air vs. moisture saturated air.

Vermiculite. A hydrous phyllosilicate mineral. Added to soil to provide root anchors, retain moisture and nutrients, and aeration.

Vernalization. The initiation of a flower by exposure to a specific period of low temperature (0 to 5 ℃). Also the promotive effects of low temperatures on flowering. Sometimes spelled “vernalisation”.

Watt. A measurement unit of electricity.

Wavelength. Measurement of the length of a light wave.

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