horticulture light

Light

Energy source for photosynthesis

Light

Often determines plant morphology

Light

Often a trigger for fall leaf color and abscission, flowering

Light

Electromagnetic radiation, both a wave and a pulse of energy (photons or quanta)

Light

Quality (wavelength λ) measured in nm (10-9 m), ranging from 380 to 770 nm; the shorter the wavelength, the higher the energy; plants don't absorb green light as well as blue or red light

Light

Quantity (intensity), how bright is the light, best measured as photosynthetic photon flux (ppf)

Light

Duration (photoperiod), how long is the day versus the night?

Photosynthesis

The process by which plants convert light energy into chemical energy in the form of glucose.

Chloroplasts

Organelles that contain chlorophyll and are the site of photosynthesis.

Thylakoids

Membranes inside chloroplasts where the light reactions of photosynthesis occur.

Grana

Stacks of thylakoids within chloroplasts.

Stroma

Colorless areas in between the thylakoids in chloroplasts.

Chlorophyll

A pigment that absorbs primarily in red and blue wavelengths and is crucial for photosynthesis.

Chlorophyll a

One of the two types of chlorophyll that makes sugar.

Chlorophyll b

An accessory pigment that assists in photosynthesis.

Carotenoids

Accessory pigments that protect chloroplasts and transfer energy to chlorophyll a.

Xanthophylls

A type of carotenoid discussed later that contributes to the color of flowers.

Light Reactions

The first stage of photosynthesis where pigments are excited by light energy.

Dark Reactions (CO2 fixation)

The process where CO2 enters leaves and is fixed into a sugar molecule.

C3 pathway

A fixation pathway that uses a three-carbon molecule and is most common in horticultural plants.

C4 pathway

A fixation pathway that uses a four-carbon molecule, found in plants like corn and bermudagrass.

CAM pathway

A fixation pathway that allows plants to fix CO2 at night to reduce water loss.

RuBisCo

The most plentiful enzyme on Earth that fixes carbon from CO2 into a 3-carbon molecule.

Photorespiration

A process that occurs when RuBisCo fixes oxygen instead of CO2, leading to no sugar production.

PEP carboxylase

An enzyme that assists RuBisCo in the C4 pathway by fixing CO2 into a 4-carbon molecule.

Cryptochromes

Blue light receptors that control stem elongation and leaf expansion.

Phototropins

Blue light receptors that control phototropisms, the bending of plants toward or away from light.

Phytochromes

Red light receptors that detect photoperiod and control seed germination.

Anthocyanin production

A process controlled by phytochromes that affects leaf movements and shade response.

Chlorosis

The yellowing of leaves due to disease, nutrient deficiency, or excessive light.

Light Quality

The spectrum of light that affects plant growth and development.

Warm season plants

Plants that thrive under warm conditions, including C4 and CAM plants.

Cool season plants

Plants that thrive under cool conditions, primarily C3 plants.

Phytochromes

Two phytochromes Pr and Pfr act like light switches.

Pr

Darkness causes the switch to move to Pr.

Pfr

Light flips it back to Pfr.

Red light receptors

Control seed germination.

Seed Germination

Weeds and other small-seeded species (lettuce) often NEED light to germinate.

Light Quality

Red light keeps internodes shorter = shorter plant.

FR light

FR light passes through leaves while red is absorbed.

Leaf Movements

Control leaf (nastic) movements.

Anthocyanin Production

Trigger anthocyanin production.

Light Compensation Point

If respiration = photosynthesis no growth occurs.

Light Saturation Point

If photosynthesis is not limited by light.

Shade Tolerance

As light levels decline, so does the number of adapted plants.

Full Sun Requirement

Veggies and fruit crops need full sun (>6 hours of full sun).

Sun vs Shade Leaves

Shade-grown leaves are larger, thinner, and darker green.

Sun vs Shade Leaves

Sun-grown leaves are smaller, thicker, and lighter green.

Grana Stacking

Shade-grown leaves have grana that are not stacked.

Grana Stacking

Sun-grown leaves have grana that are stacked.

Acclimatization of Foliage Plants

Outdoor plants moved inside often defoliate or even die.

Acclimatization

Acclimatize plants slowly to changing light levels.

Houseplants Goal

The goal for houseplants should be maintenance NOT growth.

Spacing and Orientation

Makes best use of light and increases air circulation.

Optimal Spacing

Space costs money - especially in GH.

Intercropping

Intercropping or thinning might be used to avoid wasting space.

Weed Management

Get rid of weeds using mulches, herbicides, cultivation, or mowing.

Re-spacing Efficiencies

GH and hydroponics may allow for re-spacing efficiencies.

Nursery Plant Arrangement

In nurseries, intermix rows of upright plants with rows of spreading plants.

Garden Arrangement

In gardens consider multiple rows rather than single rows.

Landscape Consideration

In landscapes, consider the final size.

Row orientation

Can make a difference in plant growth, but other logistical factors may be more significant.

GH orientation

Commonly debated; above 40° N, ridge should be East-West, below 40° N, ridge should be North-South.

Gutter-connected orientation

Should be North-South.

Sun angle shading

Considered in landscape to avoid shading from fences/buildings.

Pruning for Light Interception

Especially important for fruit trees to allow light penetration.

Raspberry thinning

Often thinned to give more cane spacing.

Pruning for shape

Common in landscape horticulture; hedges should be wider at the bottom than at the top.

Espalier

A specialized pruning technique used to train plants to grow flat against a structure.

Topiary

The art of shaping plants into decorative forms.

Bonsai

The practice of growing miniature trees in containers.

Etiolation

Occurs when plants/plant parts are exposed to darkness, leading to pale yellow or white coloration.

Chlorophyll degradation

Without light, chlorophyll degrades, causing plants to look weak and elongated.

Blanching

An intentional process to keep cauliflower heads white, produce high-quality celery or green onions, and prevent potatoes from greening.

Carotenoids

Pigments that reflect light in the yellow to red-orange range.

Betacarotene

A carotenoid that makes carrots orange.

Lycopene

A carotenoid that makes tomatoes and watermelons red.

Xanthophylls

Pigments that provide yellow coloring in flowers and leaves.

Photoperiodism

The plant's response to day length, controlling flowering, storage organ formation, stem elongation, and fall leaf coloration.

Flowering control

Controlled by NIGHT length, but the plant must still reach the adult phase.

Photoperiodism

The response of plants to the length of day and night.

Day Neutral Plants (DNP)

Bloom as long as they have reached the adult phase.

Long-Day Plants (LDP)

Bloom when nights are shorter than their critical need.

Short-Day Plants (LNP)

Bloom when nights are longer than their critical need.

Complex Photoperiodism

Some plants have to have a succession from one to the other in order to bloom.

Manipulated Photoperiodism

Interrupt night with incandescent light bulbs (lots of red) or cover plants with black cloth to promote flowering.

Formation of Storage Organs

Tubers and tuberous roots usually form as days become shorter.

Stem Elongation

Runners on strawberries and spider plants are partially controlled by photoperiod.

Fall Leaf Coloration and Abscission

As days shorter, deciduous plants start to prepare for dormancy.

Light Sources

Sun is the best source (bright and free).

Incandescent Lamps

Generate a lot of heat, mostly red wavelengths, used for night interruption to control photoperiod.

Fluorescent Lamps

Work well for germination and tissue culture, more blue light.

High-Intensity Discharge (HID) Lamps

Large, use a lot of energy, and are expensive but have a long life.

Light Emitting Diodes (LEDs)

Efficient but fixtures are expensive; can produce light in single colors.

Watt

Measures photosynthetically active radiation (PAR) and can indicate water needs.

Mole

6.02 x 10^23 units of anything is a mole.

Photosynthetic Photon Flux Density

μmoles x m-2 x s-1.

Leaf Area Index (LAI)

Ratio of area occupied by upper sides of all leaves to the ground covered.

Direct summer sun (W)

Approximately 1000 W x m-2.

Direct summer sun (μmoles)

Approximately 2000 μmoles x m-2 x s-1.