The Propagation Environment - Aggie Horticulture

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3
The Propagation
Environment
INTRODUCTION learning objectives
• Identify the environmental fac-
Propagation can be done in the field, orchard, forest, outdoor raised
tors affecting propagation.
beds, and in protected culture environments such as greenhouses, poly-
• Describe the physical struc-
covered houses, and tissue culture laboratories. The plant propagation
tures for managing the propa-
period is generally a very narrow segment of a plant’s life, ranging from
gation environment.
several weeks for fast-growing herbaceous plants to one to two years for
• Describe the containers for
woody perennials. Following propagation, the rooted cuttings, seedlings
propagating and growing
(plugs),layers,or tissue culture produced
plugs Small seedling young liner pots.
plants are transplanted as liner plants.
plants.
• Discuss the management of
The liner plants are grown in small pots
layers Plants media and nutrients in propa-
and then transplanted into larger contain-
produced asexually gation and liner production.
ers or directly transplanted into field pro-
from layering, such as • Discuss the management
duction. In other production systems
air layering or stooling. of microclimatic conditions
plants may be propagated and produced
in propagation and liner
in the same container or field location
production.
without going through a liner stage.
• Discuss the management of
To enhance the propagation of
propagule A plant biotic factors—pathogens and
structure used for plants, commercial producers manipu- pests—in plant propagation.
late the environment of propagules(cut-
regenerating plants, • Explain the post-propagation
which can include tings, seeds) by managing: care of liners.
cuttings, seeds, grafts, a. microclimatic conditions(light, water-
layers, tissue culture relative humidity, temperature, and
explants, and single gases)
cells. b. edaphic factors(propagation medium
or soil, mineral nutrition and water),
microclimatic conditions
Any environmental and
factors (relative c. biotic factors—interaction of propag-
humidity, temperature, ules with other organisms (such as
light, gases, etc.) in the beneficial bacteria, mycor-
immediate vicinity of rhizal fungi, pathogens,
the propagule during insect pests, etc.) (Fig. 3–1).
propagation. Unique ecological condi-
edaphic factors Any tions exist during propagation.
factors influenced by Commercial propagators may
the soil or propagation have to compromise to obtain
medium (substrate). an “average environment” in
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50 part one general aspects of propagation
Figure 3–1
The propagation environment: Manipulation of
microclimatic, edaphic, and biotic factors.
Modified from Landis (70).
Shading Partial which a whole range shading and stooling to maximize rooting potential of
reduction of light to 100 of species are propa- a propagule; and post propagation—hardening-off
percent light exclusion gated by cuttings, seed, (weaning rooted cuttings from the mist system and
that can occur during and/or tissue culture changing fertility regimes) to assure growth and sur-
stock plant manipu- explants (69). The vival of tender-rooted liner plants after propagation.
lationand/or environmental condi-
propagation tions that are optimum
hardening-off The for plant propagation ENVIRONMENTAL FACTORS
are frequently con-
stress adaptation process AFFECTING PROPAGATION
ducive for pests (path-
or acclimationthat
ogenic fungi, viruses, In propagating and growing young nursery plants, facil-
occurs as a propagule,
bacteria, insect, and ities and procedures are designed to optimize the
such as a cutting, is
mite development). response of plants to environmental factors influencing
gradually weaned from
Astute propagators their growth and development, such as light, water,
a high to a low relative
not only manage the temperature, gases, and mineral nutrition. In addi-
humidity environment
environment during tion, young nursery plants require protection from
during rooting; in
propagation, but also pathogens and other pests, as well as control of salinity
micropropagation(tissue
manipulate the envi- levels in the growing media. The propagation structures,
culture) acclimation is
ronment of stock equipment, and procedures described in this chapter, if
referred to as
plants prior to select- handled properly, maximize the plants’ growth and
acclimatization.
ing propagules, such as development by controlling their environment.
BOX 3.1 GETTING MORE IN DEPTH ON THE SUBJECT
LINER PRODUCTION
A linertraditionally refers to lining out nursery stock in a nurseries. Seedlings and rooted cuttings can also be trans-
field row. The term has evolved to mean a small plant pro- planted into small liner pots and allowed to become
duced from a rooted cutting, seedling, plug, or tissue cul- established during liner production, before being trans-
ture plantlet. Direct stickingor direct rootinginto smaller planted to larger containers (upcanned) or outplanted
liner potsis commonly done in United States propagation into the field.
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the propagation environment chapter three 51
BOX 3.2 GETTING MORE IN DEPTH ON THE SUBJECT
MEASUREMENT OF LIGHT
Irradianceis the relative amount of light as measured by photometric sensor, which determines foot-candles or
radiant energy per unit area. Irradiance, intensity, and lux (1 foot-candle =10.8 lux). A photometric sensor is rel-
photon flux all measure the amount of light very differ- atively insensitive to wavelengths that are important for
ently; they are not interchangeable terms. Photosynthetic plant growth; that is, it may record high light intensity
photon flux (PPF)is the best light measurement for plant from an artificial electric light source, but it does nottake
propagation, since the process of photosynthesis relies into account if the light source is rich in green and yellow,
on the number of photons intercepted, not light given or poor in red and blue light—which would lead to poor
off by a point source (intensity) or energy content (irradi- plant growth. Quantum and radiometric (pyranometer)
ance). Photosynthetic active radiation (PAR) is meas- sensors can be purchased from instrument companies
ured in the 400 to 700 nanometer (nm) waveband as (i.e., LI-COR Biosciences, www.licor.com; or Apogee
PPF in micromoles of photons per unit area per time Instruments, Inc., www.apogee-inst.com). For determin-
(µmol m–2s–1) with a quantum sensor or as watts per ing light qualityor wavelength,the spectral distribution
square meter (W/m2) with a pyranometric sensor. is measured with a portable spectroradiometer, which is
Some propagators still measure light intensity with a a very expensive piece of equipment.
Light seed collected in the fall from selected woody plant
species, such as Larix,need long-day conditions to ger-
Light is important for photosynthesis as a source of radi-
minate. Dahlia cuttings need short-day conditions to
ant energy. Light also generates a heat load that needs to
trigger tuberous root formation.
be controlled (i.e., too high a temperature can quickly
Photoperiod can be extended under short-day
desiccate and kill cuttings). The management of light
conditions of late fall and early winter by lighting with
can be critical for rooting cuttings, germinating seeds,
incandescent lights, or high intensity discharge lights
growing seedlings, or shoot multiplication of explants
(HID) (Fig. 3–14, page 65). Conversely, photoperiod
during tissue culture propagation. Light can be manipu-
can be shortened under the long-day conditions of late
lated by controlling irradiance (see Box 3.2), light dura-
spring and summer by covering stock plants and cut-
tion (daylength, photoperiod), and light quality (wave-
tings with black cloth or plastic that eliminates all light.
length). For a relative comparison of light units for
See the in-depth discussion of phytochrome and pho-
propagation, see Box 3.3 on page 52.
toperiodism in Chapter 7.
Irradiance While many propagators still measure
light intensity, determining the photon flux of light is
Light Quality Light quality is perceived by the human
more accurate because the process of photosynthesis
eye as color, and corresponds to a specific range of wave-
depends on the number of photons intercepted
lengths. Red light is known to enhance seed germination
(photosynthetic photon flux), not just the light given off
of selected lettuce cultivars, while far-red light inhibits
by a point source (intensity).
germination. Far-red light can promote bulb formation
Daylength (Photoperiod) Higher plants are classified on long-day plants, such as onion (Allium cepa). Blue
as long-day, short-day, or day-neutral, based on the light enhances in vitro bud regeneration of tomato (77).
effect of photoperiod on initiation of reproductive Using greenhouse covering materials with different
growth. Long-day plants, which flower chiefly in the spectral light-transmitting characteristics, researchers at
summer, will flower when the critical photoperiod of Clemson University (97) have been able to control the
light is equaled or exceeded; short-day plants, such as height and development of greenhouse-grown plants,
chrysanthemums, flower when the critical photoperiod rather than relying on the chemical application of
is not exceeded. Reproductive growth in day-neutral growth regulators for height control. This has applica-
plants, such as roses, is not triggered by photoperiod. tion for plant propagation, liner production, and plant
The discovery of photoperiodism by Garner and Allard tissue culture systems. Red shade cloth shifts light quality
demonstrated that the dark period, not the light towards the blue/green and is being used to enhance root
period, is most critical to initiation of reproductive development of cuttings (Fig. 3–11, page 62). Red shade
growth, even though light cycles are traditionally used cloth can also be used to increase leaf surface and branch-
to denote a plant’s photoperiod. In propagation, fresh ing, which is important in liner development (111).
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52 part one general aspects of propagation
BOX 3.3 GETTING MORE IN DEPTH ON THE SUBJECT
RELATIVE COMPARISON OF LIGHT UNITS FOR SOLAR RADIATION
AND ARTIFICIAL LIGHTING (67, 72, 117)*
Energy
Illumination
[Photosynthetic Radiation
[Light intensity]
photon plux] [Irradiance]
Light Source ((cid:2)mol m–2s–1) (watts m–2) (lux) (ft-candles)
Solar Radiation
Full sunlight 2,000 450 108,000 10,037
Heavy overcast 60 15 3,200 297
Artificial Light Source
Metal halide (400 W)
lamp @ 2 m height 19 4 1,330 124
*Photosynthetically active radiation (PAR): 400 to 700 nm. Conversions between energy, radiation, and
illumination units are complicated and will be different for each light source. The spectral distribution curve of
the radiant output must be known in order to make conversions.
Water-Humidity Control peach cuttings can be rooted under aeroponic systems,
while woody and herbaceous ornamentals can be
Water management and humidity control are critical in
rooted in modified, aero-hydroponic systems without
propagation. Water management is one of the most effec-
relying on overhead mist (108). Tissue culture explants
tive tools for regulating plant growth. Evaporative cooling
are often grown in a liquid phase rather than on a solid
of an intermittent mist
intermittent mist agar media.
system can help control
A thin film of water While leaf water potential (Ψ ) is an impor-
the propagation house leaf
produced through a tant parameter for measuring water status of seedlings
microenvironment and
pressurized irrigation and cuttings, and influences rooting of cuttings, turgor
reduce the heat load on
system that cools the (Ψ ) is physiologically more important for growth
cuttings, thereby per- p
atmosphere and leaf processes. The water status of seedlings and cuttings is a
mitting utilization of
surface of cuttings. balance between transpirational losses and uptake of
high light conditions to
water. Later in this chapter the methods to control
increase photosynthesis and encourage subsequent root
water loss of leaves of cuttings, seedlings, and con-
development. A solid support medium, such as peat-
tainerized grafted plants are discussed.
perlite, is not always necessary to propagate plants;
BOX 3.4 GETTING MORE IN DEPTH ON THE SUBJECT
PLANT WATER MEASUREMENTS IN PROPAGATION
Water potential (Ψ )refers to the difference between Moisture Corporation (www.soilmoisture.com). A psy-
water
the activity of water molecules in pure distilled water and the chrometer with a microvolt meter (LiCor, www.licor.com)
activity of water molecules in any other system in the plant. can also be used. Estimation of turgor (ΨΨ ) (or pressure
p
Pure water has a water potential of zero. Since the activity potential) requires measurement of water potential
of water in a cell is usually less than that of pure water, the (ΨΨwater)minus the osmotic potential (ΨΨπ),which is based
water potential in a cell is usually a negative number. The on the formula Ψwater = Ψp + Ψπ. Osmotic potential can
magnitude of water potential is expressed in megapascals also be determined by either a pressure chamber or a psy-
[1 megapascal (MPa) = 10 bars = 9.87 atmospheres]. chrometer. The matrix potential (Ψ ) is generally insignificant
m
Propagators can determine water potential by using a in determining Ψ but is important in seed germination.
water
pressure chamber (pressure bomb) manufactured by PMS See the discussion on water potential and seed germina-
Instrument Company (www.pmsinstrument.com) or Soil tion in Chapter 7.
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the propagation environment chapter three 53
Temperature seed coat restricts gas exchange. Likewise, gas exchange at
the site of root initiation and subsequent rooting are
Temperature affects plant propagation in many ways.
reduced when cuttings are stuck in highly water-saturated
Seed dormancy is broken in some woody species by cool-
propagation media with small air pore spaces. In leaves of
moist stratification conditions that allow the germination
droughted propagules, stomata are closed, gas exchange is
process to proceed. Temperature of the propagation
limited, and suboptimal rates of photosynthesis occur.
medium can be suboptimal for seed germination or root-
During propagation in enclosed greenhouses, ambient
ing due to seasonally related ambient air temperature or
CO levels can drop to suboptimal levels, limiting photo-
the cooling effect of mist. In grafting, heating devices are 2
synthesis and propagule development. The buildup of
sometimes placed in the graft union area to speed up graft
ethylene gas (C H ) can be deleterious to propagules
union formation, while the rest of the rootstock is kept 2 4
during storage, shipping, and propagation conditions.
dormant under cooler conditions (see Fig. 12–48).
Ethylene also plays a role in plant respiration, rooting of
It is often more satisfactory and cost-effective to
cuttings, and seed propagation.
manipulate temperature by bottom heating at the
propagation bench level, rather than heating the entire
Mineral Nutrition
propagation house (Fig. 3–2). The use of heating and
cooling systems in propagation structures is discussed To avoid stress and poor development during propaga-
further in this chapter (see Chapter 10 for heating tion, it is important that the stock plants be maintained
equipment and sensors). under optimal nutrition—prior to harvesting propag-
ules. During propagation, nutrients are generally applied
Gases and Gas Exchange
to seedlings and plugs
High respiration rates occur with seed germination and by fertigation (soluble fertigation The
plug development, and during adventitious root forma- fertilizers added to irri- application of soluble
tion at the base of a cutting. These aerobic processes gation water) or with fertilizer during the
require that O be consumed and CO be given off by controlled-release fer- irrigation of a seedling
2 2
the propagule. Seed germination is impeded when a hard tilizers that are either or rooted cutting.
(b)
(a)
Figure 3–2
Propagation house heating
systems. (a) Gas-fired infrared
or vacuum-operated radiant
heaters (arrow). (b) Forced
hot air heating system.
(c) Greenhouse, hot water
boilers. (d) Heating below the
bench for better control of
(c) (d) root zone temperature.
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54 part one general aspects of propagation
preincorporated into the propagation medium or ample light, such as a greenhouse, modified quonset
broadcast (top-dressed) across the medium surface. house, or hotbed—where seeds can be germinated, or
Cuttings are normally fertilized with a controlled- cuttings rooted, or tissue culture microplants rooted and
release fertilizer preincorporated into the propagation acclimatized. The second unit is a structure into which
medium (which is discussed later in this chapter and in the young, tender plants (liners) can be moved for hard-
Chapter 10) or with soluble fertilizer applied afterroots ening, which is preparatory to transplanting outdoors.
are initiated. The development of intermittent mist Cold frames, low polyethylene tunnels or sun tunnels
revolutionized propagation, but the mist can severely covered by Saran, and lathhouses are useful for this pur-
leach cuttings of nutrients. This is a particular problem pose. Any of these structures may, at certain times of the
with cuttings of difficult-to-root species that have long year and for certain species, serve as a propagation and
propagation periods. acclimation structure. A synopsis of how structures are
utilized in propagation is presented in Table 3–1.
PHYSICAL STRUCTURES FOR Aseptic Micropropagation Facilities
MANAGING THE PROPAGATION Aseptic micropropagation facilities are described in
ENVIRONMENT Chapter 18.
Propagation Structures Greenhouses
Facilities required for propagating plants by seed, cut- Greenhouses have a long history of use by horticultur-
tings, and grafting, and other methods include two basic ists as a means of forcing more rapid growth of plants
units. One is a structure with temperature control and (11, 41, 55, 75, 122). Most of the greenhouse area in
Table 3–1
UTILIZATION OF PROPAGATION STRUCTURES
Propagation Seedlings/ Liner production
structure Micropropagation Cuttings Plugs Grafting Layering and hardening-off
Micropropagation Yes No; except No No; except No No
facilities (indoor) microcuttings micrografting
Greenhouses Yes; during Yes Yes Yes Yes; air Yes
acclimatization layering
Closed-case No Yes Yes Yes No Yes
propagation
Hot frames
(hotbeds)
Heated sun
tunnels
Closed-case No; except Yes; hardwood and Yes Yes Yes Yes
propagation acclimatization semi-hardwood
cuttings
Cold frames
Unheated sun
tunnels
Lathhouses No; except Yes; hardwood and Yes Yes Yes Yes; used
(shade houses) acclimatization semi-hardwood extensively
cuttings for this
Miscellaneous No; except Yes; hardwood and Yes Yes; No Yes
closed-case acclimatization semi-hardwood sometimes
propagation cuttings with bench
systems in grafting and
greenhouses: acclimation
(a) Propagating
frames
(b) Contact
polyethylene
systems
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the propagation environment chapter three 55
the United States is used for the wholesale propagation greenhouse units are often attached side by side, eliminat-
and production of floricultural crops, such as pot plants, ing the cost of covering the adjoining walls with glass or
foliage plants, bedding plants, and cut flowers; fewer are polyethylene (Fig. 3–3). These gutter-connected houses,
used for nursery stock and vegetable crops (104). while more expensive to construct than independent
Greenhouse structures vary from elementary, ground-to-ground structures, allow easy access between
home-constructed to elaborate commercial installations. houses and decrease the square footage (meters) of land
Commercial greenhouses needed for propagation houses. Heating and cooling
gable-roof constructed are usually independent equipment is more economical to install and operate,
greenhouse A unit structures of even-span, since a large growing area can share the same equipment
that has more gable-roof construction, (62). Greenhouses with dou-
expensive, reinforced proportioned so that the ble-tiered, moveable benches retractable roof
upper support for space is well utilized for that can be rolled outside, greenhouse A unit
hanging mist systems, convenient walkways and and retractable roof green- with a roof that can
supplementary lights, propagating benches (55). houses reduce energy costs be opened during
or additional tiers of In larger propagation (Figs. 3–4and 3–5); they are the day and closed
potted plants. operations, several single being used in cutting and at night.
(a) (b)
(c) (d)
Figure 3–3
Gutter-connected propagation greenhouses. (a) A series of gutter-connected propagation houses. (b) The basic types of gutter-
connected propagation greenhouses: bow or truss. Bows are less expensive, but offer less structural strength. Trusses make for
a stronger house, while giving propagators the ability to hang plants and equipment, such as monorails, curtain systems, and
irrigation booms. (c) Non—load-carrying bow propagation house. (d) Load-bearing, gutter-connected truss house (arrow).
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56 part one general aspects of propagation
(a) (b)
(c) (d) (e)
Figure 3–4
(a and b) Instead of a movable bench, propagation trays are placed on rollers; notice how all
trays on rollers slant toward the middle of the propagation house for easier movement of
materials. (c) Movable benches for seedling plug production. (d and e). Propagation house with
retractable benches, which can be rolled from the greenhouse structure to the outdoors, have
reduced energy costs. (d) Inside of house with double-tiered benches that can be brought in at
night and during inclement weather. Benches slide through opening of greenhouse and can be
left outside under full sun conditions.
(a) (b)
Figure 3–5
(a, b, and c) Retractable roof
greenhouse for reducing heat
load during propagation and
liner production, and (d) a top-
vented Dutch-style glasshouse
with thermal curtains (arrow)
for shade and trapping heat
(c) (d) during winter nights.
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the propagation environment chapter three 57
seed propagation, and seedling plug production. Since aisle space and increase the usable space by 30 percent in
the liner seedlings are partly produced under full sun con- a propagation greenhouse. The benches are pushed
ditions, they are better acclimatized for the consumer (8). together until one needs to get between them, and
Quonset-type then rolled apart (Fig. 3–4). With rolling benches,
Quonset-type
construction is very pop- propagation work can be done in an ergonomically cor-
greenhouse An
ular. Such houses are rect fashion, making workers more comfortable, effi-
inexpensive propagation
inexpensive to build, cient, and productive (118). Besides increased propaga-
house made of bent
usually consisting of a tion production numbers, rolling benches allow other
tubing or PVC frame
framework of piping, automation features to be added (Fig. 3–7). Conversely,
that is covered with
and are easily covered to reduce costs, many propagation houses are designed
polyethylene plastic.
with one or two layers not to use benches, but rather cutting flats or small liner
of polyethylene (Fig. 3–6). containers are placed on the gravel or Saran-covered
Arrangement of benches in greenhouses varies con- floor (Figs. 3–6and 3–7). It all depends on the propaga-
siderably. Some propagation installations do not have tion system and units to be produced.
permanently attached benches, their placement varying In an floor ebb and flood system (flood floor),
according to the type of equipment, such as lift trucks or greenhouse benches are eliminated and plants are pro-
electric carts, used to move flats and plants. The correct duced with an automated floor watering and fertility
bench system can increase production efficiency and system. There are below-ground floor-heating pipes
reduce labor costs (124). Rolling benches can reduce and irrigation lines, a system of runoff-capturing tanks
(a) (b)
(c) (d)
Figure 3–6
Versatility of a polyethylene, saran-shaded quonset house. (a) Propagators sticking cuttings into rooting media floor
beds previously prepared and sterilized with methyl bromide. (b) Cuttings in small liner rooting pots under mist.
(c) Rooted liner crop protected under saran shade with poly sidewalls, and (d) shade removed and rooted liner crop
ready for transplanting and finishing off in larger container pots.
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58 part one general aspects of propagation
Figure 3–7
For more efficient use of
costly greenhouse
(a) (b) propagation space, movable
benches on rollers have been
installed to reduce aisle
space. (a and b) Hydraulic lift
system (arrow) to pick up and
move benches. (c) Movable
benches for maintaining
coleus stock plants. (d) To
eliminate bench space,
cuttings in liner pots are
placed on the cement
propagation house floor and
intermittent mist is applied
from mist nozzles suspended
(c) (d) from the ceiling.
with filters, and computer-controlled return of appro- are less expensive and offer less structural strength, or as
priate levels of irrigation water mixed with soluble fer- load-bearing truss-style houses, which give propagators
tilizer to the floor growing area (9, 89). While this has the ability to hang mist and irrigation booms, install
received limited use in the propagation of plants, it ceiling curtains for temperature and light control, and
does have application for liner stock plant production so on (Fig. 3–3). All-metal prefabricated greenhouses
of seedling plugs, rooted cuttings, and tissue culture with prewelded or prebolted trusses are also widely
produced plantlets (Fig. 3–8). Flood floor systems are used and are available from several manufacturers.
more efficient than conventional bench greenhouses. In any type of greenhouse or bench construction
They are highly automated, require less labor, and are using wood, the wood should be pressure-treated with a
environmentally friendly—since irrigation runoff, preservative such as chromatid copper arsenate (CCA),
including nutrients and pesticides, is recaptured and which will add many years to its life (5). The two most
recycled. The drawback of these benchless systems is common structural materials for greenhouses are steel and
the potential for rapid disease spread. aluminum. Most greenhouses are made from galvanized
Greenhouse construction begins with a metal steel, which is cheaper, stronger, lighter, and smaller than
framework covered with polycarbonate, acrylic, glass, an aluminum member of equal strength. Aluminum has
or poly (plastic) material. Gutter-connected green- rust and corrosion resistance, and can be painted or
houses can be constructed as bow-style houses, which anodized in various colors (62). With the high cost of
BOX 3.5 GETTING MORE IN DEPTH ON THE SUBJECT
SOURCES OF COMMERCIAL GREENHOUSES
For sources of commercial greenhouses, contact the commercial greenhouse manufacturers and suppliers that
National Greenhouse Manufacturers Association (www. include greenhouse structures, shade and heat retention
ngma.com). A number of trade journals such as GrowerTalks systems, cooling and ventilation, environmental control
(www.ballpublishing.com, choose the link for GrowerTalks) computers, bench systems, and internal transport systems in
and Greenhouse Beam Pro(www.greenbeampro.com) list greenhouses.