Note: Click the "Photos" link next to each entry to see photographs of
the experimental results.
Should a greenhouse, light
shelf or growth chamber be used?[Photos]
Short Answer
Growth chamber if temperature is limiting factor
Results
We conducted experiments in both spring season greenhouse lighting (May-June),
considered ideal by Arabidopsis researchers at our facility, and winter
greenhouse lighting (Nov-Jan), and compared to plants grown in growth
chambers with the same environmental setpoints. In the winter study, we
also compared with plants grown on light shelves. Light shelves are defined
here as shelves in an environmentally-controlled room with fluorescent
light suspended above the plants. Results of this study indicate that,
given that temperature is not limiting, greenhouse tables and light shelves
grew plants of equal or greater quality 'Columbia' plants than growth
chambers. This is in agreement with Ernst et al (5), though they were
using 'Landsberg' Arabidopsis. Early in the production cycle of our winter
study, plants grown on light shelves were equal in quality to other plants.
This is significant to note because the cost of this equipment, installation
and its maintenance of these units was a tiny fraction of the other two
environments. At flowering, light shelf plants were a few days behind
the other two environments, shorter, but of excellent quality. In spring
season, plants grown under natural days in greenhouse (no supplementation
with metal halide lamps) were indistinguishable from plants grown in a
growth chamber under 16 hour photoperiod. In the winter study, plants
grown under natural days did not flower but produced excellent vegetative
growth. Plants grown with 16-hour photoperiods were again indistinguishable
from plants grown in a growth chamber under same photoperiod.
Discussion
Commercially manufactured growth chambers are the "easiest" answer to
this question, as success is more likely across many mutant lines and
the conditions controlled uniformly all year long. Our pest scouting records
indicate growth chambers also tend to have fewer insect pests than other
environments (data not shown). Given the expense of these machines, however,
other environments definitely have value in Arabdiopsis production. Arabidopsis
'Columbia' will grow in any of these growth environments as long as temperature
is can be kept lower than 26C and the plants receive at least 80 µmol/m2/s
illumination from sunlight, metal halide lamps, high pressure sodium lamps,
fluorescent lamps, a combination of any of the above, or a combination
of fluorescent and incandescent lamps. Limiting factors of using light
shelves could more accurately be described as the limiting factors of
the room they are placed in. Obviously, the room needs access to water
and drains, have electrical circuits enough to power the number of fixtures
used, and should be suitably ventilated for restricted-use pesticide application
if that is part of pest management program. Other technical concerns are
the heat produced by the fixtures and humidity control of the room. Fluorescent
fixtures that utilize thin T8-style lamps and electronic ballasts produce
less heat. We strongly recommend researchers work with their physical
facilities personnel to confirm utilities, and help find cooling solutions
such as Koldwave units that utilize water to transfer heat out of the
room rather than using refrigerant. Refrigerant “air-conditioners” such
as used for offices dry the air, often to the detriment of plant growth.
It is interesting that some researchers viewing results of our formal
study were surprised that Arabidopsis can be grown in a greenhouse; no
doubt part of the "lablore" surrounding this plant. From our experience
and as reported by others (2), the only limiting factor for Arabidopsis
production in greenhouses is excessive heat during summer months, and
daylength in winter months. Greenhouses in many regions of the United
States cannot be cooled to 25C in the hottest part of the summer due to
the limitations of evaporative cooling in humid environments, and few
institutions can afford to air-condition their greenhouses. Above 25C,
we've seen some mutant lines die and wild-type plants exhibit wispy, stress-induced
flowering that often does not produce large amounts of seed. Other researchers
have reported the maximum temperature for Arabidopsis as 28C (10), 30C
for older plants (1), and even 34C (2) given adequate moisture. Purdue
produces over 6,000 square feet Arabidopsis year-round in greenhouses,
using evaporative cooling in summer and photoperiod extension to 16 hours
using metal halide lamp supplementation in winter. Greenhouse light intensity
can be as high as 1500 µmol/m2/s during summer months. This does not kill
plants absent of temperatures exceeding 25C, but could possibly contribute
to the stress-related early flowering response.
What pot size worked the best in this study?
[Photos]
Short Answer
3" square (7cm wide x 7cm deep x 7cm high)
Results
Our images show clearly that plants in the 72-cell trays and 201 "half
flats" have more chlorosis and often die when irrigation trays are kept
full of fertilizer solution compared to plants in 3" square pots or 4"
square pots. Likewise, more purpling and stress-related flowering resulted
in the trays kept full of clear water. Plants died in the middle of the
72-cell tray kept full of fertilizer solution, while surviving near the
tray edges; a pattern we have observed in our facility before.
Discussion
With proper care, just about any size container can be used for Arabidopsis.
3" square pots seem an appropriate compromise for ease-of-growing and
scalability, with 3-6 plants per pot recommended for growth to maturity.
Though not tested in our study, 3601 cellpacks (36 individual cells
per tray) are a cheaper alternative to a 3" square pot, and have similar
volume and shape. They are not as stable as a pot if separated from
the other cells, however. A general rule of thumb: The smaller the container,
the better grower you need to be. That is because the smaller the container,
the less "forgiving" a root environment is. The same soilless mix, watering
and fertilizing can result in drastically different plant growth depending
on just the pot size and shape. Small container volumes are more likely
to dry out and are less buffered against change in nutrient status or
pH. Shallow containers—such as when soilless mix is placed in a planting
tray or "flat" without pots or cell packs—have a high perched water
table (zone of saturation) relative to same soil mix in a tall container.
A high perched water table causes root environment to become anaerobic.
Root diseases are more likely under these conditions. Also, certain
nutrients such as iron become unavailable, resulting in yellowing of
young leaves and flower stems.
What soil mix worked the best in this study?
[Photos]
Short Answer
Pro-Mix PGX and MetroMix Redi-earth
Results
After planting our study, it was determined by soil analysis that the
Sunshine LA4 soilless mix we used was too low in iron for healthy growth
of Arabidopsis, though the brand had been used successfully in the past.
Also in our study, MetroMix 360 was hydrophobic; the peat component having
dried too much due to being stored too long. So this was not a fair comparison
of this brand.
Discussion
It should be noted that there are many suitable commercial soilless mixes
that are being used with great success with Arabidopsis. Thus other factors-particularly
temperature, watering and fertilizing are a most likely a better determinant
of success. It should also be noted that there have been many unsubstantiated
reports by greenhouse curators and managers of "bad lots" or "bad bags"
of soilless mixes, across all manufacturers. Some universities mix two
soilless mixes together to lessen the possibility of a problem with the
mix. We examined augmenting the soilless mix with calcined clay granules
for this and other reasons.
Can a soilless mix be augmented
to improve growth?[Photos]
Short Answer
Yes
Results
In our study, augmenting a commercial soilless media with calcined clay
granules (also called porous ceramic) ranging in diameter from 0.2-0.5
cm improved plants growing in constant sub-irrigation, across all brands
of soilless mix. Addition of these granules also improved health of plants
growing in a soilless media that was hydrophobic due to being stored too
long, and in plants from soilless media that initially tested low in iron.
Addition of the granules did not improve growth of plants compared to
plants grown in Pro-Mix PGX or MetroMix RediEarth as long as the fertilizer
was strong enough and the trays were drained following irrigation.
Discussion
We conclude from these observations that this simple augmentation (25%
by volume) of calcined clay granules might make any commercial soilless
mix more "foolproof" from manufacturing or storage problems. Augmentation
of 50% by volume is recommended if the researcher must leave water in
the tray for any period of time, such as over weekends or holidays. It
is very important to remember that these percentages and the size of calcined
clay granules are based on 3" pots, and experimentation will need to be
done for different size containers. For example, the smaller granule product
(<0.1 cm diameter) made by the same manufacturer resulted in poor plant
growth when used to augment soilless mixes in this experiment, even though
similar rates of augmentation with these granules improved growth of corn
in large 8" diameter pots or 1-gallon nursery containers at our facility.
We did not examine the benefits of using rockwool media that is used to
great success by several researchers, including the Grant Cramer lab at
University of Nevada which has trialed many systems for Arabidopsis production
(6).
What root media worked best
to cleanly remove roots?[Photos]
Short Answer
Fine-textured calcined clay granules
Discussion
Some researchers need to analyze roots, or want roots free of debris prior
to transferring into hydroponic systems. We did not examine this aspect
formally, but pulled up some plants at the termination of the study to
see if some root systems came out cleaner of debris than other. Plants
growing in fine-textured calcined clay granules (<0.1 cm diameter) were
easier to extract roots from cleanly. Note that plants growing in this
media required fertilizer at each irrigation, rather than alternating
with clear water irrigations. Early in the production cycle, plants in
this media were far behind plants in all the soilless mixes. We changed
the fertilizer frequency to every irrigation and plants were of similar
vigor and size by the end of the study. The larger granules have been
used successfully at our facility as well (no data), though the containers
needed to be nearly completely submerged when irrigated due to low capillarity.
Of course, the cleanest method would be to use hydroponic methods (5,6,12).
Does soil need to be pressed
down prior to planting?[Photos]
Short Answer
No
Results
This does not improve germination or growth, and if done when the soilless mix is wet, may ruin the soil structure.
Do seeds need to be misted to
germinate?[Photos]
Short Answer
No
Discussion
We did not examine germination rate in our study, but have made visual
observations between treatments in other studies (no data). Seeds grown
under intermittent mist (10 seconds every 8 minutes) appeared to have
better germination than plants not under mist. However, a vast majority
of our Arabidopsis is not germinated under mist due to limitations of
space in this specially-equipped propagation area. Plastic wrap or clear
plastic domes designed to fit over planting trays also appear to improve
germination, unless used in bright sunlight when temperatures under the
dome can exceed 40C. This can be remedied by covering the dome with muslin
or cheesecloth, venting the dome or propping the dome up for air circulation.
Some researchers, including the first reports on growing this species
(8) suggest using muslin kept moist by fine spraying until cotyledon expansion
begins. Mist systems are recommended for lines that seed is extremely
limited, therefore a benefit be gained from improved germination percentage.
Commercial systems can be plumbed in using copper pipes and electronic
controller units operating solenoid valves. We've constructed simple mist
systems from nozzles, pvc pipe and hose connectors that can be stored
when not in use (see image).
Can I leave plants sitting in a tray of water?
[Photos]
Short Answer
No, in most circumstances it causes growth problems
Results
Many researchers leave their plants sitting in trays of water to minimize
need for watering, or because they believe the plants require a saturated
root medium. In our study using varying sizes of pots left in 1.5-2.5
cm of standing water, plants fared poorly compared to those where the
tray was drained following irrigation. Left standing in fertilizer solution,
plants were chlorotic and stunted; in clear water, plants were purple
and stress-related flowering induced. The taller the soil column, the
less symptoms produced. Symptoms were hardly visible in 4-inch pots. Augmenting
a commercial soilless media with calcined clay granules (0.2-0.5 cm diameter)
improved plants growing in constant sub-irrigation, across all brands
of soilless mix.
Discussion
It usually takes less than 5 minutes of sub-irrigation for most containers
to absorb their full capacity of water. One report noted that water needs
of the plant greatly increase during silique filling (1), but it is still
our recommendation that trays should be drained a few minutes following
irrigation even if watering frequency is increased during this period.
Plants left sitting in trays of water grow poorly, with symptoms varying
according to solution used. Keeping plants sitting in trays of water can
lead to a suitable environment for pests such as fungus gnat and shore
fly. Accumulation of fertilizer salts is also possible.
What if water HAS to be left
in trays?[Photos]
Short Answer
Add calcined clay granules to soil mix or use a capillary mat for irrigation.
Also consider a larger (taller) pot.
Results
The taller the soil column, the less over-watering symptoms produced.
Symptoms were hardly visible in 4-inch pots.
In our study, augmenting a commercial soilless media with calcined
clay granules (0.2-0.5 cm diameter) improved plants growing in constant
sub-irrigation, across all brands of soilless mix.
Also the use of a capillary mat resulted in healthier growth than plants
left in trays of water.
Discussion
It is our recommendation that researchers who cannot manage irrigation
needs of plants over weekends or holidays keep water in trays, but augment
soil mix 1:1 with calcined clay granules. This may also be useful in 24-hour
photoperiods, when plants might suffer water stress due to high evapo-transpiration
rate under this constant light. A taller pot—even if it contains
the same volume of soil mix as a shorter, flatter pot--holds less water.
A classic demonstration of this is to saturate a rectangular kitchen sponge
while holding it out flat and allowing any excess water to drip out. When
it has all dripped out, tip the sponge upright on its end without squeezing
it. More water drips out! It holds less water when upright even though
nothing else about the sponge has changed. Some open cell inserts such
as “601cell packs” or the much-dreaded “201 half-trays”
are similar to these sponges laying flat.
Did use of capillary matting for
sub-irrigation improve growth?[Photos]
Short Answer
Only as compared to constant sub-irrigation
Results
Plants were healthier on the capillary mat than those constantly sub-irrigated,
but not as healthy as plants irrigated and drained. Algae grew on the
mat, and shoreflies and fungus gnats were visible on the mat and surface
of the soilless media. No data was taken on infestation levels of these
pests, but it appeared to be worse than other treatments.
Discussion
A simple capillary tray was designed to sub-irrigate plants without
keeping the plants sitting in water (see image). Plants in the 3" square
pot absorbed water upward through capillary action from an absorbent
mat, the mat kept wet by a reservoir of clear water or fertilizer solution.
Water was added every 3-7 days, depending on need, so was as "self-watering"
as the practice of leaving the plants sitting directly in water. Our
conclusions are that capillary mat is a viable option only relative
to leaving plants sub-irrigated continuously, and if measures are in
place to control fungus gnat population. Note that The Nottingham Arabidopsis
Stock Centre uses capillary mats for irrigation with success (10).
Was fertilizer required? [Photos]
Short Answer
Yes
Results
Good growth of Arabidopsis resulted when plants were sub-irrigated with general-use fertilizer solution alternated with clear water.
Constant use of this fertilizer solution resulted in death of some plants late in the growth cycle, most likely due to accumulation of fertilizer
salts in the soilless mix.
Discussion
Fertilizer is required for growing Arabidopsis. Without fertilizer, plants purple and stress-related, non-productive flowering occurs.
What frequency of fertilizing worked best
in this study?[Photos]
Short Answer
Every other irrigation
Results
Use of our fertilizer solution at every irrigation resulted in death of some plants late in the growth cycle, most likely due to
accumulation of fertilizer salts in the soilless mix.
Discussion
At our facility, we use a solution of 15-5-15 general-use fertilizer at a strength of 200 ppm N to accommodate a large number of species.
Use of a weaker fertilizer at each irrigation may well be possible, but was not tested.
What fertilizer strength worked best in this
study?[Photos]
Short Answer
200 ppm Nitrogen
Results
Use of the same15-5-15 fertilizer at a strength of 50 ppm (alternated with clear water irrigations) resulted in chlorotic plants.
Discussion
At our facility, we use a solution of 15-5-15 general-use fertilizer at a strength of 200 ppm N to accommodate a large number of species.
Use of a weaker fertilizer at each irrigation rather than alternating with clear may well be possible, but was not tested.
Did use of slow release fertilizer result in
healthy plants?[Photos]
Short Answer
Yes
Results
Use of 14-14-14 slow release fertilizer, incorporated into the root
medium prior to planting, resulted in good plant growth. At a rate of
1.4 grams/ 3" square pot (3.6 kg/m3)--2X the recommended rate for low-use
plants—the Arabidopsis plants were larger and greener in both early
and late stage of vegetative growth. Analysis of soilless mix taken
in late stage of vegetative growth indicated that this slow release
formulation improved soil pH (6.9) over plants irrigated alternatively
with fertilizer solution and clear water (7.6). Clear water at our facility
is highly alkaline (250-300 ppm CaCO3) and has pH of 7.5 or above, so
this soil pH benefit from slow release fertilizer use may not be realized
at facilities with better water quality.
Editor's Update: Though slow release fertilizer was effective
in this study, a subsequent study has shown that slow release fertilizer
was not effective in a fast-production system using 24-hour light.
Discussion
Effect on flowering and seed production was not studied. However, we
observed a 5-7 delay in flowering in one study on plants with slow release
incorporated, as compared to plants grown with liquid fertilizer and
no slow release fertilizer. This was in a study conducted in the greenhouse
during high light season. It begs the question whether some of the wispy,
stress-induced flowering of summer greenhouse-grown Arabidopsis can
be overcome with differing fertilization product or rate. Further study
is needed in this area.
What light intensity
worked best in this study?[Photos]
Short Answer
200 µmol/m2/s
Results
In an earlier study using continuous light, we observed damage to plants
growing at 300 µmol/m2/s but not 100 µmol/m2/s. We followed
this up with a study where we grew plants at 100, 200, 250 and 275 µmol/m2/s
using a combination of fluorescent and incandescent lamps in a growth
chamber. This time, though, the lights were on a more standard photoperiod
of 16 hours. Plants under 200 µmol/m2/s were larger and appeared
greener than those at 100 µmol/m2/s. Higher light intensities
than 200 resulted in death of some seedlings. It should be noted that
in both experiments, no barrier was placed between the lamps and the
plants, so the death could have been caused by high temperatures emitted
from the lamps.
Discussion
200 µmol/m2/s is a slightly higher intensity than suggested by
the Arabidopsis Biological Resource Center at The Ohio State University,
which recommends 130-150 µmol/m2/s for 16 hours per day. The Nottingham
Arabidopsis Stock Centre which uses 122 µmol/m2/s for 24 hours
per day in their greenhouses. We did not investigate intensities between
100-200 µmol/m2/s.
Can high intensity discharge lights
be used?[Photos]
Short Answer
Yes
Results
We compared plants grown using a combination fluorescent and incandescent
lighting with three other treatments using high-intensity discharge (HID)
lamps as sole source of illumination: high pressure sodium at 180 µmol/m2/s;
metal halide at 250 µmol/m2/s; and a mix of these two lamp types
at both 125 and 200 µmol/m2/s. All produced satisfactory plants.
The plants under HIDs appeared to have longer petioles and narrower leaves
(data not taken).
Discussion
HID lamps of 400-1000 watts are common in research greenhouses in
temperate climates. They are used for lengthening the natural photoperiod
or for supplementing sunlight. This study proves that they can be used
as sole source of illumination for this species. Warnings that Arabidopsis
cannot grow using HIDs may have resulted from damage seen after moving
plants from low-light environment of tissue culture rooms without acclimation.
What photoperiod (daylength) worked
best in this study?[Photos]
Short Answer
16 hours for flowering
Results
It is well-documented and comes as no surprise that 'Columbia' plants
grown under winter photoperiods (<12 hours) did not flower during the
duration of this experiment, whereas all plants grown under 16-hour photoperiod
flowered. We confirmed that growth was vigorous under both 16-hour and
24-hour photoperiods, and that flowering occurred 8-12 days earlier using
24-hour photoperiods. The 24-hour treatment was promising, but no data
was taken comparing seed production.
Discussion
Critical photoperiod to induce flowering has been reported as 8 hours
by Corcos (4) and as 12 hours by Tocquin et al (12). Our studies were
done with 'Columbia' wild types and will not apply to all A. Thaliana.
There may be some occasional research use for large, vegetative plants
such as we produced with short photoperiods, but the vast majority of
our researchers need seed production. We were intrigued by the use of
24-hour photoperiods used by some researchers at Purdue University and
as reported by others (1, 2, 10). We investigated how growth would compare
between 16-hour and 24-hour illumination, with an eye toward speeding
up production for large-scale, high thru-put mutant screening projects.
Conventional wisdom would suggest these the 24-hour illuminated plants
may not produce as much seed, having had less time to accumulate carbohydrates
by vegetative growth. We found only one report that suggested lower seed
yield results from this treatment, and it involved "weak mutants" (7).
Also, it was difficult for us to keep the plants irrigated because of
the increased water use of continuous-lighted plants, and many plants
died of water stress. Researchers who use this technique have to keep
their plants sub-irrigated continuously, and one wonders if this may be
the origin of the myth of Arabidopsis needing constant sub-irrigation.
Nutrition is another concern; often these plants look nutrient-starved,
with the light duration being (or mutant genetics) assumed causal. Further
study needs to be conducted using 24-hour light with increasing rates
of fertilizer to see if these deficiencies can be overcome. Other treatments
in our study suggest that 'Columbia' is more responsive to fertilization
than often prescribed.
Does growth under 24-hour light hasten production?[Photos]
Short Answer
Yes, except as compared to greenhouse summer production.
Results
In our study, Growth of Arabidopsis seedlings under continuous
fluorescent/incandescent lighting, continuous fluorescent/incandescant
lighting at 100 µmol/m2/s resulted in a healthy crop with hastened
production as compared to results documented in other studies on our
website. Note that fertilizer frequency was increased to using fertilizer
at each irrigation, and that slow release fertilizer was ineffective.
Days until 50% of plants were in flower in our studies:
24 hour light in growth chamber = 19 days
Natural day sunlight in greenhouse, summer = 18 days
Natural day sunlight in greenhouse, spring = 26 days
16 hour light in growth chamber = 26 days
Natural light in greenhouse supplemented to 16 hours, winter = 39 days
Discussion
This study suggests that a growth system can be designed to hasten production
of Arabidopsis by 1-3 weeks using 24-hour lighting and increased fertilization.
Over the course of a year, this might result in roughly 1 to 1.5 more
plant generations produced. However, there are many things that can go
wrong, as we will document in detail under other questions on this website.
Plants will purple if not fertilized enough, can be damaged if light intensity
is above 100 µmol/m2/s, and can quickly become water-stressed due
to increased evapo-transpiration. We lost plants on two occasions due
to water stress alone. It is our assumption that slow-release fertilizer
granules did not result in healthy plants under continuous high lighting
(300 µmol/m2/s) because the production period was too fast for the
nutrients to be released in sufficient quantities for vigorous growth.
Does 24-hour illumination using fluorescent/incandescent
lamps result in high-light damage?[Photos]
Short Answer
No, at 100 µmol/m2/s. Yes, at 300 µmol/m2/s.
Results
Plants under continuous light at 300 µmol/m2/s showed purpling of leaves,
leaf margin necrosis, leaf twisting, petiole elongating and more dead
plants (data not taken) after 10 days from sowing. However, the plants
that survived were further developed; 3 sets of leaves as opposed to
the 2 sets of leaves on the plants illuminated by continuous 100 µmol/m2/s,
and had increased number of leaf hairs (data not taken). Increased liquid
fertilization reduced the purpling somewhat at this 10-day stage. By
day 30, all plants under 300 µmol/m2/s had purple leaves with margin
necrosis except the plants being sub-irrigated with fertilizer solution.
These fertilized plants exhibited yellow leaf tips observed in other
24-hour light experiments late in the production cycle. Even the plants
under continuous illumination under 100 µmol/m2/s had some dying seedlings
after 10 days. Though data was not taken, we suggest it was less than
5% of the total number of seedlings. Slow-release fertilizer was no
more effective at providing nutrients than tap water controls throughout
the experiment for the plants under the higher light intensity. For
plants under the lower intensity, increasing the rate of slow release
fertilizer improved plant vegetative growth, but not as much as constant
sub-irrigation of fertilizer solution.
Discussion
We need to confirm that this intensity of illumination from fluorescent/incandescent lighting is damaging at photoperiods more commonly used in plant production, such as 14- or 16-hour. It is interesting that the plants do not exhibit such symptoms at much higher intensities (up to 1500 µmol/m2/s) of sunlight in a greenhouse. The most productive plants were grown under continuous 300 µmol/m2/s and constant sub-irrigation with fertilizer solution. However, we doubt many researchers could afford the risk to plants involved, especially if available seed were limited. We recommend the safer light intensity of 100 µmol/m2/s with constant sub-irrigation using fertilizer solution.
Can plants be transferred from low light environment
to high light?[Photos]
Short Answer
Acclimate plants to higher light over 7-10 days
Results
Purpling of leaves is the first symptom of high light damage, usually
occuring within 24 hours. We've saw this occur in our winter study. Two-month
old plants growing under natural day conditions were exposed to one 24-hour
exposure to an additional 200 µmol/m2/s of light provided by a combination
of metal halide and high pressure sodium fixtures. However, in our spring
study, 14-day old plants moved from a growth chamber of 100 µmol/m2/s
to a greenhouse where light intensity was measured in excess of 1000 µmol/m2/s
did not show any damage.
Discussion
More study is needed in this area. Either the plants are damaged by
a certain light quality or by high light at a certain developmental
age or a combination of both. We have grown plants under artificial
lighting of all kinds and have not observed a damage we associated with
the use of a certain type of high-intensity fixture, except as reported
here. However, many of our researchers have reported problems with light
higher than 300 µmol/m2/s using fluorescent lamps in growth chambers.
Since transferring plants from tissue culture to greenhouses or other
higher light environments may be required in Arabidopsis research, it
would be prudent to acclimate the plants over 7-10 days by using shading
such as muslin or cheesecloth in greenhouses or by slow ramping of light
intensity in growth chambers.
Can early, stress-induced flowering and
purpling of leaves during long, hot days be avoided?[Photos]
Short Answer
No, but more fertilizer may help.
Results
No single study we performed was thorough enough to determine the interaction
of variables that take place in a summer greenhouse on growth and flowering:
high temperature, long photoperiod and high light intensity. Taken together,
though, our opinion is that early-flowering is a long-day response while
purpling of leaves is a nutrient deficiency or a high light intensity
stress. We had some success in improving appearance of plants grown under
24-hour artificial illumination by increasing fertilization. Excellent
quality plants with less "wispy"--and presumably more productive
flower stems--were grown inside an air-conditioned bench (see details
elsewhere on this webpage), which lowered both air temperature and light
intensity of the environment. But flowering was still early on this A/C
bench.
Discussion
We often speak of this species as a “weed” not requiring
careful consideration of horticultural inputs. However, we must remember
that the wide distribution of Arabidopsis thaliana suggests
great adaptability of the species in response to its environment. Seasonal
fertilizer recommendations for Arabidopsis may be warranted, just as
they are for commercial floriculture crops grown in greenhouses.
Can a table in a greenhouse be modified with
air-conditioners?[Photos]
Short Answer
Yes
Results
We built an air-conditioned light shelf for $540 and an air-conditioned
table for $470. (For details, see Materials
and Methods for Modifying a greenhouse table and greenhouse light-shelf
with portable air-conditioners for improved cooling). This cost estimate
does not include components we had on hand such as greenhouse table, extension
cords, timers and thermometers. Both the shelf and table were located
in the same greenhouse, close to the exhaust fans to purge the additional
heat created by the a/c units before they added heat load to the greenhouse
room. The other advantage of building these in the greenhouse was the
presence of drains and the ability to safely apply pesticides. Both grew
healthy Arabidopsis crops with no signs of stress through July
when greenhouse temperatures reached a maximum of 85F (29.4C) or higher
on nine days of the experiment. Temperature on the air-conditioned table
was usually less than 70F (21.1C) , and the less than 75F (23.9C) on the
shelf.
Discussion
Though the air-conditioned table was cooler, we believe the shelf could
have been just as effective with air distribution fans to better mix the
air. The lower shelves closest to the a/c units were several degrees cooler.
Toward the end of the experiment, on July 19, we added a small fan for
vertical distribution. However, this required space on each shelf for
air to flow so the potential loss of three trays’ worth of space.
The advantages of the a/c table were that it received more light, light
was more uniform, and was easier to construct. It’s larger capacity
a/c units provided much more flexibility, as well. Those units were turned
down from their maximum setting because the day temperature was 60F (15.6C).
The advantages of the a/c shelf were that it took up much less floor space
and required smaller, less expensive a/c units.
Prior testing in summer 2005 (data not shown) had taught us that a critical
part of the construction was the insulation of the table top with foam--board
insulation and the north side of the shelf with refective insulation “bubble
wrap.” Two a/c units were installed in each area for redundancy.
They were also plugged in to separate electical circuits to reduce chance
of both of them going out at once. Having two a/c units in each area constructed,
with each a/c unit on a separate electrical circuit was a critical failsafe;
if the a/c unit fails in a system like this, the resulting temperature
under the plastic would soar much higher than the greenhouse temperatures.
Also critical was the placement of the a/c units near greenhouse exhaust
fans so that their heat load would not be added to the room.
Can a shelf unit in a greenhouse be modified
with air-conditioners?[Photos]
Short Answer
Yes
Results
We built an air-conditioned light shelf for $540 and an air-conditioned
table for $470. (For details, see Materials
and Methods for Modifying a greenhouse table and greenhouse light-shelf
with portable air-conditioners for improved cooling). This cost estimate
does not include components we had on hand such as greenhouse table, extension
cords, timers and thermometers. Both the shelf and table were located
in the same greenhouse, close to the exhaust fans to purge the additional
heat created by the a/c units before they added heat load to the greenhouse
room. The other advantage of building these in the greenhouse was the
presence of drains and the ability to safely apply pesticides. Both grew
healthy Arabidopsis crops with no signs of stress through July
when greenhouse temperatures reached a maximum of 85F (29.4C) or higher
on nine days of the experiment. Temperature on the air-conditioned table
was usually less than 70F (21.1C) , and the less than 75F (23.9C) on the
shelf.
Discussion
Though the air-conditioned table was cooler, we believe the shelf could
have been just as effective with air distribution fans to better mix the
air. The lower shelves closest to the a/c units were several degrees cooler.
Toward the end of the experiment, on July 19, we added a small fan for
vertical distribution. However, this required space on each shelf for
air to flow so the potential loss of four trays’ worth of space.
The advantages of the a/c table were that it received more light, light
was more uniform, and was easier to construct. It’s larger capacity
a/c units provided much more flexibility, as well. Those units were turned
down from their maximum setting because the day temperature was 60F (15.6C).
The advantages of the a/c shelf were that it took up much less floor space
and required smaller, less expensive a/c units.
Prior testing in summer 2005 (data not shown) had taught us that a critical
part of the construction was the insulation of the table top with foam--board
insulation and the north side of the shelf with refective insulation “bubble
wrap.” Two a/c units were installed in each area for redundancy.
They were also plugged in to separate electical circuits to reduce chance
of both of them going out at once. Having two a/c units in each area constructed,
with each a/c unit on a separate electrical circuit was a critical failsafe;
if the a/c unit fails in a system like this, the resulting temperature
under the plastic would soar much higher than the greenhouse temperatures.
Also critical was the placement of the a/c units near greenhouse exhaust
fans so that their heat load would not be added to the room.
Did any treatments reduce fungus gnat infestation?[Photos]
Short Answer
Beneficial nematodes, Steinernama feltiae
Results
Our first study involving top dressings, beneficial nematodes and different soil mixes showed a statistically significant reduction in fungus gnat larvae captured on soil surface of plants treated with one of the beneficial nematodes treatments. Silica sand at the highest rate significantly increased fungus gnat infestation. A visible layer of green algae formed on the silica sand surface at this rate, which may have explained the attraction. We repeated the nematode treatments in a larger study. 50 untreated pots and 50 treated pots were compared. Fungus gnat larvae per pot of control pots on days 0, 7 and 14 following application were 5.32, 1.54 and 1.48, respectively. Treated pots were 5.92, 1.18 and 0.22. Day 14 results were significantly different at p=.005.
Discussion
Fungus gnats (families Mycetophilidae and Sciaridae), are a common greenhouse
pest, prevalent in soilless mix kept too moist. Proper identification
is the first step to control. They are easily mistaken for shore flies
(family Ephydridae) which, though a nuisance, do not typically damage
plants. Both larvae and adult forms of these two species can be distinguished
upon close inspection. For your convenience, we've included images of
both insects' larval forms in our pictures. Damage from fungus gnat larval
feeding on Arabidopsis usually is characterized by skeletonized leaves
that are in contact with soil surface. One sound management practice for
fungus gnats is to let the soil surface dry completely in between irrigations.
This is effective because, as the name implies, fungus gnats feed on the
microscopic algae that thrive on soil surfaces where water, sunlight,
and nutrients are available. Removing water from the equation keeps the
algae from thriving. We've observed reductions in infestations when constant
sub-irrigation was ceased, though no controlled studies were performed.
Our study also examined the application of top dressings in controlling
fungus gnat infestation. The theory is that these thin layers of dressings
on the soil surface mechanically damage the fungus gnats as they burrow
into the soilless mix to lay eggs. None of the top dressings we used reduced
fungus gnat infestation with statistical significance, though we are currently
repeating the study with more replicates. Our study of treatments to reduce
fungus gnat infestation has shown that beneficial nematodes were very
effective. The nematodes were applied to soil of infested pots as a drench
and have no REI or mammalian toxicity. Of particular fascination is that
the time lapse
movie of Arabidopsis plant growth on the TAIR website (11) shows,
not only algae forming on the surface of the soilless mix, but what appears
to be fungus gnat larvae zipping across the moist surface as well! Note:
The original video was provided to TAIR by Dr. Nick Kaplinsky (Swarthmore
College, PA).
How much imidacloprid (Marathon 1G) need be
applied?[Photos]
Short Answer
If needed, 0.25 teaspoon/3" pot (4.2 kg/m3), the lowest recommended rate
Results
We tested the dose response to this chemical on Arabidopsis, and also
the effect of poor mixing, which would result in some pots being over-dosed.
Our study showed visible reduction in vegetative growth and delayed flowering
at all rates of Marathon 1G higher than the lowest recommended rate, and
non-uniform vegetative growth of poorly mixed treatments at all rates.
Some inconsequential leaf edge necrosis was photographed even on the lowest
rate. It is also interesting that plants with the lowest rate of Marathon
1G flowered several days earlier than control plant that were not treated.
However, the treatments were randomized in the irrigation trays (to keep
the product from leaching into untreated pots), so early flowering of
one treatment could have resulted from watering or fertilizer variation.
Discussion
Marathon 1G is a granular formulation of the chemical imidacloprid,
an effective control of the aphid species commonly observed in greenhouses.
We do not recommend preventative applications of one chemical class
in an environment with continuous cropping such as occurs in Arabidopsis
growth areas, lest resistance develop to the chemical. In contrast,
we spray a liquid formulation of this active ingredient (or other products)
on infested plants at first sign of the aphids. Many institutions have
mastered biological control of aphids and other insect pests. However,
some researchers still use the granular chemical incorporated into the
soil mix prior to planting.
Do any insecticides or fungicides burn
foliage?[Photos]
Short Answer
Only a few.
Results
Over the years, we've observed spray damage on Arabidopsis
from the following pesticides under certain circumstances:
-Multiple applications of insecticidal soap to the same plants
-Application of insectidal soap or horticultural spray oil to plants
in sunny, hot conditions
-PBO synergist
-Nicotine fumigant only when leaves were wet
-Dursban insecticide
-Pipron fungicide at label recommended rate
-Strike fungicide only at 6X label recommended rate
Below is a list (by US trade name) of some of the products we've applied
to Arabidopsis without observing damage. This does not mean they are
effective in controlling the target pest or to imply an endorsement
of the product:
Insectides, Miticides or insect growth regulators:
Akari
Avid
Azatin
Conserve
Distance
Duraguard
Endeavor
Enstar II
Floramite
Garlic extract
Hexygon
Marathon 1G
Marathon II
Mavrik
Mesurol
Orthene
Ovation
Overture
Pepper extract
Pylon
Sanmite
Talstar
Tame
Fungicides:
Banrot
Cleary's 3336
Cygnus
Daconil
Strike
Wettable sulfur
Discussion
Arabidopsis is susceptible to most greenhouse pests such as aphids
and western flower thrips. On rare occassions it is fed upon by whitefly,
spider mites, root aphids and even armyworms! If the planting is small
and the insect population low, a spray bottle of ready-to-use pesticide
from a garden center may suffice. These consumer-friendly products can
be applied without restricting the location. Label directions should be
followed and a few test plants should be sprayed and observed for phytoxicity
before applying to valuable plants. Spray damage appears in 24-48 hours.
Damage from granular formulations added to soil may take up to a week
to appear. Do not use any product more than twice on the same Arabidopsis
plants. Insecticidal soaps, for example, accumulate on leaves and may
lead to damage only after multiple applications. Also keep in mind that
mutant plants may be more susceptible to spray damage, particularly those
with altered leaf waxes.
I have run out of space in my facility; how
can I optimize it?[Photos]
A simple method is to move plants that are beginning to set seed (but
before seed pods can shatter) to another room or unlighted shelf so that
the valuable growing space can be used for actively growing plants. Rolling
benches are a good choice for a greenhouse. Their legs stay in place but
tops roll from side to side on rolling bars. You sacrifice permanent aisles
for about 20% more usable space. We've also taken a commercial A-frame
tiered bench (Hummert International, Inc. St. Louis, MO) and modified
if to fit Arabidopsis (and water-proof fluorescent fixtures). Placed upon
an existing bench, it doubles greenhouse space. Place upon rolling benches,
space utilization of over 100% of floor space can be achieved. Given that
the room they are located in is temperature controlled, using light shelves
is an excellent way to increase the number of plants per unit area of
floor. Units that have rolling shelves on a frame system are available
in the storage-specialty industry, especially for hospital storage. These
could be modified with fluorescent fixtures for plant growth to greatly
improve space utilization of growth rooms, perhaps tripling the useful
space more economically than building or adding new utilities to a new
room. Of course, the additional cooling and electrical requirements need
to be considered.
What is my worst Arabidopsis nightmare?[Photos]
Short Answer
Impatiens Necrotic Spot Virus (INSV)
Discussion
Impatiens Necrotic Spot Virus (INSV) is a viral disease that kills
Arabidopsis. It is vectored by western flower thrips, a common greenhouse
pest that is attracted to flowering plants. Symptoms include:
-Clearing or yellowing of leaf midrib
-Leaf collapse
-Wilted or collapsed flower stems
-Sudden death of a few plants, followed by death of more plants
INSV can infect more than 600 species, including many commercial floriculture
crops and outdoor weeds. Excellent thrips control, rogueing of infected
plants and strict quarantine procedures are required to eliminate this
disease. We recommend immunostrip test kits should be kept on hand for
quick diagnosing. They can be stored in a refrigerator for one year.
One commercial source is Agdia of Elkhart, IN.
RESPONDING TO AN INFECTION:
Once INSV is discovered in a greenhouse room, there are two practices
to choose from, “clearing” or “containment.”
For the former, all plants in an infected greenhouse room are discarded.
The room is cleaned of all plant material and rubbish that could harbor
thrips. The room is left empty for 4-7 days. Only newly seeded plants
are allowed back in the room. Of course, clearing a room is often not
possible because of irreplaceable plants.
Containment involves discarding any plants showing symptoms and any plants
that are easily replaceable even if they don’t exhibit symptoms.
The goal is to get the greenhouse empty as soon as possible while collecting
seed or other valuable tissue for the research project (the disease is
not carried in seed). A date is set that all plants will be removed, allowing
enough time for seed maturation. Aggressive pesticide spraying and sanitation
is implemented. Dedicated lab coats are kept in the room that all personnel
must wear so they don’t carry thrips out into other areas.
PREVENTING AN INFECTION:
At Purdue, we have vastly reduced incidence of the disease by implementing
a rotating greenhouse room schedule. A room is cleaned and emptied for
several days, then newly seeded or cultured plants allowed in the room
for two months. At that time, no more plants are allowed in (a new,
clean room is made available for new plants) but the plants present
in the room are allowed to remain until harvested. Following harvest,
the room is cleaned again. Avoiding the continuous culture of plants
breaks the thrips life cycle. The theory is simple but the implementation
requires multiple rooms and a great deal of communication. It will most
likely require pooling many labs into the rotating rooms, rather than
allocating each lab their own room.
Is reverse-osmosis purified water required
to irrigate plants?
Short Answer
Only if water testing indicates you have very poor water quality
Results
Though we consider our clear water to be of poor quality due to high alkalinity,
we saw no visible differences between plants irrigated with reverse-osmosis
purified water versus our tap water.
Discussion
Reverse osmosis or other purified water is necessary in laboratory culture
or hydroponics, but usually not in crop production in soilless media.
We also use it for mixing pesticides, to make them more effective.
What about growth conditions such as light intensity, light quality, cold treatments and hydroponic techniques that this study did not examine?
For your convenience, we've included excellent summaries and reports in our references section on these topics, most of them hyperlinked
to the original source.