To know a plant, grow a plant!
Variation between individuals of a particular visible trait (phenotype) is conditioned by the genetic makeup (genotype) of the individuals and their environment. Three broad categories of environmental components interact with an individual’s genotype to influence what we observe as their phenotype:
1) the physical environment, 2) the chemical environment, 3) the biological environment
Working with Fast Plants, scientists and engineers have worked together to develop optimal growing conditions that we recommend for experiments where the plants’ genotypes or general life cycle are primarily the focus.
This page is written to explain those key environmental factors that influence plant growth—in particular, Fast Plants growth.
If one or more of these factors are reduced or increased enough, then normal functioning is disrupted. When that is the case, that factor is said to be limiting . When a factor that can be quantified becomes limiting, then its observed effects can also be quantified, and that makes for great experiments.
Because Fast Plants grow so quickly, they also respond rather dramatically to environmental conditions that fall outside of their optimal conditions. This quality makes Fast Plants ideal for teaching and learning about how a particular environmental factor affects growth and development.
NOTE: Relatively small differences in your environment for Fast Plants compared to those described here can also cause a shift in the dates you expected—for example when flowering begins or when seeds your class produces will be mature.
Read on to learn more about the key physical, chemical, and biological environmental factors that typically influence Wisconsin Fast Plants’ growth and development.
THE PHYSICAL ENVIRONMENT
Appropriate lighting is perhaps the most critical component of a plant's growing environment. Plants use energy from various regions of the visible spectrum to perform a number of functions essential to their growth and reproduction. Some seeds require red light to activate germination.
Blue light is important for regulating elongation of stems and in guiding the direction of plant growth. Red and blue are the primary energy levels used for photosynthesis, whereas red and far red are important in the regulation of leaf expansion and certain pigment production systems.
Light for Fast Plants is produced by fluorescent lamps which emit a mix of photons in the visible range that appear as white with warm (red) or cool (blue) tones in the mix.
The quantity of photons reaching a surface is known as irradiance or photon flux density and is measured in micromoles (μM) or microEinsteins (μE) of photon flux per square meter per second. Irradiance of greater than 200 μ Em-2 s-1 is ideal for Fast Plants. Less than 100 μEm-2 s-1 is inadequate. As with other electromagnetic forces and gravity, the inverse square relationship applies to light. That is, if the distance between the source of light and the receiving surface doubles, the intensity of the irradiance diminishes by a factor of four.
If you are using the standard four-foot Fast Plants light bank, you can use either 40-watt cool white or the newer 32-watt high efficiency bulbs which will require different fixtures than the 40 watt bulbs. We use either 8 40-watt cool white or 6 32-watt high efficiency bulbs in a lightbank arranged like the drawing below:
Another lighting alternative that works well is the plant light box. The lightbox is lined with aluminum foil and lit with a single 40- or 42-watt compact fluorescent lamp (CFL). NOTE: this is a large CFL, it is a 150-watt equivalent high efficiency bulb, preferably of the “cool white” type to reduce heating.
Reflectors made from aluminum foil or reflective mylar (available from fabric or stationery stores) greatly increase the irradiance reaching the plants, particularly those around the edges of the lamps. Aluminum foil lined lightboxes and aluminum "curtains" that hang down to about the soil level will contribute to uniform lighting across the plants (monitor the temperature, however).
- Keeping the Fast Plants under constant 24 hour light will produce the most satisfactory results. Be sure to make arrangements (with custodians, etc.) so light banks are not turned off at any time.
- Ideally the growing tips of the plants should be kept 5 cm to 10cm from the lights. We keep newly emerging Fast Plants very close to lights until the first true leaves emerge.
The temperature of the Fast Plants' growing environment will have an important influence on the growth of your plants. Temperatures that are too high or too low can affect the timing of developmental events such as seedling emergence and flowering. Optimal temperature is between 22°C and 28°C (72°F to 82°F).
- Temperatures can be monitored under each bank using hi-low thermometers. Note fluctuations in the room temperature and variation in temperature among light banks.
- Cooling or warming your plants can be used to slow or speed growth to assist with matching growth and development to your class schedule.
The Soilless Root Medium
Fast Plants are grown in a light and airy medium with good wicking ability. Either a commercial soilless potting mix or a mixture of one part peat moss and one part vermiculite, known as peatlite, is used as the root medium that anchors the plant roots, providing support for the stem and leaves. Physical characteristics of the root medium must be such as to provide adequate capillary wicking of water to the absorptive surfaces of the root hairs and epidermal cells, yet there must also be adequate channeling within the matrix of the root medium to enable air exchange for oxygen diffusion to the growing roots.
The soilless mix used with Fast Plants has very low nutrient value because of its composition. This is particularly useful when designing experiments that involve controlling nutrient levels.
The Chemical Environment
Water functions in many ways in plants, serving as the primary solvent supporting life's metabolic processes, generating turgor pressure (water pressure) for cell enlargement and growth, maintaining ionic balance, and providing cooling via transpiration. Water is also the source of hydrogen reducing power when it is split by light energy in photosynthesis. Water enters the plant primarily through the root epidermis and hair cells, traveling through intercellular space and cortical cells to the xylem tissue where it is distributed throughout the plant.
Within the root zone, water is found adhering to soil particles as a continuous film created through the cohesive forces of the water molecules. The adhesive forces that attract water molecules to the surfaces of soil particles and plant root cells pull the water into the minute channels within the soil and plant tissues via capillarity.
Typical Fast Plants growing systems use capillary wicking material to pull water from a reservoir to the root medium which has strong capillary properties. There is an unbroken continuity of water from the soil into and throughout the plants (see figure at right).
Through this water course, the plant also gains access to inorganic nutrients. On Earth, gravity acts as a vertical counter force opposing the cohesive forces of water and adhesive forces of capillarity.
Atmospheric Relative Humidity
The atmospheric relative humidity of a classroom can affect the rate of transpiration and water uptake by plants. Under low relative humidity there can be rapid water uptake from the reservoirs. When reservoirs run dry, capillarity is broken and plants will desiccate and die. When plants begin to wilt, it is an indication that transpiration is exceeding water uptake. In some climates this occurs when there has been a rapid drop in atmospheric relative humidity. In these cases plants usually adjust by reducing transpiration and regaining their turgor pressure.
If wilting persists, check the reservoir and examine the capillary wicks and matting to be sure they have not dried out and broken the capillary connection between roots and reservoir. If the atmospheric relative humidity is very high (>95% RH), mature anthers in flowering Fast Plants may fail to open (dehisce) to expose their pollen. This occurs when plants are grown in closed containers in which the relative humidity builds up. It can be remedied by circulating air over the plants with a fan; mature anthers will then usually dehisce within a few minutes.
In addition to the elements carbon, oxygen and hydrogen that make up the main structure of organic compounds in plants, 13 other elements are required to support the range of metabolic processes that constitute life. Six elements – nitrogen, potassium, calcium, phosphorus, magnesium and sulfur – are known as macronutrients because they are required in relatively greater quantities than the seven micronutrients – iron, chlorine, copper, manganese, zinc, molybdenum and boron (Raven, Evert & Eichorn, 1992).
Inorganic nutrients are added to the root media in a balanced nutrient mixture, such as Peters® Professional All-Purpose Plant Food, water-soluble 20-20-20 N-P-K plus minor elements. Peters® Professional contains available NPK at 20% by weight (20-20-20). Primary nutrient sources are urea, ammonium, phosphate, and potassium nitrate plus minor elements. A soluble blue dye is added or mixing.
A standard Fast Plants nutrient solution contains 7 grams of Peters® 20-20-20 fertilizer powder per liter of water. The nutrient solution can be applied to the growing substrate (soilless potting mix) at the rate of 2 ml of solution on days 3, 7, 14, 21, and 28 for each plant that will be grown to maturity. If nutrients are added to the water reservoir in a continuous nutrient culture, the standard Peters® solution should be diluted to 1/8 strength.
For ease of use, we also recommend using Osmocote time-release pellets during planting. Learn more about planting and applying fertilizer here and find a shopping list that explains potting mix and fertilizer options here.
Ambient air contains nitrogen (78%), oxygen (21%), hydrogen and helium (<1%). Carbon dioxide in air is approximately 350 parts per million and is the primary source of carbon incorporated into organic molecules via photosynthesis.
In closed systems such as the Space Shuttle orbiter, where humans and other organisms are respiring, CO2 may build up to toxic levels. Plants have the potential role in space flight of extracting CO2 from the air and converting it into edible biomass. In the Space Shuttle orbiter, CO2 levels are carefully monitored and excess removed from the atmosphere by chemical trapping in filters.
The Biological Environment
Types of Organisms
There can be many types of organisms associated with the plant's environment, from algae to insects. These organisms may reside together in various symbiotic relationships, from mutually beneficial to parasitic (one partner benefits) and even pathogenic (one partner harms the other). Some symbioses may be strictly neutral. Controlling undesirable organisms in the plants' environment requires continuous attention. Possible residents include:
Controlling Undesirable Organisms
Fungi and Bacteria: Fungi and bacteria rarely attack the above-ground parts of plants as long as the relative humidity is less than 95% and there is good air flow. The best control for fungi and bacteria is sanitation. Be sure to use pathogen-free root media – most commercially available peatlite mixtures are sanitized and pathogen-free. Keep the root media well aerated and drained by not packing it in the growing containers. After growing, it is important to rinse and then soak all pots, reservoirs, capillary mats and wicks for at least 30 minutes in a 10% chlorine bleach solution. Do not reuse root media (potting mix).
Insect Pests: The continuously illuminated plants can be attractive to many insects, especially at night. Daily surveillance and removal of insects is good practice. Sticky yellow pest control cards work well to trap incoming insects and flies emerging from the soil. The sticky strips available from garden stores can be cut and stapled to bamboo grilling skewers and mounted in film cans filled with sand and placed among the plants. These are very effective for white flies, aphids, fungus gnats and thrips.
If colonies of aphids, white flies or thrips appear or evidence of larval feeding is observed (holes chewed in leaves or flowers), plants may be sprayed with insecticidal soap or another safe chemical control agent. Read labels carefully before applying chemicals. Surveillance and careful removal by hand is the best control practice.
Algae: The most common residents with Fast Plants are green and blue algae. Most do not affect plant growth but can become unsightly and occasionally will build up in reservoirs and wicking to consume nutrients and retard water flow. Algae growth can be suppressed by adding copper sulfate (CuSO4•5H2O) to the nutrient solution at a final concentration in the reservoir of between 50 and 100 parts per million (milligrams/liter). Darkening the reservoirs by spray painting the outside with black opaque paint or wrapping them with foil will retard algal growth.
Adapted from Understanding the Environment.