Section 2: Three Important Attributes of Light Affecting Plant Growth
Light Quality
Because light energy is transformed to chemical energy (and thus physiological responses) when photons interact with energy level-specific (or wavelength-specific) photoreceptors, different wavelengths of light affect different processes in the plant. Light with wavelengths of 400 to 700 nm (or photons with the corresponding energy levels) is referred to as photosynthetically active radiation (PAR) and can be used by the plant to drive photosynthesis. Within this range, red (corresponding to 650 - 700 nm) and blue (corresponding to 460 - 480 nm) light are most efficiently used by plants for photosynthesis. Green light (corresponding to 490 – 520 nm) is primarily reflected away (that is why plant leaves are usually green).
Light quality also affects plant growth through photomorphogenesis. Probably the most well known case where light quality affects photomorphogenesis is through the phytochrome photoreceptor (a specific light perceiving/reacting molecule). Phytochrome has been shown to be involved in many photomorphogenic responses including seed germination and photoperiodism. In addition to the example described above regarding flowering and photoperiodism in poinsettia, a common photomorphogenic response that is often observed is that plants grown under low light conditions such as in a forest or under other plants tend to stretch and have longer internodes than plants grown in bright light. Leaves filter red light more than far-red light thus creating a far-red light enriched environment within a leaf canopy. This high far-red light environment causes plants growing under the canopy to stretch or elongate. Additionally, plants grown under a light source high in blue light will be shorter and have darker-colored leaves than plants grown under a light source high in far-red light. These are all examples of photomorphogenic responses by plants to light and are dependent upon or controlled by the light quality perceived or encountered by the plant.
Light Quantity
Light is the energy source that powers photosynthesis. For each plant species there is an optimal light level. Below this level, photosynthetic activity is reduced to a level below the maximum photosynthetic potential of the plant. In fact, if light levels are too low, the rate of respiration (using up of carbohydrates for energy) may exceed the photosynthetic rate and result in a net loss of carbohydrates. Above the optimal level, photosynthetic rate becomes static (saturation light level) and adding light does not result in a corresponding increase in photosynthetic rate. In fact, further increases in light level can cause damage the photosystems of the plant and actually reduce the photosynthetic rate. The goal in greenhouse crops production is to optimize light levels so as to maximize the photosynthetic rate.
Light levels (or light quantity) may be measured and expressed in numerous ways:
Radiant flux is the energy (of all wavelengths) emitted by a light source and is measured in watts.
Radiant emittance is the energy received from a light source per square meter and is measured as watts per square meter.
Irradiance is a measure of the amount of light energy intercepted by a surface and is measured in watts per square meter.
Lumen is a unit of light energy output (in a directions) from a light source. A 100W incandescent bulb produces approximately 1,740 lumens.
Foot candle is equivalent to 1 lumen per square foot. Foot candles are often used to report light levels in greenhouses. However, devices designed to measure foot candles are designed to mimic the human eye (being more sensitive to yellow and green light) and are not the most appropriate for measuring or reporting light with respect to plant requirements (where blue and red light are more important).
Illumination intensity is measured as the number of foot candles striking a surface (100 lumens striking a 1 square foot area is equivalent to 100 foot candles).
Lux is equivalent to 1 lumen per square meter.
Quantum flux density is the number of photons (measured within the corresponding 400 - 700 nm wavelength range) intercepted by a surface over a period of time and is measured as Einsteins or moles per second per square meter (1 Einstein = 1 mole). An Einstein (or a mole) is a very large number so when measuring light in a greenhouse, the unit µEinsteins/m2/sec (or µmol/m2/sec) is usually used. This is the most appropriate way to measure and report light levels with respect to plant growth and development because it is an actual measure of the number of photons available (particles of light) within PAR (400-700nm) to drive photosynthesis and control some photomorphogenic responses.
If only a footcandle meter is available to measure light, and the light source is known, rough conversion factors exist for converting footcandle measurements to quantum flux density (as µmol/m2/s). These are listed in Table 1.
Table 1. General conversion factors for converting footcandles from a known light source to quantum flux density as µmol/m2/sec. |
|
Light source |
Conversion factorz |
Natural sun |
0.20 |
Incandescent |
0.22 |
Cool white fluorescent |
0.15 |
Warm white fluorescent |
0.14 |
Metal halide |
0.15 |
High-pressure sodium |
0.13 |
z (multiply footcandles to obtain µmol/m2/sec. Data from Thimijan and Heins, 1983. |
|
Another useful term related to light quantity is that of daily light integral (DLI). Where quantum flux density is a measure of light level (number of photons) at a point in time (1 second), the daily light integral measures the total sum of photons (within the 400-700nm wavelengh range) received by the plant during a 24-hour period. The daily light integral is measured as mol/day/m2 (sometimes the shorthand version of moles/day is used and per square meter is assumed) and is the number of moles (mol) of photons accumulated over a 24-hour period irrespective of the actual quantum flux density at any given point during the day. The optimal daily light integral varies by species (just as does the optimal quantum flux density). However, for low-light requiring species such as African violet, low-light foliage plants and ferns the optimal daily light integral is 5 to 10 mol/day/m2. For most bedding plant species and potted plants, a daily light integral of 10 to 20 mol/day/m2 is desirable. Lettuce and herbs require 12-17 mole/day/m2.
Photoperiod (light duration)
The duration that light is received by or provided to a plant (photoperiod) can significantly affect plant development. The major horticultural interest in controlling photoperiod is that photoperiod often controls when a plant shifts from a vegetative to a reproductive phase (when it flowers). Long-day plants (i.e. Easter lily) become reproductive when the length of the night is less than some critical value (when days are long). Short-day plants (i.e. poinsettia, chrysanthemum) become reproductive when the length of the night exceeds some critical value (when days are short). When growing greenhouse crops, it may be necessary to artificially shorten or lengthen the night in order to control plant development and when the shift from vegetative to reproductive growth occurs (which is critical to scheduling crops to flower at a specific time).
