The interaction between plants and light is a fundamental part of plant growth and crop production. Light not only powers photosynthesis, producing essential sugars for growth and development, but it is also involved in a wide range of other plant processes.
Growers play an important role in determining the amount of light each plant receives, whether it be by providing artificial lights in an indoor garden, cladding in a naturally lit greenhouse or manipulation of plant density out in the field.
The lighting subject becomes even more complex when we consider that light intensity itself interacts with a number of other factors to determine the rate of photosynthesis. Temperature, carbon dioxide concentration, humidity, moisture status, leaf area and general plant health all play a role in the net assimilation rate of a plant.
How Plants See Light
Light is part of the electromagnetic spectrum and is made up of a mixture of different-colored wavelengths: violet, indigo, blue, green, yellow, orange and red. Light has wave properties and it is the length of these waves that determines the energy and color of the light. The visible portion of this spectrum that the human eye can distinguish is within a narrow band—380-740 nanometers (nm).
Human eyes can see the green and yellow wavelengths fairly brightly, whereas plants sense and respond to a much wider spectrum, even light in the ultraviolet (UV) range and far-red wavelengths. Plants largely use wavelengths between 400 and 700 nm, which is the photosynthetically active radiation (PAR) range, or the range that carries out the vital process of photosynthesis.
On indoor grow lamps, a measure of energy in the PAR wavelength band is commonly referred to as PAR watts. A more scientific measurement is moles or micromoles of PAR per square meter per second (μmol m-2 s-1) and these are the units often seen in research reports studying the effects of artificial light sources on plant growth.
When light falls on a plant leaf, it provides the energy for the production of sugars or assimilates from water and carbon dioxide, which is absorbed from the air through the stomata on the leaf surface. Oxygen is released as a by-product, making photosynthesis the source of much of the oxygen in our atmosphere.
The glucose (sugar) produced during photosynthesis is transported around the plant and used for growth and development, or it may also be stored for later use. The most well-known light-absorbing pigments are chlorophyll a and chlorophyll b. But chlorophyll a and b are not the only light-harvesting pigments in plants, there are also a number of accessory pigments that allow photosynthesis to use a large proportion of the visible light spectrum.
Photosynthesis and CO2
Along with light, carbon dioxide (CO2) is also required for photosynthesis, but CO2 only makes up a fraction of our atmosphere—around 365 ppm, or 0.036% by volume. Despite this, dried plant materials contain on average 45% carbon, which comes from CO2 through the process of photosynthesis.
Therefore, carbon dioxide is a major plant nutrient, one that affects both photosynthesis and crop yields, and it needs to be supplied in adequate quantities to maximize plant growth. Providing indoor gardens with supplemental CO2 can result in yield increases of more than 40% under some conditions.
However, supplementation must be done carefully, as a level of CO2 enrichment suitable for a mature tomato plant under strong lights may damage small, weaker seedlings under less intense illumination. Too little CO2 will cause plants to stagnate and photosynthesis to grind to a halt; too much can cause toxicities, stunting, leaf curling and other damage, which is often hard to define or diagnose.
Carbon dioxide levels also interact with other factors to impact the photosynthesis rate. As temperature and light levels increase in the growing environment, the rate of photosynthesis and absorption of CO2 also increases, up to a maximum level. The rate will also increase with CO2 concentration, up to a point where some other factor, such as the peak speed at which plant enzymes will work, is also reached.
Indoor growers need to ensure carbon dioxide in the growing environment does not become depleted, as this would slow the photosynthesis process. Carbon dioxide deficiency is a common occurrence in many hobby greenhouses and indoor gardens because every effort has been made to prevent heat loss by sealing the growing area up tightly.
Adequately venting the growing area will prevent CO2 depletion, but the rate of air replacement required in a densely planted, mature crop in a small, confined growing area can be much higher than most growers realize when the plants are growing under strong lights and actively photosynthesizing.
Most plants respond well to CO2 levels in the 300-1,500 ppm range. Below 200 ppm, CO2 begins to limit plant growth, but if levels creep above 2,000 ppm, CO2 becomes toxic to many plants and 5,000 ppm is the threshold for human safety.
There is much debate over the level of enrichment that will give the highest rate of photosynthesis under different growing conditions. Plants will use more CO2 under warm, high-light conditions than under duller, cooler conditions, and enrichment levels can be adjusted to account for this.
Light and Humidity
So now we know that light levels, light quality and CO2 levels affect the rate of photosynthesis and a number of other plant processes. The effects of humidity are a little more indirect. Low humidity (high vapor-pressure deficit) can cause large volumes of water carried in the transpiration stream to be lost from the leaf surface to the air and force the plant to shut down its stomata to prevent desiccation.
With the stomata shut to prevent water loss, photosynthesis cannot occur, as CO2 can’t be taken in from the surrounding air. Plant growth and yield will be slowed if this occurs too often.
When humidity is low, particularly when combined with warm growing conditions, it can be difficult to tell if plants have shut down photosynthesis due to closed stomata, as under these conditions, the foliage may look normal.
One way of checking to see if the stomata are open is to measure the difference between leaf surface and air temperature using a handheld infrared thermometer. A plant with open stomata is actively cooling itself via transpiration, so the leaf surface will be a slightly lower temperature than the surrounding air. If the leaf surface is warmer than the surrounding air, then transpiration is not occurring and photosynthesis has shut down.
Light and Temperature
Along with light quality and quantity and CO2 levels, temperature also determines the rate of glucose production during photosynthesis due to the activity of enzymes that catalyze biochemical reactions in leaf tissues. These enzymes are strongly affected by temperature, so the rate of assimilate production through the process of photosynthesis is maximized when the plant is grown in its optimal temperature range.
At low temperatures, which for many plants we commonly grow is 32-50°F, the enzymes involved in photosynthesis do not work at full efficiency and this lowers the rate of sugar production and plant yields.
At optimal temperatures, which vary for different species, but are often between 50 and 75°F, photosynthetic enzymes work at their maximum rate. At these temperatures, it is often the diffusion of CO2 that becomes the limiting factor for photosynthesis.
At high temperatures (104-113°F), many plant species will experience a drop in photosynthetic rate.
Plants’ photosynthetic response to light varies considerably between species. Some plants have evolved in shaded conditions and need much less light than others. Many indoor plants are in this category and can be burned or damaged by use of high-intensity lamps.
Seedlings generally need less light than mature plants and planting density also determines how much output is needed from a light source to reach light saturation and maximize yields. Most commonly grown hydroponic vegetable and flower crops become light saturated at around half the light output of full sunlight, while some light-sensitive plants may only need a fraction of this.
Growers soon become reasonably experienced at picking up the signs of too much or too little light. Too much light causes leaf burn, bleaching or scorching, and plants become compact.
Some species, such as tomatoes and capsicums, may actually roll their outer leaf margins inwards or point their leaf tips upwards in an attempt to lower the amount of surface area receiving the high light levels. Insufficient light often results in tall, spindly plants that stretch upwards, becoming elongated and weak with soft growth.
The lower leaves may senesce and turn yellow due to a loss of chlorophyll. Flowers may abscise and fruit size may remain small in crops like peppers and tomatoes grown under low-light conditions. Fruit flavor and aroma is also negatively influenced by insufficient lighting conditions, as the plant is unable to produce sufficient sugars for maximum quality.
Optimizing the light usage of your hydroponic crops involves an integrated approach of maintaining general plant health, water status and nutrition and some environmental modifications, such as keeping temperatures within the optimal range and supplying additional CO2.
Finally, ensuring each plant reaches light saturation with sufficient illumination of the correct intensity and wavelengths also plays a role in maximizing photosynthesis, growth and yields from hydroponic crops.