Photosynthesis is the process that drives plant growth and development, and while most of us have some idea of its importance, there is a lot to understand if we are to fully optimize the potential of our plants.
We all know plants need light and this radiation energy—as light is technically known—can either be supplied by the sun or artificially, by the highly efficient grow lamps used by indoor gardeners and the plant scientists who study photosynthesis.
However, it is not just the light source that affects the rate of photosynthesis—there are a number of external and internal plant factors that influence the production of sugars for plant growth as well. Maximizing all the variables that play a role in photosynthesis at the same time can lead to some fantastic growth rates, but it takes a little understanding of plant physiology to get things just right.
What is photosynthesis?
The basic principle behind photosynthesis is fairly well understood—light energy is used to synthesize sugars (also called assimilate) from carbon dioxide and water. Light is absorbed by pigments in plants—the most common being chlorophyll—and energy provided by light is used to combine hydrogen (H) from water with carbon dioxide (CO2) from the air to form sugars, such as glucose.
The energy from light is stored within the sugar molecules, which are then also used as a raw material for the synthesis of many other compounds required by the plant for growth and development.
Photosynthesis is actually a very complex biochemical process involving many different enzymes; however, the simplified equation looks like this:
6CO2 (carbon dioxide) + 6H2O (water) in the presence of light turns to C6H12O6 (glucose) + 6O2 (oxygen)
In the equation above we can see that plants need not only a light source of sufficient intensity and the correct wavelengths for photosynthesis, but also a supply of carbon dioxide and water.
What the equation doesn’t tell us is that other factors play a role in the rate of this process as well—temperature, for example, determines the rate of photosynthesis; the nutritional status of the plant affects light harvesting; leaf area and plant pigments such as chlorophyll determine how much light can be intercepted; and stomata apertures influence the flow of CO2 into the leaf.
Just to complicate matters even more, the internal flow of water required for photosynthesis into the leaf can be affected by drought, high EC, root damage and other factors, while CO2 levels directly around the leaf surface are affected by airflow and the use of CO2 enrichment.
How much light is needed for photosynthesis?
Most indoor growers sooner or later have to grapple with the concept of how much light needs to be provided for maximum plant photosynthesis. There is no hard and fast rule for this, as different plant species have various optimal light-level requirements.
A densely planted growing area will need a higher light input than a sparsely planted one and plants’ requirements for light tend to change from seedling through to maturity. Ideally, all the plants in the growing area should receive sufficient light to allow them to reach a point termed ‘light saturation’—that is, the point when further amounts of light don’t provide any additional increase in photosynthesis.
However, what usually occurs is that most of the plants will remain at a point below the light saturation level, as leaves and plants end up shading each other as they grow and develop.
Not enough light vs. too much light for photosynthesis
A common situation in indoor gardens is insufficient light for photosynthesis. This is often because too many plants end up crammed into a small space—plant hoarding is, after all, pretty common with gardening enthusiasts—or because small plants have rapidly become large mature ones, filling the available area with dense foliage.
In this type of situation photosynthesis is limited by light availability and if the situation is severe the crop might even fall so far below the light saturation point that it reaches what is termed the ‘light compensation point.’ This is where the energy gained from photosynthesis equals that lost in the process of respiration so that no net growth can occur.
Between the light compensation point and light saturation point any increase in light will increase the rate of growth; however, the ideal situation is to remain closer to light saturation than light compensation.
Having too much light for photosynthesis can occur in indoor gardens as well—more commonly with younger plants or with those that have been grown under lower light and then planted out under a greater light intensity. Very high light intensities will break down chlorophyll, imparting a white, bleached appearance to the leaves and resulting in a decreased ability to carry out photosynthesis.
Limiting factors when it comes to photosynthesis
In an indoor garden the most likely limiting factors for photosynthesis—apart from a lack of light intensity or light of unsuitable wavelengths—are the availability of carbon dioxide and temperature control.
If temperatures become too high, plants will effectively shut down photosynthesis as the stomata (pores in the leaf that allow gas exchange) close to conserve moisture and prevent desiccation. Closed stomata prevent CO2 from the air diffusing into the leaf and so photosynthesis will stop.
Low temperatures have a similar effect—since the rate of enzyme reactions within a plant is temperature dependent, photosynthesis will slow when conditions are cooler than optimal and will be most rapid when temperatures are ideal for the particular plant species being grown.
Carbon dioxide is also a limiting factor—particularly in enclosed indoor growing spaces—and CO2 depletion in densely planted indoor gardens or even in closed greenhouses is common. Plants can rapidly strip CO2 from the air surrounding their leaves under good light conditions when high rates of the gas are being utilized for photosynthesis. Since CO2 is only naturally present in the air at rates of 390 ppm, fresh supplies will need to be vented into the area or CO2 enrichment supplied.
Many growers opt to enrich the air with CO2 up to levels of 1,400 ppm as this is proven to boost photosynthesis and growth in most plant species; however, very high levels of CO2 can cause plant damage and should be avoided—enriching with CO2 above 1,400 ppm for most crops does not tend to give any increase in photosynthesis in the long term anyway.
For growers without CO2 enrichment, ensuring there is sufficient ventilation to remove stale air and pull in fresh air to replenish CO2 levels in the indoor garden is vital for photosynthesis and the rate of this air exchange is often much higher than many growers realize.
Photosynthesis with artificial lighting
When light hits the surface of a plant, some is reflected back to the environment and some is transmitted through the leaf, but most is absorbed or harvested by the pigments in the leaf cells. The best-known light-absorbing pigments are green chlorophyll a and chlorophyll b.
However, these are not the only light-harvesting pigments in plants—there are also a number of accessory pigments, which allow photosynthesis to use quite a large proportion of the visible light spectrum. Some of these light-absorbing pigments are the xanthophylls, the carotenoids, lutein and lycopene and the number and ratios of these within the plant cells vary between different plant species.
If we look back at the history of plant lighting for indoor gardening, in the early days the light-absorption spectrum was commonly displayed as peaks for chlorophyll a and chlorophyll b. (see figure 1).
These two peaks are in the red and blue wavelength ranges. Because of this sort of graph an assumption was often made that since there are narrow bands in the red and blue wavelengths doing all the work as far as photosynthesis goes, providing full-spectrum light with wavelengths in the green and yellow bands would just be a waste of energy.
However, there is a major problem with using this sort of information—as this graph actually shows what happens when these two pigments, chlorophyll a and b, are isolated in a test tube and not how they act within a complete plant system. In a test tube it is true that chlorophyll a and b strongly absorb red and blue light and reflect yellow and green, but inside plant tissues things are different.
Figure 1: Absorption spectrum for chlorophyll a and b in vitro
Figure two is a graph we should be more familiar with these days as it reflects actual photosynthesis inside plant tissue—not what happens in a test tube, as was shown in figure one. There is still a peak in the blue and red wavelengths, but there is much more photosynthesis going on in the 500 to 600 wavelength band—the green and yellow area—than we generally recognize.
As plants reflect more green light, which is why foliage looks green to us; however, there are pigments present in the plant trapping this green light and passing the energy on to chlorophyll for photosynthesis, so green light still drives the process. This graph is an average response taken from a large number of common plant species, so it reflects accurately what occurs in most crops.
It shows that inside plant tissue the role of green and yellow light in triggering photosynthesis is actually surprisingly important. The reason behind this is that chlorophyll is not the only pigment that can absorb light—there is a range of other accessory or antenna pigments that use green and yellow light as well. For example, much of the photosynthesis occurring in the green waveband (540 nm) results from absorption by active carotenoids.
Many of the light-harvesting pigments also make use of a wide range of light wavelengths and pass the energy on to chlorophyll for photosynthesis. So while chlorophyll itself might not absorb much in the way of green or yellow light, other pigments can—and the entire spectrum can then be used by the plant.
The use of accessory pigments allows photosynthesis to use a large proportion of the spectrum—not just red and blue wavelengths—and for indoor growers that’s an exciting opportunity to take advantage of.
Figure 2: The effect of different wavelengths on photosynthesis
The Emerson enhancement effect and photosynthetic lighting
While it’s commonly known that there are a wide range of wavelengths absorbed and used by plants for photosynthesis, the enhancement effects of certain bands of the spectrum are more complex but still important to consider for indoor growers wanting to maximize photosynthetic growth effects from their lighting.
Some of the shorter light wavelengths—when combined with longer wavelengths—act to boost photosynthesis more effectively than if either of the wavelengths was present alone.
This is termed the ‘Emerson enhancement effect’ and it is an important aspect to take into consideration when deciding between different types and outputs of lamps and bulbs. This enhancement effect means there is a synergy between red and far red wavelengths and therefore a benefit to providing plants with both—even if the plants are not flowering.
Figure 3 below shows how the Emerson enhancement effect works—up at the 700 nm range, it appears as if photosynthesis drops off (this is called the ‘red drop off’), so it might appear that there is no point in providing plants with light in this waveband. However, when wavelengths in this far red range are combined with the shorter wavelengths of red light (680 nm), a photosynthetic enhancement effect occurs.
This is why we have begun to see more lighting bulbs developed featuring output in this far red range, allowing indoor plants to take advantage of a fuller spectrum in the same way that outdoor plants have always been able to do.
Figure 3: The Emerson enhancement effect
Maximizing the photosynthetic potential indoors
Maximizing photosynthesis in an indoor garden is dependent on a number of factors: the correct wavelength spectrum (as explained earlier, these days that means full-spectrum lamp outputs), sufficient intensity of light for the stage of plant development, CO2 replacement or enrichment to levels over 1,000 ppm, sufficient warmth to maximize the rate of photosynthesis, good rates of water uptake and cell turgor, overall plant health and sufficient nutrition.
Providing all these factors will allow plants to take full advantage of those cellular reactions which provide both energy and assimilate for maximum growth and development.