Hydroponic growers on both a large scale and smaller indoor enthusiasts often have the same aim: to maximize growth and yields from their production systems.
While choice of suitable cultivars, optimal nutrition, and an ideal growing climate significantly increase the potential of a hydroponic system, we are always searching for other possibilities to further push growth and productivity.
One technique that has come under increased study, particularly since the uptake of advanced and more cost-effective indoor lighting technology, has been the use of continuous light as an alternative to the normal light/dark period used for most crops.
If photosynthesis, assimilate production, and utilization by plants can be increased via continuous lighting there is potential for an increase in growth and yields. This may seem like a simple and successful formula for plant production to increase light and thus growth, but plant physiology and response to continuous light is far more complex and, it seems, also species dependent.
Light and Flowering
Light has numerous different effects on plant growth and development. Apart from the obvious use in photosynthesis to produce assimilate for growth, light plays a role in several other physiological processes.
Some plants require either short- or long-day lengths to either promote or inhibit flowering, thus continuous light is not suitable during certain phases of growth when growers are manipulating flowering times.
However, most of the widely grown, modern varieties of hydroponic crops such as lettuce, tomatoes, cucumber, and capsicum are not day-length sensitive when it comes to flowering, so they could potentially benefit from continuous day length under certain circumstances.
The amount of light intercepted by a crop is dependent not only on the number of hours per day light is provided, but also by the intensity. The amount of light received over a 24-hour period can be calculated as the daily light integral (DLI), which is a combination of intensity and duration. This is a much more meaningful way of measuring the light received by a plant than just a spot reading of intensity.
Plants grown under longer photoperiods (18 hours of light compared to 12 hours, for example), can be provided with a lower light intensity, however, the increased day length means they will receive the same overall light total as those under the shorter day length. This means plants grown under continuous light can only use so much light within a 24-hour period before they become completely light saturated at a high DLI, thus continuous lighting regimes typically use lower output lamps.
Low-level continuous lighting is currently used for some crops such as seedlings, lettuce, and leafy vegetables, however, the most potential seems to exist with those crops which require a high overall day light level and have a strong demand for assimilate to maximize fruit growth.
Continuous Light and Plant Injury
Continuous light has been reported to cause leaf and crop injury and is somewhat species dependent. Tomatoes are particularly sensitive and cultivated tomato varieties have been widely reported to develop leaf injury when grown in continuous light.
The mechanism for this damage is not completely clear, but appears to at least be partially a result of the hyper accumulation of starch and soluble sugars produced in the leaf under the prolonged lighting conditions that can cause a range of symptoms, the most common being leaf chlorosis (yellowing) and necrosis (death of leaf cells). Continuous light may also accelerate leaf aging and photo-oxidative damage due to excessive light levels and there may be other factors involved with foliar damage.
Studies have found that running a lower temperature for several hours in each 24-hour period under continuous light benefited a number of species — tomatoes grown with 82/61°F (28/16o C) alternating temperatures and cucumbers with a short-term exposure to 54°F (12oC) decreased the severity of injury symptoms caused by continuous light.
Leaf injury under continuous light appears to be not just caused by the long light exposure itself but by an interaction between light during, light intensity, and light quality as well as temperatures provided by thermoperiods.
Continuous Light and Assimilate Unloading
One of the limiting factors for the use of continuous light is the process of assimilate unloading — the transport of photosynthesis products out of the leaves where photosynthesis has been occurring to the sites where it is required for growth (such sinks for assimilate include rapidly developing cells in fruit, buds, flowers, and new root growth).
Under normal light/dark conditions, photoassimilate produced during the light period can be exported out of the leaf during both the light and the dark period when no photosynthesis is occurring, thus preventing a build-up of sugars in the leaf cells. If the assimilate produced via photosynthesis is not exported out of the leaf cells fast enough, there is a build-up of these sugars that creates a negative feedback and slows or even stops further photosynthesis.
So, even if sufficient light is provided, the build-up of assimilate in the leaf and subsequent slowing of photosynthesis means that no increases in yield and productivity will occur even under continuous light.
In fact, a reduction in growth can happen. This is seen as the major drawback in the use of continuous light for increasing the rate of food production. The other issue is this overproduction of assimilate that builds up in the leaf cells and can’t be shipped out of the leaves fast enough. This may be what is responsible for the damage and leaf injury some species experience under continuous light.
To further confuse this process of whether continuous light may be beneficial to certain crops is the fact temperature interacts with light on several different levels.
First, temperature determines the rate of photosynthesis, so more assimilate is produced under continuous light when optimal warmth is provided than when it is cooler than optimal, so an excess build-up of starch is more likely to occur in foliage under these warm conditions.
Second, temperature plays a major role in the rate of transport of assimilate out of the leaves (the source) to the sinks (fruits, flowers, buds). The temperature differences that typically occur under a normal day/night regime are warmer during the day and cooler at night. Under continuous light, temperature often remains the same right throughout the 24-hour period in order to allow maximum rates of photosynthesis to occur. Nights that are cooler than days favor phloem unloading (the transport out of sugars from the rapidly cooling leaves as darkness falls, followed by high rates of photosynthesis under warmer day temperatures).
Some research studies have found that while continuous light under the same optimal warm temperatures that allow maximum rates of photosynthesis to occur can cause an overall reduction in growth due to assimilate build-up in the leaves.
However, if the temperature is varied within a 24-hour period under the same continuous light, then growth benefits occur, and leaf damage is often prevented.
This seems to be particularly beneficial for plants such as tomatoes and eggplant, which are prone to leaf injury under continuous light.
It has been found that running temperatures of 79/61°F (26/16oC) during a 12/12 hour period under continuous light as compared to a constant 73°F (23oC), the accumulation problems of sugars in the leaves was reduced and a higher rate of dry matter production and yields occurred.
The evidence suggests the cooler temperatures run for 12 hours in every 24 under continuous light favors the metabolism of starch and sugars and translocation from the leaves and into the sinks. Studies have found that by varying the temperature within each 24-hour period, leaf injury in species prone to damage under continuous light can be largely prevented.
While low-level continuous light can give increases in growth and productivity with some crops, under certain conditions, it is not always a simple case of switching the lights on permanently and hoping for the best result.
Some species such as tomatoes are particularly sensitive to continuous light and suffer leaf injury and losses in productivity, while factors such as light intensity, spectrum, and temperatures all interact to determine the final response to this technique.
What is promising, however, is the use of temperature variations under continuous light to maximize carbohydrate metabolism and unloading of sugars from the foliage into developing cells, thus increasing the potential of light usage.
There exists a genetic potential within many commonly grown hydroponic species to breed varieties that are less prone to damage under continuous light, while at the same time be able to maximize the extra hours of photosynthesis to result in increased growth rates and yields.
For those wanting to experiment with continuous light, choosing plants that don’t require a certain day length to flower or prevent flowering is the first step, followed by careful consideration of the intensity of the light for the species chosen, and, finally, maintaining a temperature difference within each 24-hour period which provides some cooler conditions to assist with sugar unloading and carbohydrate metabolism.