Indoor gardening offers to plants conditions that are fully under the gardener’s control. So, when physiological disorders strike, they can seem strange, mysterious and even sinister… Indoor gardens can be a fairly intense environment for hydroponic plants. High levels of light, heat, carbon dioxide enrichment and a plentiful supply of water and nutrients give plants everything they could want; however, sometimes growth can still go awry with no obvious cause. Physiological disorders, which affect a plant’s development, are usually the culprit and they are caused by environmental or cultural factors. Some of these physiological disorders, such as tip burn on heading lettuce or blossom-end rot on tomatoes, are common in indoor gardens, large-scale commercial greenhouses and outdoor production, so they are relatively well-understood and easily recognizable. Other physiological conditions, however, are much more specific to the enclosed environment of indoor hydroponic system and are not as well-understood or easily diagnosed by growers. Some physiological disorder symptoms are also non-specific, meaning the same symptom could be caused by a number of different issues—like plant pathogens, nutritional problems or a complex physiological problem. In this case, having a good idea of basic plant requirements and physiology is the best place to start. Physiological problems in detail Humidity & root pressure issues Humidity levels—if not kept below a certain level—might be more often associated with an increased risk of fungal and bacterial disease; however, it also plays an important role in plant physiology. The plant must be able to transpire and lose water in order to maintain the transpiration stream within itself. If transpiration is slowed down due to high humidity and/or lack of air movement (which removes the stale and humid boundary layer sounding the leaf), then elements like calcium can’t be transported fast enough to the developing leaf tips and fruit. When this occurs, many inexperienced growers mistake the resulting brown or black tissue of blossom-end rot (BER)—in tomatoes and peppers—and tip burn—on the youngest leaves in lettuce, strawberry and other salad crops—to be a disease. However, it is in fact most often due to a localized deficiency of calcium that develops in the extremities of the plants. In well-run hydroponic systems, tip burn and BER are rarely caused by a calcium deficiency in the root zone; these calcium transport problems are more commonly found in warmer growing conditions and are usually associated with high humidity. These conditions can be improved by lowering humidity and increasing air flow over the plants to boost the rate of transpiration and, therefore, the flow of calcium out to developing fruits and leaf tips. Calcium transport disorders like these also have a genetic element, and many modern cultivars of lettuce and tomatoes have had some degree of BER or tip-burn resistance bred into them. Glassiness Another less well-known physiological disorder related to humidity levels is glassiness, most commonly seen in lettuce, young seedlings and other succulent crops.  Glassiness creates patches with a water-soaked, almost translucent appearance on the leaves of plants—and it’s often seen first thing in the morning and it might disappear later on as the plants warm up. This disorder is related to both high humidity and root pressure, which is greatest at night. Roots can take up excess moisture under the cooler conditions at night and, using root pressure, pump this up to the foliage where it can’t be lost from the leaves thanks to the low rate of transpiration that occurs when temperatures are cool and humidity is high. This condition is usually reversible (with no lasting damage) once good ventilation and air flow are introduced and humidity is lowered. However, if glassiness persists and becomes severe, leaf cells can eventually die and create dead patches of foliage that could then be infected with disease. Edema (Oedema) Edema is more common than most growers realize; however, its symptoms are often not correctly linked back to the cause. As with glassiness, edema is caused by an imbalance between the plant’s water uptake and water loss, and develops when root pressure is high and transpiration low. The enlarged cells—which are full of water—divide and rupture, causing a number of symptoms like raised blisters, galls and water-soaked swellings or protrusions on leaves, stems and veins. In later stages, the damaged tissue becomes corky with gall-like formations that harden and darken with age. In mild cases, plants can recover from edema; in severe ones, leaves can curl and become distorted and foliage will drop in the later stages. It has been suggested that edema is linked to the spectral quality of the lighting, with red light promoting the condition and ultra-violet inhibiting it. Edema, however, is usually controlled or prevented by careful management of the environment by increasing air movement, lowering humidity with high rates of ventilation, using a well-drained growing medium with optimum levels of nitrogen, ect. Light, temperature and carbon dioxide (CO2) issues specific to indoor gardens Continuous light injury or abnormal photoperiod effects Light injury is another physiological disorder often only seen in indoor gardens, where HID lighting can be run continuously or for long periods. Many plant species can tolerate continuous lighting (although running lamps for 24 hours a day may not give the growth increases expected), but some species—including tomato, potato and some ornamentals—are intolerant of extended or continuous periods of light. If exposed to such conditions, they can develop physiological disorders, such as becoming severely chlorotic, yellowed and stunted or with brown flecking of the foliage. High-light injury is also more common in environments where CO2 enrichment is used and it is thought that a high buildup of starch in the chloroplasts might play a role in this disorder. CO2 injury Enriching an enclosed growing area with CO2 can result in significant growth benefits; however, CO2 toxicity can occur when levels run too high. Some plant species are more susceptible to CO2 toxicity than others, so maximum-level recommendations are hard to define. That being said, optimum ranges for most plants are below 1,600 ppm, more commonly in the 800 to 1,200 ppm range. Sometimes, in indoor gardens, the cause of CO2 toxicity is a faulty CO2 monitor; in that case, the grower would not be aware that CO2 levels are well above optimum. It should also be noted that high levels of CO2 are toxic to humans as well—levels of 5,000 ppm can cause dizziness or a lack of co-ordination—which is another good reason for keeping CO2 monitors properly calibrated. In plants, symptoms of CO2 toxicity can include leaf rolling or deformation, chlorosis or mottling of the leaves and, in later stages, leaf drop of older foliage. Gasses as unwanted contaminants Some of the most severe physiological disorders are caused by gasses that find their way into the growing environment. Propane leaks from heating systems have been known to cause injury to indoor crops, but ethylene is a more common issue. Ethylene is a gas and a plant hormone that can originate from a number of sources, including rotting vegetation, ripening fruit, vehicle exhaust, and some plant-growth regulators. However, malfunctioning heating systems and the incorrect use of burners to generate CO2 are the most common causes of ethylene contamination in growing environments. The severity of symptoms depends on the species being grown and the level of ethylene buildup. Some sensitive plants, such as tomatoes, will show symptoms—like epinasty (downward bending of the leaves while remaining turgid), reduced growth and height and, in severe cases, leaf and flower abscission—at ethylene levels as low as 0.05 ppm. Premature bolting “Bolting” is the term used to describe the premature elongation of a plant’s compact stem. The entire plant, which is still relatively young and immature, grows upwards and forms a flower stalk. Lettuce, other salad greens and herbs can bolt or go to seed extremely early, sometimes while still in the seedling stage. Typically, this occurs when temperatures are higher than optimal, often combined with low light levels or overcrowding. Some cultivars have been bred to have some degree of premature bolting resistance, but this physiological disorder is still a common problem, particularly with lettuce. Prevention is relatively easy, however: maintain temperatures below 78oF for lettuce and other cool season salads, maintain suitably high light levels and prevent seedlings from becoming pot-bound and overcrowded before planting out. Media and nutrient issues Overwatering and under watering are the most common causes of physiological disorders in hydroponic crops; however, overwatering is far more common and misdiagnosed than under watering (we all know what a dry root system looks like, after all). Interestingly, overwatering initially looks quite similar to under watering—at least from the top of the plant. Wilting, downward hanging leaves, eventual leaf/bud/flower drop, yellowing and chlorosis are all signs of overwatering. In the most severe cases, epinasty will result because the damaged root systems will have started producing ethylene. Overwatering can be prevented by reducing the frequency and volume of nutrient applied, particularly under cooler growing conditions, and using a coarse, free-draining medium like perlite. Elephant’s foot The root zone can be the cause of other physiological disorders in hydroponics. Some of the most common of these disorders are related to salinity damage and electrical conductivity (EC) buildup. As with many physiological disorders, the symptoms of these may be confusing to new or inexperienced growers; however, they are usually never forgotten once encountered. For example, one condition that is common in pepper plants (other plants can also develop a similar appearance) is elephant’s foot—or, foot corkiness. Elephant’s foot is characterized by a swollen and sometimes cracked area on the stem close to the base of the plant, caused by injury to the stem cells by excessive amounts of salts. It is easy for nutrient salts to accumulate around the base of the plant, particularly where the nutrient is irrigated close to the stem area or when a highly free-draining growing medium is used under low-humidity conditions. Elephant’s foot is less likely to occur when seedlings are planted deeper into the growing medium and when nutrient salt buildup around the stem is prevented. Salinity damage Salinity damage in hydroponic systems is much more common in some crops than others. Tomatoes, for example, are fairly tolerant of high EC and salt buildup, whereas other more sensitive plants like cucumbers and lettuce are not. Cucumbers show a fairly distinctive disorder when the EC becomes too high in the root zone (EC levels of only 3.0 to 3.5 have been shown to cause these symptoms in some cucumber crops): wilting during the warmest part of the day, followed by a distinctive leaf halo—a thin band of yellow coloration around the leaf margin. This disorder is more common under warm growing conditions with rapid evaporation from the growing media and high rates of water uptake by the plant. These act to concentrate the EC and salts around the root zone, leading to salinity damage. Salinity damage is common under the warm, dry conditions of many indoor hydroponics gardens where EC levels in the root zone can climb far more rapidly than a grower may realize. In this case, the EC in the feed solution should be dropped right back to adjust for the increased rate of water uptake by the plants. Physiological disorders can range from mild (sometimes not even noticeable) to severe (causing plant death), so identification of the problem is the key to a quick recovery. Physiological disorders can be easily forgotten and overlooked; so, when a problem starts occurring, it pays not to just assume that every issue is either a disease or nutrient disorder. Most of these physiological conditions are induced by factors that are fully under our control and—in an indoor situation—that means keeping a constant check on light, humidity, temperature, salinity in the root zone, CO2 and heating/venting systems. Luckily, these days growers have the choice of the latest technology for monitoring and adjusting the indoor growing environment so that many physiological disorders can be prevented—using and maintaining the best equipment to monitor CO2, root zone moisture levels, pH, EC, temperature, humidity and vapor pressure deficit, air flow and light levels go a long way to preventing any unwanted growth issues.