Photosynthesis is the key to good growth and high yields. If photosynthesis is decreased, due to low light conditions, high humidity (which closes stomates and reduces gas exchange), or water stress, then the production of sugars will decline and the fruit quality, shelf life, and size will all diminish.
Because of the critical role of photosynthesis in plant growth, a one-percent decrease in light can translate to a one-percent decrease in yield. Shading from outside topography and trees, the greenhouse structure itself, or taller plants in the greenhouse can significantly reduce the amount of light reaching the crop. Both the greenhouses and the rows of plants in the greenhouse should be oriented north and south so the light is evenly distributed across each plant. Some growers reflect light back into the crop using white floorcoverings or paint. Clean white paint is more reflective than metallic or foil, although there is some indication that foil tends to "confuse" insects and slightly decrease insect pest damage.
During long periods of cloudy weather, tomato leaves become low in sugars, and may become pale and thin. Excess nitrogen at that time can be detrimental.
Some growers prefer to shade tomatoes, while others do not. Theoretically, shading will reduce photosynthesis, and therefore total yield, however, this has not always been shown in controlled studies. In fact, in some studies, total yield was improved using 30% shadecloth. Shading can improve fruit quality, since direct sunlight on fruit can cause yellow or green shoulders, cracking, and russeting. Alternatively, older leaves can be left in place to shade the individual fruit trusses. In areas of high summer temperatures and humidity, shading may be necessary to keep temperatures within a reasonable range. Ultimately, however, the decision to shade or not depends on the location of the greenhouse, the cultivar of tomato grown, the season and the overall management system employed by the grower.
Historically, the greenhouse industry has traditionally measured light in foot-candles and lumens. Foot-candles are the amount of light received on the surface and lumens are the measure of light emitted by a light source. Natural sunlight and artificial light falling on a plant are measured in foot-candles (f.c.) while the light emitted by sources such as the sun and electric lamps are rated in lumens. A clear, sunny day may measure 10,000 f.c. and an overall winter day as low as 500 f.c. To read comfortably requires about 20 f.c. The light of the full moon measures less than 1 f.c. A light meter with a scale in direct foot-candle readings is manufactured by the General Electric Company and is sold by most greenhouse supply companies.
Supplementary artificial light, from cool white, high output fluorescent or high intensity discharge sodium vapor lamps is beneficial to plants when sunlight is unavailable but is not a complete substitution. Intensity of supplementary lighting should be about 800-1000 foot-candles at the plant surface.
Today, plant scientists are primarily interested in that light which is responsible for photosynthesis. The portion of the light band most responsible for photosynthesis measures 400-700 nanometers. This band is often termed the Photosynthetically Active Radiation (PAR). Within this range, intensity is the most critical factor along with light period. Within the PAR region light is measured as the Photosynthetic Photon Flux (PPF) and is expressed in Ámol/m2/s. Daily total of PPF, expressed in mol/m2 have been shown to relate to total photosynthesis for the day.
In the southwestern region of the United States, the winter light readings are three times higher than in the northern regions such as the states of New York and Ohio. This is why the greenhouse tomato industry is growing so rapidly in Arizona and surrounding states.
Supplemental lighting is generally not economical for vegetable crops, with the exception of seedling production. However, for backyard or hobby situations, full-spectrum lighting can be effective in increasing yields by increasing the daylength to 18 hours during winter months.
Both day and night temperatures influence plant vigor, leaf size, leaf expansion rate, and time to fruit development. Under low night temperatures, the rate of leaf growth is slower, and leaf size is reduced in young plants. Day and night temperatures should be carefully monitored. A general rule of thumb for most horticultural crops is for night temperatures to be approximately 5.5░ C (10░ F) lower than day temperatures. For tomatoes, day temperatures should be 21░ -26░ C (70░ -79░ F) and night temperatures around 16░ -18.5░ C (61░ -65░ F), although many new varieties do best with little difference between day and night temperature (check with your seed company for recommended growing temperatures). For seedlings, the temperatures should be constant, 20░ -22░ C (68░ -72░ F), then gradually acclimate the plants to the diurnal temperatures before transplanting.
High temps in excess of 30░ C to 35░ C will cause many different types of damage to the plants, such as inhibition of growth and even death. The physiological nature of heat damage is thought to involve a denaturation of some protein component of plant cells. Fruit abortion may occur at these temperatures as well. Temperatures lower than optimum will alter the plant metabolic systems to slow growth and again hinder fruit set.
Fogging systems can be an alternative to evaporative pad cooling. They depend on absolutely clean water, free of any soluble salt, in order to prevent plugging of the mist nozzles. Like fan and pad cooling, fog cooling is only really efficient in low humidity environments.
In hobby greenhouses, temperatures can be measured easily with a minimum/maximum thermometer. Several thermometers should be placed throughout the greenhouse, and should be calibrated against each other and a quality thermometer at least twice per year. In large commercial operations, computer controlled systems are common. Such systems can provide fully-integrated control of temperature, humidity, irrigation and fertilization, carbon dioxide, light and shade levels.
Air Circulation and Ventilation
Good circulation is necessary for proper cooling, heating, CO2 replenishment, and removal of undesirable gases, such as ethylene. Your circulation system must work together with your heating, cooling, and CO2 systems in order to obtain peak efficiency.
Many different methods of circulating air have been developed. The vent-tube system is used quite a bit, and consists of a fan-jet connected to a perforated plastic tube running the length of the greenhouse at ceiling height. The fan forces air through the tube, which moves the warm air in the roof space downward to displace the cooler air at the floor level. This design is not very efficient. A horizontal airflow system is more efficient, and can move a larger amount of air around the plants. Large fans, hanging above the crop, are set up facing one direction in one section of the greenhouse, and in the opposite direction in the adjacent section of the greenhouse. A more complicated system is a vertical airflow system, which uses fan-jets to move air along the roof, downward at the end walls, then along the floor through the crop. This system provides the best mixing of air and brings warm air down into the plants. Various types of alternative ventilation systems have been proposed, such as up-draft and down-draft chimneys. However, it will be some time before these systems are thoroughly tested and refined.
In the tropics, natural air exchange to the outside of the greenhouse can be achieved simply through the sides of the greenhouse structure. For active or mechanical ventilation, low-pressure propeller blade fans are used for greenhouse ventilation. They are placed on the end of the greenhouse opposite the air intake, which is often covered by evaporative cooling pads and louvers. The cooling pads used in combination with fans (fan and pad cooling) can be made from a number of materials, most often they are made of a cellulose material, usually aspen wood, or a multi-celled/honeycombed material called "kool-cel". The ventilation fans for larger greenhouses (100-120 feet in length), are normally sized to allow a maximum air exchange once per minute. Small hobby greenhouses, which have a large greenhouse surface area to floor area ratio may require an air exchange of up to 2.5 times per minute.
In order for a plant to actively grow, it must be allowed to transpire freely during photosynthesis; this means plenty of available water, low to moderate humidity, and good air circulation. Humidity influences calcium uptake and hormonal distribution by controlling transpiration, ion pumping, and stomatal opening and closing. High humidity coupled with low air movement reduces transpirational cooling, and can lead to heat overload for the plant.
People tend to think of humidity in terms of relative humidity, which is the ratio of the amount of water vapor in the air to the amount of water vapor the air could hold at that temperature, expressed as a percent. Plants, on the other hand, perceive humidity in terms of vapor pressure deficit (VPD). VPD is the difference between the vapor pressure in the air and the vapor pressure inside the leaf. Water moves by diffusion from the roots through the plant and out the leaves as transpired vapor, thereby being "pumped" up the plant as the vapor moves from the higher pressure inside the leaf to the lower pressure in the surrounding air. Low VPD (high humidity, greater than 90%) is often responsible for nutrient deficiency symptoms, such as blossom end rot (calcium deficiency) because the plant is not transpiring, therefore it is not drawing water, or nutrients, into the roots. High VPD (low humidity, less than 50%) can also lead to the same symptoms, because water and nutrients are pumped too quickly through the plants, depositing nutrient ions in the leaves rather than properly in the fruit.
Greenhouse humidity can be measured with a sling psychrometer. Other equipment such as a humidistat can measure relative humidity to an accuracy within 4%. Most greenhouse supply companies sell equipment to measure humidity.
Most plants can function adequately in relative humidities of between 55 and 95%, which corresponds to VPDs of 1.0 to 0.2 kPa. For tomatoes, the ideal humidity should be between 65 and 75% during the night and 80 to 90% during the day. Tomato yields and fruit quality are lower at lower VPDs (higher humidity). Leaf size can also be reduced, and flower and fruit abortion can be significantly increased under high humidity conditions. Glassiness and "gold fleck" in tomato fruit is also attributed to high atmospheric humidity.
Misting and fogging systems are used by some growers to increase humidity and decrease temperatures. However, if used improperly, these systems can greatly increase the incidence of mildews and plant diseases, not to mention corrode metal greenhouse structures.