Plant response to Green house Environment

A) Light in Greenhouse Environments

Light Quality and Photosynthesis: The type of light is crucial in the greenhouse environment. Plants primarily use blue (400–500 nm) and red (600–700 nm) light for photosynthesis. Blue light is associated with the promotion of leaf growth, while red light influences flowering and fruiting. The green color of plants is because chlorophyll absorbs light mainly in the red and blue spectrums, reflecting green light.

Light Saturation: Some plants can become light-saturated, where the photosynthetic process reaches its maximum rate despite higher light intensities. Beyond a certain light threshold, additional light doesn’t result in increased photosynthesis.

Light Duration: The length of exposure to light (photoperiod) influences plant flowering and vegetative growth. For example, short-day plants (like chrysanthemums) need less light to flower, while long-day plants (like spinach) require longer exposure to light.

Supplemental Lighting: In areas with insufficient natural light or during the winter months, supplemental lighting can be provided to maintain optimal photosynthesis. Artificial lights (like high-pressure sodium or metal halide lamps) can be used to extend the light period and provide the necessary spectrum for growth.

B) Temperature in Greenhouses

Temperature and Enzyme Activity: Plant growth and metabolic activities are governed by enzyme-controlled biochemical reactions. Temperature directly impacts these reactions; an increase in temperature can speed up metabolic processes up to an optimum point, after which it can slow down or even cause damage.

Heat Stress and Damage: High temperatures above the plant’s optimal range can lead to heat stress, causing cell membrane damage, protein denaturation, and wilting. Similarly, too low temperatures can lead to cold stress, reducing enzyme activity and inhibiting growth.

Day vs. Night Temperature:

• • Day Temperature: The day temperature in greenhouses is generally higher due to sunlight. This encourages photosynthesis and supports active growth. For some crops, like tomatoes and peppers, higher daytime temperatures (around 25°C to 30°C) are beneficial.
  • Night Temperature: Night temperatures are equally important as they allow plants to respire and recover from the daytime processes. Ideally, night temperatures are lower than day temperatures (3–5°C difference) to promote optimal growth and energy conservation.

C) Relative Humidity in Greenhouses

Evapotranspiration: Plants lose moisture through transpiration, which increases the humidity in the greenhouse. This process is vital as it helps with nutrient transport and temperature regulation. High humidity can promote excessive transpiration and leaf dehydration if not managed properly.

Humidity Management:

• • Too High: If humidity levels exceed 80%, it may lead to fungal diseases like mildew or mold, affecting plant health. A fogging system can help by releasing tiny water droplets to maintain humidity without excessive moisture buildup.
  • Too Low: In dry conditions, plants may suffer from dehydration, and transpiration rates may decrease. This can lead to stunted growth and poor nutrient uptake. In such cases, humidifiers or misting systems can be employed.

Control Methods:

• • Evaporative Cooling: This technique, using cooling pads, is particularly useful in maintaining optimal humidity while cooling the greenhouse.
  • Dehumidification: This involves using fans or ventilation systems to expel moist air from the greenhouse. In some systems, chemical dehumidifiers may be used to absorb excess moisture.

D) Ventilation in Greenhouses

Natural vs. Forced Ventilation:

• • Natural Ventilation: Small greenhouses (less than 6m wide) typically use natural ventilation by adjusting the openings and vents to allow the cooler, fresh air to enter and warm air to exit. The effectiveness depends on the wind direction, temperature difference inside and outside the greenhouse, and the design of the structure (e.g., roof vents, sidewalls).
  • Forced Ventilation: Larger greenhouses or those in regions with less natural airflow may use fans to force air circulation. Fans can create a negative pressure environment, expelling hot air and drawing cooler air in. This also helps to circulate CO2 and maintains ideal temperature and humidity conditions.

Ventilation and CO2 Enrichment: Proper ventilation ensures that carbon dioxide (CO2) levels stay within an optimal range. Adequate ventilation combined with CO2 enrichment strategies can further enhance plant productivity.

E) Carbon Dioxide in Greenhouses

CO2 and Photosynthesis:

• • CO2 Availability: Plants use carbon dioxide in the photosynthesis process to produce carbohydrates and release oxygen. CO2 concentrations in greenhouses are generally lower than the natural ambient levels due to the photosynthetic activity of plants, which absorbs it during daylight.
  • CO2 Enrichment: In many greenhouses, CO2 enrichment is introduced to maintain optimal levels (1000–1200 ppm). Elevated CO2 levels enhance photosynthesis and plant growth. However, this requires additional energy to maintain the temperature, so it is most effective in greenhouses with supplemental lighting or when heating is already in use.

Seasonal Considerations: During winter or in colder climates, ventilation is less frequent due to heating costs, and CO2 levels may drop below optimal levels. In such cases, CO2 generators or compressed CO2 systems can be used to maintain the desired concentration.

CO2 Concentration and Crop Responses: Some plants, particularly those in the C3 photosynthetic pathway (e.g., wheat, lettuce), respond strongly to CO2 enrichment. However, C4 plants (like maize) show less response as they already use CO2 more efficiently.

Additional Considerations for Greenhouse Productivity:

Watering and Irrigation Systems: Greenhouses often use advanced irrigation systems like drip or hydroponic systems to ensure that crops receive consistent moisture. These systems prevent over-watering and under-watering, both of which can damage plants. Automated systems with moisture sensors can help regulate water application.

Nutrient Management: In a controlled environment like a greenhouse, plants receive their nutrients from either soil (in containers or beds) or a hydroponic system. Nutrient availability is crucial for maintaining healthy plant growth and maximizing productivity. Fertilization schedules are adjusted according to plant needs at different stages of growth.

Pest and Disease Management: In a greenhouse, pests and diseases can spread quickly due to the confined space. Integrated pest management (IPM) is essential in controlling pests without harming the plants. Biological control agents, beneficial insects (like ladybugs or parasitic wasps), or natural insecticides are often used in conjunction with physical barriers or traps.

F) Soil and Media in Greenhouses

Soil Quality: Soil is a fundamental growing medium for greenhouse crops. The soil’s texture, structure, and composition significantly affect root growth and plant health. A balance of sand, silt, and clay, along with organic matter, is essential for providing adequate drainage and nutrients. Soil Fertility: Greenhouse soils must be supplemented regularly with organic or synthetic fertilizers to maintain optimal nutrient levels. Commonly used fertilizers include NPK (Nitrogen, Phosphorus, Potassium) blends, along with micronutrients like calcium, magnesium, and sulfur.

Soil pH and Nutrient Availability: The pH of the soil affects nutrient uptake by plants. Most greenhouse plants thrive in slightly acidic to neutral soil (pH 6.0–7.0). Soils with low pH (acidic) can lead to nutrient deficiencies, while alkaline soils can cause problems like iron chlorosis. Soil Amendments: Lime is often added to raise soil pH in acidic soils, while sulfur can lower pH in alkaline soils.

Soil-less Growing Media: For certain greenhouse systems, especially hydroponics, soil is replaced with alternative growing media. These media, such as perlite, vermiculite, peat moss, coconut coir, or hydroponic substrates, provide excellent drainage and support for root growth while being free of soil-borne diseases.  Hydroponics: In hydroponic systems, plants are grown in a water-based nutrient solution. This method allows for precise control of nutrients and water, optimizing growth, but requires careful management of pH and electrical conductivity (EC) of the nutrient solution.

G) Air Circulation and Humidity Control

Air Circulation Systems: Air circulation in greenhouses ensures uniform temperature and humidity levels, prevents the buildup of excess moisture (which could lead to mold growth), and facilitates gas exchange around plants. In larger greenhouses, fans or air circulators are used to create air movement that helps control the microclimate.

• • Horizontal and Vertical Air Movement:
    • Horizontal Air Movement: Involves the use of fans placed along the length of the greenhouse to create a uniform temperature and humidity distribution. This method helps in uniform plant growth and aids in cooling the environment.
    • Vertical Air Movement: Ceiling-mounted fans that draw air upwards, then distribute it evenly. This method is particularly useful in minimizing temperature gradients in tall greenhouses.

Dehumidification and Drying Systems: High humidity can lead to fungal infections, but in dry climates, it can be difficult to maintain the necessary humidity levels. Specialized equipment like dehumidifiers or misting systems can either remove excess moisture or add moisture to the air as needed. Misting and Fogging Systems: These systems are often used in combination with cooling pads to maintain humidity levels between 60% and 80%, preventing moisture stress while promoting healthy transpiration.

H) Greenhouse Structure and Design

Greenhouse Shapes and Structures:

• • The shape of the greenhouse plays a role in light distribution, temperature control, and ventilation. Common designs include:
    • Gable Roof Greenhouses: Popular for their simple structure, offering good light penetration and easy ventilation.
    • Venlo Type Greenhouses: Characterized by a multi-span structure, typically used for large-scale commercial production due to their ability to allow more light and space for air circulation.
    • Dome and Geodesic Designs: These are more energy-efficient, with fewer structural materials, and are designed to maximize sunlight exposure and reduce heating costs.

Materials for Greenhouse Construction:

• • Glazing Materials: The material used to cover the greenhouse frame plays a critical role in the amount of light transmitted to the plants.
    • Glass: High light transmission but costly and fragile.
    • Polycarbonate Sheets: Lightweight, offer thermal insulation, and have a lower cost than glass.
    • Polyethylene Film: Common in commercial greenhouses due to its affordability and excellent light transmission, although it needs to be replaced periodically.

I) Energy Management in Greenhouses

Heating Systems:

• • Greenhouses require heating during the colder months to maintain optimal temperatures for plant growth. Different heating options include:
    • Hot Water Systems: These systems use boilers to heat water, which is then pumped through pipes to warm the greenhouse.
    • Forced Air Heaters: These blow warm air into the greenhouse. This is more efficient than hot water systems and is widely used in smaller greenhouses.
    • Geothermal Heating: This eco-friendly method uses the earth’s natural heat for sustainable greenhouse heating.

Energy-Efficient Techniques:

• • Thermal Blankets or Shade Curtains: These are used during the night to retain heat within the greenhouse. During the day, they can also reduce heat buildup.
  • Solar Energy: Solar panels can be used to power fans, supplemental lighting, and water pumps, reducing reliance on external energy sources and cutting costs in the long run.

Energy Conservation:

• • Double Glazing or Insulated Walls: To reduce heat loss, some greenhouses are built with insulated materials or double layers of glazing. These designs help maintain the internal temperature, especially in climates with large day-night temperature differences.

J) Water Management in Greenhouses

Water Quality and pH: In hydroponic systems, water quality is of utmost importance. The water should be clean, free from salts, chlorine, or any toxic substances that could harm plant roots. Additionally, the pH of the water must be monitored to ensure that it falls within the optimal range for nutrient absorption (usually around 5.5–6.5 for most crops).

Irrigation Systems:

• • Drip Irrigation: In this system, water is delivered directly to the roots through a network of tubes and emitters. This method reduces water waste and ensures that plants receive water at their roots rather than over the foliage, which can lead to fungal diseases.
  • Flood-and-Drain (Ebb and Flow) Systems: Common in hydroponic systems, this method involves flooding the plant beds with water and then draining it. It is particularly useful for plants with higher water requirements.

Rainwater Harvesting: Collecting rainwater in greenhouses can be an efficient way to manage water usage, especially in regions where water availability is scarce or expensive. Rainwater is typically cleaner than tap water and can be used for irrigation, reducing costs and promoting sustainability.

K) Pest and Disease Management in Greenhouses

Biological Pest Control: Greenhouses often encourage the use of beneficial insects, such as ladybugs (for aphid control), or predatory mites (for spider mite control), as part of integrated pest management (IPM) programs. These organisms help to naturally regulate pest populations without the need for harmful chemical pesticides.

Physical Barriers:

• • Insect Netting: Fine mesh screens or netting can be installed over ventilation openings and doorways to prevent pests from entering the greenhouse.
  • Sticky Traps: Brightly colored traps coated with a sticky substance can be used to capture flying insects like aphids and whiteflies.

Disease Prevention: Regular monitoring for fungal and bacterial diseases is crucial in a greenhouse. Fungicides and bactericides, while effective, are often used sparingly to avoid resistance buildup. Proper sanitation, such as cleaning tools, using disease-free seeds, and ensuring good air circulation, is the first line of defense.

Regular Inspections: Routine inspections of plants, soil, and equipment should be conducted to detect early signs of pests or diseases. Early detection ensures that any infestation is controlled before it spreads throughout the greenhouse.