In controlled protected cultivation systems, air humidity is more than a simple auxiliary index for temperature and humidity matching. It comprehensively impacts the entire process of crop photosynthesis and reproductive development by altering transpiration intensity, stomatal conductance, nutrient translocation efficiency and the risk of field diseases. Vapor Pressure Deficit (VPD), the core quantitative indicator for humidity assessment, overcomes the rough judgment based solely on Relative Humidity (RH). This paper systematically illustrates the coupling relationship between humidity, VPD and crop physiology. It sorts out various production problems caused by humidity imbalance from five dimensions: transpiration driving, photosynthetic limitation, mineral nutrient transport, disease occurrence and multi-factor coordinated regulation. Meanwhile, standardized humidity control plans by growth stage and season are provided, offering professional theoretical support for refined climate management in protected agriculture.
Relative humidity only represents the proportion of current air relative to saturation. The air’s water-holding capacity varies drastically at the same RH under different temperatures. Vapor Pressure Deficit (VPD), the difference between saturated vapor pressure and actual vapor pressure, is the precise indicator to measure crop transpiration potential.
VPD directly determines the rate of water loss from leaves. When VPD is low, the air is nearly saturated, forming a static high-humidity boundary layer on leaf surfaces that hinders outward diffusion of water vapor and drastically reduces transpiration. When VPD is excessively high, the air becomes extremely dry. Crops lose water faster than roots can absorb it, triggering protective stomatal closure and directly blocking the intake of CO₂.
Fruits and vegetables have optimal VPD ranges:
Seedling stage: 0.4–0.7 kPa. Young seedlings are fragile with poor drought tolerance and prefer moderately humid conditions.
Flowering and fruit-setting stage: 0.5–1.1 kPa. This range maintains stable transpiration flow to ensure nutrient delivery to fruits.
Long-term VPD below 0.4 kPa or above 1.2 kPa creates stressful conditions. Water vapor inside greenhouses mainly comes from crop transpiration. In airtight environments with no ventilation, water vapor accumulates continuously and VPD keeps dropping — the primary cause of low temperature and high humidity in winter and spring. Circulation fans disrupt static air layers above leaves, reduce VPD differences across the canopy and eliminate localized high-humidity dead zones.
When air is dry and VPD exceeds the threshold, crop transpiration rises sharply and triggers multiple adverse physiological responses. First, stomata constrict actively, reducing stomatal conductance and increasing resistance for CO₂ to enter mesophyll cells. Net photosynthetic rate declines continuously and dry matter accumulation decreases. Combined stress from strong light plus high temperature together with high VPD can cause rapid leaf wilting. Prolonged drought stress accelerates leaf senescence and plant aging.
Second, nutrient transport is impaired. Hard-to-mobilize mineral elements such as calcium and boron rely entirely on transpiration stream to move from roots to fruits. Under persistently high VPD with excessive transpiration, plants prioritize water supply to leaves instead of fruits. Insufficient calcium supply to fruits greatly increases the incidence of physiological disorders including blossom-end rot in tomatoes, blossom rot in sweet peppers and apical necrosis in cucumbers.
Third, plant morphology changes. Under long-term dry conditions, leaf cuticles thicken and leaf area shrinks, reducing light interception and utilization of the crop canopy. High VPD during seedling stage easily causes stunted seedlings, delayed flower bud differentiation and fewer blooms. In production, high-pressure fine mist humidification is commonly applied to raise atmospheric water vapor, lower VPD and relieve drought stress.
During low-temperature periods in winter and spring, insufficient heating and limited ventilation often lead to RH＞88% and VPD＜0.4kPa. Such high-humidity environments bring prominent negative impacts on crops.
Reduced photosynthetic efficiency: Water vapor in the leaf boundary layer approaches saturation, minimizing the vapor pressure gradient. Even when stomata are fully open, water vapor cannot escape smoothly, indirectly restraining CO₂ diffusion and lowering assimilate production. Trial data shows continuous high humidity reduces daily dry matter accumulation of tomatoes by 15%–22%, accompanied by lower fruit set rate.
Fungal disease outbreaks from persistent leaf wetness: When local temperature drops below the dew point, water vapor condenses into liquid water on leaves and petals. Spores of pathogens such as Botrytis, Peronospora and Phytophthora require over 8 hours of wetness to germinate and infect. Long-term high humidity can multiply disease incidence dozens of times and raise pesticide costs significantly.
Excessive vegetative growth and deteriorated quality: Low VPD suppresses transpiration, resulting in high cellular water content. Plants develop elongated internodes and slender petioles, showing shade-avoidance etiolation. Vegetative growth competes for assimilates, raising flower and fruit drop rate. Fruits accumulate less dry matter, sugar and flavor compounds, downgrading commercial quality.
Elevated risk of root hypoxia: High humidity is usually accompanied by persistently moist topsoil. Soil pores are filled with water, oxygen content drops and root respiration is inhibited, increasing risks of root rot.
Humidity cannot be regulated independently; it has strong interactions with other environmental factors. Temperature determines saturated vapor pressure. With fixed water vapor content, rising temperature greatly increases air water-holding capacity and lifts VPD. Therefore, moderate heating, rather than massive heat-loss ventilation, is the optimal solution for low-temperature and high-humidity conditions in winter. In summer high-temperature periods, simple ventilation tends to push VPD too high, so spray humidification is required to balance water vapor.
Light intensity indirectly affects transpiration: Under strong light, photosynthesis and transpiration rise simultaneously, releasing more water vapor. Without ventilation and dehumidification, high humidity builds up rapidly. On overcast days with weak light, crop transpiration is weak and little vapor is produced by plants. High humidity risks mainly originate from soil evaporation and nighttime condensation.
CO₂ enrichment and humidity are mutually restrictive. Closed greenhouses with CO₂ fertilization have low air exchange rates, so vapor easily accumulates and creates high humidity. Circulation fans are needed to homogenize vapor and CO₂ concentrations and prevent disease risks from combined high humidity and elevated carbon levels. If VPD is too high and stomata close, increased CO₂ concentration will barely improve photosynthetic yield.
Seedling stage: Target VPD 0.4–0.7 kPa. Moderate humidity protects tender seedling leaves while avoiding persistent leaf wetness. Short and mild ventilation for dehumidification in daytime; thermal screens at night reduce condensation dripping from the greenhouse roof.
Vegetative growth stage: Target VPD 0.6–0.9 kPa. A moderate vapor pressure difference promotes root development, suppresses excessive vegetative growth and improves leaf thickness and photosynthetic capacity.
Flowering and fruit-setting stage: Core VPD range 0.7–1.1 kPa. Stable transpiration stream guarantees calcium translocation, balancing photosynthesis and disease control. Ventilate properly on sunny days; adopt “heating + intermittent ventilation” on cold sunny days.
Late harvest stage: Appropriately reduce humidity to boost fruit dry matter accumulation and extend shelf life.
Ventilation dehumidification: The most economical and universal method. Excess vapor is removed via air exchange between inside and outside. In winter, opening windows must be limited to avoid heat loss. Insect screens reduce ventilation efficiency; wider vents are required to compensate airflow loss.
Heating for VPD adjustment: Winter-specific solution. It raises saturated vapor pressure of air, increases VPD without heavy ventilation, achieving heat preservation and dehumidification at the same time.
High-pressure spray humidification: Used mainly in hot summer when VPD tends to exceed limits. Evaporating fine mist increases air vapor content and cools the greenhouse to relieve leaf water deficit stress.
Circulation fans: Suitable year-round. They break high-humidity boundary layers on crop canopies, eliminate vertical humidity stratification, homogenize VPD across the whole greenhouse and reduce local condensation.
Condensation dehumidification units: Designed for semi-closed and artificial-light cultivation facilities. Water vapor condenses on cold surfaces; latent heat in air can be recovered. It enables precise year-round humidity control with lower energy consumption.
Air humidity should not be judged merely by relative humidity. VPD serves as the core indicator guiding production. Low VPD (high humidity) triggers diseases, excessive vegetative growth and suppressed photosynthesis; high VPD (dry air) causes stomatal closure, insufficient delivery of calcium and physiological defects. Greenhouse humidity management requires coordinated regulation together with temperature, light and CO₂. Set differentiated target VPD values according to seedling, vegetative and reproductive stages of crops. Dynamically balance water budget by combining ventilation, heating, air circulation and humidification equipment to achieve low disease incidence, high quality and high yield for greenhouse crops.
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