Invisible plant stress Part II – How VPD helps detect early stress

In the greenhouse, everything may look green and healthy. However, sometimes there’s a hidden problem that’s barely visible to the naked eye: plant stress. Stress that doesn’t show immediately but still significantly impacts the growth, health, and yield of your crop.

VPD (Vapour Pressure Deficit) indicates how much additional water vapour the air can absorb. It determines the rate of evaporation from the leaf, which influences the speed of:

• Leaf cooling

• Water transport from the roots

• Nutrient uptake (e.g., calcium)

• CO₂ absorption through the stomata (essential for photosynthesis)

For most crops, the ideal VPD range is between 0.8 and 1.2 kPa. This range allows the plant to evaporate sufficiently to absorb nutrients and CO₂ efficiently. Stomata remain open, enabling optimal photosynthesis.

Too high VPD (> 2.0 kPa):

• The plant evaporates too quickly → stress

• Stomata close to prevent dehydration

• Result: reduced evaporation, rising leaf temperature, lower CO₂ uptake, and decreased photosynthesis

Too low VPD (< 0.2 kPa):

• The air is saturated with moisture → minimal plant evaporation

• Result: poor calcium transport and increased risk of physiological disorders

During a summer period, a grower noticed that growth in one section of the greenhouse lagged, despite no visible disease or abnormalities. The plant color appeared normal, and leaves weren’t drooping, yet production was falling behind. The question arose: could invisible stress be the cause?

Data Used

In the two graphs above, three values are shown:

• Photosynthesis Efficiency – indicates how effectively the plant converts light into energy

• PAR Data (at leaf level) – shows how much light the plant can use for photosynthesis

• VPD – as explained above

What Do the Graphs Show?

On June 22 (1) and June 25 (2), the VPD graph shows an increase in vapour pressure deficit. In the graph combining photosynthesis efficiency and PAR values, we observe a decline in photosynthesis efficiency on June 22, while the PAR value increases.

A higher PAR value means more light is hitting the leaf, which raises the leaf temperature. This increase stimulates the plant’s evaporation. However, as the plant evaporates more, it begins to lose water rapidly, causing the stomata to close. When this happens, CO₂ uptake drops, which in turn lowers photosynthesis efficiency.

In summary: Evaporative stress can lead to a decline in photosynthesis efficiency.