Vapour Pressure Deficit (Air)

Relative humidity is a poor control variable because it means something different at every temperature. Air at 70% RH and 20 °C and air at 70% RH and 30 °C have very different drying power, and the plant responds to the drying power. Vapour pressure deficit states that directly: how much more water vapour the air could hold before saturating.

The calculation is saturation vapour pressure at the air temperature, multiplied by the fraction of saturation still unfilled. Saturation pressure comes from the Tetens equation, $e_s = 0.6108 \exp[17.27T/(T+237.3)]$ in kPa, which is accurate to better than a tenth of a percent across any range a growing space sees. At 25 °C that gives 3.168 kPa, so at 60% RH the deficit is $3.168 \times 0.40 = 1.267$ kPa.

The working bands are well agreed. Propagation and young plants want 0.4–0.8 kPa, since cuttings without roots cannot replace water they lose. Vegetative growth sits comfortably at 0.8–1.2. Flowering and fruiting run 1.2–1.6. Above about 1.6 kPa most crops begin closing stomata defensively, which stops carbon dioxide entering and halts photosynthesis even under full light — the plant is protecting itself, and the grower sees a mysterious afternoon slump. Below 0.4 kPa transpiration nearly stops, which matters more than it sounds: calcium moves through the plant only in the transpiration stream, so persistently low VPD produces blossom end rot and tip burn in tissue that never had a water deficit at all.