Leaf Vapour Pressure Deficit

The air-based deficit makes one assumption that is usually false: that the leaf is at air temperature. A transpiring leaf under lights is being cooled by evaporation from its own surface and typically runs 1–3 °C below the surrounding air. Since saturation pressure climbs steeply with temperature, a couple of degrees moves the true deficit substantially.

The correct form compares the saturated air inside the leaf, at leaf temperature, against the actual vapour pressure of the room. At 25 °C air, 60% RH and a leaf at 23 °C, the air-based figure is 1.267 kPa while the leaf-based figure is 0.909 kPa — nearly 30% lower. A grower steering to 1.2 kPa on an air-based controller is really running the canopy at about 0.85, which is a different regime.

Measuring it needs an infrared thermometer aimed at the canopy, which reads surface temperature directly. Two cautions on that reading: aim at leaves rather than gaps, since the instrument averages whatever is in its cone and will happily report the temperature of the floor; and take readings from several places, because leaves at the canopy edge and leaves directly under a fixture differ by more than the effect being measured.

The case that makes this worth the trouble is night-time. When the lights go off, leaves cool faster than the air because they radiate to the surfaces around them, and a leaf below the room's dew point has water condensing on it. The air-based calculation shows a comfortable positive deficit; the leaf-based one goes negative, correctly predicting wet leaves. That is the condition botrytis and powdery mildew wait for, and it is invisible to any humidity sensor in the room.