[hnuser123456]

They're just saying that hotter air can hold more water, so if the temperature rises, you need to add water to the air, to keep a constant water vapor pressure, and presumably plants have a preferred vapor pressure more than a preferred humidity %, as the same humidity % at different temperatures has a different vapor pressure, is how I read it.

[BenjiWiebe]

Doesn't humidity already take this into account? It's measured in relative humidity which is the percent saturation of the air. So RH goes down as temperature goes up, unless there's more evaporation happening.

[erutuon]

Apparently it doesn't. From what I read, relative humidity is maximum possible humidity divided by actual humidity (probably not the correct terms), whereas vapor pressure deficit is maximum possible minus actual. Fraction versus difference. So the same vapor pressure deficit could correspond to 80% relative humidity at one temperature, and 85% relative humidity at a higher temperature.