Vapour-pressure deficit the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa. In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation. To compute the VPD, we need the ambient (greenhouse) air
Amount of substance the molar mass. Molar quantities are sometimes denoted by a subscript Latin "m" in the symbol, e.g. "C", molar heat capacity at constant pressure: the subscript may be omitted if there is no risk of ambiguity, as is often the case in pure chemistry. The main derived quantity in which amount of substance enters into the numerator is amount of substance concentration, "c". This name is often abbreviated to "amount concentration", except in clinical chemistry where "substance concentration" is the preferred term to avoid ambiguity with mass concentration. The term "molar concentration" is incorrect, but commonly used. The alchemists, and