Varieties A variety may be pureline or a mixture of purelines. Natural selection and domestication over thousands of years has resulted in the development of varieties, cultured under different seasonal, soil and climatic conditions. Vapour Pressure Deficit Vapour Pressure Deficit (VPD) is the difference between the saturation vapour pressure and actual vapour pressure. It is expressed in pressure terms as bars or pascals. Vegetative apomixes : A type of apomixes where flowers are replaced in the plant by bulbis or buds which fall to the ground like seeds.