Drought relations of shrub species: assessment of the mechanisms of drought resistance.

Relatively static factors such as depth of rooting and cuticular conductance and relatively dynamic factors such as stomatol control and changes in the components of water potential were used to assess the drought resistance characteristics of six deciduous shrub species. Predawn water potential during a prolonged drought averaged-2.13 and-3.51 MPa in species known to have deep and shallow patterns of rooting, respectively. It is thus surprising that the osmotic potential at the turgor loss point averaged only-3.01 MPa in the shallow rooted group. The water potential at which irreversible cell damage occurred was the same in both groups (-4.9 MPa), and minimum values observed in the field never dropped below-4.0 MPa. There was, however, a pronounced difference between the two groups with regard to stomatal behavior. This allowed us to characterize the deep-rooted species as avoiders of stress which would cause prolonged stomatal closure whereas the shallow-rooted species had to tolerate prolonged periods of closed stomata.