Resilient tissues that maintain turgor at low water potential can confer drought tolerance in plants, and variation in turgor maintenance may help explain performance and distribution along water availability gradients. Ecological patterns and evolutionary trends in turgor maintenance are not yet well understood in wild, herbaceous plants. Using pressure-chambers, osmometry, and porometry to characterize plants in nature and in greenhouse experiments, we investigated tissue water relations in Clarkia, a genus of winter annuals distributed mostly in California.
Results/Conclusions
Among four co-occurring species, mean midday shoot water potential varied over two-fold, from -0.7 to -1.9 MPa, considerable interspecific variation in water status at peak flowering. In field sampling, these species did not differ on in water potential at turgor loss, on average, and there was wide within-species variation. Dry-down experiments revealed that these taxa and four others can adjust leaf osmotic potential at full turgor by up to 1 MPa, as plants mature and soil water declines. Thus, turgor loss points in the field would be expected to vary substantially with life stage and soil water. Accounting for osmotic adjustment, species differences appeared in turgor and stomatal conductance at low water potential. For example, of all species examined, Clarkia speciosa, which occupies micro-habitats prone to water stress, had the shallowest declines in conductance with declines in water potential. Clarkia xantiana subspecies that differ in distribution (relatively mesic v. relatively xeric, on average) and maturation time (late flowering v. early flowering) also differed in turgor maintenance, with the late-maturing subspecies being superior. Early maturation (drought escape) may relax selection for and/or trade off with turgor maintenance (drought tolerance).