Influence of Provenance on Acute Drought Tolerance of Four Native Texas Groundcover Species

From previous selection efforts, four promising native groundcovers were identified; Erigeron procumbens (Houst. ex Mill.) G.L. Nesom., Mimosa strigillosa Torr. & A. Gray, Oenothera drummondii Hook. and Phyla nodiflora (L.) Greene.  Plants were collected from an area ranging from south Texas to northwestern Louisiana and were selected on the basis of ornamental potential and observed stress tolerances.  Trachelospermum asiaticum (Siebold & Zucc.) Nakai and Sphagneticola trilobata (L.) Pruski were included as comparison species from the nursery trade.  Studies were designed to estimate the overall drought tolerance of each species.  Three genotypes were chosen to represent the most southwestern, central and northeastern provenances collected.  Plants were grown in a peat moss and pine bark-based substrate (MetroMix 900).  Acute drought was imposed by the total cessation of irrigation once plants were established in 2.51L containers.  A completely randomized design (CRD) was installed with five replicates of each of the four native species, three provenances per native species, and a single commercial genotype of the two comparison groundcovers. Volumetric water content (VWC) measurements and xylem water potentials were measured throughout the experiment.  A provenance was harvested when two of the five representatives exhibited shoot damage beyond a tolerable level to market the plants.  Total time until harvest ranged from 9d for S. trilobata to 72d for T. asiaticum while the M. strigillosa provenances remained longer than the other native materials (a mean of 29d).  Significant interactions (P ≤ 0.05) among date of measurement and provenances were observed for mid-day water potentials and the recovery from mid-day to pre-dawn measurements.  T. asiaticum and S. trilobata exhibited water potentials no lower than -0.3 MPa while potentials as negative as -3.45 MPa were observed in a provenance of P. nodiflora (TX32) at harvest.  The VWC at harvest was significantly different among species and provenances.  All E. procumbens and P. nodiflora provenances were harvested with at least 10% VWC remaining while all O. drummondii were harvested with less than 7% VWC remaining.  Mimosa strigillosa provenances from the central and southwestern regions of the collection area were harvested with 4.76 and 4.5% VWC remaining in the substrate, respectively, while substrates supporting the provenance from northwestern Louisiana held 10.68% VWC suggesting differential acute water stress tolerances among these taxa.  Taxa such as M. strigillosa appeared to survive by tolerating water deficits in the substrate, whereas T. asiaticum appeared to survive by limiting water losses thus conserving substrate moisture.