WATER-RELATIONS AND ROOT-GROWTH OF 2 POPULATIONS OF GUTIERREZIA-SAROTHRAE

We hypothesise that genotypic differences in transpiration and root gr owth in the southern and northern populations of Gutierrezia sarothrae are driven by growing season vapour pressure deficit (VPD) and that e cotypic differentiations are linked to corresponding variations in tis sue and leaf water relations. Seedlings from an Idaho (ID) and a Texas (TX) seed source were grown either in an open nursery (full sunlight) or under shade. There were no population differences in transpiration , root growth and tissue water relation parameters when the plants wer e grown under the shade. However, significant population differences w ere observed in the plants grown in the open where VPD was substantial ly higher. Transpiration in the TX population increased twice as rapid ly as the ID population in response to rising potential evapotranspira tion (PET). In addition, the TX plants grew longer and larger lateral roots than the ID plants when both populations were grown in the open. Moreover, the TX plants had more elastic cell walls (lower bulk cell elastic modulus, E) and were able to maintain leaf turgor at lower rel ative water content than the ID plants when grown in the open. Similar changes in tissue water relation parameters were observed in the New Mexico (NM) and the ID seedlings subjected to soil water deficit. Cell elastic modulus (E), relative water content at zero turgor (RWC0) and apoplastic water fraction (W-a) were significantly lower in the NM po pulation than in the ID population in response to soil water deficit, suggesting a higher turgor maintenance ability in the NM population. R WC0 was highly correlated with E in the NM (r = 0.92, P < 0.0001) and the TX (r = 0.91, P < 0.05) plants, but poorly correlated with osmotic potential at full turgor (Pi(100)), indicating significance of cell w all elasticity in turgor maintenance. Rigid cell walls and poor turgor maintenance ability in the ID plants subjected to drought may lead to restricted growth, which reflected hydraulic limitation of plant root systems. Higher water use efficiency (WUE) and higher apoplastic wate r fraction in the ID population reflected conservative water use at th e leaf and tissue levels, which was consistent with the water use patt ern at the canopy level. (C) 1998 Elsevier Science B.V. All rights res erved.