Water relations and root growth of two populations of Gutierrezia sarothrae

Abstract

We hypothesise that genotypic differences in transpiration and root growth in the southern and northern populations of Gutierrezia sarothrae are driven by growing season vapour pressure deficit (VPD) and that ecotypic differentiations are linked to corresponding variations in tissue 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 were grown under the shade. However, significant population differences were observed in the plants grown in the open where VPD was substantially higher. Transpiration in the TX population increased twice as rapidly as the ID population in response to rising potential evapotranspiration (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 relative 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 (RWC 0) and apoplastic water fraction ( W a) were significantly lower in the NM population than in the ID population in response to soil water deficit, suggesting a higher turgor maintenance ability in the NM population. RWC 0 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 (Π 100), indicating significance of cell wall 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 water fraction in the ID population reflected conservative water use at the leaf and tissue levels, which was consistent with the water use pattern at the canopy level.