LATERAL ROOT DEVELOPMENT AND HYDRAULIC CONDUCTANCE IN 4 POPULATIONS OF GUTIERREZIA-SAROTHRAE

We hypothesize that drought-avoidance in Gutierrezia sarothrae populat ions is characterized by well-developed lateral roots. Root growth and hydraulic conductance were studied in four populations of Gutierrezia sarothrae. Seedlings from a Malta, Idaho (ID) seed source had four ti mes higher root/shoot ratio (P < 0.05), but only 17% hydraulic conduct ance of those from Tahoka, Texas (TA) seed source. Consequently, trans piration surface area of the ID seedlings was 17% that of the TA seedl ings. There was no difference in total root length between the seedlin gs, but hydraulic conductance per unit root length was 3.8 fold greate r (P < 0.05) in the TA seedlings, which was accompanied by a lower spe cific lateral root length (P < 0.05), thus greater lateral root diamet ers. A parallel experiment with adult plants from New Mexico (NM) and Plains, Texas (PL) and the above two populations showed that whole-pla nt hydraulic conductance was ordered as NM > TA=PL > LD. The NM and TA populations had lower specific lateral root length (P < 0.05), thus l arger-diameter laterals, than the ID population. The NM population had similar transpiration surface area and total root length as that of t he TA population, but a greater lateral root biomass (P < 0.05) than t he latter. Because of higher hydraulic conductance in the NM populatio n, its xylem water potential declined more slowly than that of the Tex as and Idaho populations as soil moisture deficit developed. Stem hydr aulic conductance was positively correlated with xylem water potential . Therefore, the NM population was able to maintain a higher stem cond uctance during drought, which was associated with its higher whole-pla nt hydraulic conductance and greater carbon partitioning in the stems. The drought avoidance mechanism in the NM plants was characterized by an efficient water transport system due to greater lateral root bioma ss and more stems per unit leaf area. The higher root/shoot ratio in d esert populations (NM and LD) as compared with that of the semiarid ra ngeland populations (TA and FL) does not ensure more effective water a cquisition. Rather, root morphological modifications such as accelerat ed secondary thickening of lateral roots may play an important role in enhancing water-acquisition capability of the root systems of Gutierr ezia sarothrae.