Lateral root development and hydraulic conductance in four populations of Gutierrezia sarothrae

Abstract

We hypothesize that drought-avoidance in Gutierrezia sarothrae populations 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 times higher root/shoot ratio ( P<0.05), but only 17% hydraulic conductance of those from Tahoka, Texas (TA) seed source. Consequently, transpiration surface area of the ID seedlings was 17% that of the TA seedlings. There was no difference in total root length between the seedlings, but hydraulic conductance per unit root length was 3.8 fold greater ( P<0.05) in the TA seedlings, which was accompanied by a lower specific lateral root length ( P<0.05), thus greater lateral root diameters. A parallel experiment with adult plants from New Mexico (NM) and Plains, Texas (PL) and the above two populations showed that whole-plant hydraulic conductance was ordered as NM > TA=PL > ID. The NM and TA populations had lower specific lateral root length ( P<0.05), thus larger-diameter laterals, than the ID population. The NM population had similar transpiration surface area and total root length as that of the TA population, but a greater lateral root biomass ( P<0.05) than the latter. Because of higher hydraulic conductance in the NM population, its xylem water potential declined more slowly than that of the Texas and Idaho populations as soil moisture deficit developed. Stem hydraulic conductance was positively correlated with xylem water potential. Therefore, the NM population was able to maintain a higher stem conductance during drought, which was associated with its higher whole-plant 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 biomass and more stems per unit leaf area. The higher root/shoot ratio in desert populations (NM and ID) as compared with that of the semiarid rangeland populations (TA and PL) does not ensure more effective water acquisition. Rather, root morphological modifications such as accelerated secondary thickening of lateral roots may play an important role in enhancing water-acquisition capability of the root systems of Gutierrezia sarothrae.