Genotypic differences in root anatomy affecting water movement through roots of soybean

The ability of root systems to absorb water was determined as the root hydraulic conductance for five exotic genotypes (PI 416937, H2L16, N95-SH-259, PI 407859-2, and PI 471938) and the commercial cultivar Young of soybean (Glycine max [L.] Merrill). The genotypes were grown for 28 d in flowing hydroponic culture to minimize possible variations in physical or chemical constraints on root development and functioning. Root hydraulic conductance was determined in response to applied hydrostatic pressure to the solution inside a pressure vessel to induce solution flow through the root system to the nonpressurized cut-stem surface. Almost twofold differences in hydraulic conductance of from 0.43 to 0.79 x 10(-7) m(3) s(-1) MPa-1 among the six genotypes were statistically significant. External root surface area and surface area of the stele were determined as estimates of the dimensions of exodermal and endodermal Casparian bands as barriers to radial movement of water. Volume of the cortex was considered to be proportional to the possible resistance of the symplastic pathway through the cortical cells themselves. Abundance of large metaxylem elements with radii 20 mum or greater was determined for comparison of relative axial conductance through root sections. Root hydraulic conductivity based on external surface area, which ranged from 2.20 to 3.82 x 10(-7) m s(-1) MPa-1, did not account for a statistically significant portion of the genotypic difference in water conductance. The relationship between root hydraulic conductance and surface area of the stele, however, accounted for 35% of the genotypic variation in conductance. The endodermis thus appears to be a limiting barrier to water conductance with dimensions relative to the exodermis that vary among the genotypes. Although statistically significant differences occurred among the genotypes for cortex volume and relative axial conductance, these differences were not correlated with differences in conductance. The diversity among the six genotypes for root anatomical traits that apparently influence water movement through the root system under well-watered conditions is sufficiently large to justify exploration of the relationship between root hydraulic conductance and performance of soybean under water-limiting conditions.