| 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', 'N92-SH-447' and 'PI 471938') and the commercial cultivar 'Young'. These genotypes, which were selected on the basis of rooting pattern and shoot desiccation tolerance under field conditions, were grown in flowing hydroponic culture to eliminate physical or chemical constraints on root development and functioning. Statistically significant differences among genotypes for root hydraulic conductance, as measured by the slope of the linear regression between volumetric flow rate and applied pressure, were not related solely to differences in mass of roots.; Potential anatomical barriers to radial and axial water flow through roots were examined for their contribution to differences in water transport among the genotypes. Differences in water flow among genotypes were not statistically proportional to root surface areas per plant (e.g. hydraulic conductivities were different); thus the exodermal Casparian band is not the definitive barrier to water flow. Since relative sizes of the cortex and the stele were statistically different among the genotypes, either the volume of the cortex or the endodermal Casparian band could be a primary barrier to water flow. The genotypic differences in conductance were not significantly related to cortical volume. The correlation between root hydraulic conductance and surface area of the stele, however, was significant and explained 35% of the variation in conductance among genotypes. The endodermis does appear to be an important barrier to water flow across genotypes although it does not account for the majority of the differences in water movement. Finally, while genotypic differences in diameters of xylem elements contributed to an order of magnitude range in capacities for axial flows, differences in capacity for axial flow were not significantly related to differences in conductance. Thus, the deviation in hydraulic conductance among the six genotypes appears to result from the interaction among the anatomically defined barriers with no signal barrier as paramount. |
