Developmental and Functional Morphology of Crop Root System in Relation to Hydraulic Characteristic

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This study aimed to examine several developmental and histological root traits that are closely related with water uptake and conductance of a seminal root system of sorghum seedling. In addition, the site of water entry and water uptake rate were determined by using sulphorhodamine G(SR) as an apoplastic tracer. It was then attempted to establish a hydraulic architecture of the root system based on these information.Results showed that heterorhizy exists in lateral roots traits examined such as branching, elongation and expansion ability, Casparian band, exodermis and endodermis development, xylem dimension and vascular connection to seminal root axis. The lateral roots accounted for approximately two third of both root surface area and total amount of SR accumulated in the root system. The accumulation of SR per unit root surface was different between root types and different portions along the seminal root axis. SR accumulated most on 0-15 cm portion from the tip. Among lateral roots, those emerged 15 cm away from the tip of seminal root axes accumulated SR most densely. These results indicate that this part is most active in water uptake in the root system. However, late metaxylem vessel (LMX) in the seminal root axis was not yet opened at that portion, indicating high axial hydraulic resistance. Furthermore, the accumulation drastically dropped basipetally from that portion on, whereas no clear difference in Casparian strip development and exo/endodermis maturity was detected between 15 cm and 20 cm portion from the tip.These facts suggest that the histological features that were determined in this study may not be the sole major factor to determine the hydraulic conductivity but some other traits like the permeability of cell membrane that is closely related with age and thus portion in a root system may also play important roles.