Abstract
There are many resistances to radial water flow in plant root. Water may move from root surface to xylem vessels via apoplastic pathway and cell-to-cell pathway. By using root pressure probe, some measurements suggest that water flow via apoplastic pathway is restricted at hydrophobic tissues (endodermis and exodermis) where Casparian bands and suberin lamellae are developed. However, the water flow might not be completely blocked at these tissues. The hydraulic conductivity in apoplastic pathway of rice root is lower than that of corn root. This may be one reason for lower water uptake rate by rice root system. Although rice root lacks many cortical cells because of development of aerenchyma, the influence of aerenchyma on water flow via cell-to-cell pathway might be very small. Solute permeabilities and reflection coefficients also can be measured by using root pressure probe. Rice root has lower solute permeabilities and reflection coefficients than corn root. This suggests that rice root may not discriminate solute from water strictly but permieabilities of water and solute are restricted simultaneously.
