ABSTRACT

Sucrose is the major photosynthate transported through the phloem sieve tube from source to sink organs in most plants. It is now agreed by many plant physiologists that in sieve tube sucrose is moved in bulk with water and other solutes by hydrostatic pressure difference between source and sink side based on the concentration gradients of solutes. Because of extremely higher levels of sucrose than other solutes in sieve tube, concentration gradient of sucrose in sieve tube may determine the bulk flow rate of various solutes, including sucrose. By using a reliable physical equation, we calculated flow rates of sucrose solutions with various linear sucrose concentration gradients in a simplified cylindrical sieve tube. Results suggest that it is not true that in the plant phloem sieve tube the more sucrose concentration gradient becomes steep, the more bulk flow rate of various solutes becomes large.