Specific Mass Transfer and Sink-Controlled Phloem Translocation in Castor Bean

Abstract

The specific mass transfer (SMT) into the fruits of Ricinus communis was measured in three different systems: (1) the intact truss, (2) fruit-thinned truss (all but one fruit removed), and (3) exuding peduncle. We found that the SMT in an intact system accords well with values reported in previous studies. But in the single-fruit and exuding systems the values were 2 and 18 times higher than the intact system, respectively. The inability of a single fruit to achieve a much higher SMT value shows that the assimilate unloading is sink-controlled. The high SMT value of the exuding system demonstrates the capacity of the sieve tubes to transport large volumes of phloem sap when turgor pressure in the sieve tubes at the sink ceases to be a limiting factor. This can be contrasted with the single-fruit system where the sink probably exerts a 'back pressure' while the assimilate supply is unlimited. The twofold increase in SMT shown by a single-fruit system is probably caused by hormone-stimulated assimilate unloading. SMT values have been used also to calculate the velocity of sap flow in the sieve tubes.