Callose Deposition and Photoassimilate Export in Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc

Callose accumulated on sieve plates of phloem of white bean seedlings exposed to excess Co, Ni, or Zn. The callose deposits ranged in thickness and were most pronounced in midribs of unifoliate leaves and their subtending petioles. Lesser callose deposits were found in stems. Although translocation of (14)C was reduced drastically in seedlings exposed to excess metal, no correlation was found between translocated (14)C and the amount of callose in the petioles. It is concluded that the inhibition of phloem translocation in seedlings exposed to excess metal is due to effects other than callose deposition.