Is there a relationship between tracheary element formation and lignification in soybean (Glycine max var. Wayne) callus cultures?

Abstract
The possible relationship between tracheary element number and lignin content was studied in cultured soybean (Glycine max L. var. Wayne) cotyledon callus. Callus initiated on 4.5 μM 2,4-dichlorophenoxyacetic acid contained 3.0 × 104 tracheary elements per gram fresh weight and 41 μg lignin per milligram dry weight after 10 days incubation, and these values did not vary significantly after two subsequent transfers (7 days each) to a medium containing 0.1 μM α-naphthaleneacetic acid and 0.01 μM kinetin. Transfer of this callus to a medium supplemented with 60 μM indole-3-acetic acid and 0.5 μM kinetin resulted in significant increases in tracheary element number and lignin content (290 and 56%, respectively). A time-course study revealed that both tracheary element number and lignin content reached a maximum 5 to 6 days after transfer to the medium containing indole-3-acetic acid and kinetin. However, when total callus lignin content was plotted against total tracheary element number, no statistically significant relationship was found. The formation of lignin not associated with tracheary elements may have been a factor. These results indicate that the induction of tracheary element formation and lignification in soybean callus have similar hormonal requirements, but lignification occurs independently of tracheary element formation in this system.