The type of transport mechanism involved in the accumulation of K+ (potassium) against its concentration gradient in the vacuoles of elongating cotton fibers was investigated by means of ultrastructural enzyme localization. Adenosine triphosphatase (ATPase) enzyme activity was observed in the elongating cotton fiber from one day preanthesis to ten days postanthesis by using a heavy metal simultaneous capture reaction technique.

Cotton ovules of a linted cultivar (Gossypium hirsutum 'Hancock') and lintless hybrid line 9SO x HG) were fixed in 1.5% glutaraldehyde buffered with 0.1 M cacodylate buffer, pH 7.2, for 4 hours at 4 C. The samples were incubated in an enzyme reaction medium containing ATP as a substrate and lead as a capturing agent for 2 hours at 31 C, pH 7.0. The enzyme activity was observed as electron dense deposits of lead phosphate at the site of enzyme activity. The samples were post-fixed with 1% OsO₃ for 2 hours at ice bath temperature. The controls included: 1) Samples incubated in a reaction medium minus the substrate, ATP. 2) Samples pretreated with enzyme inhibitor N, N-Dicyclohexylcarbodiimide (DCCD), followed by incubation in a complete medium plus inhibitor. 3) Samples incubated with and without K+ in the reaction medium to demonstrate K+ - activated ATPase activity.