Relationship between the activity of the orotic acid pathway and glucose content of roots of Cucurbita pepo

. Apical 3‐cm root segments excised from 2‐d‐old squash seedlings ( Cucurbita pepo L. cv. Early Prolific Straightneck) that were germinated and grown between sheets of paper towelling moistened with H 2 O 2 incorporated 144±10 nmol (± SE. n = 15) NaH 14 CO 3 into uridine nucleotides (∑UMP) per gram intact roots during the 3‐h incubation period (Lovatt, Albert & Tremblay, 1979). Continued culture of squash seedlings in this manner for an additional 24 or 48 h had no effect or slightly increased (30%) the activity of the orotic acid pathway. However, transfer of 2‐d‐old seedlings to Shive's nutrient solution reduced the activity of the orotic acid pathway within 15 h to 2.3 nmol NaH 14 CO 3 incorporated into ∑UMP per gram apical 3‐cm root segments during the 3‐h incubation period. The observed decrease in capacity to synthesize pyrimidine nuclotides de novo paralleled the reduction in glucose content of the roots and was reversed by supplying glucose during hydroponic culture in Shive's nutrient solution. Root elongation was not affected by the reduced activity of the orotic acid pathway nor by the decreased level of available glucose.