Sink development, sucrose metabolising enzymes and carbohydrate status in turnip (Brassica rapa L.)

Accumulation of 60-70 % of biomass in turnip root takes place between 49-56 days after sowing. To understand the phenomenon of rapid sink filling, the activities of sucrose metabolising enzymes and carbohydrate composition in leaf blades, petiole and root of turnip from 42-66 days of growth were determined. An increase (2-3 folds) in glucose and fructose contents of roots accompanied by an increase in activities of acid and alkaline invertases was observed during rapid biomass accumulating phase of roots. The observed decrease in the activities of acid and alkaline invertases along with sucrose synthase (cleavage) in petiole during this period could facilitate unrestricted transport of sucrose from leaves to the roots. During active root filling period, a decrease in sucrose synthase (cleavage) and alkaline invertase activities was also observed in leaf blades. A rapid decline in the starch content of leaf blades was observed during the phase of rapid sink filling. These metabolic changes in the turnip plant led to increase in hexose content (35-37 %) of total dry biomass of roots at maturity. High hexose content of the roots appears to be due to high acid invertase activity of the root.