Amelioration of sucrose-metabolism and yield changes, in storage roots of NaCl-stressed sugarbeet, by ascorbic acid

NaCl-stress caused a great reduction in sucrose accumulation and yield, and biomass production by roots of sugarbeet plants grown for 6 months in greenhouse. Irrigating plants with saline water (100 and 200 mM) reduced the leaf concentration of total carbohydrates and proteins, particularly at 200 mM NaCl. In contrast. soluble sugars and proteins, and total free amino acids (TAA), including proline, were progressively accumulated as NaCl level increased. 100 mM NaCl induced a marked increase in the activity of sucrose-degrading enzymes [alkaline and acid invertases (IVs), and sucrose synthase (SS)] and hexose concentration in the storage roots, but a reverse trend was noticed at 200 mM NaCl. However, sucrose-phosphate synthase (SPS) activity and sucrose accumulation were progressively suppressed as salinity level increased. Applying ascorbic acid, especially with 4 mM, to sugarbeet has alleviated the adverse effects by increasing the salt-tolerance of the plant.. This counteraction was associated with no change in SS activity, but with a distinct increase in all other test characters, except leaf proline and root hexose accumulation, and IV activities that were evidently retarded. This response may indicate a specific role of ascorbic acid in sucrose and protein metabolism. It may also explain the role of soluble sugars, proteins, TAA and proline in osmotic adjustment. Results showed also a negative correlation between sucrose accumulation in roots, and IV and SS activities that, in turn, were positively correlated with hexose concentration. Conversely, SPS activity was directly correlated with sucrose accumulation confirming a decisive role of this enzyme with IV, in the regulation of sugar storage by sugarbeet roots. Possible regulation and energetic differences between all sucrose-metabolizing enzymes are discussed.