Enzymes Involved in the Postharvest Degradation of Sucrose in Beta vulgaris L. Root Tissue

Abstract

The reducing sugar content of sugar beet (Beta vulgaris L.) roots increased during 30 days of storage at 21 C and 160 days at 5 C as a result of an increase in acid invertase activity. Sucrose synthetase and neutral invertase activities were high at harvest but declined during storage, thus showing no relationship with postharvest reducing sugar accumulation in sugar beet roots. Acid a-glucosidase activity was detected in fresh roots but showed no activity with sucrose as a substrate.The sucrose content of sugar beet roots declines after harvest as a result of sucrose conversion to reducing sugars and oligosaccharides (20). At storage temperatures above 5 C reducing sugars (glucose and fructose) accumulate, while at temperatures below 5 C both raffinose and reducing sugars accumulate (20). The objective of this study was to determine which enzymes are involved in sucrose degradation in sugar beet roots during storage.Acid invertase activity is normally very low in fresh sugar beet roots, but induction of invertase activity by aging beet disks in water has been well documented (1,15,19 operations were carried out at 0 to 5 C. Roots were peeled, diced, and homogenized in a VirTis homogenizer' with an equal w/v ratio of grinding medium (0.05 M potassium phosphate, pH 7.2; 0.1 mm EDTA; 1 mM 2-mercaptoethanol; 10 mM Na2SO.). The homogenate was strained through two layers of dacron ninon and centrifuged at 16,000g for 20 min. The supernatant liquid was dialyzed overnight against 10 volumes of either grinding medium for neutral invertase and sucrose synthetase assays, or acetate buffer (0.05 M acetate, pH 5.0; 0.1 mM EDTA; 1 mm 2-mercaptoethanol; 10 mm Na,SO8) for acid invertase assay. The extract dialyzed at pH 5 was centrifuged at 1 6,000g for 10 min before assay and the pellet discarded.Assay. The standard assay mixture for neutral invertase contained 0.16 ml of buffer (8 ltmoles of potassium phosphate, pH 7.2; 0.016 lumole EDTA; 80 jumoles of sucrose) and 0.04 ml of the enzyme preparation. Normal assay time was 30 min at 35 C. Assay mixtures for sucrose synthetase were the same as for neutral invertase except that 10 ,u1 of 0.05 M UDP were added and only 0.03 ml of the enzyme preparation was used. Assay mixtures for acid invertase were the same as for neutral invertase except that the phosphate buffer was replaced by acetate at pH 5.0. The reactions were stopped by adding 1 ml of Somogyi's (18) copper reagent and reducing sugars determined by the method of Nelson (14). Glucose liberation, in assays in which maltose was the substrate, was determined by the glucose oxidase/peroxidase method with a commercial reagent (Glucostat, Worthington2).Protein was determined by the method of Lowry (9). The reducing sugar content of the stored roots was determined on a clarified extract prepared by the method of Dexter et al. (4). RESULTS AND DISCUSSIONIn a preliminary experiment a survey of enzymes which may degrade sucrose in fresh sugar beet roots was made. Sucrose, raffinose, and maltose were used as substrates and a...

Impact of storage temperature and duration on sucrose catabolism in harvested sugarcane stalks

“…The activity of SAI in harvested sugarcane stalks was barely detectable at harvest but increased 5 to 21 fold or 12 to 48 fold when they were stored for 15 days at 25 o C or ambient temperature. This is in good agreement with earlier reports (Wyse, 1974, Mao et al, 2006Singh et al, 2008). The increase in SAI activity in sugarcane stalks after harvest may indicate its necessity for the postharvest generation of respiratory substrates.…”

“…Several scientists have reported on the increment of reducing sugars in stored canes (Solomon, 2000;Singh et al, 2008;Uppal, 2003;Larrahondo et al, 2006). In addition, Wyse (1974) found that reducing sugar content of sugarbeet roots did not change during 30 days in storage at 2 o C, but doubled at 21 o C.…”

Sucrose Uptake by Sugar Beet Tap Root Tissue

“…Although no direct evidence is available to substantiate a cytoplasmic bypass, the uptake of a major portion of sucrose without prior hydrolysis would suggest such a pathway. The sugar beet root contains high levels of soluble sucrose synthetase activity (25); thus, any sucrose transported through the cytoplasm would likely be broken down by this enzyme unless the sucrose moving through the cytoplasm and into the vacuole were highly compartmentalized.…”

Comparative Enzymic Studies of Sucrose Metabolism in the Taproots and Fibrous Roots of Beta vulgaris L

“…The in vitro invertase activity of soluble and particulate fractions of taproot and fibrous root homogenates was examined to determine whether differences in enzyme solubility of the two root forms were correlated with differences in root form and function. The soluble protein fraction from the taproot homogenate represented about two-thirds of the total recoverable protein, but fibrous root protein was about equally distributed between the soluble and particulate fractions (Table I) (3,5,6,11,16,19,23,24) and other 1,13,18,19). Changes in the soluble to particulate ratio of plant organs (17,18) of several species.…”