RESPIRATION, SOLUBLE CARBOHYDRATES AND ENZYMES OF CARBOHYDRATE-METABOLISM IN TIPS OF HARVESTED ASPARAGUS SPEARS

We held asparagus (Asparagus officinalis L.) spears at 20 degrees C for up to 5 days after harvest and examined changes in respiration rate, soluble carbohydrates and the activities of enzymes concerned with glycolysis and carbohydrate breakdown in the tips. Respiration rate declined after harvest, to reach 25% of its initial rate after 2 days. The gas exchange (respiratory) quotient was 1.05 at harvest and declined in parallel with respiration rate for 2 days. There was a rapid loss of sucrose from spear tips over the first 6 h, and a more gradual loss after that. Glucose concentration declined gradually throughout, but fructose concentration barely changed over 3 days. We conclude that a major change in the metabolism of asparagus spear tips occurs soon after harvest. The change is characterised by a switch from sucrose-derived hexose phosphate to another primary respiratory substrate, probably derived from protein and/or lipid. The activities of hexokinase (EC 2.7.1.1), fructokinase (EC 2.7.1.4), phosphofructokinase (EC 2.7.1.11), sucrose synthase (EC 2.4.1.13) and alkaline invertase (EC 3.2.1.26) were generally low compared with the activities of the other enzymes assayed. Overall, enzyme activities declined over 5 days of starvation. The exceptions were hexokinase, phosphofructokinase and pyruvate kinase (EC 2.7.1.40), where activities were reasonably steady.