Carbon metabolism in developing spears of two asparagus (Asparagus officinalis) cultivars with contrasting yield

Abstract

To assess the relative importance of sucrose-cleaving enzymes in the regulation of carbon accumulation in developing asparagus spears (growing shoots), we investigated spear elongation, carbohydrate accumulation and enzyme activities of acid invertase (AI), neutral invertase (NI) and sucrose synthase (SS) in two asparagus (Asparagus officinalis L.) cultivars with contrasting yield. The greater elongation rate measured in the high-yielding cultivar ASP-69 was associated with a significantly higher hexose accumulation (P < 0.05) in spear tissue in comparison with the low-yielding cultivar ASP-03. However, sucrose content was similar in the two cultivars, suggesting a more efficient machinery for transport and catalysis of carbohydrate in spears of ASP-69. Biochemical evidence indicated that the greater elongation rate in ASP-69 was associated with a significantly higher AI activity (P < 0.05) in the elongation zone, whereas SS activity was not significantly different between the two cultivars. There was little NI activity detected in either cultivar. These results strongly suggest that it is AI, and not SS or NI, that is an important determinant of the difference in sucrose metabolism between the two asparagus cultivars in metabolising imported sucrose in the elongation region, which in turn plays a part in regulating the import of sucrose into spear tissue. The profile of sucrose-cleaving enzyme activities along spear sections indicated that SS was the dominant enzyme in both the tip and base of spears, whereas AI was the dominant enzyme in the elongation zone. Apart from sucrose-cleaving enzymes, the associated biochemical processes for structure and component synthesis in spear tissues also contributed to the regulation of carbohydrate accumulation. It is most likely that carbohydrate metabolism in the developing spears is a whole spear property influenced by sucrose degradation (AI and SS activity) and its utilisation in building spear structure and storage materials. The overall data substantiate the conclusion that changes in the activity of sucrose-cleaving enzymes are correlated with sink functions in developing spears.