SOLUBLE ACID INVERTASE ACTIVITY IN LEAVES IS INDEPENDENT OF SPECIES-DIFFERENCES IN LEAF CARBOHYDRATES, DIURNAL SUGAR PROFILES AND PATHS OF PHLOEM LOADING

Leaf sucrose, starch, hexose and maximum extractable soluble acid inve rtase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplast ic or symplastic). Four species which represent the different phloem-l oading types (tomato, barley, maize and Fuchsia) were studied in detai l. Using this information we wished to determine whether a positive co rrelation between foliar carbohydrates and acid invertase activity exi sts in leaves from different species and, furthermore, whether this re lationship is determined by phloem-loading type. Acid invertase activi ty was relatively constant throughout the day in all species. The exte nt of sucrose, hexose and starch accumulation and the sucrose:starch r atio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexo se, sucrose or starch content of the leaves in any of the species, reg ardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low s ucrose, low invertase; and (3) low sucrose, high invertase. The absenc e of an inverse relationship between leaf sucrose, hexose or starch co ntents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.