The base of the leaf acts as a localized sink for photosynthate in mature barley leaves

The gradients in photosynthetic and carbohydrate metabolism which persist w ithin the fully expanded second leaf of barley (Hordeum vulgare) were exami ned. Although all regions of the leaf blade were green and photosynthetical ly active, the basal 5 cm, representing approximately 20% of the leaf area, retained some characteristics of sink tissue. The leaf blade distal from t he leaf sheath exhibited characteristics typical of source tissue; the;acti vities of sucrolytic enzymes (invertase and sucrose synthase) were relative ly low, whilst that of sucrose phosphate synthase was high. These regions o f the leaf accumulated sucrose throughout the photoperiod and starch only i n the second half of the photoperiod whilst hexose sugars remained low. By contrast the leaf blade proximal to the leaf sheath retained relatively hig h activities of sucrolytic enzymes (especially soluble, acid invertase) whi lst sucrose phosphate synthase activity was low. Glucose, as well as sucros e, accumulated throughout the photoperiod. Although starch accumulated in t he second half of the photoperiod, a basal level of starch was present thro ughout the photoperiod, by contrast with the rest of the leaf. The (CO2)-C- 14 feeding experiments indicated that a constant amount of photosynthate wa s partitioned towards starch in this region of the leaf irrespective of irr adiance. These findings are interpreted as the base of the leaf blade actin g as a localized sink for carbohydrate as a result of sucrose hydrolysis by acid invertase.