EFFECTS OF APOPLASTIC SUCROSE ON CARBOHYDRATE POOLS AND SUCROSE EFFLUX OF MESOPHYLL PROTOPLASTS OF PEA (PISUM-SATIVUM)

The effects of the apoplastic, i.e. external, concentration of sucrose (0-30 mM) on O-2 evolution, O-2 consumption, starch, sucrose, glucose and fructose content, and uptake and efflux of sucrose in mesophyll p rotoplasts of Pisum sativum L. cv. Fenomen were studied. Neither photo synthesis, dark respiration, sucrose nor starch content change with in creased apoplastic sucrose concentration. The contents of glucose and fructose in the protoplasts increase with increased apoplastic sucrose concentration. The sucrose efflux increases with increased external s ucrose concentrations between 1 and 5 mM, but above this range the eff lux decreases with increased external sucrose concentrations. These fi ndings indicate that although external sucrose does not enter the prot oplasts, there is a relationship between the external sucrose pool and the internal pools of sugars in the mesophyll protoplasts. The result s suggest an active sucrose efflux from the protoplasts at physiologic al concentrations of apoplastic sucrose (max 5 mM) and a simple diffus ion mechanism at higher concentrations.