GAS-EXCHANGE AND CARBON PARTITIONING IN THE LEAVES OF CELERY (APIUM-GRAVEOLENS L) AT VARIOUS LEVELS OF ROOT-ZONE SALINITY

Both mannitol and sucrose (Suc) are primary photosynthetic products in celery (Apium graveolens L.). In other biological systems mannitol ha s been shown to serve as a compatible solute or osmoprotectant involve d in stress tolerance. Although mannitol, like Suc, is translocated an d serves as a reserve carbohydrate in celery, its role in stress toler ance has yet to be resolved. Mature celery plants exposed to low (25 m M NaCl), intermediate (100 mM NaCl), and high (300 mM NaCl) salinities displayed substantial salt tolerance. Shoot fresh weight was increase d at low NaCl concentrations when compared with controls, and growth c ontinued, although at slower rates, even after prolonged exposure to h igh salinities. Gas-exchange analyses showed that low NaCl levels had little or no effect on photosynthetic carbon assimilation (A), but at intermediate levels decreases in stomatal conductance limited A, and a t the highest NaCl levels carboxylation capacity (as measured by analy ses of the CO2 assimilation response to changing internal CO2 partial pressures) and electron transport (as indicated by fluorescence measur ements) were the apparent prevailing limits to A. Increasing salinitie s up to 300 mM, however, increased mannitol accumulation and decreased Suc and starch pools in leaf tissues, e.g. the ratio of mannitol to S uc increased almost 10-fold. These changes were due in part to shifts in photosynthetic carbon partitioning (as measured by C-14 labeling) f rom Suc into mannitol. Salt treatments increased the activity of manno se-6-phosphate reductase (M6PR), a key enzyme in mannitol biosynthesis , 6-fold in young leaves and 2-fold in fully expanded, mature leaves, but increases in M6PR protein were not apparent in the older leaves. M annitol biosynthetic capacity (as measured by labeling rates) was main tained despite salt treatment, and relative partitioning into mannitol consequently increased despite decreased photosynthetic capacity. The results support a suggested role for mannitol accumulation in adaptat ion to and tolerance of salinity stress.