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PLANT PHYSIOLOGY , Vol 106, Issue 1 281-292, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Gas Exchange and Carbon Partitioning in the Leaves of Celery (Apium graveolens L.) at Various Levels of Root Zone Salinity
J. D. Everard, R. Gucci, S. C. Kann, J. A. Flore and W. H. Loescher
Department of Horticulture, Michigan State University, East Lansing, Michigan 48824-1325 (J.D.E., S.C.K.,J.A.F., W.H.L.)
Both mannitol and sucrose (Suc) are primary photosynthetic products in
celery (Apium graveolens L.). In other biological systems mannitol has been
shown to serve as a compatible solute or osmoprotectant involved in stress
tolerance. Although mannitol, like Suc, is translocated and serves as a
reserve carbohydrate in celery, its role in stress tolerance has yet to be
resolved. Mature celery plants exposed to low (25 mM NaCl), intermediate
(100 mM NaCl), and high (300 mM NaCl) salinities displayed substantial salt
tolerance. Shoot fresh weight was increased at low NaCl concentrations when
compared with controls, and growth continued, although at slower rates,
even after prolonged exposure to high 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 at the highest NaCl levels carboxylation
capacity (as measured by analyses of the CO2 assimilation response to
changing internal CO2 partial pressures) and electron transport (as
indicated by fluorescence measurements) were the apparent prevailing limits
to A. Increasing salinities up to 300 mM, however, increased mannitol
accumulation and decreased Suc and starch pools in leaf tissues, e.g. the
ratio of mannitol to Suc increased almost 10-fold. These changes were due
in part to shifts in photosynthetic carbon partitioning (as measured by 14C
labeling) from Suc into mannitol. Salt treatments increased the activity of
mannose-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. Mannitol biosynthetic capacity (as measured by labeling rates) was
maintained despite salt treatment, and relative partitioning into mannitol
consequently increased despite decreased photosynthetic capacity. The
results support a suggested role for mannitol accumulation in adaptation to
and tolerance of salinity stress.
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