Both mannitol and sucrose (Suc) are primary photosynthetic products in celery (Apium graveolens 1.). 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 NaCI), intermediate (100 mM NaCI), and high (300 mM NaCI) 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 CO, assimilation response to changing internal CO, 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 "C labeling) from SUC into mannitol. Salt treatments increased the activity of mannose-6-phosphate reductase (MCPR), a key enzyme in mannitol biosynthesis, 6-fold in young leaves and 2-fold in fully expanded, mature leaves, but increases in MCPR 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.The polyols were the first class of compounds to be termed compatible solutes (Brown and Simpson, 1972), and many of these compounds (acyclic polyols, e.g. sorbitol, mannitol, and glycerol, and substituted cyclic polyols, e.g. pinitol) and several related derivatives (e.g. glycerol glucoside) play roles in stress protection in almost all classes of living organisms,

The two commercially important fruiting cherries in North America are Prunus cerasus L. (sour) and P. avium L. (sweet). Carbon partitioning between individual organs, reproductive and vegetative, is dependent upon photoassimilate supply from photosynthesis and storage carbohydrate, as well as the ability of the translocation system to deliver these resources to sinks (Layne and Flore, 1995). Sink strength of individual organs varies with time of year, and age of the plant, as the sink demands and seasonal development pattern of the plant changes with time. Herein we will review assimilate supply and storage, seasonal partitioning patterns, and the impact that biotic and abiotic stress has on supply, partitioning, and control.

The source-sink ratio of l-year-old, potted `Montmorency' sour cherry (Prunus cerasus) trees was manipulated by partial defoliation (D) or continuous lighting (CL) to investigate the phenomenon of end-product inhibition of photosynthesis. Within 24 hours of D, net CO2 assimilation rate (A) of the most recently expanded source leaves of D plants was significantly higher than nondefoliated (control) plants throughout the diurnal photoperiod. Between 2 and 7 days after D, A was 30% to 50% higher and stomatal conductance rate (g,) was 50% to 100% higher than in controls. Estimated carboxylation efficiency(k) and ribulose-1,5-bisphosphate (RuBP) regeneration rate increased significantly within 2 days and remained consistently higher for up to 9 days after D. Leaf starch concentration and dark respiration rate decreased but sorbitol and sucrose concentration increased after D. The diurnal decline in A in the afternoon after D may have been due to feedback inhibition from accumulation of soluble carbohydrates (sucrose and sorbitol) in the cytosol. This diurnal decline indicated that trees were sink limited. By 9 days after D, photochemical efficiency was significantly higher than in control plants. In the long term, leaf senescence was delayed as indicated by higher A and gs in combination with higher chlorophyll content up to 32 days after D. CL resulted in a significant reduction of A, gs, k, variable chlorophyll fluorescence (Fv), photochemical efficiency, and estimated RuBP regeneration rate of the most recently expanded source leaves within 1 day. During the exposure to CL, A was reduced 2- to 3-fold and k was reduced up to 4-fold. The normal linear relationship between A and gs was uncoupled under CL indicating that A was not primarily limited by gs and since internal CO2 concentration was not significantly affected, the physical limitation to A imposed by the stomata was negligible. The decrease in Fv and photochemical efficiency indicated that leaves were photoinhibited within 1 day. The decrease in instantaneous chlorophyll fluorescence after at least 1 day of CL indicated that there was a reversible regulatory mechanism whereby the damage to photosystem II reaction centers was repaired. Leaf chlorophyll content was not altered by 1,2, or 3 days of exposure to CL, indicating that photooxidation of chlorophytl did not occur. The time to full photosynthetic recovery from CL increased as the duration of exposure increased. CL plants that were photoinhibited accumulated significant starch in the chloroplast in a companion study (Layne and Flore, 1993) and it is possible that an orthophosphate limitation in the chloroplast stroma was occurring. D plants that were continuously illuminated were not photosynthetically inhibited. After 7 days of CL, plants that were then partially defoliated yet remained in CL photosynthetically recovered within 5 days to pre-CL values. Under the conditions of this investigation, end-product inhibition of A occurred in young, potted sour cherry trees but the mechanism of action in D plants was different than in CL plants.

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