No abstract

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,

No abstract

BackgroundAlthough the effect of salicylic acid (SA) on photosynthesis of plants including grapevines has been investigated, very little is yet known about the effects of SA on carbon assimilation and several components of PSII electron transport (donor side, reaction center and acceptor side). In this study, the impact of SA pretreatment on photosynthesis was evaluated in the leaves of young grapevines before heat stress (25°C), during heat stress (43°C for 5 h), and through the following recovery period (25°C). Photosynthetic measures included gas exchange parameters, PSII electron transport, energy dissipation, and Rubisco activation state. The levels of heat shock proteins (HSPs) in the chloroplast were also investigated.ResultsSA did not significantly (P < 0.05) influence the net photosynthesis rate (Pn) of leaves before heat stress. But, SA did alleviate declines in Pn and Rubisco activition state, and did not alter negative changes in PSII parameters (donor side, acceptor side and reaction center QA) under heat stress. Following heat treatment, the recovery of Pn in SA-treated leaves was accelerated compared with the control (H2O-treated) leaves, and, donor and acceptor parameters of PSII in SA-treated leaves recovered to normal levels more rapidly than in the controls. Rubisco, however, was not significantly (P < 0.05) influenced by SA. Before heat stress, SA did not affect level of HSP 21, but the HSP21 immune signal increased in both SA-treated and control leaves during heat stress. During the recovery period, HSP21 levels remained high through the end of the experiment in the SA-treated leaves, but decreased in controls.ConclusionSA pretreatment alleviated the heat stress induced decrease in Pn mainly through maintaining higher Rubisco activition state, and it accelerated the recovery of Pn mainly through effects on PSII function. These effects of SA may be related in part to enhanced levels of HSP21.

The acyclic polyol sorbitol is a primary photosynthetic product and the principal photosynthetic transport substance in many economically important members of the family Rosaceace (e.g. almond [Prunus dulcis (P. Mill.) [Pyrus communis]). To understand key steps in long-distance transport and particularly partitioning and accumulation of sorbitol in sink tissues, we have cloned two sorbitol transporter genes (PcSOT1 and PcSOT2) from sour cherry (Prunus cerasus) fruit tissues that accumulate large quantities of sorbitol. Sorbitol uptake activities and other characteristics were measured by heterologous expression of PcSOT1 and PcSOT2 in yeast (Saccharomyces cerevisiae). Both genes encode proton-dependent, sorbitol-specific transporters with similar affinities (K m sorbitol of 0.81 mm for PcSOT1 and 0.64 mm for PcSOT2). Analyses of gene expression of these transporters, however, suggest different roles during leaf and fruit development. PcSOT1 is expressed throughout fruit development, but especially when growth and sorbitol accumulation rates are highest. In leaves, PcSOT1 expression is highest in young, expanding tissues, but substantially less in mature leaves. In contrast, PcSOT2 is mainly expressed only early in fruit development and not in leaves. Compositional analyses suggest that transport mediated by PcSOT1 and PcSOT2 plays a major role in sorbitol and dry matter accumulation in sour cherry fruits. Presence of these transporters and the high fruit sorbitol concentrations suggest that there is an apoplastic step during phloem unloading and accumulation in these sink tissues. Expression of PcSOT1 in young leaves before completion of the transition from sink to source is further evidence for a role in determining sink activity.Sorbitol (an acyclic polyol) and Suc are the primary photosynthetic products and the major phloemtranslocated components in a number of economically important taxa in the family Rosaceae, in particular in the subfamilies Pomoideae (e.g. (Loescher and Everard, 1996). Sorbitol is often the dominant translocated photosynthetic product. In mature apricot (Prunus armeniaca) leaves, for example, 65% to 75% of the translocated carbon is sorbitol (Bieleski and Redgwell, 1985). In these species, understanding the factors involved in facilitating and regulating sorbitol transport, including export from the leaf, long-distance distribution via the phloem network, and import into various sink tissues, is at least as important as those of Suc and other sugars.In the past 10 years, sugar transporters have been extensively studied in various sink and source tissues with the isolation of two distinct families of sugar carriers: the disaccharide transporters that primarily catalyze Suc transport and the monosaccharide transporters that mediate transport of the hexose sugars (for review, see Weise et al., 2000; Williams et al., 2000). Active uptake of Suc across the plasma membrane in all known cases involves an H ϩ -Suc symporter (Lemoine, 2000). The biochemical properties of the Suc transporters ...