Storage In Sinks

The carbon derived from translocated sugar alcohols is utilised in nonphotosynthetic tissues (vegetative sinks, fruits and possibly perennial parts) for the immediate production of energy and carbon skeletons for synthesis, or is stored for later use. Parasitic Angiosperms can be considered as a special type of sink.

5.1. Vegetative storage organs

One organ in which the storage of a sugar alcohol has been extensively studied is celery petiole [75]. In mature celery leaves, the mannitol svnthesised in mesophyll cells can be temporarily stored in petiole parenchyma cells, to be remobilized during senescence. Since the activity of the mannitol synthesising enzyme is very low in petiole parenchyma tissue, it has been concluded that mannitol is accumulated from the phloem translocation stream. The thick and fleshy petioles of celery leaves act as reversible carbohydrate sinks, storing mainly mannitol, glucose and fructose but very little sucrose in their parenchyma. Mannitol (about 20 mg/ml petiole sap), glucose and fructose varied little in the petiole parenchyma during leaf development. Soluble acid invertase activity was closely related to development with a sharp decrease during leaf maturation. Glucose and fructose, the main storage sugars of protoplasts obtained from petiole parenchyma, were vacuolar, whereas mannitol was both cytosolic (19%) and vacuolar (81 %).

5.2. Fruits

In fruits, sugar alcohols are not major storage carbohydrates. In olive fruits, glucose was the predominant sugar, followed by fructose and mannitol. The relative amounts of mannitol varied between 2 and 10% depending on the cultivar [99]. Mannitol levels and oil accumulation seem to be related.

The sorbitol content of juice from peaches, black currants, red currants, raspberries and elderberries was shown to be in most cases under 1% [100]. The soluble sugar content of fruits of 9 different species (including some Rosaceous trees) was compiled from the literature by Wrolstad and Shallenberger [101], These authors showed that, despite varietal and environmental effects, the individual fruits show characteristic patterns in terms of their sorbitol content and glucose:fructose ratio. This descriptive study was aimed at determining criteria for verifying the authenticity of fruit juices and concentrates.

In stone fruits of Rosaceae trees, the sugar content and the proportion of the 4 major sugars (sucrose, glucose, fructose and sorbitol) at maturity depend greatly on the species and may be influenced by the variety [102]. The same appears to be true for pome fruits. The early works of Bieleski on apple [79] and De Villiers et al. on prune [80] concluded that, although sorbitol is the main photosynthate tranlocated from leaves to fruits in this family, it is metabolised to other sugars within the fruit. Using incorporation of l4C sorbitol or 14C-sucrose in growing apple fruit, Beruter et al. [103] showed that when l4C-sorbitol was fed, fructose was preferentially labelled. They also showed that glucose derived from sucrose enters into the hexose phosphate pool more readily than fructose derived either from sucrose or sorbitol.

In French prune, sorbitol and glucose were the predominant carbohydrates in mature fruits, followed by sucrose [104], The mature peach fruit is characterised by a high sucrose content (up to 60 percent dry mass) and a very low sorbitol content in flesh (under 8 percent dry mass) [105], In peach, glucose and fructose in nearly equal amounts were the predominant sugars detected during the early stage of development. Sucrose subsequently began to accumulate and was the predominant sugar in mature fruits. The sorbitol concentration remained low and constant in peach fruit during the entire season [57, 106].

In pear fruits, sorbitol was the major carbohydrate during all the stages of development until harvest. Sorbitol content increased sharply during fruit development, up to 24 percent dry mass, whereas the contents of glucose, sucrose and fructose remained relatively constant during the growing period [107], In mature apple fruit, fructose is the prominent soluble sugar and sorbitol is the minor one [108], Sorbitol content changes little during ripening and storage. Although sorbitol is a minor soluble sugar of apple, a higher concentration of sorbitol characterises apple fruits with watercore symptoms [109], However. Marlow and Loescher [109] suggest that this sorbitol accumulation is a consequence rather than a cause of this physiological disorder.

The activity of sugar metabolising enzymes has been described in different Rosaceae fruits. Most investigations were aimed at determining variations in the activity of the enzymes involved in sucrose and sorbitol metabolism during fruit development, in relation to fruit growth and sugar accumulation. However, the physiological consequences of the correlations between enzyme activity and soluble sugar content remain to be elucidated.

In Japanese pear (Pyrus pyrifolia) fruit [110], the activity of NAD-dependent sorbitol dehydrogenase, which converts sorbitol to fructose, was higher than that of sorbitol oxidase, NADF' -dependent sorbitol dehydrogenase or sorbitol-6-phosphate dehydrogenase throughout the growing season. The activity increased in June, decreased with fruit expansion and rose again during maturation. Fluctuations in enzyme activity could be related to changes in fructose accumulation. Sorbitol oxidase activity, which was one-tenth that of NAD-dependent sorbitol dehydrogenase activity, showed a similar developmental pattern. Acid invertase activity was distinctly higher than that of the sorbitol-related enzymes.

In apple, NAD-dependent sorbitol dehydrogenase had the highest activity out of 4 sorbitol-related enzymes [62]. Later, the seasonal changes in the amounts of the NAD-dependent sorbitol dehydrogenase protein were determined in developing apple fruits by immunoblotting analysis [52], The amounts of the enzyme protein were very low in young fruits and rose as fruits matured. The weak correlation observed between enzyme protein and activity, and also the changes in enzyme specific activity, suggested that there may be post-translational modifications to the pre-existing enzyme or isoenzyme(s) of NAD-dependent sorbitol dehydrogenase. The role of this enzyme in sink activity in apple fruits could not be explained simply by the amount and activity of the enzyme. In young apple fruits, other enzymes may be more directly related to fruit growth. Recently. Archbold [60] concluded that carbohydrate availability may modify the sorbitol dehydrogenase activity of apple fruit.

In peach [57], fruits contained appreciable sorbitol oxidase activity, while other sorbitol-related enzymes were barely detectable. It was suggested that transported sorbitol was predominantly converted to glucose and that the supply of glucose and fructose depended on acid invertase and sorbitol oxidase. The accumulation of sucrose seemed to depend on sucrose synthase.

In loquat fruit [111], sorbitol concentration remains very low, suggesting that sorbitol is metabolised by sorbitol dehydrogenase, consistent with the observed high sorbitol dehydrogenase activity. However, in mature loquat fruit, aldose-6-phosphate reductase activity was shown to be unexpectedly high compared to apple, japanese pear, and peach fruits [111]. The authors hypothesised that this enzyme must play a metabolic role in loquat fruit by synthesising sorbitol.

5.3. Perennial parts of woody species

The importance of starch as a storage carbohydrate in woody perennials is widely accepted [112]. However, some soluble carbohydrates, such as rafftnose [113], also behave like storage carbohydrates besides starch. Lewis [4] (and references therein) have already-reviewed the seasonal variations in sugar alcohol concentration in the bark of olive. Gardenia, apple and Itea. Seasonal variations in carbohydrate contents (including sorbitol) in perennial parts is well documented in Rosaceous trees [113 . 114. 115 , 116]. It is also described in olive tree [95]. Although sorbitol or mannitol do not behave like typical storage carbohydrates, they exhibit some significant seasonal variations.

In cherry trees [113], in the bark and wood of one and 2-year old shoots, sorbitol and sucrose were the main soluble carbohydrates, with sucrose often exceeding sorbitol concentrations during dormancy, and sorbitol usually predominating during active growth. In these organs, sorbitol concentration varied between 2 and 4 percent dry weight, the maximum being observed in late summer. In the wood of roots, sorbitol concentration also varied by a factor two with a minimum observed in June. In the trunk bark of prune scions or rootstocks, sorbitol concentration fluctuated between 0.25 and 0.7 percent dry weight over a whole year [115].

In apple tree, starch is the main reserve carbohydrate [114]. During flowering and fruit setting [116], sorbitol concentration varied less than starch concentration: it remained between 22 and 41 mg g'1 DW in woody spurs, and 15 and 25 mg g'1 DW in two year old wood. When the cluster expanded, the pool of soluble carbohydrates rapidly increased in the flowers and in the other parts of the cluster, while it remained fairly stable in spurs and shoots up to full bloom. The simultaneous decrease in starch level in both spurs and shoots is consistent with a rapid conversion of starch to soluble sugars translocated to the different parts of the cluster. Although sorbitol is the main photosynthetic product in apple tree, there seems to be a preferential utilisation of sucrose rather than sorbitol for anabolic processes and respiration during fruit setting in this species [116].

In olive trees, the seasonal variations in starch and soluble carbohydrates in bark and xvlem tissues were described during a complete annual cycle [95]. Mannitol content did not exhibit any significant fluctuation in xylem. The pattern of changes in mannitol content in bark was similar to those observed in leaves. The highest mannitol content in bark tissues was observed in winter and the lowest content in May.

5.4. Parasitic Angiosperms

Sugar alcohols have been detected in several parasitic flowering plants [117]. The sugar alcohol present in the parasite may be derived from the host [13] or be synthesised in the parasite itself [14], depending on the species. Traces of sorbitol were only found in mistletoes parasitising host plants containing sorbitol [118]. Host specific compounds may be stored in the parasite or not detected in the parasite due to rapid utilisation [58, 118]. In parasitic Angiosperms, the roles of sugar alcohols seem to be the same as in other higher plants: translocation of carbon through phloem [15], and resistance to osmotic and drought stress [119. 120] for instance. Parasite plants may lower their osmotic potential through the accumulation of a sugar alcohol, thus facilitating the flux of resources to the parasite [117]. High mannitol concentrations in Orobanche, Lalhraea and Striga were implicated in generating an osmotic gradient between host and parasite [117]. Moreover. Lewis and Harley [121] and Lewis and Smith [7] (cited by [122]) suggested that in symbiotic and parasitic associations, the rapid conversion of the sugar transferred from the host into a sugar alcohol maintains the sugar concentration gradient and prevents the sugar from diffusing back to the host plant.

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  • mehari yusef
    What are the main carbohydrate sinks in flowering plants?
    3 months ago

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