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The surface-enhanced Raman scattering (SERS) of sodium alginates and their hetero-and homopolymeric fractions obtained from four seaweeds of the Chilean coast was studied. Alginic acid is a copolymer of β-D-mannuronic acid (M) and α-L guluronic acid (G), linked 1 → 4, forming two homopolymeric fractions (MM and GG) and a heteropolymeric fraction (MG). The SERS spectra were registered on silver colloid with the 632.8 nm line of a He-Ne laser. The SERS spectra of sodium alginate and the polyguluronate fraction present various carboxylate bands which are probably due to the coexistence of different molecular conformations. SERS allows to differentiate the hetero-and homopolymeric fractions of alginic acid by characteristic bands. In the fingerprint region, all the poly-D-mannuronate samples present a band around 946 cm −1 assigned to C-O stretching, and C-C-H and C-O-H deformation vibrations, a band at 863 cm−1 assigned to deformation vibration of β-C 1 -H group, and one at 799-788 cm −1 due to the contributions of various vibration modes. Poly-L-guluronate spectra show three characteristic bands, at 928-913 cm −1 assigned to symmetric stretching vibration of C-O-C group, at 890-889 cm −1 due to C-C-H, skeletal C-C, and C-O vibrations, and at 797 cm −1 assigned to α C 1 -H deformation vibration. The heteropolymeric fractions present two characteristic bands in the region with the more important one being an intense band at 730 cm −1 due to ring breathing vibration mode.

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Tobacco plants were treated with structurally unrelated oligosaccharides obtained from Chilean marine macroalgae. These oligosaccharides were prepared by chemical depolymerization of native polysaccharides extracted from brown and red algae and correspond to pure polymers of around 20 units of guluronic acid (Poly-Gu), mannuronic acid (Poly-Ma) and sulphated galactan (Poly-Ga). These oligosaccharides were solubilized in water, at a final concentration of 500 μg mL −1 , and sprayed on tobacco leaves, once a week for a month. Their effects on the stimulation of growth and defense against tobacco mosaic virus (TMV) were determined 7 and 15 days after the final spraying, respectively. The activities of several defense and antioxidant enzymes and the levels of water-soluble antioxidant compounds were determined. Plants treated with Poly-Ga and Poly-Ma showed an increase in height of 23% and 49%, respectively, whereas Poly-Gu did not stimulate growth. Plants treated with Poly-Gu, Poly-Ma and Poly-Ga showed an increase in defense against TMV corresponding to decreases in the number of necrotic lesions of 9%, 22% and 74%, respectively. The stimulation of plant growth correlates with activation of the antioxidant enzyme ascorbate peroxidase (AP) and with a decrease in ascorbate level. On the other hand, the stimulation of defense against TMV is correlated with the activation of the defense enzyme phenylalanine ammonia-lyase (PAL). These results indicate that algal oligosaccharides differentially stimulate growth and defense against TMV in tobacco plants and that these processes involve the activation of the enzymes AP and PAL, respectively.

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