Physiological aspects of skin adhesion in sweetpotato (Ipomoea batatas (L) Lam).

Abstract

One of the problems in postharvest handling of sweetpotato roots is the loss of the skin from the surface of the roots, referred to as 'skinning'. During skinning, cuticle, epidermis, and a portion of the outer layers of the periderm separates from underlying tissue, resulting in an increased rate of moisture loss, weight loss, shriveling of the root surface, susceptibility to pathogen attack and poor appearance. Thus, skinning results in a loss of crop value and a reduction in profits for the grower. The following characteristics were measured to determine if they influenced skin adhesion and/or peeling resistance in sweetpotato roots: pectin, lignin, and anthocyanin content, polygalacturonase (PG), pectinmethylesterase (PME) and cellulase activity, periderm dry matter content and root weight. Studies were conducted to determine if there was variation in skin adhesion, cell wall enzyme activity and cell wall components in the periderm of roots (a) during storage, (b) at different physiological ages, (c) when grown under several temperature regimes and (d) when grown in different locations in the Southeastern U.S. The anatomy and histochemical properties of roots grown at different temperatures and locations were characterized. Correlations between the variables were established to determine if skin adhesion and/or peeling resistance might be explained based on cell wall enzyme activity, cell wall composition, and/or anatomy of root periderm.

Results indicated that skin adhesion in sweetpotato was highly variable and was affected by cultivar, temperature, humidity, origin and physiological age of the root, and storage. Skin adhesion, cell wall enzyme activity, and pectin content in sweetpotato vary with physiological age of the root. In field experiments, the effect of soil temperature on skin adhesion was not clear, possibly due to the interaction with other environmental conditions. Under controlled conditions, skin adhesion, cell wall enzyme activity, anthocyanin content, periderm and biomass dry matter content, yield, root weight, and root diameter were affected by growth temperature. A growth temperature of 34/31 ºC yielded roots that were more resistant to skin loss, were smaller, and had a thicker periderm, composed of more cell layers and lower dry matter content than roots grown at lower temperatures. A growth temperature of 20/17 ºC increased anthocyanin content of the root periderm. In roots grown in the Southeastern U.S., the degree of skin adhesion and peeling resistance varied depending on root origin. Skin adhesion during storage of sweetpotato roots varied depending on cultivar. Skin adhesion of the roots seemed to increase during the early weeks of storage, but this effect did not appear to be permanent and appeared to be reversible at any time during storage. Curing improved skin adhesion and peeling resistance of the roots. The effect of curing was dependent on the root growth temperature. A similar interaction was detected between curing and growth temperature for some of the studied variables. Skin adhesion did not appear to be directly linked to cell wall enzyme activity. However, enzyme activity levels were affected by cultivar, environmental factors, storage and origin of the roots. Periderm dry matter, lignin, and anthocyanin content did not appear to be significant factors in skin adhesion or peeling resistance. The histochemistry of the periderm of sweetpotato indicated a different anatomical and structural composition of the cell walls depending on growth temperature and where the roots were grown.