Study of the Early Events Leading to Cassava Root Postharvest Deterioration

Abstract

Cassava (Manihot esculenta Crantz) roots, the fourth most important food crop of the world, is the major carbohydrate source for more than 600 million people in Africa, parts of Latin America, Oceania, and Asia. Besides being a rich source of starch (∼80% of root), the root is also rich in vitamin C, some carotenoids, calcium, and potassium. Upon harvest, roots begin a process of physiological decay within 24-36 h called postharvest physiological deterioration or PPD. The early events leading to PPD are not known. Research to date concerning the study of PPD has mostly focused on the signaling events several hours after harvest. Upon examination of physiological and biochemical changes occurring 3 or 4 h after cassava root detachment, changes in the nature and type of volatile compounds emitted, secondary metabolites accumulated, and changes in the expression of key genes in reactive oxygen species (ROS) turnover were observed along with a correspondent increase in tissue cytoplasmic singlet oxygen presence using radical-specific fluorescent imaging of tissue samples. It is likely that these findings have significant implications to help us understand and assist in dissection of the early events leading to the postharvest deterioration of cassava root.

Extending Cassava Root Shelf Life via Reduction of Reactive Oxygen Species Production

“…Previous studies have shown that mechanical injury of cassava storage roots triggers cyanogenesis (McMahon et al, 1995) and an associated burst of acetone from cyanohydrins detected throughout the storage root (Iyer et al, 2010), followed by the production of ROS (Reilly et al, 2004). Cyanogenesis in cassava is induced during rupture of the vacuole, where linamarin is stored, followed by the production of cyanide in a two-step process initially catalyzed by the cell wall-localized enzyme linamarase and in leaves accelerated by hydroxynitrile lyase activity (Mkpong et al, 1990;White et al, 1998).…”

“…However, the short shelf life of the roots (only 2-3 d) limits cassava's economic and industrial potential. Harvested cassava roots undergo rapid postharvest physiological deterioration (PPD), which reduces their quality for market and consumption (Booth, 1976;Wenham, 1995;Buschmann et al, 2000;Reilly et al, 2001Reilly et al, , 2004Westby, 2002;Iyer et al, 2010). Therefore, cassava processing facilities, if any, must be at or near the site of production to reduce postharvest losses.…”

“…PPD is initiated by mechanical damage, which typically occurs during harvesting and progresses from the proximal site of damage to the distal end, making the roots unpalatable within 72 h (Wenham, 1995;Buschmann et al, 2000;Iyer et al, 2010). This deterioration is an active process distinct from the secondary deterioration caused by microbial infection (Booth, 1976).…”

“…Several studies have been carried out to understand the biochemistry of PPD (Buschmann et al, 2000;Huang et al, 2001;Reilly et al, 2001Reilly et al, , 2004Iyer et al, 2010). These studies have placed reactive oxygen species (ROS) production as one of the earliest events in the deterioration process.…”

“…Other early events include increased activity of enzymes that modulate ROS levels, such as catalase, peroxidase, and superoxide dismutase (Buschmann et al, 2000;Reilly et al, 2001;Iyer et al, 2010). Further evidence in support of a role of oxidative stress in PPD comes from the observation that cassava cultivars that have high levels of b-carotene (which quenches ROS) are less susceptible to PPD (Sanchez et al, 2005).…”