Abstract:
Sweet potato (Ipomoea batatas Lam) is the third most important food commodity in Sierra Leone and global demand for the commodity is growing. To meet this demand requires the use of effective storage methods due to the perishability (shrivelling, sprouting and disease) of the root after harvest. This study therefore investigated the efficacy of controlled atmosphere (CA) and ethylene on the physiological, biochemical, mycological and molecular dynamics of sweet potato during storage. This was done by storing sweet potato samples (cultivar: 06-52; known as Belle Vue in the USA) at 20˚C as follows: CA (5 kPa CO₂ and 8 kPa O₂), air (0.003 kPa CO₂ and 21 kPa O₂), CA supplemented with 0.001 kPa ethylene and continuous exogenous ethylene (0.001 kPa) for a maximum of 12 weeks. A transition phase was established at six weeks of storage which involved swapping of treatments from CA with and without supplemented ethylene to air storage and vice versa (Experiment 2) and that from ethylene to air
storage and vice versa (Experiment 4).
Results showed that in as much as the ethylene supplementation was successful in supressing sprouting, the contents of phenolics and sugars increased as well as weight loss and respiration rates, which undermine its positive aspects. Storage in CA reduced weight loss and respiration rates but did suppress sprouting as well. The complete inhibition of sprouting during storage could also be attributed to decreased biosynthesis
of the cytokinin, trans-zeatine riboside (trans-ZR) in the sweet potato due to ethylene supplementation. Also ethylene induced rise in phenolics corresponded with an increased relative expression of ethylene response factors (ERF) in the proximal section unlike the reducing sugars. The ERF gene was more expressed towards the end of storage whilst ethylene insensitive 2 (EIN2) was more expressed at the beginning which suggested that EIN2 was actively repressed during storage particularly under CA treatment.
Furthermore, continuous CA storage was effective in mitigating disease development on the sweet potato as well as in controlling Penicillium development but not that of Fusarium. Major diseases identified on the sweet potato were Fusarium surface rot and Rhizopus soft rot predominantly at the proximal and distal sections mainly during storage under ethylene supplemented CA. The mycotoxin, aflatoxin G1 was the most predominant potentially produced aflatoxin on the sweet potato and continuous CA was very effective in inhibiting the potential contamination of all the studied aflatoxins: aflatoxin B1 (AFB1), aflatoxin B2 (AFB2) and aflatoxin G1 (AFG1) during storage.
