Effect of Storage Temperature and Postharvest Tuber Treatment with Chemical and Biorational Inhibitors on Suppression of Sprouts During Potato Storage

The production and consumption of potato (Solanum tuberosum L.) has increased in Kenya and other tropical countries in recent years, due to the introduction of disease and insect resistant cultivars adapted to tropical highland environments (Lung’aho et al. 2006; Olanya et al. 2012). Postharvest losses of agricultural commodities have been associated with poor or inadequate storage conditions and often accounts for approximately 30% of losses of total harvested crops (Gustavsson et al. 2011). In the developing countries, the losses of harvested agricultural commodity such as potato (Solanum tuberosum L.) have often been exacerbated due to the elevated storage temperatures at on-farm locations and the suboptimum post-harvest processing and produce handling conditions (Wustman & Struik 2007).

With the increase in market potential of potato tubers in Kenya (Scott et al. 2000), the evaluation of storage temperature conditions that could impact sprout development, tuber weight loss and produce quality have been of considerable importance. Low storage temperatures have often been used to prolong the storability of potato by lengthening the natural dormancy (Wiltshire & Cobb 1996). Published reports have indicated that storage temperatures of 2–4 °C are suitable for long-term storage of potato, since there is limited sprout development (Paul et al. 2016). However, low temperature storage of ware potatoes destined for either fresh consumption or post-harvest processing have shown accumulation of reducing sugars (glucose and fructose) and some degree of sweetening of potato tubers (Sonnewald 2001).

To limit the cold-induced sugar accumulation in potato tubers, long-term storage of potato is done at 8–12 °C. However, this storage temperature is favorable for sprouting, hence treatment with sprout inhibitors becomes essential for successful long-term storage. For newly developed potato cultivars earmarked for utilization in tropical environments, there are limited published data on storage potential and the effects of storage temperatures and sprout inhibitors on tuber sprout suppression. It has been hypothesized that in the absence of postharvest treatment of potato tubers with sprout suppressants, the rate of sprout development on tubers increases with temperature. In general, at 3–20 °C temperature range during storage, the length of tuber dormancy is inversely proportional to temperature (Wiltshire & Cobb 1996). However, the effects of temperature on tuber dormancy depends on cultivar, with different cultivars breaking dormancy at different times even when stored at the same temperature. Respiration rates of potato tubers have been reported to have a minimum at about 4 °C and increase at higher temperatures (Wustman & Struik 2007). In the highland tropics where temperature variation occurs rapidly and extensively, the relationship between temperature and tuber dormancy has not been demonstrated.

Chlorpropham (CIPC) – isopropyl N-(3-chlorophenyl)carbamate – mitosis inhibitor, is the most commonly used sprout suppressant on potatoes (Paul et al. 2016; Smith & Bucher 2012). However, there has been an increasing concern about the toxicity of its residue and degradation of products, which negatively impact the environment and human health (Smith & Bucher 2012). As a result, in several countries, the use of CIPC and other chemicals are restricted with ban of CIPC in the EU by 2020 (EU 2019/989 of 17 June). There is thus a considerable interest in finding alternative effective potato-sprouting suppressants that have a negligible environmental impact (Paul et al. 2016). One of the CIPC alternatives is 1,4 dimethylnaphthalene (DMN), a naturally occurring compound in the potato tissues and other plants, and has been shown to be effective in controlling sprout development of stored ware potato tubers and is currently being used in many countries in the world (Campbell et al. 2010; de Weerd et al. 2010; Kleinkopf et al. 2003). DMN mode of action is still not clear, but it has been suggested that it may suspend/suppress normal cell division by inhibiting expression of genes that promote cell division or by enhancing expression of genes that inhibit cell division, which causes extension of dormancy period (Campbell et al. 2010, 2012; Kleinkopf et al. 2003). Other CIPC alternatives that have been found to suppress sprouting and sprout growth include natural compounds containing volatile monoterpenes and essential oils, such as cineole and eucalyptus oil (Vaughn & Spencer 1991, 1993; Knowles & Knowles 2008), caraway oil (Gómez-Castillo et al. 2013; Oosterhaven et al. 1995; Şanli et al. 2010; Teper-Bamnolker et al. 2010), peppermint, spearmint, clove, and mint oils (Gómez-Castillo et al. 2013; Kleinkopf et al. 2003; Teper-Bamnolker et al. 2010), menthe oil (Mehta & Kaul 2002), essential oils from aerial parts of Mentha spicata (Chauhan et al. 2011) and essential oils from Chenopodium ambrosioides and Lippia multiflora (Owolabi et al. 2010). Peppermint oil also has antioxidant and antimicrobial actions (Coleman et al. 2001; Frazier et al. 2004). Peppermint oil acts by physically burning or damaging the meristematic tissue to cease or disrupt cell proliferation (Frazier et al. 2004; Kleinkopf et al. 2003; Vaughn & Spencer 1991). These plant-derived natural compounds leave no residue on the tubers and hence are not toxic. But the efficiency of these alternative inhibitors depends on the cultivar, temperature of storage, progression of tuber dormancy, concentration, time and the number of applications, and the mode of the chemical action. Recently, we have shown that sprout suppression on tubers of tropical-adapted potato cultivars could be achieved for extended storage duration (Nyankanga et al. 2018). However, there are few published reports available in the literature that document the efficacy of these sprout suppressants at different storage temperatures, when applied on tubers of tropical-adapted potato cultivars. Similarly, the suppressive effects of peppermint oil at different storage temperatures, applied to tubers at postharvest as a biorational inhibitor relative to chemicals for sprout suppression in a tropic environment have not been adequately quantified. Therefore, the objective of this research was to determine the effects of temperature (10 °C and 23 °C) on the efficacy of CIPC, DMN and peppermint oil on sprout development on potato tubers of tropical-adapted cultivars following storage for 24 weeks.

Storage temperatures significantly affected sprout development of potato tubers and lower percentage of tubers with sprouts was recorded at 10 °C than at 23 °C. Therefore, in the absence of inhibitor applications, tubers from tropical-adapted potato cultivars can be stored at 10 °C for some duration. Application of synthetic and natural inhibitors delayed and decreased sprouting but both storage temperature and genotype influenced these parameters. In a previous study (Nyankanga et al. 2018), differences in dormancy period of cultivars were recorded and shown to vary with the application of chemical inhibitors under ambient tropical conditions.

The most dramatic effect was achieved with CIPC, which inhibited the germination of all tubers at 10 °C and up to week 20 at 23 °C, but at this storage temperature, an effect of genotype was observed. Our results are similar to the previous study, which showed the efficacy of CIPC on potato tubers (Mehta & Kaul 1991; Mehta 2004; Nyankanga et al. 2018; Şanli et al. 2010); however, also some toxicity of CIPC on tubers was observed (Mondy et al. 1992). The same authors reported that tubers treated with CIPC and stored at 5 °C contained higher residue levels than those stored at 20 °C. The residue levels of CIPC decreased with storage duration (Sakaliene et al. 2008; Singh & Ezekiel 2010). We hypothesize that the single application of CIPC required for the entire duration of potato storage may be economically feasible for resource-constrained potato farmers in the highland tropics under ambient conditions under which it has been shown to have less residue levels. Therefore, it may be feasible to remove potato tubers treated with CIPC from storage and expose it to elevated temperatures (>25 °C) for several days to minimize residue levels on tubers destined for consumption. In this research, we did not assess the residue levels of CIPC on potato tubers, but hypothesize that due to the longer duration of storage (>20 weeks) under elevated temperatures, residue levels may be quite low (Mehta & Ezekiel 2003).

Also, DMN and peppermint essential oil were more effective in delaying sprouting at 10 °C than at 23 °C. On tubers of ‘Kenya Mpya’ stored at 10 °C, DMN treatment completely inhibited sprout development, suggesting that it could provide complete sprout suppression for tubers of this cultivar but not at 23 °C as sprout development was recorded. Their inhibiting influence was also dependent on storage temperature and tuber genotype. Inhibition caused by PPO was weaker than DMN at 10 °C for ‘Asante’ and ‘Kenya Mpya’. At 23 °C, PPO inhibited sprouting weaker than DMN in ‘Kenya Mpya’ but stronger in ‘Asante’ and ‘Shangi’. The efficacy of essential oil derived from peppermint for sprout inhibition on potato has been previously documented (Song et al. 2008). It has been demonstrated that essential oil (derived from plants), whose composition consisted mainly of monoterpenes and carvone, are effective in reducing sprouting, tuber weight loss and tuber rot over a 225-day storage (Alamar et al. 2017). The sprout inhibition by compounds derived from essential oils of carvone, acts by preventing dormancy break and inhibits sprout growth after dormancy initiation (Oosterhaven et al. 1995). However, because of the volatility of essential oil, it has been suggested that multiple applications of essential oils are needed for inhibition of sprout development on tubers. Additional mode of actions of essential oils were postulated to be associated with disruption of microbial cell membranes. Furthermore, it has been shown that some essential oil contains compounds of the unsaturated ketone group, which inhibits sprout growth (Capelle et al. 1996). Other mechanisms of essential oils have been associated with the degradation of enzymes that are crucial for the biosynthesis of cytokinin, gibberellic acid and abscisic acid, and membrane components of the cell, thereby affecting sprout development.

Sprouts of tubers treated with DMN and stored at 10 °C had sprout length of <2.6 mm throughout the storage duration, while tubers at 23 °C had greater sprout length (≥6 mm). In this research, we noted that even though tubers of ‘Asante’ and ‘Shangi’ treated with DMN sprouted at the same time as untreated tubers following storage at 23 °C, the percentage of sprouting and sprout length were greatly reduced. In a similar research, it was reported that two applications of DMN at 200 and 300 mg·kg⁻¹ were equal to CIPC treatment in the dose 22 mg·kg⁻¹ in the suppression of sprout elongation after 295 days in storage at 7 °C (Lewis et al. 1997).

In this study, it was observed that the number of sprouts at the completion of storage experiment was reduced considerably for ‘Shangi’ and ‘Asante’ tubers exposed to DMN treatment. This may have been a consequence of the dry necrotic sprouts dropping from tubers, perhaps due to tuber handling. Therefore, tubers of cultivars that exhibited short dormancy under ambient (23 °C) storage conditions would behave similarly under cold storage (10 °C), when compared to the tubers of cultivars with longer dormancy duration. However, previous researchers noted reduced sprout elongation of tubers subsequent to treatment with DMN and storage at 10 to 17 °C compared to the untreated tubers (Lewis et al. 1997). Therefore, our research results imply that tuber treatment with DMN suppressed sprout growth and greatly reduced sprout elongation at the two storage temperatures.

DMN and peppermint oil, the use of which is not prohibited, also provided comparatively good sprouting inhibition under storage of potato tubers in ambient temperature, indicating that they could be used by resource-constraint potato farmers in tropics as an alternative to CIPC.