Study on Storability of Afghan Local Onions (Allium cepa L.) at Two Thermal Conditions

Introduction Storage potential is an important characteristic for onion. Annually, a large quantity of worldwide onion production deteriorated during storage. The storage ability of onion is influenced by various endo- and exogenous factors including cultivar and storage conditions specially, temperature. Based on literature, there is considerable genetic variation for onion storability in onion germplasms from different origins. Rivera Martínez et al. (2005) evaluated the storability of 18 local and 4 commercial onion cultivars in Spain. Their results showed that there was significant variation for storability among different onion cultivars, and interstingly the local cultivars had more storability than commercial ones. Onion storage could also affected by environmental conditions such as temperature. Benkeblia et al. (2000) studied the effects of various temperatures on respiratory parameters of onion. Their finding showed onion respiration were raised by increasing temperature. Likewise, the positive effect of higher temperature on enhancing the growth of sprouts and elevating transpiration has been reported (Miedema, 1998; Yoo et al., 1997). Although, Afghanistan is considered to be one of the origins of onion (Brewster, 1994) there are no reports on estimating the storability of Afghan local cultivars under different temperature conditions. So this research has been conducted to study the effects of various temperatures on storability of native onion cultivars of Afghanistan.Materials and Methods In this research, the plant materials consisted of 10 onion cultivars consisting eight local varieties “Hanaaie Harat”, “Zard-e-Harat”, “Ghermrz-e-Harat”, “Mazar-e-Sharif”, “Kabul”, “Sar-e-Pul”, “Ghonduz” and “Balkhaab” originated from Afghanistan, an Iranian cultivar (“Ghermez-e-Azarshahr”), and a commercial cultivar “Sweet Grano”. The cultivars were stored at two different temperature regimes (5 ºC and ambient temperature), during six-time intervals of 20 days. The average temperature of ambient storage was 13 °C ranging from 8-18 °C. The experiment lasted from November 6th, 2011 to March 5th, 2012. This research was performed in a completely randomized design based on split factorial design, with three replications. For each replication, 50 healthy non-sprouted bulbs were packaged in plastic nets. Storage potential related characteristics such as sprouting percentage, weight loss percentage, sprout length, and sprout weight/bulb weight ratio in 20 days intervals were measured. All data were subjected to analysis of variance (ANOVA) using SAS statistical software. Mean comparisons were calculated at 5% probability level using LSD when the F-value was significant (Fisher’s protected LSD).Results and Discussion The results showed that there were significant differences among cultivars for storage characteristics (weight loss percentage, sprouting percentage, length of sprouts and sprout weight/bulb weight ratio). “Ghonduz” and “Ghermrz-e-Harat” local cultivars had the most and least storage ability, 50% of bulb sprouting after 105 and 35 days, respectively. Rivera Martínez et al. (2005) and Ramin (1999) also reported Spanish and Iranian local cultivars had more storability compared to commercial cultivars. In 5 °C, weight loss of onion bulbs followed a linear model during storage period. But, in ambient storage, it was higher at the beginning and the end of storage period, showing the adverse effects of high temperatures on bulbs weight loss during storage. The interaction between cultivar and time was also significant. It showed onion weight loss differs in various intervals.  It could be attributed to the genetic variation and morphological difference such as number and thickness of dry scales of onion bulbs. In ambient storage condition, onion sprouting percentage showed a sharp increase in early storage intervals and became stable in the late intervals. Under 5 °C treatment, it raised as a linear model with very slow slope. It seems that weight loss and sprouting of onion bulbs in ambient storage is dependent on temperature and relative humidity of environment. This finding is in agreement with previous reports (Baninasab and Rahemi, 2006; Forudi, 2005). The sprout length and the ratio of sprout weight to bulb weight was lower in 5 ºC than ambient temperature. Abdalla and Mann (1963) and Yoo et al. (1997) also reported that smaller sprouts were observed in lower temperatures. Conclusion In this experiment, the effects of temperature on storability of some local cultivars of onion originated from Afghanistan were studied in different intervals. It was found that the storage potential of onion bulbs could be influenced by genetic factors and environment conditions such as temperature. Findings of this research showed that sprouting percentage, weight loss percentage and onion sprout growth were less under 5 °C treatment than ambient temperature. Therefore, it was concluded that storing onion bulbs in low temperature conditions can reduce the onion spoilage and improve their storage life. Additionally, cultivars showed highly significant variations for all characteristics related to storage ability. Our findings showed that some local cultivars are more storable than commercial cultivars. Among the cultivars, “Ghonduz” cultivar has more storage potent as compared to all studied cultivars. Our results confirmed that local cultivars can be a good source of desirable genes related to storability. Subsequently, they can exploited to broaden the genetic base of breeding matreials.