Onion (Allium cepa) is a globally important vegetable crop. It is grown in more than 160 countries and on more than 4.9 million ha. Global onion production has been increasing steadily since 1990 (FAO, 2016). The most widely produced marketable product of common onion is a dormant bulb. It is important for the bulbs to remain unsprouted while in storage because premature sprouting results in poor-quality bulbs with low consumer acceptance. Dormancy plays an integral role in onion bulb storage longevity and sprout suppression, but can also delay growth when it is desired. Dormancy release, along with vernalization, are key processes for seed production during which rapid, uniform growth and development are valuable. There is great economic interest in controlling dormancy release for both breeding and seed production.
Dormancy is a temporary suspension of visible growth of any plant structure containing a meristem (Lang et al., 1987). There are three classifications of dormancy, which are defined by the origin of initial growth suppression cues: endodormancy, paradormancy, and ecodormancy. Endodormancy occurs when the signal to suspend growth originates within the affected structure. Paradormancy is a response to a signal that originates in an organ other than the affected structure; ecodormancy is a response to unfavorable and extreme environmental conditions, such as very high or low temperatures or water availability (Lang et al., 1987). All three types of dormancy may occur simultaneously within a plant, but the type present in each structure depends on the life stage, physiology, and environmental conditions. For onion bulbs that have been harvested and put into cold storage, endodormancy and ecodormancy are the primary processes that regulate growth (Chope et al., 2012a).
Bulb endodormancy is prevalent in long-day storage onion germplasm and begins to take effect in the weeks before harvest. As onions growing in the field complete their bulbing phase, they cease production of new leaves and the remaining leaves topple as they senesce. This senescence is often used by growers as an indication that the bulbs are ready to be harvested. When removed from the field, bulbs are taken into storage where they remain dormant. After ≈3 weeks, the bulbs resume preharvest levels of cellular division and transcription (Chope et al., 2012b; Pak et al., 1995). Pak et al. (1995) found that as cellular division resumes, bulbs are capable of rooting when exposed to high moisture. This marks the transition from endodormancy to ecodormancy.
In onion, endodormancy is typically broken through exposure to cold temperatures. These cold treatments also serve to vernalize the bulbs. Little is known about the mechanism of endodormancy release in most plant species, but in seeds, which have been investigated most thoroughly, there is generally a strong association with abscisic acid degradation and gibberellic acid synthesis as dormancy is broken (Née et al., 2017). Successful use of chemical treatments to break endodormancy in seeds and the buds of woody plants using compounds such as hydrogen cyanamide have been reported (Horvath et al., 2003; Mohamed et al., 2012; Vergara and Pérez, 2010). Hydrogen cyanamide acts as a catalase inhibitor, which is a key enzyme that removes reactive oxygen species from plant cells. When administered to endodormant grapevine buds, hydrogen cyanamide was shown to increase levels of hydrogen peroxide in cells before breaking dormancy (Mazzitelli et al., 2007; Mohamed et al., 2012; Pérez and Lira, 2005). Pérez and Lira (2005) hypothesized that hydrogen peroxide acts as a secondary messenger to signal the release of endodormancy in grapevines (Vitis vinifera L.). This hypothesis was later supported by Mohamed et al. (2012) upon finding that, before dormancy release, an accumulation of hydrogen peroxide causes a temporary oxidative stress in grape buds following an exogenous application of hydrogen cyanamide. Similar hypotheses have been posited regarding hydrogen peroxide’s role as a signal in seed dormancy release (Oracz et al., 2007). Chemical signaling under stress is a common role for reactive oxygen species, including hydrogen peroxide. Specifically, hydrogen peroxide has been shown to play a role in a range of cellular processes including programmed cell death, response to wounding, and abscisic acid-mediated stomatal closure (El-Maarouf-Bouteau and Bailly, 2008). An exogenous application of hydrogen peroxide has also been used successfully to break dormancy in seeds (Liu et al., 2011). Much like the findings in endodormant buds and seeds of other plant species, transcriptional analysis of onion bulbs during dormancy release shows that transcripts associated with genes related to defense and stress response are highly upregulated during the transition from endodormancy to ecodormancy (Chope et al., 2012b).
The hypothesis by Pérez and Lira (2005), in combination with the demonstrated success in using hydrogen peroxide to break seed dormancy, made it an interesting candidate for use on onion bulbs. Through this work, we sought to test the effects of an exogenous application of hydrogen peroxide on endodormant onion bulbs and to determine whether these treatments could serve as an effective way to break endodormancy.
We thank Ned, John, and Joan Crescio of Jack’s Pride Farms in Randolph, WI, for their assistance and for donating the bulbs used in this study.
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