Changing oxygen concentration around the rootstock alters spear elongation and sprouting of white asparagus

• White asparagus cultivated under 5, 10, 20, 40, 60, and 80% O2 concentrations.
• Sprouting seemed to be delayed when the O2 concentration around the roots was <15%.
• Sixty and 80% of O2 atmosphere delayed spear elongation.
• Controlling O2 atmosphere around rootstock may be a new method for growth regulation.

We investigated the effects of manipulating the atmospheric oxygen (O2) concentration around white asparagus rootstocks planted in pots to evaluate the applicability of this technique for growth regulation in white asparagus production. The O2 concentrations around the pots were adjusted to 5%, 10%, 20%, 40%, 60%, and 80%, and plants were exposed to these conditions over a 38-d period. We found that the time of both initial sprouting (i.e., when the length of the first emerging spears reached >30 mm) and harvesting were delayed when the O2 concentration around the pots was 5%; moreover, decreases in the soluble solids contents (SSCs) of the storage roots were inhibited at 5% O2 relative to the control (20% O2). These delays were likely due to a hypoxic effect, which inhibits the metabolism of some respiratory substrates such as sugars and/or organic acids. Under the 5% O2 treatment, the O2 concentration during the gaseous phase of the soil in the pot was estimated to be 12.4–14.4%; therefore, sprouting should be delayed when the O2 concentration around the roots is <15%. Our results also showed that the rate of spear elongation decreased when the O2 concentration around the spears was within the range of 60–80%. Considering that the 40% O2 concentration did not affect the rate of spear elongation, it appears that the threshold for this phenomenon is an O2 concentration of 40–60%. When the rootstocks grown under each O2 treatment were replanted and cultured under normal atmospheric conditions for 25 d, no significant differences were found in growth and yield ability, such as the time of initial harvesting, number of harvested spears, yield, and spear weight. Therefore, the effects of different atmospheric O2 concentrations did not continue after cessation of the experimental treatments. Our results suggest that controlling the O2 concentration around rootstocks may be a new method for the growth regulation of white asparagus. We also expect that the new method can be applied in a closed cultivation system as well as in a plant factory.