Exogenous ethylene inhibits sprout growth in onion bulbs

Abstract

Background and aims: Exogenous ethylene has recently gained commercial interest as a sprouting inhibitor of onion bulbs. The role of ethylene in dormancy and sprouting of onions, however, is not known.

Methods: A cultivar (Allium cepa 'Copra') with a true period of dormancy was used. Dormant and sprouting states of onion bulbs were treated with supposedly saturating doses of ethylene or with the ethylene-action inhibitor 1-methylcyclopropene (1-MCP). Initial sprouting was determined during storage at 18 degrees C by monitoring leaf blade elongation in a specific size class of leaf sheaths. Changes in ATP content and sucrose synthase activity in the sprout leaves, indicators of the sprouting state, were determined. CO(2) and ethylene production of onion bulbs during storage were recorded.

Key results: Exogenous ethylene suppressed sprout growth of both dormant and already sprouting onion bulbs by inhibiting leaf blade elongation. In contrast to this growth-inhibiting effect, ethylene stimulated CO(2) production by the bulbs about 2-fold. The duration of dormancy was not significantly affected by exogenous ethylene. However, treatment of dormant bulbs with 1-MCP caused premature sprouting.

Conclusions: Exogenous ethylene proved to be a powerful inhibitor of sprout growth in onion bulbs. The dormancy breaking effect of 1-MCP indicates a regulatory role of endogenous ethylene in onion bulb dormancy.

Fig. 1.

Photographs and digital scans of sprout leaves isolated from onion bulbs after different treatments. (A) Longitudinal cut through a dormant onion bulb showing the stem plate, with the approximate location of sprout leaves. Insert: dormant bud with sprout leaf visible in the size class (leaf sheath length 2·0–3·5 mm) used in this study. LS, Leaf sheath; LB, leaf blade. (B) Sprout leaves (leaf sheaths 2·0–3·5 mm) isolated from onion bulbs stored at 18 °C and continuously treated with air or 7·2 ± 1·4 µL L⁻¹ ethylene until 14 weeks after harvest. Ethylene treatment was started during bulb dormancy 2 weeks after harvest. (C) Sprout leaves (leaf sheaths 2·0–3·5 mm) isolated from sprouting onion bulbs after 4 weeks continuous treatment at 18 °C with air or 7·2 ± 1·4 µL L⁻¹ ethylene. Ethylene treatment was started 12 weeks after harvest.

Fig. 2.

Changes in (A) percentage initial sprouting, (B) sucrose synthase activity and (C) ATP content in sprout leaves of onion bulbs during storage at 18 °C in air or 7·2 ± 1·4 µL L⁻¹ ethylene. Each data point represents the mean of 40 bulbs (A; n = 40) or ten sprout leaves (B, C; n =10). Vertical bars indicate ± s.e.

Fig. 3.

Changes in (A) percentage initial sprouting and (B) CO₂ production of onion bulbs during storage at 18 °C in air or 10·6 ± 1·4 µL L⁻¹ ethylene. Batches of bulbs were transferred at 4 weeks or 9 weeks after harvest from ‘ethylene’ into ‘air’. Dashed lines indicate time of 50 % initial sprouting. Each data point in (A) represents the mean of 40 bulbs (n = 40); each data point in (B) represents the mean of four storage barrels (n = 4). Vertical bars indicate ± s.e. In the case of the air treatment error bars are too small to be visible.

Fig. 4.

Changes in (A) percentage initial sprouting of onion bulbs and (B) sucrose synthase activity in sprout leaves after treatment with 1-MCP or untreated (control). Treatment with 1-MCP was 4 weeks after harvest. Dashed lines indicate 50 % initial sprouting. Each data point represents the mean of 40 bulbs (A; n = 40) or ten sprout leaves (B; n =10). Vertical bars indicate ± s.e.