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, 149, 221-234

Fructans Redistribution Prior to Sprouting in Stored Onion Bulbs Is a Potential Marker for Dormancy Break

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Fructans Redistribution Prior to Sprouting in Stored Onion Bulbs Is a Potential Marker for Dormancy Break

I C Ohanenye et al. Postharvest Biol Technol.

Abstract

Continuous supply of high quality onion bulbs to meet year-round demand is dependent on maintaining dormancy and bulb quality during storage. Sprouting impacts negatively on the storage quality of onion bulbs. Ethylene supplementation has previously been revealed to inhibit sprout growth in stored onion bulbs. Fructans content, especially those at higher degree of polymerisation (DP), are reported to positively correlate with delayed sprouting. However, little is known about the impact of pre-harvest irrigation regimes on fructans accumulation and redistribution in relation to onion bulb dormancy and quality in store. Across two seasons, onion plants of cultivars 'Red Baron' and 'Sherpa' were subjected to full irrigation (FI) (100% replenishment of crop evapotranspiration) or deficit irrigation (DI) (50% of FI treatment) from bulb initiation to harvest. Bulbs were harvested at full maturity and stored at 1 °C for five months. Bulbs were treated with or without 1-MCP (1 μL L-1) for 24 h before storage under continuous ethylene supplementation (10 μL L-1) or air. DI had no effect on dormancy-break, sprout emergence, total fructans content and total sugar content. In contrast, ethylene delayed sprout emergence and suppressed sprout growth; added 1-MCP enhanced this effect. The concentration of DP3-8 fructans were higher in top and bottom sections compared to the baseplate. Before sprout emergence, fructans of DPs 7-8 were no longer present in the top and bottom wedges, while they accumulated in the baseplate; irrespective of pre- or postharvest treatments. This redistribution of fructans within the bulb suggested a transition in dormancy state and could be used as a predictive marker for sprouting in stored onion bulbs.

Keywords: Allium cepa; Deficit irrigation; Fructans; Sprouting.

Figures

Fig. 1
Fig. 1
Sprout length in stored onion bulbs. Sprout length was measured as sprout length in proportion to bulb height represented as a percentage for onion bulbs of cultivar ‘Red Baron’ (RB) and ‘Sherpa’ (SH) grown in 2015 (A and B) and 2016 (C and D) under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. After curing, bulbs were either treated with 1-MCP at 1 μL L−1 for 24 h or untreated before storage; and then, stored at 1 °C under continuous ethylene supplementation at 10 μL L-1 or air. DIA and FIA are DI and FI bulbs in air, DIE and FIE are DI and FI bulbs stored under ethylene, DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. LSD bar at 95% confidence shown.
Fig. 2
Fig. 2
Pre- and postharvest total fructans content in the top wedge, bottom wedge and baseplate of stored onion bulbs treated or untreated with 1-MCP. Total fructans contents (sum of fructans of DP3-8 content) per dry weight for the top wedge (A), bottom wedge (B) and baseplate (C) of onion bulbs of cultivar ‘Sherpa’ grown in 2016 under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were treated with 1-MCP at 1 μL L−1 for 24 h before storage or untreated with 1-MCP, and were stored at 1 °C under continuous ethylene supplementation at 10 μL L-1 or air. DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. Where pre-harvest = –8; harvest = –6; mid-curing = –3; and end of curing = 0 in weeks. LSD bar at 95% confidence shown.
Fig. 3
Fig. 3
Pre- and postharvest kestose and nystose contents in the top section, bottom section and baseplate of stored onion bulbs treated with and without 1-MCP and ethylene. Kestose and nystose contents per dry weight for the top wedge (A and D), bottom wedge (B and E) and baseplate (C and F) of onion bulbs of cultivar ‘Sherpa’ grown in 2016 under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were treated with 1-MCP at 1 μL L−1 for 24 h before storage, and were stored at 1 °C under continuous ethylene supplementation at 10 μL L-1 or air. DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. LSD bar at 95% confidence shown.
Fig. 4
Fig. 4
Pre- and postharvest fructans of DP7 and 8 contents for the top section, bottom section and baseplate of stored onion bulbs. Fructans of DP7 and 8 contents per dry weight for the top wedge, bottom wedge and baseplate of onion bulbs of cultivar ‘Sherpa’ grown in 2016 under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were stored at 1 °C under continuous ethylene supplementation at 10 μL L−1 or air. DIA and FIA are DI and FI bulbs stored in air and DIE and FIE are DI and FI bulbs stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. LSD bar at 95% confidence shown.
Fig. 5
Fig. 5
Pre- and postharvest total sugar content in the top section, bottom section and baseplate of stored onion bulbs treated with and without 1-MCP and ethylene. Total fructans contents (sum of fructose glucose and sucrose content) per dry weight for the top wedge (A), bottom wedge (B) and baseplate (C) of onion bulbs of cultivar ‘Sherpa’ grown in 2016 under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were treated with 1-MCP at 1 μL L−1 for 24 h before storage or untreated with 1-MCP, and were stored at 1 °C under continuous ethylene supplementation at 10 μL L-1 or air. DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. Where pre-harvest = –8; harvest = –6; mid-curing = –3; and end of curing = 0 in weeks. LSD bar at 95% confidence shown.
Fig. 6
Fig. 6
Pre- and postharvest sugar content in stored onion bulbs treated with and without 1-MCP and ethylene. Fructose, glucose and sucrose contents per dry weight for the top wedge (A, B and C), bottom wedge (D, E and F) and baseplate (G, H and I) of onion bulbs of cultivar ‘Sherpa’ grown in 2016 under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. After curing, bulbs were treated with 1-MCP at 1 μL L−1 for 24 h before storage and were stored at 1 °C under continuous ethylene supplementation at 10 μL L-1 or air. DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. LSD bar at 95% confidence shown.
Fig. 7
Fig. 7
Real-time respiration rates as CO2 production (mg kg–1 h–1) of stored onion bulbs. Onion bulbs of cultivar ‘Red Baron’ (RB) and ‘Sherpa’ (SH) were grown in 2015 (A and B) and ‘Sherpa’ in 2016 (C and D) under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were treated with or without 1-MCP at 1 μL L−1 for 24 h before storage; and then, stored at 1 °C under continuous ethylene supplementation at 10 μL L−1 or air. DIA and FIA are DI and FI bulbs in air, DIE and FIE are DI and FI bulbs stored under ethylene, DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. LSD bar at 95% confidence shown.
Fig. A1
Fig. A1
Depiction of sprout assessment.
Fig. B1
Fig. B1
Onion bulb cut into top wedge, bottom wedge and baseplate.
Fig. C1
Fig. C1
Postharvest curing weight-loss (A and B) and storage weight losses (C and D) in stored onion bulbs. Onion bulbs of cultivar ‘Red Baron’ (RB) and ‘Sherpa’ (SH) were grown in 2015 (A and C) and 2016 (B and D) under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were either treated with 1-MCP at 1 μL L−1 for 24 h or untreated before storage; and then, stored at 1 °C under continuous ethylene supplementation at 10 μL L-1 or air. DIA and FIA are DI and FI bulbs in air, DIE and FIE are DI and FI bulbs stored under ethylene, DIMA and FIMA are DI and FI bulbs treated with 1-MCP and stored in air and DIME and FIME are DI and FI bulbs treated with 1-MCP and stored under ethylene. LSD bar at 95% confidence shown.
Fig. D1
Fig. D1
Pre- and postharvest fructans DP6 content in stored onion bulbs. DP6 fructans contents per dry weight for the top wedge (A and B), bottom wedge (C and D) and baseplate (E and F) of onion bulbs of cultivar ‘Sherpa’ grown in 2015 under full irrigation (FI) or deficit irrigation (DI); where FI amounted to 100% replenishment of crop evapotranspiration (ETc) and DI was ETc replenishment at 50%. Bulbs were harvested at full maturity (100% fall-down) and cured under glass for six weeks for both years. Post-curing, bulbs were stored at 1 °C under continuous ethylene supplementation at 10 μL L−1 or air. DIA and FIA are DI and FI bulbs stored in air and DIE and FIE are DI and FI bulbs stored under ethylene. SpA and SpE are sprout emergences for bulbs stored in air and ethylene, respectively. LSD bar at 95% confidence shown.

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