Onions and climate change

Studies of the effect of increasing temperatures and higher CO2 levels in the atmosphere on the growth of cvs 'Hysam' and 'Sito' were performed in the UK, in polyethylene-covered tunnels (Daymond et al., 1997). Mean temperatures warmer than ambient by 2.5°C reduced yields (by 3.4-4.4% °C-1 and 8.7-11.8% °C-1 in cvs 'Hysam' and 'Sito', respectively), presumably because they shortened the duration of growth. Enrichment with CO2 at 532 ^mol (vs. ambient concentration of 374 ^mol) increased bulb dry weight (by 29.0-37.4% and 35.3-51.0% in cvs 'Hysam' and 'Sito', respectively) because it increased the rate of leaf expansion and the rate of photosynthesis until bulbing and extended the duration of bulb growth. From comparison of the temperature rise needed to offset entirely the yield increases of each cultivar due to elevated CO2 (8.5-10.9°C and 4.0-5.8°C for cvs 'Hysam' and 'Sito', respectively), it was concluded that a future concentration of 560 ^mol mol-1 CO2 associated with a 2.1°C rise in global temperatures should be beneficial for bulb onion production in the UK, particularly for long-season cultivars.

The advantages of climate change for the commercial production of bulb onions (cv. 'Hysam') in Britain were corroborated by Wurr et al. (1998) in a more comprehensive experiment. However, Wheeler et al. (1998) concluded that warmer crop-production temperatures would be detrimental to postharvest bulb quality in the UK because sprouting in storage might increase.

In New Zealand, in the context of agricultural onion practices and greenhouse gas fluxes, van der Weerden et al. (2000) started a new onion research line focusing on N2O emissions and showed that onion yield averages 10 t kg-1 N2O-N.

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