The growth, development and yield of onion (Allium cepa L) in response to temperature and CO2

Stands of two cultivars (cv. Hysam and Site) of onion (Allium cepa L.) were grown in the held within polyethylene-covered tunnels along which a temperature adient was imposed. Pairs of tunnels were maintained at either 374 or 532 mu mol mol(-1) CO2. The rates of progress from transplanting to bulbing, and from bulbing to harvest maturity, were positive linear functions of mean temperature for each cultivar. At a given temperature, the time of bulbing was earlier, but the duration of bulb growth longer, at elevated compared with normal CO2. Canopy architecture was not affected by CO2, temperature or cultivar; an estimate of 0.25 for the canopy light extinction coefficient was common to all treatment combinations. Radiation use efficiency was greater at elevated compared with normal CO2 in the period up to bulbing, but was the same at both CO2 concentrations during subsequent bulb growth. Total crop dry weight at bulbing was increased by 32-44% due to elevated CO2. Bulb yields at harvest maturity declined with progressively warmer temperatures and to a greater extent in cv. Site than cv. Hysam. Enrichment with CO2 increased bulb yields by 29-37% and by 35-51% in cvs Hysam and Site, respectively. From comparison of the temperature rise needed to offset entirely the yield increases of each cultivar due to elevated CO2, it is concluded that current estimates of climate change should be beneficial for bulb onion production, particularly for long-season cultivars.