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 whic h a temperature adient was imposed. Pairs of tunnels were maintained a t either 374 or 532 mu mol mol(-1) CO2. The rates of progress from tra nsplanting to bulbing, and from bulbing to harvest maturity, were posi tive linear functions of mean temperature for each cultivar. At a give n temperature, the time of bulbing was earlier, but the duration of bu lb growth longer, at elevated compared with normal CO2. Canopy archite cture was not affected by CO2, temperature or cultivar; an estimate of 0.25 for the canopy light extinction coefficient was common to all tr eatment combinations. Radiation use efficiency was greater at elevated compared with normal CO2 in the period up to bulbing, but was the sam e at both CO2 concentrations during subsequent bulb growth. Total crop dry weight at bulbing was increased by 32-44% due to elevated CO2. Bu lb yields at harvest maturity declined with progressively warmer tempe ratures 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 a nd Site, respectively. From comparison of the temperature rise needed to offset entirely the yield increases of each cultivar due to elevate d CO2, it is concluded that current estimates of climate change should be beneficial for bulb onion production, particularly for long-season cultivars.