Bulb Development in Onion (Allium cepa L.) IV. Influence on Yield of Radiation Interception, its Efficiency of Conversion, the Duration of Growth and Dry-matter Partitioning

Increases in total dry-matter yield during bulb growth in well-irrigatedonion crops were correlated with the total solar radiation interceptedby the leaf canopy. The mean efficiency of conversion of interceptedradiation to dry-matter was 1.58 g MJ^–1. However, efficiencieswere significantly different between seasons and in two yearsout of three they were lower for spring-sown crops than forautumn-sown crops. The lower efficiencies coincided with periodsof high mean temperature and irradiance. The percentage of thetotal irradiance intercepted by the canopy during bulbing (1%)was increased by higher plant densities, earlier sowing andin later-maturing cultivars. It was higher in spring-sown thanin autumn-sown crops especially at low plant densities. Theduration of bulb growth was negatively correlated with 1% andwith the mean air temperature during bulb growth. Consequentlyautumn-sown crops had a longer duration of bulb growth thanspring-sown crops and produced exceptionally high bulb yieldsat high plant densities. Non-irrigated crops had a lower 1%and a shorter duration of bulb growth than irrigated crops,and a lower conversion efficiency in a season of high mean temperatureand irradiance. Onion, Allium cepa L., bulb, irradiance, efficiency, partitioning, plant population, temperature, irrigation, growth-analysis, water-stress, leaf-area-index