Growth, Dry Matter Partition and Radiation Interception in an Overwintered Bulb Onion (Allium cepa L.) Crop

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Abstract
Growth, bulb development, partition of dry weight between leaf blades and bulbs, and the interception of solar radiation were measured in overwintered crops of five cultivars of bulb onion with different maturity dates sown on successive dates in three seasons. The onset of bulbing was later the later maturity of the cultivar. Later sowing did not delay the onset of bulbing but it did delay maturity. There was little mean difference between cultivars in the duration of bulb growth defined as the interval between onset of bulbing and maturity, but there were considerable differences between cultivars within a season, and between seasons for a given cultivar. Duration of bulb growth ranged from 11 to 46 days with a mean of 35 days. Increases in total shoot dry weight during bulb development and, in the absence of much bolting, bulb dry-matter yields were linearly related to the total radiation intercepted during bulb growth. These relationships were similar to those reported for other crops in Britain. Radiation interception during the phase of bulb growth was low compared with other crops, with a mean value of 49 per cent and a maximum of 65 per cent. The percentage of solar radiation intercepted during bulb development was higher from early sowings than from later ones, particularly in early maturing cultivars. The harvest index was high, with typically more than 80 per cent of the shoot dry weight in bulbs at maturity.

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