| heal.abstract |
Cos (Parris Island), butterhead (White Boston), looseleaf (Grand Rapids) and crisphead (Great Lakes) lettuce were sown in autumn, winter and spring and the plants were cultivated for seed under plastic cover. The seed yield of all varieties was significantly higher from the autumn sowing in comparison with the winter and spring sown crops. Differences in seed yield related to the longer period of vegetative growth from the autumn sowing, which resulted in a significantly higher head weight prior to bolting. The low seed yield of spring-sown crops resulted from premature bolting due to the warm climatic conditions prevailing at the early stages of plant growth. The application of gibberellic acid at the 8-leaf stage promoted bolting, but only in the autumn sown crops of Parris Island and Grand Rapids did this bring about a significant increase in seed yield. The application of gibberellin at head maturity increased the height of the seed stalk, as did gibberellin application at the 8-leaf stage, but did not affect seed yield. Unlike field grown seed crops in cooler climates, where spring sowings are favoured by a progressive increase in ambient temperature up to about 25°C, under the warm climatic conditions of southern Greece, autumn sowing is indicated to avoid premature bolting. |
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