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Introduction

The germination of lettuce seed‡ (Lactuca sativa) is inhibited at certain temperatures, above the optimum. These temperatures may not prevent growth, however, of seeds which have already started to germinate. This inhibition seems to be largely a varietal characteristic, for the temperature at which one variety will germinate satisfactorily may completely inhibit the germination of another variety. Furthermore, it usually requires a higher temperature to inhibit the germination of old seed than it does that of freshly harvested seed of the same variety. By freshly harvested seed is meant that which is not more than five weeks old. In the case of most varieties, almost complete failure of the seed to germinate occurs at 30° C, regardless of age, although there are a few varieties that germinate fairly well at this temperature.

In the Imperial Valley of California, and in other sections with similar climate, where lettuce seed is planted in late summer and early fall, unsatisfactory stands are often obtained because of low germination. It is believed that this low germination results from high soil temperatures which prevail at germination time.

These studies have to do principally with the relation of temperatures to lettuce seed germination, but the effect of certain other environmental factors is also considered. They attempt to throw light upon the causes of inhibition of germination at high temperatures; and, they suggest practical methods of overcoming the difficulties of germinating lettuce seed at temperatures somewhat above the optimum.