The growth of three crisp lettuce varieties from different sowing dates

Abstract

Summary: Three American crisp lettuce varieties Ithaca, Pennlake and Saladin, formerly called Salinas, were sown on three occasions between April and July in 1980 and 1981 and on six occasions in 1982 between March and July. The rate of emergence of all three varieties increased with mean soil temperature at seed depth. The number of leaves was linearly related to air temperature measured on an accumulated day-degree scale > 0 °C from emergence. Pennlake had the highest rate of leaf production and Saladin the lowest rate. The relative growth rate of Ithaca increased with later sowing, while that of Pennlake declined. The time of maturity of one variety relative to another changed with sowing date but in all three varieties a quadratic curve relating the time of maturity to the time of sowing accounted for a high proportion of the variance in the time of maturity. When the times of sowing and maturity were both expressed on a day-degree scale > 0 °C a linear relationship accounted for more than 98% of the variance in time of maturity. These data provide the basis for planning a continuity programme. Within a variety there was considerable variation in mean head weight between sowings in the same year and different responses of head weight to sowing date from year to year. Using data from all 12 sowings, there was shown to be a significant association between head weight and the mean temperature up to 42 days from emergence but there was no association with radiation. Low mean temperatures (< 12 °C) were associated with lower head weights of Ithaca but higher head weights of Saladin. The opposite was true for mean temperatures greater than 16 °C. The reasons for this are discussed.