1966 Volume 35 Issue 4 Pages 387-394
1. From 1961 to 1965, the author carried out an investigation on lettuce planting throughout the year, using varieties Great Lakes 366, Great Lakes 54 and Imperial 847, and observed their ecological adaptation.
2. The above investigation suggests that in Okayama district it is impossible to grow crisp varieties such as Great Lakes during April to August because of high temperature inducing early flower bud formation. From the view point of quality and harvesting time, the time of sowing in spring will be limitted by the middle of March.
3. The proper time for sowing in late summer is very short and varies with the year around August 20, although lettuce culture sowed in late August or early September was successful under the condition covered with vinyl film.
4. Winter culture in the field condition was also possible in this district by sowing in late September nor early October.
5. In the lettuce culture under green house without heating, the sowing should be done between late September and early November, because they will be injured with high temperature during the growing period.
6. Large and well heading lettuces were harvested from spring or green house culture, whereas poor quality, small and loose heading lettuces were got from late summer culture.
7. It was found that there was a correlation between the index of w1/W or w2/W and the mean temperature during the growing period. (w1; the weight of outer 10 leaves of the heading part. w2; the weight of inner 10 leaves of the heading part. W; the total weight of the heading part). In the former, negative correlation was found and in the latter positive correlation was found.
8. Within the range of temperature that allows the growth of the lettuce, it was found that below 17-18°C the higher the temperature during the growth being the larger the mean weight of a leaf, but the temperature exceeded that the reverse relation was observed.
9. Flower bud formation is ready to occur above 20°C in the growing season.
10. The lettuces grown in warm temperature tend to have long stems and flat form in the head, while those grown in cool temperature have short stems and glovular forms.
11. The lettuces grown in winter season and harvested in cool season have high sugar contents and high osmotic pressures, while those grown in warm season have low values of them.
12. Sugar content and osmotic pressure of the outer leaves were higher than those of the heading part. However, it is observed that those values in the outer leaves of the head were lower and become higher in the inner leaves of the head.
13. It is very interested to note that concentric circle lines in the values of sugar content and osmotic pressure were found within a leaf. The highest and isometric value was recorded in a line along the margin of the leaf, and then the values gradually decreased in the inner part of the leaf. These values in the outer leaves and inner leaves varied remarkably.
14. Heat summation from seedling to flower bud formation was about 1, 700°C in Great Lakes, and varied greatly with season.