Ecological studies of vegetables in the regions of different altitudes. 7

Ecological behaviors of lettuce in hot season

Abstract

To find out the proper seasons for lettuce growing in the regions of different altitudes, two varieties, New York No. 12 and Great Lakes, were planted successively at Nagano and Sugadaira in 1951 and 1952. Factors affecting flower bud differentiation and its development, favorable temperature range for vegetative growth, and damages to diseases were also examined.
It was revealed that Great Lakes, a slow bolting variety, could be grown even in midsummer in the region of 800-900m high above the sea level. In the regions of lower altitudes, lettuce should be sown early in the spring so as the compact head would be formed before the hot season came in or should be grown late enough the head be formed in the cool period of the fall.
Flower bud differentiation was induced in the lettuce when heat summation above 5°C attained to about 1, 700°C for Great Lakes or to about 1, 500 °C for New York No. 12. Bolting was accelerated by high temperature and were checked by low temperature below 15°C.
There was no difference between the two varieties in the speed of seed stalk development, but the time of flower bud differentiation was later in Great Lakes, a slow bolting variety, than New York No. 12, an early bolting variety.
Length of photoperiod seemed to have no effect on the time of flower bud differentiation or its development.
In the early stage of growth (about 75 days after germination) lettuce was rather insensitive to temperature or diseases, but in the late stage of growth when it grew vigorously and formed head, it became sensitive to temperature and susceptible to bottom rot and tipburn. Most favorable growing temperature range for the lettuce in its late growing stage was proved to be 18-21°C in daily mean.