ANALYSIS OF THE SUBSEQUENT GROWTH AND DEVELOPMENT OF WINTER GLASSHOUSE LETTUCE IN RESPONSE TO SHORT PERIODS IN GROWTH CHAMBERS DURING PROPAGATION
Growth was analysed in terms of relative growth rate, net assimilation rate and leaf area ratio; also stem lengths and numbers of leaves were recorded. Raising plants in artificial conditions not only produced an increase in dry weight at planting time (as much as 16 times greater), but influenced the type of head produced (more compact than usual for the time of the year) in spite of the intervening weeks of low light conditions in the glasshouse. The gain in time to heading amounted to about 20 days as a result of 12 days in the growth cabinets at 17 klux provided in 16-hour days.
Subsequent floral initiation accompanied by bolting was also promoted when conditions in the growth cabinets were excessive. Although floral initials were not generally microscopically visible until several weeks after planting out, a propensity for early bolting appeared to have been 'built in' during the propagation period, especially if light was given in 24-hour days rather than 16-hour days: an effect that was not merely one of total quantity of radiation, although, in general, the higher the radiation level and the longer the time spent in the growth cabinets, the higher the proportion of early bolting.
The tendency to bolt early was accompanied by increased stem and mean internode lengths, which could be detected at an early stage before floral initiation was microscopically visible. These stem characteristics may be an indication that floral induction has taken place.
The use of a lower temperature of 16°C during the propagation period, instead of the 20°C used mainly in these experiments, delayed early bolting, even when it was dropped for only 8 hours during the 24 hours of continuous light.