CONCLUSIONS
We found in these studies that there was a clear significant increase in the relative
growth rates (RGR) of lettuce by increasing the air temperatures from 20-25° C, and a
similar increase by increasing the CO2 level from 500-1000 ppm, but that no further
increase at either 30° C or at 1800 ppm of CO2 occurred. There was no significant interaction between the two major factors. Increasing the light intensity resulted in an increase in RGR at all temperatures and CO2 levels. It should be noted that 500 ppm far exceeds the normal level of carbon dioxide in a greenhouse, and that even when supplementary CO2 is provided the level tends to be only 600 ppm due to leakage through the structure. It is possible that at the higher temperatures and light levels there was some moisture stress, and it will be a part of future studies to examine this,
and also the reason why there was no CO2 response above 1000 ppm.