Stimulation of Lettuce Productivity by Manipulation of Diurnal Temperature and Light

'Salad Bowl' and 'Waldmann's Green' leaf lettuce (Lactuca sativa L.) were exposed to photosynthetic photon flux densities (PPFD) of 444 or 889 micromoles s-1 m-2 for 20 hours day-1 under a diurnal temperature regime of 25 degrees C days/15 degrees nights or 20 degrees days/15 degrees nights. Leaf dry weight of both cultivars was highest under the high PPFD/warm temperature regime and lowest under the low PPFD/cool temperature regime. 'Waldmann's Green' yielded more than did 'Salad Bowl' at 889 micromoles s-1 m-2 and 25 degrees days/20 degrees nights. Under high PPFD, both cultivars yielded better with 25 degrees days/25 degrees nights than with 25 degrees days/20 degrees nights, although relative growth rates were the same under both temperature regimes.