To be technically accurate: heat without bolting will cause bitter lettuce, bolting will cause bitter lettuce, heat will cause bolting will cause bitter lettuce. The exact numbers are VERY specific to cultivars.

UF Research Study: Lettuce Cultivars for Warm Season 99-13

> Common name: lettuce.
> Latin name: Lactuca sativa L.
> Family name: Compositae (Asteraceae)
> The Latin name (Lactuca) is derived from the Latin root word "lac" or milk.
> "Lettuce" derived from the French "laitue" meaning "milk".
> "Sativa" means grown from seed.

Bitterness associated with the production of latex, the milky juice still found in the cultivated varieties when they flower.

http://www.hort.purdue.edu/rhodcv/hort4 ... e00001.htm



Also, from our Master Gardener's in AZ: Lettuce withstands light frost; however, sunlight and high summer temperatures usually cause seedstalk formation (bolting) and bitter flavor.

http://ag.arizona.edu/pubs/garden/mg/ve ... ttuce.html

and a fun fact...

What is the best time of the day to harvest lettuce?

A. Research has shown that lettuce harvested at 7 a.m. contains up to 100% more sugar than leaves picked at 2 p.m.

http://www.ag.ndsu.edu/hort/info/vegetables/tips.htm