Q What is tipburn?

A The edges of the leaves, especially those on hearting varieties of lettuce, discolour and die, making the crop very unappetising. The symptoms are less common on outer leaves, and are most common in years when temperatures are high and conditions particularly sunny and bright.
Tipburn is also more likely if the soil is too dry (so uptake of nutrients is impeded) or deficient in calcium. 

Q Can lettuce plants affected by tipburn be helped?

A Unfortunately, once the damage has occurred, there's nothing much you can do. The best advice is to discard affected leaves and use what you can of the remainder.

Q How can I prevent tipburn?

A There is no easy way to control tipburn, but keeping the soil just moist at all times, and perhaps providing some temporary shading during the hottest part of the day, will be of benefit.