Physiological disorders related to heat stress
Tip burn
Tip burn is characterized by necrosis on the ends and edges of young leaves in development. This disorder occurs mainly a few days before harvest and is attributed to localized calcium deficiency. Environmental conditions that promote rapid plant growth (warm temperatures, high light intensity and long photoperiod) or reduce transpiration (high relative humidity, low air movement, water stress) favor tip burning.
The presence of necrotic leaves in a lettuce affects its appearance and shelf life.
Rib discoloration
Symptoms develop when lettuce matures and becomes firmer. This physiological disorder is directly associated with heat stress. The onset of symptoms begins with small brown streaks along the midribs of the leaves beneath those surrounding the head lettuce. Ranging from light brown to black, the lesions darken over time and are often followed by rotting during storage, thus reducing lettuce quality and marketing.
Bolting
Also known as "going to seed", bolting is the rapid elongation of the main stem in the center of the plant, before the lettuce has reached an optimal weight for the market. The quality of the lettuce is thus strongly altered and no longer meets the marketing standards. This is especially true in romaine lettuce, where the length of the heart should not exceed one third of the plant's total size.
Bolting can result from several factors, including day length exceeding 13 hours of sunlight, as is the case in Quebec, the prevalence of high temperatures at particular stages of growth and certain stresses, such as lack of water.
Since they have no control over the weather in the fields, the selection of resistant varieties adapted to our conditions is a major asset for producers. Depending on the physiological disorder, crisphead lettuce varieties such as Hochelaga, Estival, AAC Champlain, AAC Canicula and AAC Richelieu have good tolerance levels and even resistance. Always in search of innovation, the Lettuce Foundation continues its research. Thus, new varieties of head and romaine lettuce should emerge in the coming years.
