S. Jenni, C. Vigneault, J. Boutin, J.F. Dubuc, F.W. Takasu, T. Rennie
Rib discoloration is a physiological disorder appearing as brown spots in the midribs of the head leaves of crisphead lettuce (Lactuca sativa). Symptoms generally occur with warm summer temperatures, when the heads are close to commercial maturity. A direct relationship between rib discoloration and heat stress applied after the heading stage was previously established in a controlled environment. The two-week period after the heading stage was identified as the critical stage for the generation of rib discoloration by adverse temperature conditions. However, this relationship has not been validated in field conditions. The present study proposed to relate rib discoloration with field temperature recorded at particular phenological stages. A four-depth copper-constantan thermocouple needle was specifically designed to simultaneously measure tissue temperature at 1, 5, 10 and 40 mm below the head surface. One four-depth needle was placed inside each of eight uniform heads per field according to the 14:00 h sunlight vector line crossing the centre of the head, and was connected to a multiplexed data logger recording head and air temperatures simultaneously. At commercial maturity, heads from 12 fields were evaluated for the number of leaves showing rib discoloration symptoms. Lesions were scattered between the 1st and 13th leaves forming the head and more frequently occurred on the 4th leaf, which corresponded to a depth of 5 mm. Significant correlations were found between the number of leaves with rib discoloration and the time-temperature integration recorded between BBCH (Biologische Bundesanstalt, Bundessortenamt and CHemical industry) stages 45 and 47, using tissue temperature at 5 mm below the head surface or air temperature at 2 m above the ground as measured by a nearby meteorological station. These findings 1) confirmed the hypothesis of a developmental period that is more sensitive to heat stress inducing rib discoloration symptoms, and 2) represented the first step towards the development of a model predicting rib discoloration from field temperature recorded at specific phenological stages.
Jenni, S., Vigneault, C., Boutin, J., Dubuc, J.F., Takasu, F.W. and Rennie, T. (2012). TOWARDS PREDICTING RIB DISCOLORATION FROM FIELD TEMPERATURE AND PHENOLOGY IN CRISPHEAD LETTUCE. Acta Hortic. 936, 113-117
DOI: 10.17660/ActaHortic.2012.936.12
Lactuca sativa, heat stress, high temperature, thermocouple, modelling, physiological disorder, phenology

Acta Horticulturae