|Jenni, Sylvie -|
Submitted to: Euphytica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 2009
Publication Date: February 1, 2010
Citation: Jenni, S., Hayes, R.J. 2010. Genetic variation, genotype x environment interaction, and selection for tipburn resistance in lettuce in multi-environments.. Euphytica, Feb 2010, Vol 171 (3), pages 427-439. Interpretive Summary: Tipburn is a physiological disorder causing dark necrotic lesions on the edges of lettuce leaves that reduces the quality and shelf-life of lettuce (Lactuca sativa L.). The environmentally induced disorder is likely related to localized calcium deficiencies in lettuce leaves, and breeding new lettuce cultivars with resistance to tipburn is the most efficient control method. The objective of this work was to determine whether lettuce cultivars have similar levels of tipburn resistance across diverse production environments, with the goal of establishing whether cultivars must be developed for specific production regions. Tipburn incidence was recorded over two years in field experiments in Salinas, CA, and Yuma, AZ, for 21 romaine, 17 crisphead, 11 green leaf, and six red leaf type cultivars, and over three years in field experiments in Quebec for 15 romaine cultivars. Analysis of the California and Arizona dataset revealed that the environment, cultivars, and unpredictable interactions between environment and cultivar contribute to tipburn development. The results indicate that tipburn resistance in lettuce cultivars should be selected based on overall mean performance, and the stability of cultivars across multiple testing locations and growing seasons. Among the lettuce types, crisphead was the only one demonstrating any significant improvement for tipburn resistance, reflecting the greater breeding effort applied to this type compared to the romaine, green leaf or red leaf types. In separate experiments to improve tipburn resistance in romaine type cultivars, single plant selection for the absence of tipburn was conducted in three romaine breeding populations. This effort was ineffective for developing tipburn resistant romaine cultivars, a result that is consistent with our findings of limited genetic variation for tipburn resistance among romaine cultivars and the existence of interactions between cultivars and environments. Identification and selection of morphological characters associated with resistance may be an effective alternative method for developing tipburn resistant cultivars.
Technical Abstract: Tipburn is a physiological disorder causing major economic damage in all areas of lettuce (Lactuca sativa L.) production. Tipburn is known to be the result of a localized calcium deficiency, but the environmental conditions leading to the expression of the disorder are largely unpredictable. The objectives of this work were to determine the genotype (G) × environment (E) interaction (GE) for tipburn incidence, with the goal of establishing whether cultivars must be developed for target environments, and to study the variation in tipburn incidence in a wide range of lettuce cultivars. Tipburn incidence was recorded over two years in Salinas, CA, and Yuma, AZ, for 21 romaine, 17 crisphead, 11 green leaf, and six red leaf type cultivars, and over three years in Quebec for 15 romaine cultivars. Analysis of the California and Arizona dataset revealed that E, G, and GE affected (P < 0.0001) tipburn incidence, with GE explaining 40.8%, G explaining 34.5%, and E explaining 24.8% of the total variation in tipburn incidence. Crossover interaction was present but not repeatable over years, indicating that Salinas and Yuma belong to the same complex mega-environment and that cultivar/breeding line evaluations should be based on mean performance and stability. Among the lettuce types, crisphead was the only one with a significant genetic variability for tipburn resistance, reflecting the greater breeding effort applied to this type compared to the romaine, green leaf or red leaf types. This result was further confirmed by the analysis of 15 romaine cultivars tested in three Quebec sites over three years, where G (P < 0.05) contributed only 5.0% of the total variation in tipburn incidence, whereas E (P < 0.0001) contributed 49.4% and GE (P < 0.0001) contributed 45.6%. Again, crossover interaction was present but not repeatable over the three years. Analysis of a dataset with five romaine cultivars in eight environments in California, Arizona, and Quebec for two years revealed significant E (P < 0.0001) and G (P < 0.05) effects but no GE effect. Yuma in 2006 and Saint-Blaise in 2005 were highly correlated (r = 0.923, P < 0.05) and were the most discriminating and most representative environments for tipburn evaluation. Single plant selection for tipburn resistance in three F2 romaine populations did not result in genetic improvement for tipburn resistance, a result that is consistent with the presence of GE and a lack of genetic variation for tipburn resistance in the romaine gene pool. Further, the degree of head closure was significantly associated with tipburn incidence. Numerous multi-location trails are needed to identify cultivars and breeding lines with tipburn resistance that is effective across a wide diversity of environments. Selection in early segregating generations is ineffective in lettuce, particularly in romaine populations with low genetic diversity. However, identification and selection of morphological characters associated with resistance may be an effective alternative.