Biochemical Aspects of Resistance to Leafminers and Aphids In Lettuce

Host plant resistance is an important component of integrated pest management while understanding the biochemical mechanisms could speed up the resistance breeding.  To study the biochemical mechanisms of lettuce plant resistance to its major insect pests, leafminer (Liriomyza langei) and aphids, sixteen lettuce genotypes with various degree of resistance were planted in growth chambers and a field with four replications.  Protein, sugars, phenolics, carotenoids, and chlorophylls were extracted from plant leaves, and damage by leafminers and aphid numbers of different species were recorded.  Leafminer stings per cm² leaf area of young plants were highly correlated with stings/cm² of mature plants.  This suggests that leafminer-resistant varieties for mature lettuce plants may also be deployed for baby leaf production. Sucrose concentration of young plants was correlated with leafminer stings per cm² leaf (r = 0.730, p = 0.01) and mines per plant (r = 0.790, p = 0.01), suggesting that low sucrose content could be used as a marker for early screening or selection for leafminer resistance.  Leafminer stings/cm² were negatively associated with contents of flavonoids (r = -0.560, p = 0.05) and chlorogenic acid (r = -0.598, p = 0.05), while mines per plant were negatively associated with concentrations of flavonoid (r = -0.700, p = 0.01), chlorogenic acid (r = -0.594, p = 0.05) and rutin (r = -0.603, p = 0.05), implying a role of phenolics in the leafminer resistance.  Total number of apterous aphids including Myzus persicae, Macrosiphum euphorbiae and Aulacorthum solani per 100 g plant weight was correlated with contents of protein (r = 0.890, p = 0.01), β-carotene (r = 0.828, p = 0.01), lutein (r = 0.819, p = 0.01), chlorophyll (r = 0.739, p = 0.01), and glucose (r = -0.771, p = 0.01) of young lettuce plants, which could serve as early indicators of aphid resistance.  Aphids per 100 g plant weight were also associated with chicoric acid concentration of mature lettuce plant (r = 0.750, p = 0.01).  This study provides insights into the mechanisms of resistance to leafminers and aphids as well as screening and selection tools for breeding insect-resistant lettuce.