Chlorophyll fluorescence imaging to facilitate breeding of Bremia lactucae-resistant lettuce cultivars

During recent years, the progressive variability of Bremia lactucae pathogens has increasingly challenged the breeding of new downy mildew-resistant lettuce cultivars. However, conventional plant breeding is a complex and long-lasting process. Especially resistance-testing of newly obtained lettuce lines is still based on visual rating procedures and, hence, labour-intensive and time-consuming. In this manuscript, a straightforward chlorophyll fluorescence imaging (CFI)-based approach will be presented that is capable of objectively and rapidly distinguishing resistant and susceptible butterhead and batavia lettuce lines. To meet the practical demands of plant breeders as close as possible, the proposed technique was based on actually applied conventional visual rating procedures. To rapidly assess potential changes in plant vitality due to successful infections, local distributions and temporal dynamics of the potential maximum photochemical efficiency of electron flow through photosystem II (Fv/Fm) was pixel-wise statistically analysed. Various tests were performed to evaluate the B. lactucae susceptibility in young plants of ten various sensitive and resistant butterhead and batavia lettuce lines. From leaves of one to three months old plants, discs were punched out, inoculated with B. lactucae race BI:18 and incubated at 15 C in a humid chamber for 12 h. To optimize conditions for infection, open Polyurethane (PUR) plastic foam were tested as bases of leaf discs in addition to conventional filter paper. Routinely, degree and intensity of infection, extent of contamination with secondary pathogens and occurring leaf damage were recorded by visual rating. In image analyses from CFI, infected leaf discs are mostly characterized by considerable patchy distribution patterns of respective photosynthetic efficiency. Presented results indicate that the proposed approach of pixel-wise analyses of Fv/Fm images is effective and precise to rapidly, objectively and, after improvement of underlay and adaption of inoculation time in darkness, automatically differentiate B. lactucae-susceptible and resistant lettuce cultivars.