Higher seed maturity levels, darker pericarp, and smaller seed size relate to improved damping-off tolerance in spinach

Abstract

With the growing demand for baby-leaf spinach (Spinacia oleracea L.) and an increasing ban on chemical seed treatments, problems of damping-off diseases become more apparent. Pythium ultimum has been shown to be the most common damping-off pathogen of spinach. Previous studies showed a large variation in pre-emergence damping-off tolerance levels among seed lots of spinach cultivars, which did not correlate with the rate of seedling emergence. In this study, we analysed if individual seed vigor-related traits, such as seed size and maturity, can influence seed tolerance towards pre-emergence infection by P. ultimum. Seeds of the same seed lots were measured for their morphological and multispectral properties on the level of a single seed. The seedling emergence from those individual seeds was assessed at different P. ultimum doses and for each individual seed, the characteristics were correlated with its emergence success. The results showed that relatively higher levels of chlorophyll fluorescence, an indicator of lesser seed maturity, had a negative association with emergence success. With increasing P. ultimum dose, the seed size and light reflectance from the pericarp, particularly with violet-blue light, showed an increasingly negative association with emergence. This indicated that smaller seeds and seeds that reflected less (violet-blue) light were more tolerant towards P. ultimum infection. A validation study with seed size and maturity fractions of a single seed lot (excluding potential genotypic and seed production effects) confirmed that seed maturity had a positive association with seedling emergence, and that seed size had a negative association with the emergence in the presence of P. ultimum. Seed size and light reflectance accounted for a small part of the variation in P. ultimum tolerance among the seed lots. Selecting the most mature seeds with relatively less light reflectance (darker pericarp) or mature seeds with smaller sizes is recommended to obtain seed lots with improved pre-emergence damping-off tolerance levels.