From seed to seedling - damping-off tolerance in Spinacia oleracea L

Spinacia oleracea L. (spinach) is an economically important crop that is increasingly cultivated for the consumption of its young leaves, also called baby-leaf spinach. Damping off in spinach is a worldwide problem that can be caused by multiple soilborne pathogens, of which the oomycete Pythium ultimum seems to be most prevalent in causing pre- and post-emergence damping off. Pre-emergence damping off involves the rotting of seeds or roots that have become infected before a seedling emerges above the soil level, while post-emergence damping off involves the wilting or even mortality of seedlings after emergence. Symptoms are most severe under wet soil conditions and suboptimal soil temperatures that hamper germination and seedling emergence, which results in poor and non-uniform crop stands. In conventional spinach production, chemical treatments of seeds and fields are widely applied to manage damping off. However, there is a growing demand for organically produced or non-chemically treated crops, requiring alternative solutions. This dissertation describes the studies that were performed on the possibilities to improve damping-off tolerance in spinach through plant or seed breeding, focusing on seed traits that underlie this tolerance.

Seeds from two or three seed lots of commercial spinach cultivars and from one seed lot fractionated into different seed sizes and/or maturity levels were tested. The spinach seed is botanically an indehiscent fruit with a ‘true seed’ surrounded by a fruit wall, called the pericarp. This anatomy brings a challenge for the germination potential under stressful conditions, e.g., high moisture and/or low oxygen levels. The research on moisture sensitivity of spinach seeds in the absence of P. ultimum showed that larger seeds (>3.50 mm), which had a thicker pericarp, germinated less at high moisture level compared to smaller seeds, which had a thinner pericarp and, more frequently, openings in the pericarp. The sensitivity to high moisture conditions was reduced by germinating spinach seeds under lower temperatures or elevated oxygen levels, or by removing the pericarp from the seed, also called dehulling. These results indicate that the pericarp interferes with the oxygen availability to the embryo, necessary for germination.

When screening eight spinach cultivars in fields with damping-off occurrence and in the greenhouse with naturally-infested field soil, genotypic variation in damping-off tolerance could not be confirmed due to a large variation among the replicates and among seed lots of the same cultivars. These results emphasised the need for a more standardised phenotyping assay. The developed assay consisted of a relatively dry substrate of sand, perlite, and vermiculite in equal volume ratios, inoculated with a cornmeal/sand-based inoculum of a pathogenic P. ultimum isolate. The assay showed reproducible results for pre-emergence damping-off tolerance levels, but variation in tolerance levels among seed lots of the same cultivars confounded the search for cultivar variation.

The large variation among seed lots was further investigated, and no correlation was found between the emergence rate and damping-off tolerance of these seed lots. However, by pre-treating the seeds using priming or dehulling methods, the rate and uniformity of seed germination and seedling emergence increased. Primed seed lots showed a higher rate of seedling emergence and pre-emergence damping-off tolerance levels compared to untreated seed lots. After dehulling, some seed lots had a higher rate of seedling emergence and damping-off tolerance levels, while other seed lots from the same cultivars showed only one of the two effects. These results indicated that rate of germination and emergence is a potential driver but not the only driver of pre-emergence damping-off tolerance.

When seed lots were characterised for specific seed traits, including seed size, roundness, maturity (measured by the negatively correlating chlorophyll fluorescence level), and pericarp colour (measured by light reflectance at a broad spectrum of wavelengths), only a small part of the variation in pre-emergence damping-off tolerance levels could be attributed to differences in seed size and pericarp colour. The large variation among and within seed lots urged to zoom in on the individual seed level, which was possible with the developed assay and sowing the seeds in individual cells. Generally, more mature seeds showed higher P. ultimum tolerance than less mature seeds. Of those seeds, the smaller seeds and the seeds with a darker pericarp were more tolerant than the larger seeds and lighter-coloured seeds. The light reflectance in the violet to blue light spectrum showed the strongest negative association with P. ultimum tolerance, which may relate to the presence of phenolic compounds.

In conclusion, to develop spinach cultivars with improved tolerance to P. ultimum, seed vigour of spinach seeds can be enhanced. Preferred seed traits are a smaller seed size, thinner pericarp, and darker pericarp in addition to high levels of seed maturity. Further research is needed to confirm a genetic base for one or more of these traits, which is necessary for breeding purposes. In the meantime, seeds can be pre-selected for these traits, or they can be pre-treated by dehulling (removal of the pericarp) or priming (increasing their germination rate). A combination of damping-off tolerant cultivars, post-harvest seed selection or seed treatments, and site-specific field measures will remain necessary for a sustainable control of damping off in both organic and conventional spinach production.