Histology of quantitative resistance in flax to the flax rust fungus (Melampsora lini)

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Abstract
Macroscopical and histological research is useful to characterize quantitative resistance. Analysis of histological components of the resistance to the flax rust fungus (Melampsora lini) was carried out on seedlings of seven flax (Linum usitatissimum) accessions: five fibre flax accessions with various levels of quantitative resistance and two near-isogenic lines of linseed with gene K or NI. Flax seedlings were inoculated with uredospores. Collected leaves of the flax accessions were stained with Chlorazol Black E. It was found that in the pathosystem flax - M. lini, a lower number of pustules (lower infection frequency) on quantitatively resistant accessions did not result from mechanisms acting before stomatal penetration by the fungus. Reduced infection frequency was associated with increased rates of early abortion of the rust fungus but rates of this abortion were not large enough to explain the low infection frequency on the quantitatively resistant accessions Afganistan and Solido. In both of the accessions the number of haustoria formed 2 days after inoculation and the size of established colonies after 5 days were reduced. Hyphae of the fungus in the accession Solido stained intensely, and appeared stunted and crinkled. Most of the established colonies in the quantitatively resistant accessions were associated with plant cell necrosis, and at the macroscopic level, pustules were interspersed with some necrotic flecks. This indicates that quantitative resistance of flax to flax rust was caused by incomplete hypersensitivity. Various intensities of the hypersensitive reaction were involved with the resistance of some wild Linum species against M. lini, although in some other accessions it was based on prehaustorial arrestment of the infection units.