SUSCEPTIBILITY OF CARROT CULTIVARS TO MYCOCENTROSPORA-ACERINA AND THESTRUCTURE OF CELL-WALL POLYSACCHARIDES

The role of cell wall polysaccharides in pre-formed resistance to necr otrophic fungi is poorly understood. The relationships between cell wa ll polysaccharides, especially the middle lamella pectin, of four carr ot cultivars and their susceptibility to Mycocentrospora acerina, a co ld storage pathogen, were investigated. Amounts of cell wall material and composition of the pectic fraction (galacturonic acid, neutral sug ars) were not correlated with cultivar resistance. However, amounts of acid-insoluble pectin (protopectin) increased with the resistance of the cultivar. Maceration in vivo of carrot tissues from different cult ivars by M. acerina enzyme preparations, mainly polygalacturonases, co rrelated with their susceptibility to the pathogen. In the same way, t he solubilization rate in vitro by M. acerina enzymes of pectic materi al isolated from the most susceptible cultivar was 1.8 times higher th an from the least susceptible cultivar. Hydrolysis of purified middle lamella pectin by endopolygalacturonase confirmed this correlation. Al though the degrees of methoxylation of carrot cultivar pectins were si milar. their behaviour towards endopolygalacturonase was different, in dicating a variation in the distribution of the methoxyl groups along the pectin chain. The results showed a similar composition of cell wal l polysaccharides in the different cultivars but the solubility and es terification characteristics of pectins suggest that pectic substances have an important role in constitutive resistance of carrot to M. ace rina.