Seedlings of four different experimental hybrids H1, H12, H37, H44, at the age of 28 days after sowing, were subjected to two different concentrations of culture filtrate (oxalic acid) from Sclerotinia sclerotiorum Lib. De Bary. while control plants were grown in the absence of toxins. Forty-eight hours after the treatment, the following characters were observed: plant height difference PHD, plant dry weight difference (PWD), basal stalk diameter difference (BSDD), oxalic acid content (OAC) in plants and shikimate dehydrogenase (SKDH) activity. Among the hybrids considered, genotype H1 appeared to be the most tolerant to Sclerotinia sclerotiorum culture filtrate, as confirmed by its genetic background. PWD and BSDD were unaffected by oxalic acid treatments and a significant increase in OAC and SKDH activity was observed with respect to the control.

The results of this study indicated a positive relation between oxalic acid content of fungal culture filtrate and the enzymatic activity of SKDH in sunflower plants subjected to the treatment. The oxalic acid content in plants attacked by the pathogen increases, reaching a threshold limit beyond which damage becomes visible or defense mechanisms are triggered, which implies an increase of SKDH activity, presumably related to the resistance mechanism.

The proteic structure of SKDH was also investigated and the hypothesis that SKDH alloenzyme is a monomer encoded by a single gene in a biallelic codominant locus was confirmed.

Second dimension SDS-PAGE, using a 3% ß-mercaptoethanol solution, revealed that SKDH is an isozyme characterized by two allelic forms: SkDH-a (64 kDa) and SkDH-b (57 kDa). When a 6 % ß-mercaptoethanol solution was used in correspondence of the SkDH-b band, two bands (40 and 34 kDa) were observed in the treated and control genotypes