PHYSIOLOGICAL-ASPECTS RELATED TO TOLERANCE OF SPRING WHEAT CULTIVARS TO SEPTORIA-TRITICI BLOTCH

The susceptible wheat cultivar Miriam exhibited tolerance under severe infection of Septoria tritici blotch (STB). Nethouse and greenhouse t rials confirmed former field results in which losses in grain weight o f 'Miriam' wheat due to STB infection were significantly lower than th ose of the susceptible cultivar Barkai, under equivalent severity and the same disease progress curve. Several physiological mechanisms that may explain this tolerance of 'Miriam' wheat were studied. A comparis on between protected and infected plants proved that carbohydrate rese rves in the culms and other vegetative plant parts did not account for the lower losses in grain weight of 'Miriam'. Each tiller was shown t o be independent in its supply of carbohydrates to its grains, and no import from secondary tillers was observed. Differences in the ratio b etween grain weight and vegetative biomass could not explain the susta ined grain filling of infected plants of 'Miriam'. The daily balance o f CO2 exchange of the ears was negative, since carbon fixation by the spike in the light was more than counterbalanced by night time spike r espiration. Radioisotope studies revealed that mature, infected 'Miria m' plants maintained as large a percentage of the carbohydrates fixed at the vegetative stage and early grain filling as healthy plants. On the other hand, under the same conditions, infected 'Barkai' plants lo st a larger fraction of these carbohydrates. The rate of carbon fixati on per unit of chlorophyll and per residual green leaf area of infecte d 'Miriam' was higher than in healthy plants. It is proposed that this enhancement of photosynthesis in residual green tissue of infected pl ants of the tolerant cultivar Miriam compensates for the loss of photo synthesizing tissue due to STB.