Nitrogen availability and susceptibility of tomato leaves to Botrytis cinerea

The aim of this study was to investigate the effect of nitrogen availabilit y on susceptibility of tomato leaves to the fungal pathogen Botrytis cinere a. Plants with varying nitrogen availability were grown by adding N daily i n exponentially increasing amounts to a nutrient solution at different rate s. Leaves of plants grown at low nitrogen availability had a high leaf C/N ratio (21 g g(-1)) and were about 2.5 times more susceptible to primary les ion formation by B. cinerea compared to plant grown at high nitrogen availa bility, which had a low leaf C/N ratio (11 g g(-1)). Leaf C/N ratio account ed for 95% of variation in susceptibility. This relationship between C/N ra tio and susceptibility persisted when plants were grown with exponential P addition and optimal N supply, and was thus independent of plant growth rat e or related factors. We could not explain the effect of nitrogen availabil ity by variation in the most obvious N-based resistance compound alpha-toma tine because more susceptible leaves with a high C/N ratio contained more a lpha-tomatine. These leaves also contained more soluble carbohydrates. The level of soluble carbohydrates correlated positively with susceptibility, i ndependent of the growth method. We therefore suggest that the effect of N availability on susceptibility must be explained by variation in levels of soluble carbohydrates and speculate about the role of these carbohydrates i n the infection process.