Abstract
1. In the present paper, an investigation has been made on the susceptibility to Helminthosporium leaf spot of rice plants grown in various nutritional conditions with different amounts of elements. The growth of plants, the content of inorganic elements and carbohydrates, the intensity of respiration and the intensity of photosynthesis in leaves were also measured considering the relation to the disease susceptibility.
2. The susceptibility was expressed as the total number of spots per unit length of leaf and the index of spot enlargement (per cent of large spots to the total number). The index of spot enlargement correlated positively with the respiratory intensity of leaves, but negatively with the growth of the plants or with the P-R ratio (the ratio of photosynthetic-to respiratory intensity). However, the total number of spots was not related to any of the above three factors. Judging from the above results, the index of spot enlargement is more significant to the susceptibility than the total number of spots. On the other hand, the susceptibility showed no clear relation to the content of inorganic elements and carbohydrates in leaves.
3. The susceptibility was decreased in plants grown in nutritional solutions with an excess nitrogen (+N) or potassium (+K). The presence of manganese (+Mn), iodine (+I) and zinc (+Zn) also decreased the susceptibility. On the contrary, excess in phosphorus (+P), deficiency of nitrogen (-N), potassium (-K) and magnesium (-Mg), or the presence of cadmium (+Cd) and cobalt (+Co) increased the susceptibility.
