Growth stage-dependent effects of ozone on the permeability for ions and non-electrolytes of wheat leaves in relation to the susceptibility to Septoria nodorum Berk

Spring wheat plants (cv. Turbo were exposed for 3 days to 0-240 micrograms m-3 ozone (0-120 ppb v/v; 7 h day-1), at different growth stages (GS) from the three-leaf stage (GS 13) to anthesis (GS 65), in environmentally controlled climate chambers. There were three ontogenetic phases distinctly differing in ozone sensitivity. Young plants from tillering until beginning of stem elongation (GS 21-31), and mature plants from ear emergence through anthesis (GS 55-65) were most sensitive to ozone leaf injury, whereas plants during stem elongation (GS 32-39) exhibited high tolerance to the pollutant. The same influence of growth stage was found for the leakage of electrolytes, nitrate, amino-N compounds and soluble carbohydrates from whole leaves after exposure to ozone. Significant ozone effects generally occurred at concentrations above 160 micrograms m-3, whereas ozone at 80 micrograms m-3 did not affect any of the parameters measured. Content of nitrate and carbohydrates in the leaf homogenate was unchanged after ozone exposure, whereas the amino-N fraction was significantly increased. The latter might have partially contributed to the enhanced leakage of amino-N. Ozone effects on the susceptibility of wheat to Septoria nodorum showed the same dependence on plant growth stage as leaf-injury and leakage. Necrosis, formation arid germination of conidia of the pathogen in leakage solutions were increased exclusively with plants at tillering stage or after car emergence but not during stem elongation. However, ozone (160 microgram m-3) did not enhance the leakage of the three solutes into the apoplast. This indicates that permeability properties of the cytoplasmic membranes were not affected and suggests the prevalence of an ozone-induced increase of the symplastic permeability. This is the first report to show that permeability changes in leaves exposed to non-injurious doses of ozone are not necessarily based on an enhanced membrane leakiness. It is concluded that activation of symplastic leaf permeability may play a significant role in ozone-induced leakage of nutrients from leaves, and that this is one likely mechanism for improved growth of S. nodorum on and in its host.