A physiological study of Panicum maximum infected by Phyllachora paspalicola
Detailed morphological studies of the leaf pathogen Phyllachora paspalicola (tarspot) on its host Panicum maximum Jacq. have been previously described. This research reports physiological changes in leaf tissue infected by P. paspalicola. Aspects of photosynthetic and nitrogen metabolism were studied in healthy and tarspot-infected leaf tissue. Concentrations of glucose, fructose, and sucrose in infected and noninfected leaves were also measured. Gross and net photosynthetic rates declined significantly in tarspot-infected leaves at high disease intensities (>25%). Chlorophyll content declined in leaves exhibiting >25% infection. Transmission electron microscopy revealed disruption of lamellae and formation of large lipid bodies in infected mesophyll and bundle sheath cells. Significant changes in some C4 enzymes, such as NADP malate dehydrogenase, aspartate aminotransferase, ribulose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, were noted in tarspot-infected leaves. Small differences in the activities of NADP–malate enzyme and alanine aminotransferase were also observed. Activity of the respiratory enzyme succinate dehydrogenase was also stimulated in intensely infected leaves. An increase in nitrate levels was measured as ascopore sporogenesis occurred. At sporulation and during postsporulation, total nitrogen declined in leaves exhibiting intense infection (>25%). A small increase in glucose and fructose content occurred in leaves exhibiting mature fruiting bodies, whereas a significant (p = 0.05) increase in sucrose content was observed in infected leaves.