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Limitations to Photosynthesis in Leaves of Wheat Plants Infected by Pyricularia oryzae

Blast, caused by Pyricularia oryzae, has become an economically important disease in wheat in Brazil, but little effort has been devoted to understanding the wheat–P. oryzae interaction. This study was intended to determine the effects of P. oryzae infection on the photosynthetic process in wheat plants using a susceptible (BR 18) and a partially resistant cultivar (BRS 229). It was found that the net carbon assimilation rate (A), stomatal conductance (g_s), and transpiration rate were dramatically reduced in both cultivars due to P. oryzae infection but to a lesser degree in BRS 229. Photosynthesis was impaired in asymptomatic leaf tissues, indicating that blast severity is not an acceptable indicator for predicting P. oryzae-induced reductions in A. The proportionally larger decreases in A than in g_s, in parallel with increases in internal CO₂ concentration (C_i), suggest that the lower influx of CO₂ into the diseased leaves caused by stomatal closure was not a prominent factor associated with the reduction in A. Additional support for this conclusion comes from the nonsignificant correlation between A and g_s, the negative correlation between A and C_i and the positive correlation between blast severity and C_i. Both the maximum rate of carboxylation and the maximum rate of electron transport were dramatically depressed at advanced stages of P. oryzae infection, mainly in BR 18, although the reduction in A was not closely related to the decrease in the electron transport rate. In conclusion, biochemical limitations likely related to the reduced activity of Rubisco, rather than diffusive limitations, were the main factor associated with decreases in A during the infection process of P. oryzae on wheat leaves.