Alteration of photosynthetic performance and source-sink relationships in wheat plants infected by Pyricularia oryzae

Two experiments were carried out to assess the changes associated with photoassimilate production and partitioning in the source-sink relationship of flag leaves and spikes of wheat plants infected with Pyricularia oryzae, the causal agent of blast. Flag leaves and spikes were inoculated at 10 and 20 days after anthesis (daa) with a conidial suspension of P. oryzae. Analysis of chlorophyll a fluorescence using maximal photosystem II quantum efficiency (F-v/F-m), fraction of energy absorbed that is used in photochemistry (YII), quantum yield of non-regulated energy dissipation (Y(NO)) and quantum yield of regulated energy dissipation (Y(NPQ)), showed an impairment of the photosynthetic performance in both infected flag leaves and spikes, coupled with reduced concentrations of chlorophyll a + b and carotenoids. Compared to non-inoculated controls, there was lower capacity for CO2 fixation by RuBisCO in the infected flag leaves. Similarly, in the infected flag leaves and grains (obtained from infected spikes), there were lower concentrations of soluble sugars, while the hexoses-to-sucrose ratio increased in infected flag leaves. Compared to non-inoculated controls, infected flag leaves showed lower sucrose phosphate synthase (SPS) activity and lower expression of the sucrose synthesis (SuSy) gene, while higher expression and activity of acid invertases also occurred. At the advanced stages of fungal infection, the concentration of starch in grains decreased but remained high for the infected flag leaves. There were reductions in ADP-glucose pyrophosphorylase activity and the expression of ADP-glucose pyrophosphorylase genes and a down-regulation of beta- and alpha-amylase expression at the advanced stages of fungal infection on flag leaves and spikes. In conclusion, the effect of blast on both grain quality and yield can be associated with alterations in both production and partitioning of carbohydrates during the grain filling process.