Effect of leaf rust (Puccinia triticina) on photosynthesis and related processes of leaves in wheat crops grown at two contrasting sites and with different nitrogen levels

Leaf rust is one of the main diseases affecting wheat yield production. Considering the physiological variables that determine yield, diseases could affect radiation capture and/or radiation use efficiency. Reductions in radiation use efficiency may be mediated through effects on photosynthesis rate and related variables (i.e. dark respiration rate, stomatal conductance or photosynthesis events per se). The aim of this study was to analyze the effects of leaf rust on wheat leaves photosynthesis rate and to understand which processes determining photosynthesis are affected by this pathogen. Gas exchange measurements were taken on flag leaves with various rust severity levels in experiments carried out on two locations which included different nitrogen fertilization rates and sowing dates. Leaf rust reduced net photosynthesis rate at light saturation through reductions in gross photosynthesis (average reduction: 6.1 mu mol CO2 m(-2) green area s(-1)) rather than through increases in dark respiration rate (average increase: 0.7 mu mol CO2 m(-2) green area s(-1)). Changes in leaf nitrogen concentration did not modify the effects of leaf rust on net photosynthesis rate. Although net photosynthesis rate at light saturation was reduced, no effects were observed at low irradiance levels. The reduction of photosynthesis was due to effects on non-stomatal processes: indeed, important reductions of SPAD units (i.e. chlorophyll) were observed on green areas of diseased leaves. SPAD values on diseased leaves were 26.4 +/- 0.98 and 27.6 +/- 1.05 for No and NI treatments, respectively: while on healthy leaves, values were 32.0 +/- 0.83 and 38.6 +/- 0.41, respectively. (C) 2011 Elsevier B.V. All rights reserved.