The impact of larval feeding by wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), and reduced water availability on the photosynthesis and primary metabolism of wheat, Triticum aestivum (L.), was evaluated at the grain-filling developmental stage. Photosynthetic parameters measured included photosynthesis (Ps), stomatal conductance (gs), and transpiration (E) in the flag leaves. The parameters were measured at 4 wks after the treatments were imposed. Additional concomitant chlorophyll a fluorescence measurements were taken using both dark- and light-adapted tests. Photosynthesis was significantly affected by C. cinctus injury and suboptimal water availability. However, no significant interaction was observed between the two treatment factors. Plants under a reduced or suboptimal watering regime had Ps rates that were 43.7% lower than plants that were watered daily. We also observed a 12% higher Ps rate in uninfested plants compared to plants infested by C. cinctus. Several chlorophyll a fluorescence parameters also were affected by C. cinctus. Specifically, reductions in the photochemical efficiency of photosystem II (PSII) of C. cinctus infested plants were observed for plants under reduced water availability. This study demonstrates that wheat plants at the grain filling stage have reduced photosynthetic capacity when watered less frequently or when subjected to C. cinctus larval feeding injury. Less frequent watering and larval feeding injury did not have significant impacts on yield in this greenhouse study.

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