Daily photosynthetic and C-export patterns in winter wheat leaves during cold stress and acclimation

Diurnal patterns of whole-plant and leaf gas exchange and (14) C-export of winter wheat acclimated at 20 and 5degreesC were determined. The 5degreesC-acclimated plants had lower relative growth rates, smaller biomass and leaf area, but larger specific leaf weight than 20degreesC plants. Photosynthetic rates in 20degreesC and 5degreesC-acclimated leaves were similar; however, daytime export from 5degreesC-acclimated leaves was 45% lower. Photosynthesis and export remained steady in 20degreesC and 5degreesC-acclimated leaves during the daytime. By comparison, photosynthesis in 5degreesC-stressed leaves (20degreesC-acclimated plants exposed to 5degreesC 12 h before and during measurements) declined from 70 to 50% of the 20degreesC-acclimated leaves during the daytime, while export remained constant at 35% of the 20degreesC-acclimated and 60% of the 5degreesC-acclimated leaves. At high light and CO2 , photosynthesis and export increased in both 20degreesC and 5degreesC-acclimated leaves, but rates in 5degreesC-stressed leaves remained unchanged. At all conditions daytime export was greater than nighttime export. Taken together, during cold acclimation photosynthesis was upregulated, whereas export was only partially increased. We suggest that this reflects a requirement of cold-acclimated plants to both sustain an increased leaf metabolic demand while concomitantly supporting translocation of photoassimilates to overwintering sinks.