The lack of photosynthetic response to UV exposure in combination with the increased or no transpiration response suggests that the water use efficiency (WUE) of the plants may change due to UV exposure. The mean WUE of the field study Essex control plants (under Llumar) was 0.0023 (SD = 0.0005) at V5 stage and 0.0026 (SD = 0.0008) at R3. The mean WUE of the field study Williams control plants was 0.0021 (SD = 0.0005) at V5 stage and 0.0023 (SD = 0.0004) at R3. The mean WUE of the 2007 Essex and Williams control plants at R2 through R4 was 0.0074 (SD = 0.0026) and 0.0064 (SD = 0.0034) respectively, higher than those at R3 in the field. The mean WUE of the Essex plants at VC through V4 ranged from 0.0019 to 0.0065. The WUE of four to five pairs of photosynthetic rate and E measurements made on each plant during daylight hours in 2007 suggested a diurnal periodicity, but the pattern was not consistent for all days and cultivars.
Results are not conclusive, but indicate that the WUE of Essex was not affected by UV exposure (Fig. 16.8(a)), while that of Williams 82 was affected by UV exposure. The 2007 study showed the UV exposure did not significantly affect (t-test at P = 0.10) the WUE in Essex within a given measurement period or as a pooled sample. The UV exposure in 2007 did not significantly (t-test at P = 0.10) affect WUE in Williams 82 within a given measurement period or as a pooled sample, although there was a tendency for reduced WUE associated with UV exposure (Fig. 16.8(a)).
The WUE of Essex plants at VC stage increased with UV exposure until after the second day of exposure. The relationship between UV exposure and WUE of Essex plants at the V1 through V4 stages was ambiguous. Generally it appears that the WUE is unaffected by UV exposure, but plants at V4 stage may have reduced WUE under exposure for the duration of the measurements (Fig. 16.8(b)).
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