Leaf photosynthetic rates of the control plants typically varied from 1 to 11 mmol m-2s-1 (Fig. 16.7(a), Table 16.2) and were not significantly different over the course of the experiment (Fig. 16.7(a)). Four or five photosynthetic rate measurements were made on each plant during daylight hours in 2007. A diurnal periodicity was suggested in the measurements, but was not conclusive. Measurement variability, limiting the ability to determine treatment effects, is partly a result of variability in stomatal openings across the leaf.

Photosynthetic rates of both Williams 82 and Essex cv. were not significantly affected (t-test at P = 0.10) by the UV exposures within a given measurement period or as a pooled sample in the 2007 study. In the 2008 study, A of the fully-expanded leaves from the VC was non-significantly (t-test at P = 0.10) higher than the control leaves for the first four days of the exposure. Plants at V1, V2, V3, and V4 stages, however, showed a non-significant trend of decreasing A with increasing exposure (also showed no effect of UV exposure (Fig. 16.7(b)). As a pooled sample of all hours of measurements, the UV exposure resulted in a statistically significant increase in A at VC (t-test P = 0.01) and no statistically significant change at V1, V2, V3 or V4. This is in agreement with recent studies showing no reductions in photosynthesis under current or future UV-B exposures in the presence of ambient PPFD (Fiscus and Booker, 1995; Allen et al., 1998).

In the 2007 study on leaves from R2 through R4, CPPsn was not significantly different relative to the control (t-test at P = 0.10) for both cultivars. Initially the 0Psn was higher than the control for Essex and lower than the control for

24 48 72 96 120 144 Hours into experiment (a)


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I lours into experiment (b)

I lours into experiment (b)



24 48 72 96 120 144 "0 24 48 72 96 120 Hours into experiment Hours into experiment

Figure 16.7 Influence of UV exposure on Essex leaf photosynthesis at development stages VC through V4. Measurements of the efficiency of photosystem II (PS II) are indicated in panel (a) while measurements of photosynthetic rate (a) are indicated in panel (b). Differences indicated are UV-Control. Error bars are standard error of the difference. All measurements were made in greenhouse experiments

Williams 82 after the first day of exposure, but showed no significant affect at the end of the first night and in the subsequent two days of exposure. This suggests that initial exposures to UV radiation may result in a response by the plant that is compensated for by the second day through metabolic responses to the initial exposure.

The Williams 82 A, during the first day of exposure in the 2007 experiment, were greater for the control than the UV-exposed leaves while CPPSn was greater for the UV-exposed plants than the controls. Subsequent UV exposure did not show effects on A. The Essex leaves had the exact opposite relationship during the first two hours of exposure, yet responded like the Williams 82 leaves by the end of the first exposure period.

The ¥pSn of leaves of Essex during vegetative growth stages was examined in 2008 to determine if there is evidence of decreased efficiency under UV exposure. There was no discernible difference in the control plant leaf ^PSn measurements over the course of the experiment (Fig. 16.7(d)). During VC stage, the UV-exposed plants were uniformly, but not significantly, more efficient than the control (Fig. 16.7(c)). This tendency continued for the leaves at V1 and V2 stages. At V3 and V4 stages, the UV-exposed leaves were negatively affected by the first exposure day, with exposure during the subsequent days having less effect on the efficiency (Fig. 16.7(c)). This suggests that the plant has responded to the exposure by changing metabolic pathways that protect the plant from further damage with additional exposure. If true, this might explain why the decreased CPPSn at R2 through R4 did not appear to affect A in Essex. The general finding of insensitivity of yPSn and CPPSn to UV exposure extending beyond one day is consistent with the lack of A response to the UV exposure described above.

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