Transpiration

The E of the control plants in the 2007 and 2008 experiments varied between 0.5 and 2.6 mmol H2O m s (Figs. 16.4(a) and 16.5(a)). At VC stage, the Essex control leaves increased transpiration over the course of the experiment which corresponded to the increases in gs (Fig. 16.2(c)). There was no trend evident in the E of the 2007 or 2008 experimental control leaves with increasing PPFD (Fig. 16.5(b)). Although four to five E measurements were made on each plant during daylight hours in 2007, there was no evident diurnal periodicity in the measurements. Measurement variability, limiting the ability to determine treatment effects, is partly a result of the variability in stomatal openings across the leaf.

In the 2007, E measurements on plants in reproductive stages R2 through R4 showed non-significant (t-test at P = 0.10) increases for both cultivars under UV exposure (Fig. 16.4(b)). For Williams, E increased less than 27 hours into the experiment. For Essex, E increased after the first day of exposure. The UV-exposed leaves of Essex cv. transpired more than control plants on the first two days of the experiment when the sky was clear, but less than the control plants on the third day of exposure (around hour 50 in Fig. 16.4(b)) when the sky was overcast. The control plants had lower average E (Fig. 16.4(a)) on the overcast day than the clear days. The sky cover however did not affect gs (Fig. 16.2(a)). As a pooled sample of all hours of measurement, the UV exposure resulted in a statistically significant increase in E at R2 through R4 in Essex (t-test at P = 0.10), but did not indicate a statistically significant increase in Williams 82. Similarly the 2008 greenhouse experiment on Essex leaves at vegetative stages VC through V4 showed that E was initially significantly decreased by UV exposure at the V1 and V2 stages, but not at the V3 stage (Fig. 16.5(c)). Leaves at VC had significantly

Figure 16.4 Influence of UV exposure on Essex and Williams 82 leaf transpiration at development stages R2-R4. Difference is indicated as UV-Control. Essex cv is indicated by the open circle. Williams 82 is indicated by the filled circle. Error bars are standard error of the difference. All measurements were made in greenhouse experiments

I lours into experiment

Figure 16.4 Influence of UV exposure on Essex and Williams 82 leaf transpiration at development stages R2-R4. Difference is indicated as UV-Control. Essex cv is indicated by the open circle. Williams 82 is indicated by the filled circle. Error bars are standard error of the difference. All measurements were made in greenhouse experiments

48 72 % Hours into experiment

Figure 16.5 Influence of UV exposure on Essex leaf transpiration at development stages VC through V4. Transpiration of control plant leaves are indicated in panel (a) with the corresponding relationship between PPFD and transpiration in panel (b). Difference in transpiration due to UV exposure is indicated as UV-Control in panels (c) and (d). Error bars are standard error of the difference. All measurements were made in greenhouse experiments greater transpiration under the UV exposure than those unexposed; however, this effect decreased with days of exposure (Fig. 16.5(c)). As a pooled sample of all hours of measurements, the UV exposure resulted in a statistically significant increase in gs at VC (t-test P = 0.01), and statistically significant decreases in gs at V1 (t-test P = 0.1) and V4 (t-test P = 0.01), but not at V2 or V3. This suggests that the plant does not initially have a mechanism for UV-B protection, but develops this protective mechanism by the third day of exposure at VC stage. For all other stages, it appears that the plant reduces transpiration under increased UV exposure. This is in agreement with the results of Teramura et al. (1980) in a greenhouse study of Hardee cv. after at least two weeks of UV exposure.

The temperature difference between the leaf and the air (AT) was significantly less for the UV-exposed plants than the control plants for all but a few measurements over the growth stages VC through V4 (Fig. 16.6(a)). The AT was significantly less because the leaf temperature was significantly higher for the UV-exposed leaves z i l 0 -1

♦ c

D

UV-Control

Hours into experiment (a)

-100

40 80

Hours into experiment (a)

Figure 16.6 Influence of UV exposure on Essex temperature difference between the leaf and the air. The difference between the control and UV-exposed leaves is indicated as UV-Control. The difference between air and leaf temperature with UV exposure is indicated in panel (a). The relationship between the differences in leaf stomatal conductance and transpiration due to UV exposure are indicated in panels (b) and (c) respectively. Symbols are indicated in Fig. 16.5. Error bars are standard error of the difference. All measurements were made in greenhouse experiments than the control leaves. However, the AT did not correspond with differences in either gs or E between the UV-exposed and control plant leaves (Fig. 16.6(b), (c)). This suggests that the UV-exposed plants could not entirely compensate for elevated leaf temperatures through the normal process of increasing E through decreased gs.

Transpiration measurements made in the field in 2004 showed non-significant (t-test at P = 0.10) decreases in Essex at V5 and non-significant differences at R3 (Table 16.2). Similarly, transpiration of Williams 82 leaves indicated nonsignificant decreases at V5 and non-significant differences at R3 (Table 16.2). This tendency was in contrast to the 2007 greenhouse study of Williams 82 (under a much higher exposure rate) that showed an enhanced E (non-significant) with UV exposure. In general, the magnitudes of the transpiration differences due to short term UV exposures in the greenhouse were smaller than those for longer term exposures in the field. These results suggest that the response of both cultivars to UV exposure was decreased transpiration in the vegetative stage. Essex showed a decrease in transpiration with UV exposure shortly after UV exposure begins. Williams 82 showed a decrease in transpiration that develops over time.

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