Stomatal Conductance

The 2004 gs measurements of the control plant leaves at V2 stage were significantly higher than that of the R2 through R4 stages in 2007 (Fig. 16.2(a)), but were comparable to the measurements of the control plant leaves at V2 stage in 2008 (Fig. 16.2(c)). The gs of Essex leaves at VC stage consistently increased with time into the experiment while that of the other stages showed no trend over the experimental time period (Fig. 16.2(c)). The Williams 82 and Essex control plant gs measurements were relatively insensitive to the PPFD in 2004 and 2008 (Fig. 16.2(b)). The Essex control plant gs measurements in 2007 were similarly relatively insensitive to PPFD, but showed a greater range of gs for the same range in PPFD (Fig. 16.2(d)). Even though four to five gs measurements were made during daylight hours on each plant 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 variability in stomatal openings across the leaf.

In the 2004 study, the gs measurements on V2 leaves were only significantly different from the control (students t-test, hereafter referred to as "t-test" at P = 0.10) on the first day of exposure (Fig. 16.3(a)). Note that the PPFD over the UV-exposed plants was only 10 |imol m s - 40 ^mol m s less than that over the control plants. Starting at 3 hours into the UV exposure, the gs of UV-exposed Essex was greater, yet not significantly, than those of the control plants. Similarly,

0 40 80 120 I lours into experiment (c)

Figure 16.2 Influence cultivar and leaf node on leaf stomatal conductance. Essex and Williams stomatal conductances at R2-R4 stage (circles indicating 2007 measurements) in panel (a), and V2 stage (diamonds indicating 2004 measurements and squares indicating 2008 measurements). The relationship between the PPFD and stomatal conductance of the control leaves in 2007 are indicated in panel (b). Filled symbols represent Williams 82 while open symbols represent Essex cv. Essex leaf stomatal conductances at development stages VC through V4 (2008) are illustrated in panel (c) with corresponding relationship between the PPFD and stomatal conductance of the control leaves in 2007 are indicated in panel (d). Error bars are standard error of the difference. All measurements were made in greenhouse experiments the gs of the UV-exposed Williams 82 was significantly lower (t-test at P = 0.10) than the control plants only after 6 hours of exposure (the end of the first day). As the UV exposure increased beyond the first day, the exposure to UV had less effect on gs (Fig. 16.3(a)). Solar irradiance on the third day of measurements (120 ^mol m-2s-1 -140 ^mol m-2s-1) was less than the prior days (120 |imol m-2s-1 - 200 ^mol m-2s-1), but did not appear to influence the plant response.

In the 2007 study, many of the gs measurements of UV-exposed Essex leaves from reproductive R2 through R4 stages had slightly greater, but non-significant, differences (t-test at P = 0.10) from the control plants mostly occurring after 31 hours into the experiment (end of the second day of exposure). The increased

Stomatal Conductance

Figure 16.3 Influence of UV exposure on leaf stomatal conductance. Essex and Williams stomatal conductances at R2-R4 stage (circles indicating 2007 measurements), and V2 stage (diamonds indicating 2004 measurements and squares indicating 2008 measurements). The relationship between the difference in PPFD and stomatal conductance between UV and control leaves in 2007 are indicated in panel (b). Filled symbols represent Williams 82 while open symbols represent Essex cv. Essex leaf stomatal conductances at development stages VC through V4 (2008) are illustrated in panel (c). Difference is indicated as UV-Control. Error bars are standard error of the difference. All measurements were made in greenhouse experiments

Figure 16.3 Influence of UV exposure on leaf stomatal conductance. Essex and Williams stomatal conductances at R2-R4 stage (circles indicating 2007 measurements), and V2 stage (diamonds indicating 2004 measurements and squares indicating 2008 measurements). The relationship between the difference in PPFD and stomatal conductance between UV and control leaves in 2007 are indicated in panel (b). Filled symbols represent Williams 82 while open symbols represent Essex cv. Essex leaf stomatal conductances at development stages VC through V4 (2008) are illustrated in panel (c). Difference is indicated as UV-Control. Error bars are standard error of the difference. All measurements were made in greenhouse experiments gs associated with UV exposure on Williams 82 leaves at R2 through R4 was not significant at P = 0.10. As a pooled sample of all hours of measurement, the UV exposure resulted in a statistically significant decrease in gs at R2 through R4 in Williams 82 (t-test at P = 0.05), but not a statistically significant increase in Essex. In the 2008 study, the UV-exposed Essex plants at the VC stage had a tendency for greater gs (t-test at P = 0.10) than the controls for the first 2 days (corresponding to low gs values for control leaves); however, the differences between exposed and unexposed leaves disappeared by the third day (Fig. 16.3(c)). At all subsequent vegetative stages (V1 though V4), the Essex leaves exposed to UV had an increased or similar gs as the controls (Fig. 16.3(c)). As a pooled sample of all hours of measurement, the UV exposure resulted in a statistically significant increase in gs at VC (t-test P = 0.01) and statistically significant decreases at V1, V2, and V4 (t-test P = 0.01), but not at V3.

In the 2004 field study both, cultivars showed non-significant differences in gs between the UV-exposed and control plants at V5 (Table 16.2). Although such results might be expected given the trend of decreasing impact of UV exposure on gs as the plants have increased UV exposure indicated in the greenhouse experiments (Fig. 16.3(a)), the wide variability of measured gs in the replicates prevents any definitive statement.

Table 16.2 2004 field study results

UV-Control

V5 Stage

R3 Stage

Table 16.2 2004 field study results

UV-Control

V5 Stage

R3 Stage

Cultivar

Measurement

Mean

SE

Mean

SE

Essex

gs (mmol m 2 s ')

-140

158.1

52.9

187.5

E (mmol m 2 s ')

-1.35

0.558

0.859

0.830

A (pmol m 2 s ')

- 2.33*

1.28

-1.15

2.25

WUE ( x 10 4)

2.28

3.29

- 4.12

3.07

Williams 82

gs (mmol m 2 s 1)

- 23.6

142

322

367

E (mmol m 2 s 1)

-1.44

0.863

0.395

1.06

A (pmol m 2 s 1)

- 2.07

1.91

0.204

1.21

WUE( x 10 4)

3.89

2.95

0.0268

2.39

* = statistically significant difference at P = 0.10.

* = statistically significant difference at P = 0.10.

In general, the effects of UV exposure on Essex corresponded to an increase in gs at the beginning of an exposure with the plants (at V1 through V4 and R2 through R4) compensating to some degree as the exposure duration increased. This is consistent with the field study results of no significant difference in gs at V5 or R3. Williams 82 at V2 and R2-4 initially appeared to have a decreased gs which changed to a non-significant (t-tests at P = 0.1) increase throughout the duration of exposures in the greenhouse. The field study showed negligible changes in gs at V5 and R3. Several studies have demonstrated reduced gs in response to UV-B radiation (Middleton and Teramura, 1993; Dai et al., 1995), while others revealed an increase in gs (Musil and Wand, 1993). Eisinger et al. (2000) have shown that UV exposure caused stomatal opening in broad bean at two major peaks, which are in 280 (UV-B) nm and 360 (UV-A) nm. Negash (1987) showed that UV radiation from 255 nm - 295 nm caused rapid stomatal closure in Eragrostis tef after 15 and 45 minutes of UV exposure. Battaglia and Brennan (2000) observed a decrease in stomatal aperture following brief UV-B exposure in the cotyledons of cucumber (Cucumis sativus L. cv. Poinsett), but not in sunflowers (Helianthus annuus L cv. Gray Stripe). Some studies suggest that the gs in plants vary greatly because of the difference in their light requirement

(Kozlowski and Pallardy, 1997). The 2004, 2007, and 2008 greenhouse studies showed no evidence of influence of PPFD on gs (Figs. 16.2(b), (d)). The difference in the amount of UV-B received between the cultivars, with Essex receiving 7% greater UV exposure than Williams 82, may have partly contributed to the differences in plant response found in this study and could be an important factor under prolonged UV-B exposure.

Stomatal conductance generally exhibited a response pattern similar to that of reductions in guard cell length (Gitz et al., 2005). Nogues et al. (1998) found that reductions in conductance in pea (Pisum sativum) exposed to high levels of UV-B were largely the result of altered guard cell. We have shown that UV caused an increase in guard cell length in Williams 82, while a reduction in guard cell length was observed in Essex (unpublished data). It remains to be conclusively shown whether the observed alteration in guard cell length are the result of UV damage to developing stomatal initials or the result of developmental process as Essex and Williams 82 exhibit differential morphological responses.

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