UVB Absorbing Compounds and Leaf Pigments

The 2004 greenhouse study evaluated UV-B absorbing compounds and leaf pigment concentrations after 23.4 kJ m-2 UV-Bbe of exposure. The initial content of UV-B absorbance of acidified methanolic leaf extracts of Essex at 300 nm - 320 nm

Hours into experiment

Figure 16.8 Changes in water use efficiency with UV exposure. Differences indicated are UV-Control. Error bars are standard error of the difference. All measurements were made in greenhouse experiments was higher than in Williams 82 and Essex had significantly higher UV-B absorbance than Williams 82 after UV-B exposure. The UV-B absorbance of Essex and Williams 82 cultivars showed a peak spectral absorbance at the minimum wavelength of measurement (280 nm) after 18 hours of UV-B exposure. The difference in bulk UV-B absorbance of leaf extracts of UV exposed plants and unexposed control plants of Essex was higher than those of Williams 82 (4.36 and 1.80 respectively). Essex plants had an average of 2.4 times the absorbance of Williams 82. This is in contrast to the greenhouse study results of Murali et al. (1988) who found that Williams plants had a greater concentration of UV-absorbing compounds than Essex plants after a much longer period of 34 days at 11.5 kJ m d UV-Bbe. Since the spectral irradiance distribution of the lamps differed from natural solar radiation, the change in bulk UV-B absorbance with exposure was weighted according to the spectral irradiance distribution of the lamps. The spectrally-weighted difference in bulk absorbance between UV-exposed and control plants of Essex (3.57) was 3.5 times that of Williams 82 (1.02).

The 3 days of UV exposure (totaling 23.4 kJ m UV-Bbe) resulted in an increase in the concentration of several leaf pigments in Williams 82 and Essex (Table 16.3). The chlorophyll a and carotenoid concentrations were significantly higher (t-test at P = 0.10) for the UV-exposed Williams 82 plants compared to control plants. In contrast, UV did not significantly affect chlorophyll b concentrations in both cultivars (Table 16.3). Chlorophyll a is the most widely distributed photosynthetic pigment in plants and is mainly responsible for the absorption of light that powers photosynthesis. Similar to the results of our present study, chlorophyll a has been observed to be more affected than chlorophyll b (Teramura 1983; Strid et al., 1990). Williams 82 also responded to the exposure to UV radiation by increased production of carotenoids while Essex did not (Tables 16.3 and 16.4). Carotenoids play an essential role in both light harvesting and photoprotection (Taiz and Zeigler, 2002). Similarly, Steel and Keller (2000) observed an increase in carotenoid concentrations in Vitis vinifera following UV-B exposure.

Table 16.3 Leaf pigment concentrations at V2 stage in 2004 greenhouse study

UV- Control

UV- Control

Table 16.3 Leaf pigment concentrations at V2 stage in 2004 greenhouse study

CV

Carotenoids (mg/cm2)

Chlorophyll a (mg/cm2)

Chlorophyll b (mg/cm2)

Anthocyanins (mg/cm2)

Essex Mean

0.008

1.29

- 0.22

0.00012

SE

0.054

1.07

0.35

0.00068

Williams 82 Mean

0.121*

4.22*

0.20

- 0.00017

SE

0.043

1.88

0.57

0.00063

* — statistically significant difference at P — 0.10.

* — statistically significant difference at P — 0.10.

Table 16.4 Leaf pigment concentrations at V5 in 2004 field study

UV- Control

Table 16.4 Leaf pigment concentrations at V5 in 2004 field study

UV- Control

CV

Carotenoids (mg/cm2)

Chlorophyll a (mg/cm2)

Chlorophyll b (mg/cm2)

Essex

Mean

- 0.014

- 0.23

- 0.26

SE

0.046

1.45

0.49

Williams 82

Mean

0.017

0.57

0.18

SE

0.034

1.10

0.40

Ultraviolet exposure did not appear to alter anthocyanin concentrations in either the Essex or Williams 82 cultivars (Table 16.3). Consequently, the enhanced UV absorbing compounds do not apparently include the anthocyanins. While

Chalker-Scott (1999) suggests that anthocyanins are linked to UV-B tolerance in some cultivars, our results did not indicate that anthocyanins were produced in plants of the studied cultivars at the V2 stage in the presence of UV-B radiation.

The 2004 field study evaluated UV-B absorbing compounds and leaf pigment concentrations at V5 after approximately 150 kJ m UV-Bbe (35 days from planting) of exposure and R3 after approximately 488 kJ m UV-BBE (84 days from planting) of exposure. As in the greenhouse experiment, the UV-B absorbance of Essex and Williams 82 cultivars showed a peak spectral absorbance at the minimum wavelength of measurement (280 nm) at V5 as well as R3. At the V5 stage, the difference in bulk UV-B absorbance of leaf extracts between UV exposed plants and unexposed control plants of Essex was 4.7 times higher than the Williams 82 (2.35 and 0.50 respectively), while at the R3 stage, there was no significant difference in this same comparison (- 0.10 and - 0.03, respectively). The spectrally-weighted difference between leaf absorbances of the UV-exposed and control plants of Essex was 2.6 times greater than that of Williams at V5 and 4.3 times smaller than that of Williams at R3. The tendency for greater spectrally-weighted UV-B absorbance in Essex than Williams leaves at V5 is similar to the results at V3 in the 2004 greenhouse study. The change in cultivar with the greatest UV-B absorbing compound amount with development is a result of a decrease in weighted UV-B absorbance in Essex leaves with UV exposure at R3 (0.007) compared to at V5 (0.065), while the change in weighted UV-B absorbance in Williams leaves at R3 (0.27) was similar to that at V5 (0.25). It appears that by the time the plants are in reproductive phases, the earlier differences in UV absorbing compounds may be less important.

The control plants in the 2004 field study had lower concentrations of all pigments than the plants in the 2004 greenhouse study. Differences in chlorophyll content between greenhouse and field studies are consistent with commonly reported responses to increased blue and UV-A radiation in the greenhouse study relative to the field. This reduction may also have been a result of additional stresses on the plants in the field. Plants in the field were not supplementally watered beyond natural precipitation. In a comparison with the control plants, the UV-exposed plants did not have significantly different concentrations of carotenoids, chlorophyll a, or chlorophyll b (Table 16.4). The lack of statistical significance was not a result of greater variability in the field plants as the coefficient of variation of chlorophyll a and b in the field study was very similar to that of the greenhouse study. Variability in leaf carotenoid concentration in the field study was greater than that in the greenhouse study.

Was this article helpful?

0 0

Post a comment