Correlations among the UVB Related Variables within and among the Species

This study has shown that with increased UV-B radiation during the growing season (Fig. 18.9(a)), there are cumulative increases in leaf concentration of UV-B absorbing compounds (Fig. 18.9(b)), leaf total thickness (Fig. 18.9(c)), and leaf chlorophyll content (Fig. 18.9(d)), in all the 35 species combined. However, these parameters are all delayed responses from increasing solar UV levels. Although the leaves from each species were sampled in the first week of each month to represent that month, they perhaps better represent the previous month in relation to their responses to the UV-B radiation. Even so, there are still delays in the responses in UV-B absorbing compounds and other parameters as shown in Fig. 18.9. Perhaps a cumulative response might be in effect. The increased UV-B absorbing compound content over the growing season helps enhance the plants defense against the enhanced UV-B level. Correlation analyses of these variables (Table 18.2) indicated that during the growing season, there is a good correlation between total leaf thickness and total concentration of leaf UV-B absorbing compounds (r = 0.9). Also, considering that chlorophyll synthesis is very dynamic throughout the season, while UV-B absorbing compounds are probably only produced at very specific developmental stages it would make sense that chlorophyll concentrations would correlate much better with solar irradiance over the long haul than would UV-B absorbing compounds, which may be locked in at the time of synthesis and are not continuously being recycled. Thus, it is not surprising that the ambient UV-B radiation was correlated better to leaf chlorophyll content (r = 0.94) than to the concentration of UV-B absorbing compounds (r = 0.45). This may also be attributed to the fact that the UV-B radiation follows the trend of the total solar radiation, within which the PAR is well-correlated to chlorophyll development during the growing season. The chlorophyll content declined significantly in October (Fig. 18.9(d)) while the UV-B absorbing compound concentration remained at a steady state during the latter part of the growing season from August to October (Fig. 18.9(b)).

As we have demonstrated in Figs. 18.7 and 18.8, considerable inter-specific variations exist in the leaf total UV-B absorbing compound concentration, leaf epidermal thickness, and depth of UV-B penetration, as well as the epidermal transmittance among the species investigated. In order to reveal if there is any inter-specific correlation between these variables, we also performed the correlation analyses (Table 18.3). The results show there is a good inter-specific correlation between leaf total thickness and leaf epidermal thickness (r = 0.84), meaning that the species with thicker leaves may have thicker epidermis. Some positive interspecific relationships seem to exist between the leaf total thickness and its chlorophyll content (r = 0.66), and between the epidermal thickness and the depth of UV-B penetration into leaves (r = 0.53). The correlation coefficients between the epidermal transmittance and depth of UV-B penetration (r = 0.36) and between

Figure 18.9 Monthly summation of ambient UV-B radiation in Baton Rouge, LA in year 2000 provided by the USDA-UV-B Monitoring Program Baton Rouge Station (a); and seasonal trends in leaf UV-B absorbing-compound concentration (b); leaf thickness (c); and leaf chlorophyll content (d); of the 35 species combined. Within each chart, means with unlike letters differ significantly according to Duncan's multiple range tests, p 0.05. The error bars represent ± 1SE

Figure 18.9 Monthly summation of ambient UV-B radiation in Baton Rouge, LA in year 2000 provided by the USDA-UV-B Monitoring Program Baton Rouge Station (a); and seasonal trends in leaf UV-B absorbing-compound concentration (b); leaf thickness (c); and leaf chlorophyll content (d); of the 35 species combined. Within each chart, means with unlike letters differ significantly according to Duncan's multiple range tests, p 0.05. The error bars represent ± 1SE

Table 18.2 Correlation coefficients during leaf growth and development in all the species combined

Correlation coefficient

UV-B radiation (kJ/m2/month)

Leaf thickness

Leaf chlorophyll (p.mol/m2)

Leaf UV-B absorbing compounds

(A280 nm -320 nm/cm )

UV-B radiation (kJ/m2/month)

1

Leaf thickness (p.m)

0.48

1

Leaf chlorophyll (^mol/m2)

0.94

0.66

1

Leaf UV-B absorbing-compounds

A280 nm - 320 nm/cm

0.45

0.90

0.72

1

leaf total thickness and the depth of the penetration (r = 0.36) were too low to depict any inter-specific relationships. No inter-specific relationships were discovered between the leaf total UV-B absorbing compound concentration and any other variables, including leaf total thickness, epidermal thickness, epidermal transmittance, and depth of UV-B penetration (r values ranged from -0.02 to 0.06, as shown in Table 18.3). This may be attributed to the individualistic nature (randomness) of the species, or is perhaps due to the fact that the trees selected in this study,

Table 18,3 Correlation between the selected UV-B related properties in mature leaves across all the species studied

Correlation coefficient

Leaf total thickness

Leaf upper epidermal thickness (Mm)

Leaf upper epidermal transmittance to

310 nm UVB

Depth of 310 nm Leaf concentration of

UVB penetration total UV-B absorbing into leaves compounds

Leaf chlorophyll content (jimol/nr)

Leaf total thickness (|iin) 1.00

Leaf upper epidermal 0.84

thickness (nm)

Leaf upper epidermal -0.23

transmittancc to 310ntu UVB {%)

Depth of 310 nm UVB 0.36

penetration into leaves (fim)

Leaf concentration of total 0.04

UV-B absorbing compounds

Chlorophyll content 0.66

1,00

0,02

0.53

0.06

0.52

1,00

0.36

1,00

0.02

0.10

1,00

1.00

including 4 evergreen and 31 deciduous broadleaf species, generally belong to two life forms (evergreen angiosperms and deciduous dicotyledon trees) that may not be able to warrant for any significant correlations. A study involving species from 6 plant life forms (evergreen gymnosperms, evergreen angiosperms, deciduous dicotyledon trees, deciduous dicotyledon shrubs/vines, herbaceous dicotyledon, and grasses) did reveal better correlations between some of the UV-B related variables across the species (Day, 1993).

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