Functional significance and induction by solar radiation of ultraviolet-absorbing sunscreens in field-grown soybean crops - PubMed (original) (raw)
Functional significance and induction by solar radiation of ultraviolet-absorbing sunscreens in field-grown soybean crops
C A Mazza et al. Plant Physiol. 2000 Jan.
Abstract
Colorless phenylpropanoid derivatives are known to protect plants from ultraviolet (UV) radiation, but their photoregulation and physiological roles under field conditions have not been investigated in detail. Here we describe a fast method to estimate the degree of UV penetration into photosynthetic tissue, which is based on chlorophyll fluorescence imaging. In Arabidopsis this technique clearly separated the UV-hypersensitive transparent testa (tt) tt5 and tt6 mutants from the wild type (WT) and tt3, tt4, and tt7 mutants. In field-grown soybean (Glycine max), we found significant differences in UV penetration among cultivars with different levels of leaf phenolics, and between plants grown under contrasting levels of solar UV-B. The reduction in UV penetration induced by ambient UV-B had direct implications for DNA integrity in the underlying leaf tissue; thus, the number of cyclobutane pyrimidine dimers caused by a short exposure to solar UV-B was much larger in leaves with high UV transmittance than in leaves pretreated with solar UV-B to increase the content phenylpropanoids. Most of the phenylpropanoid response to solar UV in field-grown soybeans was induced by the UV-B component (lambda </= 315 nm). Our results indicate that phenolic sunscreens in soybean are highly responsive to the wavelengths that are most affected by variations in ozone levels, and that they play an important role in UV protection in the field.
Figures
Figure 1
Effects of solar UV-B on the concentration of UV-absorbing compounds (per unit leaf area) in the leaves of eight soybean cultivars arranged in order of increasing maturity group (MG); from MG III to VIII: Williams, A4423RG, CX458, A5634RG, A5308, A6445RG, Charata-76, and A8000RG. Notice that for the Dekalb and Nidera varieties we used the MG information provided by the breeders, which is indicated by the first two digits in the alpha-numeric cultivar name (e.g. Nidera A5308 belongs to maturity group 5.3). Samples were taken from the central leaflet of the youngest fully-expanded trifoliate at midday on January 12, 1999 (2 months after crop seeding). Each datum point is the average of five independent field plots. ○, +UV-B (_r_2 = 0.64; P = 0.02); ●, −UV-B (_r_2 = 0.49; P = 0.05). The UV-B effect is significant at P < 0.0001.
Figure 2
Effects of solar UV-B radiation on chlorophyll fluorescence induced by UV radiation (RFUV). The figure shows representative fluorescence images of soybean leaves (abaxial surface; central leaflet of the youngest fully expanded leaves; cv Williams) exposed to the indicated UV treatments in the field (darker tones indicate less fluorescence).
Figure 3
Effects of solar UV-B radiation on the intensity of chlorophyll fluorescence induced by UV (RFUV) and UV-B (RFUVB) in leaves of eight soybean lines arranged in order of increasing maturity group (MG); from MG III to VIII: Williams, A4423RG, CX458, A5634RG, A5308, A6445RG, Charata-76, and A8000RG. Notice that for the Dekalb and Nidera varieties we used the MG information provided by the breeders, which is indicated by the first two digits in the alpha-numeric cultivar name (e.g. Nidera A5308 belongs to maturity group 5.3). Samples were taken from the central leaflet of the youngest fully-expanded trifoliate (abaxial surface) at midday on January 12, 1999 (2 months after crop seeding). Each datum point is the average of five independent field plots. The slope of the RFUVB/MG relationship is significant at P = 0.04 (average of the two UV-B treatments). Notice that because the geometry of fluorescence excitation in this experiment was slightly different from the one used to produce the data reported in Figures 7 and 8, the absolute values RFUV values cannot be directly compared. ○, +UV-B; ●, −UV-B; in both panels the UV-B effect is significant at P < 0.0001.
Figure 4
Relationship between the content of extractable UV-absorbing leaf phenolics (_A_305) and UV-B-excited chlorophyll fluorescence (RFUVB) in eight soybean genotypes exposed to two contrasting levels of solar UV-B in the field (original data in Figs. 1 and 3B). ○, +UV-B; ●, −UV-B. _r_2 = 0.86; significance of the slope: P < 0.0001. B, Effects of exposure to solar UV-B on the RFUVB:RFUV ratio. P = 0.0001 (n = 576).
Figure 5
Fluorescence images obtained using individual leaves of WT and transparent testa mutants of Arabidopsis (adaxial surface).
Figure 6
Relationship between the fluorescent signals induced by UV (RFUV) and blue light (RFB) in a group of soybean samples that differed greatly in their RFUV values.
Figure 7
Effects of solar UV-B radiation on CPD density in leaf DNA and the impact of constitutive and UV-B-induced variations in sunscreen levels. A, Effect of solar UV-B on CPD density in DNA extracted from soybean leaves harvested at noon on January 12, 1999 (2 months after sowing; youngest fully expanded leaf, cv A8000 RG; UV-B [305 nm] at sampling time approximately 9 μW cm−2 nm−1). The digital image shows representative slots; the difference between treatments in CPD density was significant at P < 0.0001; n = 5 independent plots per treatment. B, Effect of a 150-min exposure to midday sunlight on CPD density in DNA extracted from WT and tt5 Arabidopsis plants. Before the exposure, the plants were grown in a growth chamber under 100 μmol m−2 s−1 PPFD. The experiment was carried out on March 30, 1999; the average UV-B irradiance (305 channel) during the course of the experiment was 5 μW cm−2 nm−1. Representative images of chlorophyll fluorescence excited by UV-B are given for WT and tt5 leaves. C, Protective function of UV-B-induced sunscreens in field-grown soybean (cv A8000 RG). The youngest fully expanded leaves of +UV-B and −UV-B plots were harvested on March 24, 1999, and placed in flower pots with their petioles kept under water. The leaves (three replicate leaf groups per treatment) were sampled for CPD and RFUVB determinations (abaxial surface), and then placed under fluorescent light (200 μmol m−2 s−1 PPFD) to drive DNA photorepair. After 150 min the leaves were placed outdoors and exposed to direct sunlight for 45 min (average UV-B irradiance [305 nm] during the exposure = 5 μW cm−2 nm−1). At the end of this exposure (15:15 h) the leaves were sampled again for CPD and RFUVB determinations. In all panels, one unit of damage is the CPD level induced by 1 J m−2 of 254-nm radiation in 1 ng of purified herring DNA (see “Materials and Methods”). Non-irradiated herring DNA gave no signal in the blots. Pretreatments: ○, +UV-B; ●, −UV-B.
Figure 8
Effects of filters that cut off different portions of the solar UV spectrum on the accumulation of UV-absorbing compounds in field-grown soybean plants of the cv Williams. The curves represent representative measurements of the spectral irradiance under the different filters obtained on March 26, 1998. The bars represent the average RFUV values for each of the radiation treatments; fluorescence images of representative samples are shown on top of each bar (darker tones indicate less fluorescence). Notice that each RFUV bar is positioned at the wavelength in which the spectral irradiance for the relevant treatment is ≈1% of the spectral irradiance at 400 nm. All samples were taken from the central leaflet of the youngest fully expanded leaf on March 20, 1998; each bar is the average of four true replicates (independent field plots). ●, Aclar filter; ○, Mylar filter; ▪, glass filter; □, Lexan filter.
References
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