UV Effects on Early Development and Survival of Young Soybean

Soybeans are generally planted close to the surface of soil; however, little UV radiation is said to penetrate any soil type. Organisms are most prone to cell and genetic alteration during periods of rapid call division and growth, as would be the case for plants in the first 3 to 4 days after planting, when the seed is germinating. We have been intrigued by the reports that plants appear to be the most affected by UV radiation in the first 3 weeks of growth, particularly in terms of the leaf's ability to synthesize screening compounds and in the activities of key enzymes in the phenylpropanoid pathway (Cosio and McClure, 1984; Liu and McClure, 1995; Sullivan et al., 2007). As our primary work is evaluating the photobiological responses of very young seedlings in response to different qualities of light (i.e., photoinduction of various pathways), we treated germinating seeds with very small doses of different qualities of UV to follow the early (first 3 to 4 weeks) seedling responses, from germination out to 28 days post-treatment, in a number of experiments designed to look at the development of critical tissues in a young seedling.

On Day 3 after planting Williams and Forrest seeds in darkness, sets of germinating seeds were given either a single dose of UV radiation (total dose 104 ^mol m-2, equivalent in dose to the strength of 1 sec of sunlight at midday (Warpeha et al., 2008)) at various wavelengths between 300 nm and 368 nm or a mock pulse of UV (Untreated = Control). The germinating seedlings were then transferred to a light growth chamber (14 h light: 10 h dark, i.e., 14:10; maintained at 23 °C (Warpeha et al., 2009)). At 7 days post-planting (4 days post-germination), we scored seed germination. We observed that 20% - 30% of planted seeds had mortality during the germination process; i.e., cessation of growth processes and decomposition, after treatment with 300 nm, 305 nm, or 311 nm in both varieties (Table 17.1).

Table 17.1 Survival of germinating seed post UV-irradiation at 7 days after planting. Germinating seeds were treated with UV-B radiation (Untreated = mock pulse; UV treatments = total dose 104 prnol m 2 (Warpeha et al., 2008)) wavelengths as shown on Day 3 after planting, and scored for germination and survival 4 days later

Table 17.1 Survival of germinating seed post UV-irradiation at 7 days after planting. Germinating seeds were treated with UV-B radiation (Untreated = mock pulse; UV treatments = total dose 104 prnol m 2 (Warpeha et al., 2008)) wavelengths as shown on Day 3 after planting, and scored for germination and survival 4 days later

Variety

300 nm

305 nm

311 nm

317 nm

Untreated

Forrest

73%

79%

80%

81%

100%

Williams

68%

71%

78%

78%

100%

A number of Forrest and Williams seedlings died within the first week post-treatment, as shown in Table 17.1. This phenotype of severe damage after exposure to UV radiation may be due to epigenetic factors. For other abiotic stresses experienced in germinating seedlings/young plants, the structure of the shoot apical meristem may be adversely affected or development of the photosynthetic apparatus is impaired. Structural damage to the shoot apical meristem appears to be a major reason for death and decline in the growth of very young plants in both Arabidopsis and crop plants of interest in the case of salinity stressors (Sacks et al., 1997; West et al., 2004; Mahmoodzadeh, 2007), but is still poorly investigated for other stressors.

In order to understand what happens to seedlings surviving the 4-day milestone as shown in Table 17.1, we assessed young seedling responses to brief doses of UV administered at the time of germination. On Day 3 after planting Williams and Forrest seed in darkness, at least 3 sets of germinating seeds were given a single dose of UV radiation (total dose 10 ^mol m- (Warpeha et al., 2008)) at 300 nm, 317 nm, or 368 nm, or were left Untreated (Controls). Immediately post-irradiation, at least 3 replicated sets of 20 seedlings were transferred to soil in order to follow growth effects under "summer" growth conditions (14 h light: 10 h dark; 23 °C (Warpeha et al., 2009)). Surviving seedlings were observed up to 28 days after planting, and a representative example of each is depicted in Fig. 17.1. The seedlings that were given a 368 nm treatment appeared no different than those which were not treated (Controls) with UV. Seedlings treated by 300 nm or 317 nm did have surviving plants that grew at a much smaller rate in the first week (one example each of Williams is shown in Fig. 17.1), but then appeared to "recover" and grow at a steady rate as can be observed by seedling heights (Fig. 17.2). The 300 nm-treated seedlings in particular had smaller leaves and thickened stems at the very base; however, not all seedlings displayed these characteristics.

Untreated 300 nm 317 nm 368 nm

Figure 17.1 Photographs of Williams, 21 days after planting. Growth characteristics were observed for seedlings that were Untreated, or treated with a UV wavelength: 300 nm, 317 nm, and 368 nm. Plants were grown as described in Methods for seedling growth. Depicted above are some of the surviving seedlings. Delay and abnormal growth shown for example, by the yellow outline on the photographs. Treatments with 368 nm appeared highly similar to Untreated seedlings, same age (above)

Figure 17.2 The response of soybean varieties Williams and Forrest to UV radiation treatment: a comparison of growth responses as assessed by height over 28 days post-planting. Sets of seedlings were treated with the UV wavelength shown (Untreated = mock pulse) and height measured every week from Day 3 after planting. Each data point is the result of 3 independently grown replicates of at least 20 seedlings. The SEMs are less than 10% for all points

- planting

Figure 17.2 The response of soybean varieties Williams and Forrest to UV radiation treatment: a comparison of growth responses as assessed by height over 28 days post-planting. Sets of seedlings were treated with the UV wavelength shown (Untreated = mock pulse) and height measured every week from Day 3 after planting. Each data point is the result of 3 independently grown replicates of at least 20 seedlings. The SEMs are less than 10% for all points

When compared to untreated seedlings, there was only one clear difference in the manner of growth for the variety of Williams. Seedlings treated with 300 nm experienced an immediate slowing of growth not observed for other treated and control seedlings of either variety. From 14 to 28 days post-irradiation, the rate of growth (i.e., slope of growth of sets of seedlings) was similar for all treatments. However, the main change in growth that occurred prior to 7 days post-irradiation persisted to 28 days after treatment for the 300 nm-treated seedlings (Fig. 17.2). The mean height of Williams "untreated" 35.95 ± 1.0 cm compared to the height of Williams "300 nm" at 29.9 ± 1.5 cm was significantly different (paired Student t test; mean height of 3 paired sets; p = 0.04). The 317 nm and 368 nm final height differences are less pronounced when compared to untreated seedlings (34.5 + 0.9 cm and 35.8 +1.2 cm, respectively), and are not statistically significant. Growth severely declines in response to 300 nm within the first week after exposure. The resumption of growth over time may be due to repair mechanisms and deployment of screening pigments (Warpeha et al., 2008; 2009).

The data of Table 17.1, and Figs. 17.1 and 17.2, indicate that the impact of UV radiation comes in the first week which could translate to losses of yield in the field for certain varieties. For example, while experiencing mortality in germinating seeds, the Forrest variety did not have as great a sensitivity in growth as those seedlings that survived the first week (Fig. 17.2). Williams experienced both high mortality in the germinating stage and delay in growth in the seedling stage. In soybean, there seems to be a great sensitivity to UV radiation, even with brief treatments early in the plant's life cycle, compared to Arabidopsis (Warpeha et al., 2008; 2009). There are also varietal-specific sensitivities as well as a variation to different wavelengths of light, as shown in our data. Recent reports show that the specific pigment responses in leaves to UV can vary, dependent upon the genetic variety of the seedlings (Sullivan et al., 2007), and occurs for a number of characteristics in soybean varieties (Biggs et al., 1981; Teramura and Murali, 1986; Reed et al., 1992), and Pinus species (Sullivan and Teramura, 1988). We are unsure of the mechanism of these intraspecific differences. More research is required at the ultrastructural level to look at the early development in vulnerable, fast-growing cells of young seedlings (i.e., meristems).

The first set of leaves inside the cotyledons is the site of rapid cell division and chloroplast development in the 7 days post-germination. As the cotyledons separate after coming up through the soil, the developing first pair of leaves is exposed to incident UV. Since there were seedlings that survived the first week of post-irradiation, but died by 28 days post-irradiation, it was of interest to determine if the brief UV radiation treatments had an effect on the young leaves, not immediately apparent in the germination or growth (height) characteristics, particularly for the more sensitive variety, Williams. Three-day-old dark-grown

seedlings were given a brief (10 ^mol m ) treatment of a specific UV radiation, then returned to darkness for 5 more days. The young leaves were then harvested to examine the effects on leaf expansion for Williams (Fig. 17.3; Forrest did not vary; data not shown).

Untreated J00 urn 3ttS nm 31! nm 317 nm 325 nm 332 nm 368 nm (no UV)

"all SLMs less than 10%

Figure 17.3 The response of soybean variety Williams to UV radiation treatment: a comparison of fresh weight accumulation. Seedlings were treated with the UV wavelength shown (Untreated = mock pulse) on Day 3 as described in Methods, returned to darkness, then harvested 5 days later to determine the effect of UV-B radiation on fresh weight accumulation. Each data point is the result of 3 independently grown replicates. The SEMs are less than 10% for all points

Untreated J00 urn 3ttS nm 31! nm 317 nm 325 nm 332 nm 368 nm (no UV)

"all SLMs less than 10%

Figure 17.3 The response of soybean variety Williams to UV radiation treatment: a comparison of fresh weight accumulation. Seedlings were treated with the UV wavelength shown (Untreated = mock pulse) on Day 3 as described in Methods, returned to darkness, then harvested 5 days later to determine the effect of UV-B radiation on fresh weight accumulation. Each data point is the result of 3 independently grown replicates. The SEMs are less than 10% for all points

It is apparent from the data shown in Fig. 17.3 that UV-B wavelengths can prevent expansion of developing leaves in the first week post-irradiation. Conversely, UV-A wavelengths induce an increase in leaf expansion manifesting in increase in overall fresh weight compared to Untreated seedlings. The leaves are larger in appearance so it is not merely an increase in density and in soybean treated with UV-A wavelengths, but the plastid is more developed compared to Untreated seedlings (Warpeha et al., 2009). For wavelengths under 317 nm, the effect on live tissue is to prevent accumulation of structural materials and expansion. We know from experiments in Arabidopsis and soybean that cuticular function and expansion is adversely affected by UV-B wavelengths (Warpeha et al., 2008; 2009). Moreover, wavelengths below 317 nm cause aberrant changes to cell ultrastructure, including the structures of the developing plastid, the major site with more synthesis of phenylpropanoids (Warpeha et al., 2008; 2009).

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