Surviving Soybean the Affect on Seed Production

The yield of soybean exposed to UV-B radiation can vary for different varieties but a mechanism for this effect has not been identified (Teramura and Murali, 1986). We examined the effects of early exposure of seedlings to UV-B on seed viability for both Williams and Forrest varieties.

On Day 3 after planting Williams and Forrest seed in darkness, different sets of germinating seeds were given a single dose of UV radiation (300 nm; 317 nm; 368 nm; total dose 104 ^mol m-2), or a mock irradiation (Untreated). The exposed seedlings were then transferred to soil to grow outdoors in summer until seed set. Only Williams data for actual seed appearance is discussed herein as Forrest was unremarkable under these conditions (Fig. 17.4).

3 17 rim

Figure 17.4 The response of soybean variety Williams to UV radiation treatment: a comparison of seed development. Seedlings were treated with the UV wavelength shown (Untreated = mock pulse) on Day 3, then grown until seed set. After natural ripening of seed pods, the pods were opened and the seeds were evaluated for appearance and level of development. The pod shown for Untreated is representative of pods and seeds of Untreated seedlings. The pods shown for 300 nm and 317 nm treatments are not the majority, occurring less than 10% of the time in the overall seed harvest. The seed that failed to develop to maturity (i.e., the aborted embryos shown for 300 nm and 317 nm treatments) were not used in the viability study

Mature pods were harvested and seeds were compared to the Control plants. Untreated seedlings produced uniform round seeds. Less than 1% of the seeds were deformed. Visible changes in appearance were observed for some seeds that were produced from seedlings treated with 300 nm or 317 nm (Fig. 17.4). The most common features observed in seed production by exposure to UV-B radiation were aborted embryos, or misshapen or arrested seed development. However, these effects were seen in only 10% or less of all seeds harvested from the experimental plants. Otherwise, seed morphology appeared normal and ripened at the same time as Untreated seeds.

Seeds harvested from plants producing seed, where some examples are shown in Fig. 17.4, were planted and assessed for viability and any abnormal growth characteristics in growth chamber studies under summer conditions. Seeds obtained from different varieties were planted similarly to those in the germination studies (see Sections 17.1 and 17.4), but were not treated on Day 3. The seeds were then transferred to 14:10 growth chambers and observed over a 10-day period (no additional treatments were given). From the data shown in Table 17.2, it is apparent that there is some negative effect on viability of the surviving mature seeds that were harvested from plants originally treated with 300 nm or 317 nm (on Day 3 of their growth cycle), even if they look normal. Only the viability of seed obtained from Williams seedlings treated with 300 nm was statistically different (paired Student t test; mean germination (79% vs. 100%) of paired sets; p = 0.049) from Untreated seed viability. If seedlings survive to yield seed, then the mature seed produced appears viable. There were no great losses in germination, even though there was a reduction in viability for seedlings that originally experienced 300 nm or 317 nm treatments. The shoot apical meristem is most vulnerable in the first 7 days of growth (and we irradiate on Day 3); and indeed, plants are germinated containing all the meristems of the future tissues that will become the reproductive parts. We are currently investigating what layers of the meristems are most affected by UV and the scoreable differences of the changes induced.

Table 17.2 Viability of seed produced from plants treated with UV radiation on Day 3 (see Fig. 17.4). Seeds harvested from plants treated (Fig. 17.4) were planted in soil and assessed for germination under normal light (14 h light: 10 h dark), temperature (23°C) and water conditions at 10 days after planting (normal germination is between Days 2 and 4)

Table 17.2 Viability of seed produced from plants treated with UV radiation on Day 3 (see Fig. 17.4). Seeds harvested from plants treated (Fig. 17.4) were planted in soil and assessed for germination under normal light (14 h light: 10 h dark), temperature (23°C) and water conditions at 10 days after planting (normal germination is between Days 2 and 4)

Variety

300 nm

317 nm

368 nm

Untreated

Forrest

81%

88%

96%

100%

Williams

79%

84%

97%

100%

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