3. Propuesta didáctica
3.3. Secuencia didáctica
Calystegiasilvatica subsp. disjuncta can produce seed (Section 4.2) and it is possible those seeds might be affected by herbicide application. Herbicides may be absorbed by the seeds as they are developing and herbicides applied at initial seed set can affect seed germination. This was found to be the case with glyphosate, where applications to Xanthium strumarium and Sesbania exaltata during the initial seed set stage reduced the number of the seeds, the seed weights and seedling emergence (Clay & Griffin 2000). Metsulfuron has also been found to affect ragwort (Senecio jacobaea)
and nodding thistle (Carduus nutans) seed production and germination (James et al 1999). Thus the herbicides applied to the established plants in the field (Chapter 2) may have affected the C. silvatica seeds. The objective for this experiment was to test whether the seeds that were found on the C. silvatica plants eight weeks after
treatment had been affected by the herbicide application.
4.4.1 Materials and methods
Seeds were collected from the field trial (Chapter 2), treated and untreated plots, on 16 June 2012, eight weeks after herbicide application. Seeds could not be collected from three plots as in two of the plots, one metsulfuron treatment and one
aminopyralid treatment, there were no seeds and the other plant, an untreated
control, had been earlier removed by cows. The treatment information can be found in Section 2.2.3.
Any seed capsules collected that were not full or were open were discarded. Each of the full unopened seed capsules were opened and the seeds counted and assessed as either being healthy, shrivelled, very shrivelled, underdeveloped, insect damaged or had evidence of fungi.
Due to a limited amount of healthy seeds available only four replicates of 10 seeds of the untreated control could be used. All other treatments had four replicates of 25 seeds made up of all the plots for that treatment in a completely randomised design. The seeds were placed onto anchor germination paper (45.5 x 15.0 cm) that was moistened with water, the seeds were dusted with thiram and scarified with a scalpel. The seeds were then kept at 25oC and assessed 7, 14, 21 and 28 days after incubation and the number of normal germinated seeds, abnormal seedlings, dead seeds, hard seeds and fresh ungerminated seeds were counted.
SAS was used to perform an analysis of variance of the data collected and least significant differences were calculated where significant treatment differences were found.
4.4.2 Results
The proportion of normal, underdeveloped, shrivelled or very shrivelled seeds in the treatments, and those affected by other factors such as fungi or insects (Table 4.5), was similar to those sampled from the waste area (Section 4.2). There were also no significant differences between the proportion of the normal, shrivelled or
underdeveloped seeds between the herbicide treatments and the untreated control. There was a lower proportion of normal seeds found in the metsulfuron treatment but
this was not significantly different from the untreated control. The proportion of herbicide treated seeds with fungi or insect presence was also not significantly different from the untreated controls. The proportion of very shrivelled seeds in the glyphosate and triclopyr/picloram/aminopyralid was significantly less than the proportion of very shrivelled seeds found in untreated control plots. There were also no significant differences in the average numbers of seeds per pod between the herbicide treatments and the untreated control and the average number of seeds per pod ranged from 1.9 – 2.9.
Table 4.5: The proportion of normal, shrivelled, very shrivelled and underdeveloped C.
silvatica seeds across different herbicide treatments. The proportion of seeds with the presence of either insects or fungi is also included.
Treatment Normal Shrivelled Very
shrivelled Insect presence Fungi presence Under- developed 2,4-D/dicamba 56.7 5.6 14.8 2.9 20.1 0.0 aminopyralid 57.0 7.3 9.7 9.0 17.0 0.0 triclopyr/picloram /aminopyralid 71.3 4.1 6.6 0.8 17.1 0.0 metsulfuron 39.9 1.2 24.2 17.0 13.6 4.2 glyphosate 79.0 3.0 5.5 0.5 12.0 0.0 clopyralid 62.4 5.1 9.0 8.7 4.4 10.3 untreated 63.0 5.3 19.7 6.8 5.1 0.0 LSD (P<0.05) NS1 NS 12.0 NS NS NS 1NS = not significant (P<0.05)
When the percentage of germinated seeds is compared (Table 4.6), it can been seen that both the aminopyralid and metsulfuron treatments had only 62% and 64% normal seed germination whereas the glyphosate treatments had 79% normal seeds
germinate. These treatments were not significantly different though and show the variability of normal seed germination.
Table 4.6: Percentage of normal and abnormal seedlings and dead, fresh ungerminated or hard C. silvatica seeds in a germination trial of seeds collected from different
herbicide treatments. Treatment Normal seedlings Abnormal seedlings Dead Fresh ungerminated Hard 2,4-D/dicamba 72.0 0.0 28.0 0.0 0.0 aminopyralid 62.0 0.0 38.0 0.0 0.0 triclopyr/picloram/ aminopyralid 72.0 0.0 27.0 1.0 0.0 metsulfuron 64.0 0.0 35.0 1.0 0.0 glyphosate 79.0 0.0 20.0 1.0 0.0 clopyralid 75.0 0.0 23.0 2.0 0.0 untreated 70.0 0.0 30.0 0.0 0.0 LSD (P<0.05) NS1 NS NS NS NS 1NS = not significant (P<0.05) 4.4.3 Discussion
The herbicides didn’t appear to have any effect on the number of seeds which
appeared to be normal or the number of seeds which were shrivelled, underdeveloped or showed the presence of other organisms. The differences in the proportions of very shrivelled seeds are likely to be because of the variability of the seeds collected. The herbicide treatments also didn’t influence the germination of the seeds. Although the number of normal seeds which germinated in both the aminopyralid and
metsulfuron treatments was lower than other treatments and the untreated control, the results were not significant and this shows the variability in the results. Also the percentage of normal germination was similar to those sampled from the waste area in Section 4.3.
It was therefore shown that the herbicides did not affect the germination of C. silvatica
seeds. This could be because the herbicides were applied to plants which already had flowered and were beginning to senesce, as they were dying back before winter and therefore most of the seeds would have already have been formed and filled and it is unlikely that the plant would have been translocating sugars to the seeds. Therefore the herbicides were also unlikely to have been transported, by the plant, to the seeds. The amount of seed from these plants would not likely provide a major increase to the soil seed bank.