Model simulations

Simulations and field experiments showed that bolting date has a large effect on the time required to produce seeds. This is because bolting date determines the

temperature and photoperiod conditions that are experienced during seed development. It was also suggested that not all possible bolting dates would be relevant to Arabidopsis growing in the wild, since photoperiod and vernalisation requirements would favour bolting in spring or summer. Therefore, bolting dates were first predicted using the model described by Wilczek et al (2009). These predictions were used as input for the seed set model, which then calculated the date of seed dispersal and the temperature during seed maturation. The two models combined could therefore be used to examine the effect of germination date on seed set timing, and also to determine the range of seed maturation temperatures that are most relevant for primary dormancy establishment.

In addition to predicting plant growth in York, simulations were carried out using temperature data from a further 15 locations. These consisted of locations in Europe, Asia and North America, which were chosen to represent the distribution of

Chapter 3: Modelling Seed Dispersal Timing

55

Arabidopsis ecotypes. Daily temperature records were collected from weather station

archives, and the mean annual temperature cycle in each location was calculated using data spanning 10 years. This was also combined with times of sunrise and sunset in each location. Bolting and seed set dates were predicted as described

above, and the maturation temperature was estimated as the mean temperature during the week prior to seed set. Output from Col-0 and Ler models were similar, therefore only results from Col-0 are presented. Parameter values used were those defined for Col-0 in Wilczek et al. (2009), and seed set model 1 listed in Table 3.1.

Results of simulations using temperature data from York are presented in Figure 3.8. Germination in late summer resulted in bolting and seed set during autumn.

However, germination from mid September onwards resulted in an over wintering life history, in which bolting was delayed until the following calendar year. This causes the discontinuous appearance of the graph in Figure 3.8A. The steep gradient in this region also indicates an area of extreme sensitivity to germination date. Similar findings were reported by Wilczek et al. (2009), who demonstrated high sensitivity to germination dates in late summer and early autumn in Norwich and Cologne. In contrast however, Figure 3.8 shows that germination dates ranging from September to March resulted in very little variation in predicted seed set dates, which tended to fall in mid to late May. This indicates that the sensitivity of bolting and seed set timing to germination date dramatically reduces in autumn and spring, after the critical window of high sensitivity in early autumn. This was not reported previously in Wilczek et al. (2009), who showed only the predicted time taken to reach bolting, rather than bolting date as a function of germination timing.

As a consequence of the low variation in seed set timing, predicted seed maturation temperatures were relatively constant at 12°C for germination dates spanning from

56 September to March (Figure 3.8B). As germination dates progressed into late spring and summer, generation times became shorter as warm temperatures and long days resulted in rapid flowering. Predicted seed maturation temperatures also increased until a cut-off in late summer. Germination at this time resulted in autumn bolting, when seed maturation could no longer be completed before the onset of cooler temperatures, causing a sharp drop in the predicted maturation temperatures.

Figure 3.8 Simulations of bolting and seed set timing in York, UK

(A) Bolting date (black) and seed set dates (red) were predicted for Col-0 plants germinating on successive days. (B) Seed maturation temperatures were predicted from the mean temperature during the week prior to seed set.

Jul Sep Nov Jan Mar May Jan Mar May Jul Sep Nov Bo lt in g d a te Se e d se t d a te A

Jul Sep Nov Jan Mar May 0 5 10 15 20 25 B

Simulated germination date

Ma tu ra ti o n t e mp e ra tu re (° C )

Chapter 3: Modelling Seed Dispersal Timing

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Figure 3.9 Simulations of bolting and seed set timing for in a range of locations

Bolting dates (black) and seed set dates (red) were predicted for Col-0 germinating on successive days in each location.

Jan Apr Jul Oct

Oulu, Finland Moscow , Russia Vilnius, Lithuania

Jan Apr Jul Oct

Landsberg, Germany Gorzow , Poland Edinburgh, UK

Jan Apr Jul Oct

Vancouver, Canada Dijon, France Nantes, France

Jan Apr Jul Oct

Columbia, USA Naples, Italy Valencia, Spain

Jul Oct Jan Apr Jan

Apr Jul Oct

Catania, Italy

Jul Oct Jan Apr Gran Canaria, Spain

Simulated germination date

Jul Oct Jan Apr Sal, Cape Verde

B o lt in g d a te S e e d s e t d a te

58

Figure 3.10 Simulations of seed maturation temperatures in a range of locations

Seed maturation temperatures were predicted from mean temperatures during the week prior to seed set. Seed set dates were predicted for each maternal germination date using combined bolting and seed set model simulations (see Figure 3.9).

0 10 20 30

Oulu, Finland Moscow , Russia Vilnius, Lithuania

0 10 20 30

Landsberg, Germany Gorzow , Poland Edinburgh, UK

0 10 20 30 Vancouver, Canada M a tu ra ti o n t e m p e ra tu re ( °C )

Dijon, France Nantes, France

0 10 20 30

Columbia, USA Naples, Italy Valencia, Spain

Jul Oct Jan Apr 0

10 20 30

Catania, Italy

Jul Oct Jan Apr Gran Canaria, Spain

Simulated germination date

Jul Oct Jan Apr Sal, Cape Verde

Chapter 3: Modelling Seed Dispersal Timing

59 Similar results were obtained from simulations in different locations (Figure 3.9 and Figure 3.10). A trend related to mean annual temperature was also observed. In general, the degree of synchronicity in bolting and seed set dates for autumn and spring germination was greater in cooler climates. This was because the switch to over wintering occurred earlier in autumn, and also because temperatures remained below for longer in the spring. This meant all plants germinating within a wider time frame would not accumulate any photothermal units until the start of the growth season, and would therefore fulfil their photothermal requirements at the same time. In locations with increasing mean annual temperatures, winters were milder and synchronicity in over wintering and spring annual plants decreased. In equatorial climates such as Gran Canaria and Cape Verde, growth was not inhibited during winter leading to continuous rapid cycling.