HTT model fitted

The hydrothermal time model has been frequently used to predict seed germination behaviour based on the interaction between the physiological responses to temperature and WPs (Bradford, 1995, Rowse and Finch-Savage, 2003). Using this model, seed germination times across the range of sub- optimal temperatures and WPs can be described with accuracy (e.g. Dahal and Bradford 1994). In the current study also, although germination behaviour of all grass species was predicted reasonably well

(R2_{≥91%) at sub-optimal temperatures, in certain cases, the predicted germination curve fitted poorly}

with the observed germination. There was an overestimation in predicted germination at the higher temperature treatments at sub-optimal temperatures when WP was decreased. As an example the HTT model overestimated germination time and the final germination% of cocksfoot when WP decreased to -0.63 MPa at 20 °C. This means that the assumption of having a normal distribution in Ψb (g) has been rejected in this case. Larsen et al. (2004), also reported an overestimation in the predicted germination curves when WP was –1.5 MPa at 25 °C in two red fescue cultivars. They assumed that the reason for this may possibly be that the actual WP in the germination medium (which was not measured frequently during the experiment), was different than the expected WP, probably due to evaporation of water during the experiment. However, if their argument is correct, and evaporation is the case, it would happened equally for all WP treatments and so, estimation of germination in WPs higher than -1.5 MPa (including -0.2, -0.4, -0.6, -0.8, -1 and -1.25 MPa) which were incubated at the same temperature (25 °C) should also be overestimated by the model at the same level. In the current study, this may not be the case since WPs of the solutions were checked every four days (Section 3.2). Therefore, it can be concluded that inability of the HTT model to predict germination in the supra- optimal temperatures, specifically under decreased WP can be the reason for over predicting germination rate and final germination percentage in their study as well as the current research. The specified model by Equations 2-12 and 2-13 applies at sub-optimal and optimum temperatures (Section 2.10).

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Models of Alvarado and Bradford (2002) and Rowse and Finch-Savage (2003) assumed a normal distribution of Ψb (g) and a linear upward shift in Ψb (g) with increasing temperature at supra-optimal temperatures. As described in Section 3.4.1 the probit-based model described the thermoinhibition response as a function of raising the threshold WP for seed germination (seed base WP, Ψb (g)). These models assume a normal distribution of Ψb (g) and a linear upward shift in Ψb (g) with increasing temperature. However, the assumption that Ψb (g) is normally distributed within these models has not been interrogated (Watt et al., 2010). The current study examined whether the decline in germination percentage and germination rate (GR) in brome, cocksfoot, perennial ryegrass and tall fescue seeds observed at supra-optimal seedbed temperatures could be accurately predicted using a similar hydrothermal model to that proposed by Alvarado and Bradford (2002) and Rowse and Finch- Savage (2003), where Ψb (g) is adjusted upwards with increasingly supra-optimal temperatures. Based on the results of this research, increasing Ψb (g) of the species used in this study did not follow a normal distribution and there was not a linear upward shift in Ψb (g) with increasing temperature at supra- optimal temperatures. Results show that, at supra-optimal temperatures, thermo-inhibition of all species in this study appeared to be sensitive to decreasing WP (less than -0.37 MPa) at the range of optimum temperatures, as well as supra-optimal temperatures, and this possibility needs further research. This suggests that both at optimum and supra-optimal temperatures defined for each species, there might be an interaction between temperature and WP treatments caused an unexpected rapid decline in the final germination% and GR of the seeds, once WP decreased to ≤-0.37 MPa.

The results of the current study are consistent with the results reported by Watt et al. (2011) for B.

davidii Franch. (buddleja), P. radiata D. Don (radiata pine), Allium cepa L. cv.Hyton (onion) and Daucus carota L. cv.Narman (carrot). Watt et al. (2011) showed that at supra-optimal temperatures, not

considerable thermoinhibition happened for the earliest germinating seeds under 0 MPa. This means that the seeds germinated very rapidly under moist conditions at supra-optimal temperatures. However, for the rest of the population, decreasing WP below 0 MPa caused a progressive rise in Ψb (g) at supra-optimal temperatures so they have suggested that the slower germinating seeds were thermoinhibited.

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3.7

Conclusions

 The maximum final germination percentage for cocksfoot was 20-40% when T=30 °C and

moisture was not limited. Decreasing WP (increasing moisture stress) at 30 °C resulted in ≤20% germination. Therefore, under dryland conditions, sowing cocksfoot is not recommended when the average soil temperature is 30 °C or more.

 There was a wider range of optimum temperatures for perennial ryegrass (20-30 °C) compared

with cocksfoot (20-25 °C). The highest thermal time requirement for 50% of the final germination (Tt50) was 114 °Cd for cocksfoot and the lowest was 90 °Cd for perennial ryegrass.

 The base temperatures were between 1.5 °C for perennial ryegrass and 4 °C for brome. Ceiling

temperatures were between 35-40 °C for all species.

 Cocksfoot was relatively sensitive to increasing temperature and decreasing WP. More than

50% of cocksfoot seeds germinated only under a narrow range of 10 ≤T≤20 °C when 0≥WP≥- 0.37 MPa.

 Except when temperature was 15 °C, under all temperature treatments, final germination

percentage of cocksfoot was 20-40% when WP=-0.63 MPa. The optimum temperatures for cocksfoot were 20 and 25 °C when moisture was not limited. However, 15 °C is the optimum temperature in which 41-60% of cocksfoot seeds germinated when the level of moisture stress was 14 and 16% soil moisture for silt loam and clay soil types respectively (WP=-0.63 MPa).

 Tall fescue germination was accurately predicted by the HTT model.

 Prediction of GR and the final germination percentage using the hydrothermal time model was

accurate at sub-optimal temperatures. However, there were some exceptions. The HTT model also overestimated GR and the final germination percentage under decreased WP at To.

 Both GR and final germination percentage of brome was unaffected by decreased WP at -0.18

MPa when temperature was 15 and 20 °C. This was probably due to the different seed shape and size for brome compared with the other species which resulted in increasing the hydraulic conductivity under a certain WP.

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Chapter 4 Agronomic performance of brome, cocksfoot, perennial