Thermal thresholds as predictors of seed dormancy release and germination timing: altitude-related risks from climate warming for the wild grapevine Vitis vinifera subsp. sylvestris

Abstract

Background and aims: The importance of thermal thresholds for predicting seed dormancy release and germination timing under the present climate conditions and simulated climate change scenarios was investigated. In particular, Vitis vinifera subsp. sylvestris was investigated in four Sardinian populations over the full altitudinal range of the species (from approx. 100 to 800 m a.s.l).

Methods: Dried and fresh seeds from each population were incubated in the light at a range of temperatures (10-25 and 25/10 °C), without any pre-treatment and after a warm (3 months at 25 °C) or a cold (3 months at 5 °C) stratification. A thermal time approach was then applied to the germination results for dried seeds and the seed responses were modelled according to the present climate conditions and two simulated scenarios of the Intergovernmental Panel on Climate Change (IPCC): B1 (+1·8 °C) and A2 (+3·4 °C).

Key results: Cold stratification released physiological dormancy, while very few seeds germinated without treatments or after warm stratification. Fresh, cold-stratified seeds germinated significantly better (>80 %) at temperatures ≥20 °C than at lower temperatures. A base temperature for germination (T(b)) of 9·0-11·3 °C and a thermal time requirement for 50 % of germination (θ(50)) ranging from 33·6 °Cd to 68·6 °Cd were identified for non-dormant cold-stratified seeds, depending on the populations. This complex combination of thermal requirements for dormancy release and germination allowed prediction of field emergence from March to May under the present climatic conditions for the investigated populations.

Conclusions: The thermal thresholds for seed germination identified in this study (T(b) and θ(50)) explained the differences in seed germination detected among populations. Under the two simulated IPCC scenarios, an altitude-related risk from climate warming is identified, with lowland populations being more threatened due to a compromised seed dormancy release and a narrowed seed germination window.

Fig. 1.

Final germination percentages (mean ± s.d.) of fresh seeds for the four investigated populations at different temperatures for each pre-treatment: 0 = control, W = warm stratification (25 °C for 3 months) and C = cold stratification (5 °C for 3 months). Data are the mean of three replicates. For each seed lot one-way ANOVA, followed by post-hoc Fisher's LSD test, was carried out; bars with the same letters are not different at P < 0·05.

Fig. 2.

Final germination percentages (mean ± s.d.) of dry seeds for the four investigated populations at different temperatures for each pre-treatment: 0 = control, W = warm stratification (25 °C for 3 months) and C = cold stratification (5 °C for 3 months). Data are the mean of three replicates. For each seed lot one-way ANOVA, followed by post-hoc Fisher's LSD test, was carried out; bars with the same letters are not different at P < 0·05.

Fig. 3.

Base temperatures for germination (T_b) for the four populations, calculated after cold stratification (3 months at 5 °C) and incubation at constant temperatures (10–25 °C). Within each population, the linear regressions for the different percentiles were constrained to the common value of T_b. For percentiles for which regression lines had a P > 0·05, T_b values were not calculated. Statistical differences among populations were analysed by one-way ANOVA followed by post-hoc Fisher's LSD test; values with the same letters are not significantly different at P < 0·05.

Fig. 4.

Germination in probits of dried seeds as a function of thermal time requirement (θ, °Cd), according to the equation: probit(g) = K + θ_g/σ. Thermal times were calculated assuming base temperatures for germination of 11·3, 9·0, 10·5 and 10·4 °C for BA1, IG1, SA1 and SU1 seed lots, respectively. Thermal time to reach 50 % of germination [θ₅₀, corresponding to probit(g) = 5] are showed for each population. The germination percentages on a probit scale are also reported for reference on the right.

Fig. 5.

Dependency of thermal time to reach 50 % of germination (θ₅₀, °Cd) with altitude (m a.s.l.). Points correspond to the minimum and maximum altitude of BA1, IG1, SA1 and SU1 populations.

Fig. 6.

Environmental heat sum for non-dormant seeds calculated for each population for the present data and under two different IPCC scenarios (B1, +1·8 °Cd and A2, +3·4 °Cd ) as the sum of mean monthly temperature – base temperature for germination (T_m – T_b) per the number of days (t_m), until reaching the median thermal times for germination (θ₅₀, °Cd). Base temperatures for germination (T_b) are assumed to be 11·3, 9·0, 10·5 and 10·4 °C for BA1, IG1, SA1 and SU1 seed lots, respectively (dashed lines). Maximum stratification temperature for dormancy release (T_Smax) set to 15 °C, according to Wang et al. (2009). Annual trends of temperature and rainfall are also reported for each population, starting from the month of natural dispersal (10 = October).