Predicting the consequences of global warming on Gentiana lutea germination at the edge of its distributional and ecological range

Abstract

Background: Temperature is the main environmental factor controlling seed germination; it determines both the percentage and the rate of germination. According to the Intergovernmental Panel on Climate Change, the global mean surface temperature could increase of approximately 2-4 °C by 2090-2099. As a consequence of global warming, the period of snow cover is decreasing on several mountain areas. Thermal time approach can be used to characterise the seed germination of plants and to evaluate the germination behaviour under the climate change scenarios. In this study, the effect of different cold stratification periods on seed dormancy release and germination of Gentiana lutea subsp. lutea, a taxon listed in Annex V of the Habitats Directive (92/43/EEC), was evaluated. Furthermore, the thermal requirements and the consequences of the temperature rise for seed germination of this species were estimated. In addition, a conceptual representation of the thermal time approach is presented.

Methods: Seeds of G. lutea subsp. lutea were harvested from at least 50 randomly selected plants in two representative localities of the Gennargentu massif (Sardinia). Germination tests were carried out under laboratory conditions and the responses at 5, 10, 15, 20, 25 and 30 °C were recorded. Different cold stratification pre-treatments at 1 ± 1 °C (i.e. 0, 15, 30, 60 and 90 days) were applied. Successively, the base temperature (T _b) and the number of thermal units (θ, °Cd) for germination were estimated. Additionally, this study examined the consequences of an increase in temperatures based on the Representative Concentration Pathways (RPC) scenarios.

Results: The results indicated that from 0 to 30 days of cold stratification, the germination was null or very low. After 60 and 90 days of cold stratification the seed dormancy was removed; however, 25 and 30 °C negatively affected the germination capacity of non-dormant seeds. Seeds cold-stratified for 90 days showed a lower T _b than those stratified for 60 days. However, 60 and 90 days of cold stratification did not cause great variations in the thermal time units. Analysing the RPC scenarios, we detected that the number of days useful for dormancy release of seeds of G. lutea may be less than 30 days, a condition that does not permit an effective dormancy release.

Conclusions: We conclude that seeds of G. lutea need at least 60 days of cold stratification to remove dormancy and promote the germination. The thermal time model developed in this work allowed us to identify the thermal threshold requirements of seed germination of this species, increasing the knowledge of a plant threatened by global warming. Our results emphasise the need for further studies aiming at a better characterisation of germination efficiency, especially for species that require cold stratification. This would improve the knowledge on the germination mechanisms of adaptation to different future global warming conditions.

Keywords: Base temperatures; Cold stratification; Dormancy; Future climatic scenarios; Gentianaceae; Thermal time.

Figure 1. Modelling of thermal time approach to study the germination process of non-dormant seeds.

The conceptual figure represents the modelling of thermal time (θ) approach to study the germination process of non-dormant seeds*. Cardinal temperatures representatives of this process are base (T_b), optimal (T_o) and ceiling (T_c) temperatures. Estimation of these values contributes to detecting the number of thermal time units need to be accumulated for germination to occur. Under global warming, seeds could reach the thermal time allowing germination (Scenario A) or could be limited due to the reduction of the accumulated thermal time (Scenario B). *Including seeds in which dormancy was previously removed.

Figure 2. Soil temperatures recorded by data loggers.

Mean daily temperatures in both the studied localities from 01/08/2013 to 01/08/2016. Data obtained by data loggers buried in the soil at a depth of ca. 3 cm.

Figure 3. Germination percentages at the end of each treatment.

Final germination percentages achieved at the end of the germination tests after each different period of cold stratification (from 0 to 90 days of pre-treatment (C0, C15, C30, C60 and C90)) for (A) IS ‘Is Terre Molentes’ and (B) TM ‘Trainu Murcunieddu’. Data are the mean of four replicates (±SD). Post hoc pairwise t test comparisons in each locality were carried out for each germination temperature, and bars with different letters indicate significant (P < 0.05) differences among pre-treatments.

Figure 4. Base temperatures for seeds germination of G. lutea.

Base temperatures (T_b) for G. lutea seeds germination of the two studied localities ((A and B): IS ‘Is Terre Molentes’ and (C and D): TM ‘Trainu Murcunieddu’), calculated after the C60 (60 days at 1 ± 1 °C; A and C) and C90 (90 days at 1 ± 1 °C; B and D) pre-treatments. Within each locality and pre-treatment, linear regressions for the different percentiles were constrained to the common value of T_b. T_b values were not calculated for percentiles whose regression lines had P > 0.05. T_b values with different letters are significantly different at P < 0.05.

Figure 5. Thermal time requirement after 60 and 90 days at 1 ± 1 °C.

Germination after C60 (60 days at 1 ± 1 °C) and C90 (90 days at 1 ± 1 °C) for the two studied localities (A: IS ‘Is Terre Molentes’ and B: TM ‘Trainu Murcunieddu’) as a function of the thermal time requirement (θ, °Cd). Thermal times were calculated from germination time courses, assuming the common T_b reported in Fig. 3. Thermal times that required reaching 50% of germination (θ₅₀) are shown with dashed lines.