A hydrothermal time model explains the cardinal temperatures for seed germination

Temperature (T) and water potential (y) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination (T_b), an optimum T at which germination is most rapid (T_o), and a maximum or ceiling T at which germination is prevented (T_c). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of T_b multiplied by the time to a given germination percentage (t_g). Similarly, germination at reduced y can be characterized on a hydrotime basis, or t_g multiplied by the y in excess of a base or threshold y that just prevents germination (y_b). Within a seed population, the variation in thermal times to germination among different seed fractions (g) is based on a normal distribution of y_b values among seeds (y_b(g)). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds T_o. We report here that supra‐optimal temperatures shift the ψ_b(g) distribution of a potato (Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above T_o. A modified hydrothermal time model incorporating changes in ψ_b(g) at T > T_o describes germination timing and percentage across all T and ψ at which germination can occur and provides physiologically relevant indices of seed behaviour.     

Thermal niche for seed germination of Xyris species from Brazilian montane vegetation: Implications for climate change

Understanding how climate change will affect regeneration from seeds is important for developing conservation strategies. We evaluated seed germination requirements for sympatric species of Xyris from montane rupestrian grasslands (campo rupestre) in Brazil to determine their thermal niche and thermal requirements for seed germination. We also assessed whether projected temperature increases would affect seed germination of the species. Seed germination was evaluated at a wide range of constant temperatures (10–40°C) under light (12-hr photoperiod) and dark conditions. Base temperatures (T_b) and thermal times for 50% germination (θ₅₀) were calculated for three species. The effects of projected mean temperature increase on seed germination percentage and timing were evaluated. All species revealed an absolute light requirement for germination. Thermal germination niche breadth was greatest for X. asperula (15 to 35°C) and narrowest for X. seubertii (20 and 25°C). Base temperatures for X. asperula, X. pilosa and X. trachyphylla were 9.0, 12.8 and 11.1°C, respectively. In the scenario with the highest temperature increase (A2), the greatest reductions in seed germination are observed for X. pilosa and X. seubertii. The lowest projected temperature increase (2°C) was sufficient to decrease by 1 day the germination time of X. asperula and X. pilosa. Species of Xyris do not present a pattern for thermal germination niche and thermal requirements values, indicating that the effects of climate warming on the regeneration of these seeds will probably vary among species.     

Effect of temperature and cold stratification on seed germination of the Mediterranean wild aromatic Clinopodium sandalioticum (Lamiaceae)

A germination study was carried out on seeds of Clinopodium sandalioticum (Bacch. & Brullo) Bacch. & Brullo ex Peruzzi & Conti (Lamiaceae), a wild aromatic plant endemic to Sardinia. Seeds were incubated at a range of constant (5–25°C) and an alternating temperatures regime (25/10°C), with 12 hours of irradiance per day. The results achieved at 10°C were also compared with those obtained after a period of cold stratification at 5°C for three months. Final seed germination ranged from ca. 28% (5°C) to ca. 72% (25/10°C). A base temperature for germination (T_b) of ca. 5°C and a thermal constant for 50% germination (S) of 89.3°Cd were identified and an optimal temperature for germination (T_o) was estimated to be comprised between 20 and 25°C. Cold stratification negatively affected seed viability and germination at 10°C. Although a typical “Mediterranean germination syndrome”, could not be detected for C. sandalioticum seeds, these results were coherent with those previously reported for other Mediterranean Lamiaceae species.     

Modelling the Effects of Water Stress and Temperature on Seed Germination of Radish and Cantaloupe

Cantaloupe (Cucumis melo L.) and radish (Raphanus sativus L.) are considered as important vegetables with potential for national and international markets due to their sugars, vitamins and minerals. This study arranged, therefore, to simultaneously investigate the effect of temperature (T) and water potential (ψ) on seed germination (SG) of these plants using two hydrothermal time (HTT) models and to determine cardinal Ts and base water potential (ψ_b(50)) for both species. The results indicated that SG of both species was more affected by ψ than T (p ≤ 0.001). At Ts below an optimum temperature (T_o) the ψ_b(50) was constant (− 0.582 and − 0.760 MPa for radish and cantaloupe, respectively) and then increased linearly by 0.0481 and 0.0446 MPa °C⁻¹ as T increased above T_o (as thermoinhibition) until 0 MPa at the ceiling temperature (T_c), respectively. As the first report, however, we observed that the T at which ψ_b(50) begins to change was the same here (that is, T_d = T_o), when determined by either model for both species. This result suggests that the assumption in Rowse and Finch-Savage’s model (T_d is often less and or very close to T_o) may be invalid in some cases. For both species, the base temperature (T_b) and T_o were not affected by ψ and were constant while there was an exception only for T_c for which the value declined with decreasing ψs (more negative). In general, the estimated T_b, T_o and T_c were 9.64, 21.3 and 33.0 °C for radish and 11.8, 28.3 and 45.7 °C for cantaloupe in the control condition (ψ = 0 MPa), respectively. The HTT models used here and their parameters, each with strengths and weaknesses, can be used as a predictive tool in both cantaloupe and radish SG simulation models. However, at first, we need to select an appropriate HTT model based on SG behavior of plant species and then use the best model for quantifying the response of SG across Ts and ψs.

     

Germination and emergence of Sorghum bicolor. genotypic and environmentally induced variation in the response to temperature and depth of sowing

Abstract The germination of Sorghum bicolor seeds of 9 genotypes was tested at temperatures between 8°C and 48°C on a thermal gradient plate. Samples were tested from three regions of the panicle expected to differ in temperature during grain filling. Seeds of a tenth genotype, SPV 354, produced in controlled-environment glasshouses at different panicle temperatures, were tested similarly. In addition, the emergence of SPV 354 was measured from planting depths of 2 and 5 cm at mean soil temperatures of 15, 20 and 25°C. Four methods of calculating mean germination rate for the nine genotypes were compared. Germination characters like base, optimum and maximum temperature (T_b, T_o, T_m), thermal time (θ)and the germination rate at T_o(R_max showed only small differences between methods. There was a range of genotypic variation in all characters: T_b 8.5–11.9°C; T_o, 33.2–37.5°C; T_m, 46.8–49.2°C; θ, 23.4–38.0°Cd; R_max, 0.69–1.14-d_-1. In contrast, mean germinability (G) was between 90% and 100% over the temperature range 13–40°C. Panicle temperature had no effect on any germination character in SPV 354. However, deeper burial increased θ for emergence and decreased G, irrespective of soil temperature except at 5 cm. Increasing panicle temperature, by reducing seed size, reduced G and increased θ by about 10% only at 15°C and 5 cm depth.     

Modeling seed germination in Melisa officinalis L. in response to temperature and water potential

Seed germination is greatly influenced by both temperature (T) and water potential (ψ) and these factors largely determine germination rate (GR) in the field. Quantitative information about T and ψ effects on seed germination in lemon balm (Melisa officinalis L.) is scarce. The main objective of this study was to quantify seed germination responses of lemon balm to T and ψ, and to determine cardinal temperatures in a laboratory experiment. A segmented model was used to describe the effects of ψ (i.e., T) on GR and other germination parameters. The segmented model estimates were 7.2 °C for base (T _b), 28.9 °C for optimum (T _o), 40.1 °C for ceiling temperature (T _c) and 1.64 physiological days (f _o) (equivalent to a GR_max of 0.610 d^?1 and a thermal time of 35.6 °C days) to reach 50 % maximum germination in the control (0 MPa) treatment (R ² = 0.99, RMSE = 0.005 day^?1). The inherent maximum rate of germination (days) was calculated by the [GR_max = 1/f _o] model. ψ affected cardinal temperatures. From 0 to ?0.76 MPa, when ψ increased, T _b was a constant 7.2 °C to ?0.38 MPa and increased linearly to 20.1 °C as ψ decreased. T _o and f _o increased linearly from 28.9 to 30 °C, and from 1.64 to 5.4 day^?1, respectively as ψ decreased. However, there was no signification difference in T _o as ψ decreased nor did T _c decrease from 40.1 to 35 °C as ψ decreased. T _b, T _c and GR_max were the sole parameters affected by ψ and could be used to characterize differences between ψ treatments with respect to GR at various Ts. Therefore, the segmented model and its parameters can be used in lemon balm germination simulation models.     

Thermal time and ecological strategies - a unifying hypothesis

Rates of embryogenesis and of development and growth in several nematodes are linearly related to temperature over a considerable range. On this basis, published data on the thermal time requirements are compared for a tropical and a temperate species of plant parasitic nematode Meloidogyne javanica and M. hapla respectively, the two being closely related and morphologically and biologically similar. M. hapla has a lower base temperature (T_b) and a higher thermal constant (S) than M. javanica with the relative values being almost inversed. Consequently, above their respective T_b values the slope of the relationship between rate of development and temperature was greater for the tropical species than that for the temperate species. A mathematical exploration of the relationship between T_b and S was made assuming that, over a narrow range, T_b×S was a constant. With this assumption, for any given average environmental temperature (T_e) the optimum base temperature for minimum developmental duration was T_e/2, and the temperature at which the duration of development was equal for the otherwise identical species was shown to be the sum of their base temperatures. The practical effect of the differences in T_b and S was to give M. hapla, the temperate species, a shorter life cycle and hence a competitive advantage at temperatures below 21ÅC and M. javanica, the tropical species, the advantage above that temperature. It is argued that a negative correlation between T_b and S is likely to be widespread, and provides a mechanism for regulating the distributions of related, competing organisms. Support for the hypothesis that the value of S tends to decrease as T_b increases is derived from data on the embryogenesis of an animal parasitic nematode Haemonchus contortus and from seed germination studies. Contrary results and exceptions are also briefly discussed. The observed interaction between T_b and S may be fundamental to many poikilothermic organisms and plants and provides an explanation for tropical species generally having higher T_b values than temperate ones. The ecological implications of different values of T_b and of S, including their relationship with organisms which are “r” or “K” strategists are briefly discussed.     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: II. INTRASPECIFIC VARIATION IN CHICKPEA (Cicer arietinum L.) AT CONSTANT TEMPERATURES

Positive linear relationships were shown between constant temperaturesand the rates of progress of germination to different percentiles,G, for single populations of each of five genotypes of chickpea(Cicer anetinum L.). The base temperature, T_b, at which therate of germination is zero, was 0·0°C for all germinationpercentiles of all genotypes. The optimum temperature, T_o(G),at which rate of germination is most rapid, varied between thefive genotypes and also between percentiles within at leastone population. Over the sub-optimal temperature range, i.e.from T_b to T_o(G), the distribution of thermal times within eachpopulation was normal. Consequently a single equation was appliedto describe the influence of sub-optimal temperatures on rateof germination of all seeds within each population of each genotype.The precision with which optimum temperature, T_b(G), could bedefined varied between populations. In each of three genotypesthere was a negative linear relationship between temperatureabove T_b(G) and rate of germination. For all seeds within anyof these three populations thermal time at supra-optimal temperatureswas constant. Variation in the time taken to germinate at supra-optimaltemperatures was a consequence of normal variation in the ceilingtemperature, T_o(G)—the temperature at or above which rateof progress to germination percentile G is zero. A new approachto defining the response of seed germination rate to temperatureis proposed for use in germplasm screening programmes. In two populations final percentage germination was influencedby temperature. The optimum constant temperature for maximumfinal germination was between 10°C and 15°C in thesepopulations; approximately 15°C cooler than the optimumtemperature for rate of germination. It is suggested that laboratorytests of chickpea germination should be carried out at temperaturesbetween 10°C and 15°C. Key words: Chickpea, seed germination rate, temperature     

Dissecting seed dormancy and germination in Aquilegia barbaricina,through thermal kinetics of embryo growth

• Threshold‐based thermal time models provide insight into the physiological switch from the dormant to the non‐dormant germinating seed.
• This approach was used to quantify the different growth responses of the embryo of seeds purported to have morphophysiological dormancy (MPD) through the complex phases of dormancy release and germination. Aquilegia barbaricina seeds were incubated at constant temperatures (10–25 °C) and 25/10 °C, without pre‐treatment, after warm+cold stratification (W+C) and GA₃ treatment. Embryo growth was assessed and the time of testa and endosperm rupture scored. Base temperatures (T_b) and thermal times for 50% (θ₅₀) of embryo growth and seed germination were calculated.
• W+C enabled slow embryo growth. W+C and GA₃ promoted rapid embryo growth and subsequent radicle emergence. The embryo internal growth base temperature (T_be) was ca. 5 °C for W+C and GA₃‐treated seeds. GA₃ treatment also resulted in similar T_b estimates for radicle emergence. The thermal times for embryo growth (θ_e50) and germination (θ_g50) were four‐ to six‐fold longer in the presence of GA₃ compared to W+C.
• A. barbaricina is characterised by a multi‐step seed germination. The slow embryo growth during W+C reflects continuation of the maternal programme of development, whilst the thermal kinetics of both embryo and radicle growth after the removal of physiological dormancy are distinctly different. The effects of W+C on the multiphasic germination response in MPD seeds are only partially mimicked by 250 mg·l^?1 GA₃. The thermal time approach could be a valid tool to model thermal kinetics of embryo growth and radicle protrusion.

     

Seed dormancy,germination and soil seed bank of Lamiophlomis rotata and Marmoritis complanatum (Labiatae), two endemic species from Himalaya–Hengduan Mountains

Seed dormancy and germination characteristics are important factors determining plant reproductive success. In this study, we aimed to explore the characteristics of seed dormancy and germination of two endemic Labiatae species (Lamiophlomis rotata and Marmoritis complanatum) in the Himalaya–Hengduan Mountains. Germination was first tested in the light using freshly matured seeds at 25/15 and 15/5°C, and then again after dry after-ripening. Dried seeds were incubated in the light at a range of constant temperatures (1–35°C). The effects of dark and GA₃ on germination were tested at several different temperatures. Base temperature (T_b) and thermal times for 50% final germination (θ₅₀) were calculated. Seeds were also buried at the collection site to test seed persistence in the soil. Increased final germination after dry after-ripening indicated that the seeds of the two species exhibited non-deep physiological dormancy; however, they exhibited different germination characteristics and soil seed bank types. In L. rotata, GA₃ only promoted germination at 5°C, producing no significant effect at other temperatures. Dark conditions decreased germination significantly at all temperatures. T_b and θ₅₀ values were 0.6 and 82.7°C d. The soil seed bank of this species was classified as persistent. In M. complanatum, GA₃ significantly promoted germination at all temperatures except 15°C. Dark conditions depressed germination significantly at warmer temperatures (20 and 25°C) but had no effect at lower temperatures. T_b and θ₅₀ values were 0.1 and 92.3°C d. The soil seed bank was classified as transient. Our results suggest that the seed dormancy and germination of the two co-existing species share some commonalities but there are also species-specific adaptations to the harsh alpine environment.     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: I. A COMPARISON OF CHICKPEA, LENTIL, SOYABEAN AND COWPEA AT CONSTANT TEMPERATURES

For a single seed population of each of four species of grainlegume positive linear relationships were shown between temperatureand rate of germination for different fractions (G) of eachpopulation, from a base temperature, T_b(G), at which germinationrate is zero, to an optimum temperature, T_o(G) at which germinationrate is maximal. At constant temperatures warmer than T_o(G)there were negative relations (probably linear) between temperatureand rate of germination to the maximum temperature for germination,T_m(G), Within each population T_b(G) did not differ, but it didvary between species, viz.0.0?C, 0.25?C, 4.and 8.5?C for chickpea(Cicer arietinum L.), lentil (Lens culinaris Medic.), soyabean(Glycine max [ Merr.) and cowpea (Vigna unguiculata [L.] Walp.),respectively. In contrast, T_o(G) varied both within each populationand also between the four species: 80% of seeds in each populationhad T_o(G) values within the range 31.8?C to 33.8 ?C, 24.0?Cto 24.4?C, 34.0?C to 34.5?C and 33.2?C to >40?C, respectively.Values of T_m(G) were much more vanable: the 80% population rangewas 48 .0?C to 60.8?C for chickpea, 31.8?C to 34.4?C for lentiland 46.8?C to 55.2?C for soyabean; reliable estimates couldnot be made for cowpea, but the results suggest higher and morevariable values of T_m(G) than in the other three species. Atsub-optimal temperatures the distribution of thermal time forthe different fractions of each population was normal, exceptfor lentil where it was log-normal. A single equation is proposedto describe the influence of sub-optimal temperatures on ratesof germination for whole seed populations. At supra-optimaltemperatures, variation in thermal time for the different fractionsof each population was only slight. The implications of thesefindings for the adaptation of grain legume crops to differentenvironments, and for the screening of germplasm, are discussed. Key words: Seed germination rate, temperature, grain legumes     

Understanding biological and ecological factors affecting seed germination of the multipurpose tree Anogeissus leiocarpa

• Anogeissus leiocarpa (DC.) Guill. & Perr. (Combretaceae) has important economic and cultural value in West Africa as source of wood, dye and medicine. Although this tree is in high demand by local communities, its planting remains limited due to its very low propagation via seed.
• In this study, X‐rays were used to select filled fruits in order to characterise their morphology and seed germination responses to treatment with sulphuric acid and different incubation temperatures.
• Morphological observations highlighted a straight orthotropous seed structure. The increase in mass detected for both intact and scarified fruits through imbibition tests, as well as morphological observations of fruits soaked in methylene blue solution, confirmed that they are water‐permeable, although acid‐scarified fruits reached significantly higher mass increment values than intact ones. Acid scarification (10 min soaking in 98% H₂SO₄) positively affected seed germination rate but not final germination proportions. When intact fruits where incubated at a range of temperatures, no seeds germinated at 10 °C, while maximum seed germination (ca. 80%) was reached at 20 °C. T₅₀ values ranged from a minimum of ca. 12 days at 25 °C to a maximum of ca. 34 days at 15 and 35 °C. A theoretical base temperature for germination (T_b) of ca. 10 °C and a thermal requirement for 50% germination (S) of ca. 195 °Cd were also identified for intact fruits.
• The results of this study revealed the seed germination characteristics driven by fruit and seed morphology of this species, which will help in its wider propagation in plantations.

     

Support for the thermal coadaptation hypothesis from the growth rates of Sceloporus jarrovii lizards

The thermal coadaptation hypothesis posits that ectotherms thermoregulate behaviorally to maintain body temperatures (T_b) that maximize performance, such as net energy gain. Huey's (1982) energetics model describes how food availability and T_b interact to affect net energy gain. We tested the thermal coadaptation hypothesis and Huey's energetics model with growth rates of juvenile Yarrow's spiny lizards (Sceloporus jarrovii). We compared the preferred (selected) T_b range (T_sel) of lizards in high and low energy states to their optimal temperature (T_o) for growth over nine weeks, and determined whether the T_o for growth depended on food availability. We also measured the same lizards’ resting metabolic rate at five T_bs to test the energetics model assumptions that metabolic cost increases exponentially with T_b and does not differ between energy states. The T_sel of lizards on both diets overlapped with the T_o for growth. The assumptions of the energetics model were verified, but the T_o for net energy gain did not depend on food availability. Therefore, we found support for the thermal coadaptation hypothesis. We did not find support for the energetics model, but this may have been due to low statistical power.     

The Influence of Temperature on Seed Germination in Cultivars of Common Bean

Common bean (Phaseolus vulgaris L.) is grown over a wide rangeof environments, including sites with low or high soil temperaturesat sowing time. To describe the temperature responseof seedgermination, 20 bean genotypes were evaluated using a rolledpaper towel system with 11 constant temperatures ranging from12 to 34 °C. Germination response was characterized by fittingcumulative counts using a maximum-likelihood analysis. Rateof germination increased from abase temperature (T_b) typicallynear 8 °C to an optimal development temperature (T_o) of29 to 34 °C. T_b did not differ among common bean genotypes.Mesoamerican germplasm showed slightlyhigher T_o than Andeangermplasm, but there was large variation in T_o within each ofthe two gene pools. The single accession of tepary bean (P.acutifolius) evaluated appeared to be the mosttolerant to highgermination temperatures. Key words: Common bean, seed germination rate, temperature     

Evaluating thermal resource partitioning

Summary The field thermal biology of sympatric Anolis cooki and A. cristatellus were evaluated in January and in August in desert scrub forest at Playa de Tamarindo near Guanica, Puerto Rico. Data on randomly positioned copper models of lizards, each equipped with a built-in thermocouple, established null hypotheses about basking frequency and operative temperatures (T _e) against which the behavior and body temperatures (T _b) of live lizards were evaluated. Both species exhibited non-random hourly basking rates (more marked in cristatellus than in cooki), and cristatellus was virtually inactive during the warm mid-day hours. The relationship between lizards' T _b and randomly sampled T _e differed between the species: cristatellus's mean T _b was 2° to 3° C lower than randomly sampled mean T _e in both months, whereas cooki's mean T _b was slightly higher than mean T _e in January and slightly lower in August. Although cooki's mean T _b was higher than that of cristatellus in both months, the T _b's of the two species overlapped substantially over an annual cycle. Given the similarities in their field active T _b and the low thermal heterogeneity among microsites at Playa de Tamarindo, these species appear not to partition the thermal environment there in a coarse-grained way. Instead, the relatively small differences in their field active T _b probably result from small differences in their use of similar microhabitats within their mutually exclusive territories. Thermal resource partitioning by territorial animals is unlikely unless thermal heterogeneity is coarse-grained in relation to territory size.     

Hydrothermal time analysis of watermelon (<Emphasis Type="Italic">Citrullus vulgaris</Emphasis> cv. ‘Crimson sweet’) seed germination

This study evaluated the ability of a hydrothermal time model (HTT) to describe the kinetics of watermelon (Citrullus vulgaris cv. ‘Crimson sweet’) seed germination under different temperatures (T) and water potentials (ψ) and also to determine the cardinal temperatures of watermelon. Results indicated that ψ influenced germination rate and germination percentage. For this seed lot, cardinal temperatures were 10 °C for T _b, 28.34 °C for T _o and 40.8 °C for T _c in the control (0 MPa) treatment. There was a decrease in hydrotime constant (θ _H) when T was increased to T _o and then remained constant at supra-optimal temperatures (30 MPah^?1). Also, at temperatures above T _o, ψ _b(50) values increased linearly with T. The k _T value (the slope of the relationship between ψ _b(50) and T exceeds T _o) of this seed lot was calculated as 0.076 MPa°Ch^?1. Results this study show that when the HTT model is applied, it can accurately describe ψ _b(g) and the course of germination around Ts (R ² = 0.82). Moreover, the ψ _b(50) was estimated to be ?0.96 MPa based on this model. Consequently, the germination response of watermelon for all Ts and ψs can be adequately described by the HTT model and enabling it to be used as a predictive tool in watermelon seed germination simulation models.     

Plasticity of the thermal requirements of exotherms and adaptation to environmental conditions

In exothermal organisms, temperature is an important determinant of the rate of ecophysiological processes, which monotonically increase between the minimum (t_{d min}) and maximum (t_{d max}) temperatures typical for each species. In insects, t_{d min} and t_{d max} are correlated and there is a approximately 20°C interval (thermal window W_T = t_{d max} − t_{d min}) between them over which insects can develop. We assumed that other exotherms have similar thermal windows because the thermal kinetics of their physiological processes are similar. In this study, we determined the thermal requirements for germination in plants. Seeds of 125 species of Central European wild herbaceous and crop plants were germinated at nine constant temperatures between 5 and 37°C, and the time to germination of 50% of the seeds D and rate of germination R (=1/D) were determined for each temperature and the Lactin model used to determine t_{d min}, t_{d max}, and W_T. The average width of the thermal windows for seeds was significantly wider (mean 24°C, 95% CI 22.7–24.2°C), varied more (between 14.5 and 37.5°C) and development occurred at lower temperatures than recorded for insects. The limiting temperatures for germination, t_{d min} and t_{d max}, were not coupled, so the width of the thermal window increased with both a decrease in t_{d min} and/or increase in t_{d max}. Variation in W_T was not associated with taxonomic affiliation, adult longevity, or domestication of the different species, but tends to vary with seed size. Plants are poor at regulating their temperature and cannot move to a more suitable location and as a consequence have to cope with wider ranges in temperatures than insects and possibly do this by having wider thermal windows. Synthesis: The study indicated specificity of W_T in different exotherm taxa and/or their development stages.     

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

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 (θ₅₀) 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 θ₅₀) 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.     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): I. CONSTANT TEMPERATURE

The germination of pearl millet (Pennisetum typhoides S. &H.) seeds was investigated at constant temperatures between12 ?C and 47 ?C on a thermal gradient plate. The rate of germination increased linearly with temperaturefrom a base T_b to a sharply defined optimum T_o beyond whichthe rate decreased linearly with temperature, reaching zeroat T_m. The linearity of the response both above and below T_oallowed time and temperature to be combined in a thermal timeat which a specified fraction of the seeds germinated. Withinthe population T_b and T_m were constant.     

Can we predict ectotherm responses to climate change using thermal performance curves and body temperatures?

Thermal performance curves (TPCs), which quantify how an ectotherm's body temperature (T_b) affects its performance or fitness, are often used in an attempt to predict organismal responses to climate change. Here, we examine the key – but often biologically unreasonable – assumptions underlying this approach; for example, that physiology and thermal regimes are invariant over ontogeny, space and time, and also that TPCs are independent of previously experienced T_b. We show how a critical consideration of these assumptions can lead to biologically useful hypotheses and experimental designs. For example, rather than assuming that TPCs are fixed during ontogeny, one can measure TPCs for each major life stage and incorporate these into stage‐specific ecological models to reveal the life stage most likely to be vulnerable to climate change. Our overall goal is to explicitly examine the assumptions underlying the integration of TPCs with T_b, to develop a framework within which empiricists can place their work within these limitations, and to facilitate the application of thermal physiology to understanding the biological implications of climate change.