Seed germination depends on both intrinsic and environmental factors. Among the latter, water, oxygen and temperature are the most important, but for some photoblastic species, light is also crucial, and this is termed photoblastism ^[1][2][3][1,2,3]. Seed response to light can be positive (germination stimulation) or negative (germination inhibition). While considering the seed size, many small seeds are positively photoblastic, and most pioneering plants produce such seeds ^[1][2][1,2]. Among plant-life form categories, hemicryptophytes and sprouters often produce positively photoblastic seeds, while chamaephytes often produce negatively photoblastic seeds [3].

According to the experimentally proven phytochrome response theory, red light (R) is responsible for the germination of photoblastic seeds. This is perceived by a protein-bilin photoreceptor, phytochrome, which also controls blossoming and other physiological responses ^[4][5][6][4,5,6]. The impact of blue light (B) on germination is considered to be negative [7]. However, the germination of a green vegetable of the Brassicales order, Cleome gynandra, is stimulated by blue light [8]. B is perceived by seed cryptochromes and phototropins, and the molecular mechanism of germination under B is under investigation ([9], and the references therein).

The germination stimulation of photoblastic seeds by smoke was first described in the 1990s. Preliminary studies involved positively photoblastic seeds of ‘Grand Rapid’ lettuce, as well as some Fabaceae and Cistaceae species that were typical of fire-prone environments ^[10][11][12][10,11,12]. The first substance of physiological activity, 3-methyl-2H-furo[2,3-c]pyran-2-one, belonging to butenolides, was discovered later and named karrikin 1 (KAR1) ^[13][14][13,14]. However, the existence of smoke-responsive plant species that did not react to KAR1 led to the discovery of other biologically active smoke chemicals: cyanohydrins glyceronitrile (2,3-dihydroxypropanenitrile) and mandelonitrile (MAN), hydroquinones, nitrates, and syringaldehyde ^[15][16][15,16]. These were found to be responsible for germination, and many papers and reviews described the use of some of these compounds as biostimulants ^{[8][14][15][17][18][19][20][21][22]}[8,14,15,17,18,19,20,21,22]. Moreover, a smoke-derived inhibitor of germination, 3,4,5-trimethylfuran-2(5H)-one or trimethylbutenolide (TMB), was identified ^[17][23][24][17,23,24].

2. The Impact of Smoke Formulations and Isolated Smoke Compounds on the Germination of Photoblastic Seeds

The first data linking smoke with its ability to substitute light for positively photoblastic seeds of ‘Grand Rapids’ lettuce come from Drewes et al. [10] and Jäger et al. [11]. They laid the foundation for further research with the use of seed lots of different origin and to check the red/far red (R/FR) seed response to establish the involvement of the phytochrome system in smoke-stimulated seed germination. Merrit et al. [25] provided evidence that KAR1 acts in a similar manner as gibberellic acid when stimulating the germination of some Australian plants. Over the last thirty years, the research also included species of different habitats. Do smoke compounds and light act in concert, or can smoke emulate the impact of light? This is difficult to answer based on the literature data. The use of photoperiod for seeds of unknown or undefined photoblastism was probably due to the involvement of natural daylength occurring in the specific area. Light source, that is, light spectrum and intensity, varied in different experiments. The seeds were kept under natural light, artificial white light or fluorescent light, which introduced variability among the compared studies. This is discussed in Section 5. While looking for other factors differentiating the experiments on photoblastic seeds, smoke formulations and isolated compounds were noticed. When an experimental treatment involves smoke-infused water or smoke fumigation, the question arises as to what should be used as a control. The first attempt toward this was made by Jäger et al. [11]. Aqueous smoke extracts were prepared from a range of plants, and the extracts from agar and cellulose were used. All of them stimulated the germination of lettuce, and a chromatographic analysis indicated the presence of the same compounds, which was a big step towards the discovery of KARs. Today, it is known that the chemical composition of the smoke varies due to specific secondary metabolites and different amounts of various carbohydrates, that is KAR and TMB precursors ^[26][27][28][26,27,28]. In paper [29], smoke water from two plants, white willow and lemon eucalyptus, was used, and the authors reached the same results in the seeds of different horticultural crops. The author of [30] reported no difference in the effects of smoke that was generated by burning laboratory filter paper or dried meadow sward, which eliminated the potential stimulatory impact of coumarins, secondary metabolites that are abundant in grasses, on germination [31]. In another experiment, [32] the smoke water of an Australian grass, Themeda triandra (Poaceae), was used to treat the seeds of a cosmopolitan persistent weed, Avena fatua, of Eurasian origin. As reported by Long et al. [33], KAR1 is a smoke-derived compound that is physiologically active toward the seeds of A. fatua, so the impact of smoke water on its seeds is independent of the type of plant material that is used for smoke generation [33]. Experiments employing liquid chromatography/mass spectrometry systems for qualitative and quantitative analysis proved that smoke that was obtained from different plant residues may have different properties ^[34][41]. Taking this into account, Gupta et al. [27] proposed a method to standardize the SW composition for seed germination and plant growth stimulation based on a ‘Grand Rapids’ lettuce bioassay, and to estimate the levels of stimulatory (KAR1 and KAR2) and inhibitory (TMB) compounds using a UHPLC-ESI(+)-MS/MS analysis. Considering the specificity of the response of positively photoblastic seeds, seven global weeds: Avena fatua, Lolium rigidum, Eragostis curvula, Phalaris minor, Hordeum glaucum, Ehrharta calycina, and Bromus diandrus to karrikinolide (probably KAR1) seem highly interesting [33]. The germination of A. fatua non-dormant seeds was consistently stimulated by KAR in different experiments. On the contrary, the seeds of L. rigidum were not stimulated by KAR in any case. A different response was observed in E. curvula. Its seeds were unaffected by KAR when freshly collected from the maternal plant, but the dormant ones responded to KAR after cold stratification. All the findings pointed to the conclusion that the response of grasses depended on the species, temperature, presence or absence of light, seed storage history (which implies seed burial and low temperature), and KAR concentration. Another important conclusion was that the so-called window of suitable conditions for responding to KAR was narrow, which is discussed in detail in Section 3. Hidayati et al. ^[35][36] revealed a stimulating effect of both KAR1 and aerosol smoke on the seeds of Hibbertia sp. (shrubs of Dilleniaceae family), whose germination is very slow (1–2 months for seedling emergence). Some Hibbertia plants producing positively photoblastic seeds responded to both treatments, but aerosol smoke provided better results. This means that not only KAR1, but also other smoke-derived compounds stimulated seed germination. In this resea rch and a paper [18], the interest is in the mode of action of different compounds acting individually (KARs, benzaldehydes, cyanohydrins and nitrates) or in concert when aerosol smoke or smoke-saturated water (smoke water, SW) is used. Technically, aerosol smoke and SW contain the same compounds as gaseous products of combustion. However, during storage, various chemical reactions may occur in SW and alter its chemical composition. From a cognitive point of view, such an approach seems interesting, as emphasized already in 2010 by Dayamba et al. [26]. On the other hand, in the research focusing on the cellular mechanism of smoke action in seeds, and in experiments of practical importance, special attention is paid to the impact of substances that are isolated from smoke to obtain reproducibility. This seems justified, as smoke formulations produce different results due to the variable content of germination stimulants and inhibitors ^[17][23][24][17,23,24]. Following this path of thinking raises another question: do different smoke-derived, physiologically active chemicals act in the same way in photoblastic seeds? To address this query, Tavşanoğlu et al. (2017) [21] studied both KAR1 and a cyanohydrin analogue, mandelonitrile (MAN), in the seeds of an annual plant, Chaenorhinum rubrifolium, that was characterized by strong physiological dormancy. Other factors, such as mechanical scarification, heat shock, aqueous smoke, nitrogenous compounds, gibberellic acid (GA), darkness, and photoperiod conditions were also considered. KAR and MAN stimulated the germination of Ch. rubrifolium, used both individually and in combination, and the highest germination rate was achieved by a joint treatment with KAR1 and light. Moreover, heat shock and smoke combinations also had positive synergistic and additive effects on germination under light conditions. Therefore, not only the smoke-specific molecules but also environmental factors characteristic of the local environment must be considered (Figure 1). The additive and/or synergistic effect of the external factors was also investigated by Jurado et al. ^[36][42]. They found that environmental features including cold storage, heat, smoke and charcoal (post-fire carbon residue), used individually and in combinations, increased the seed germinability of a Mexican shrub, Arctostaphylos pungens. However, the germination rate was relatively low (approx. 30%), even for a combination of cold storage and fire-simulating variables. Some cacti produce photoblastic seeds, and it is possible that smoke mimics or modulates the impact of light during their germination ^[37][43]. Similarly, Bączek-Kwinta [30] described a stimulating effect of smoke from burning local meadow plants on the positively photoblastic seeds of Matricaria chamomilla and Solidago gigantea from a European, non-fire prone habitat. The seeds germinated better in darkness after smoke fumigation than in the light. However, the germination of smoke-treated seeds that were exposed to light was delayed. The reason for this could be the generation of reactive oxygen species by some secondary seed metabolites [30]. To summarize, the literature on the stimulation of photoblastic seed germination by smoke-derived compounds shows that the species specificity, smoke formulation, concentration of active smoke chemicals (both stimulants and inhibitors), and environmental factors that are typical of local habitats, as well as the physiological window for seed germination, are important (Figure 1).

3. Physiological Window of KAR1 Perception by Seeds

A limited possibility of KAR perception by seeds was indicated for the first time by Nelson et al. [5]. They proved that KAR1 triggers the germination of the primarily dormant seeds of a model plant species, Arabidopsis thaliana, more effectively than the phytohormones of a well-known stimulatory function in germination (GA, EBR (24-epibrassinolide) and ethylene precursor, ACC (1-aminocyclopropane-1-carboxylate)). KAR1 was not equally effective across different Arabidopsis ecotypes, which was explained by different depths of the primary seed dormancy. Interestingly, the concentration of ABA, a natural phytohormone keeping seeds in the dormancy state, was unaffected by KAR1. As Arabidopsis is a plant that does not grow in fire-prone environments, the authors suggested that KARs belong to the natural plant hormones (or growth regulators) that are involved in seed germination. KARs can also be produced by microorganisms, and a strong selective advantage for species that perceive KARs, such as Arabidopsis, is possible [5]. Up to now, six KARs have been discovered in plant-derived smoke, and more than 50 analogues have been synthesized ^[38][44]. The most often studied natural KAR is KAR1, but the cited research studies revealed the different impact of various KARs on plant seeds ^[5][20][39][5,20,45]. Another interesting fact is that KARs, together with another group of specific biocompounds, strigolactones (SLs), belong to butenolides. Some responses to KARs and SLs can be similar because both groups share common signaling pathways ^[40][41][42][46,47,48]. This creates new possibilities but also poses new research challenges.