The successful establishment of a plant species in a location is closely related to rapid germination. Different genotype and/or environmental factors can affect this process by increasing or decreasing it.
Light is a factor that does not prevent the germination of seeds if it acts as a signal(25) that could cause a change in the germination rate and final germination(31) and, therefore, in thermal time. This factor is necessary for the germination of many species(31), mainly in plants with small seeds(31,50), because large seeds can emerge from a much greater depth than light can penetrate(51). Light is one of the main determinants of the accumulation of a persistent soil seed bank of numerous weeds(52).
Exposure to light can be short, less than a minute, or long; short exposure is more common in small seed weeds than in large weeds(31). Light seems to influence the germination of different Typha species(5,53). This would explain why there was no germination in the treatments with photoperiods = 20 d (total darkness), even though cattail seeds absorbed water and began to swell; they did not break the coats for germination. In these circumstances, the dormancy of cattail seeds does not break. However, treatments with a photoperiod ≤ 10 d had a germination response greater than 50%, and this response increased when the number of days in darkness decreased (Table 1), with significant differences among different photoperiods (Table 2). These results seem to indicate that light is necessary for germination to occur in cattail seeds and that the longer the light period of the seeds is, the greater the germination response.
Treatments of the same population had an increase in θT(50) as the number of days in darkness increased. It seems that there was a relationship between θT(50) and photoperiod (Table 4). Initially, a linear increase in thermal time was expected as the number of days in darkness increased. Indeed, there was an increase, but was not proportional; for example, in the case of the population of Cu with constant temperatures, θT(50) at 0 d was 267 °h, and the value corresponding to 10 d is 411 °h, with a 10 day light period. This means a 50% increase in thermal time, not a 100% increase as expected. This modification would indicate that T. domingensis seeds accumulate hours of temperature and that when receiving light, the dormancy is broken, and the germination response occurs more quickly than expected. Dormancy broken in the presence of light has been studied(31) and is common in small seeds as cattails.
Water depth is another factor that seems to be related to the amount of light and the ease of germination of cattail seeds. Some authors have stated that flooded areas increase the germination of Typha species, and this increase in germination has a direct relation with depth(17,54). This seems to be caused by the decrease in the level of oxygen, not by the lower intensity of the light in these situations(33). However, there are other studies that show no relation between the increase in germination and depth(34,55). Some authors have established the limit of germination in Typha species in clear water deeper than 40 cm(2) or in sediment deeper than 1 cm(56). There is an extreme case where cattail seeds germinated under 80 cm water (survived 8 weeks)(57). Depth was not a factor in this study, but in our own experience, germination in cattail seeds is greater and faster when seeds are located on a saturated substrate than on a flooded substrate, although the depth used was small (< 0.4 cm). This seems to indicate a relationship between light and depth, but this relationship needs to be further examined.
The germination response in plants of distinct origins could be different(58). Differences related to the origin of a population are frequent in numerous species of plants, whether crops(59) or weeds(27,60). Successfully colonizing a new location is related to the greater adaptive capacity of these populations than other populations to harsher environmental conditions(61), allowing these populations to have greater flexibility and adaptability to different locations or future climate change scenarios(17).
The ambient temperatures of the different populations used (Fig. 5) show that the northern populations (Cu and Ma) have similar values but lower values than the southern populations (Ba and Se) (Table 5). The results of this study showed that in comparison to southern populations, northern populations have a lower thermal time, and a higher germination response (Tables 2 and 3) despite similar temperatures and photoperiods.
These differences among populations are consistent with the results of other studies carried out with Typha latifolia L. in fifteen European populations(17) or USA populations(62); in both studies, in comparison to northern populations, southern populations germinated at a lower temperature. However, in our study, the opposite scenario occurred. Before providing conclusions, some points of these studies must be clarified. For example, in comparison to T. latifolia, T. domingensis is a species more adapted to warmer areas. In the European study, only two Mediterranean populations were used, and both populations germinated more rapidly than northern populations; the distances among the origins of the populations were greater than those in our study. Other authors mention that other factors, such as temperature or nutrient supply(63), seem to be more important than the origin of the seeds in the case of neighbouring populations(17).
In this study, the estimated mean Tb was 16.4 and no differences greater than 0.6 °C were observed regardless of origin, photoperiod, level or amplitude of temperatures. We could have considered that Tb was constant; however, other studies with crops(46) or weeds(64) estimated different Tb values for the different amplitudes of temperatures. There were significant differences in the germination responses both in terms of the level and amplitude of temperatures (Table 2). In comparison to treatments with other mean temperature, treatments with mean temperatures close to Tb achieved a lower germination response in all treatments (Table 2). No data were found for the calculated Tb for Typha species, but the estimated values of Tb for cattail seeds in this study were very similar to those obtained in other studies with summer weeds(29,65). Steinmaus (2000) established a relation between the slope of the line used to estimate Tb and germination rate; this rate will be greater with a higher slope. In our study, higher slopes occurred in Cu in thermal regimes with both constant and alternating temperatures and coincided with the lower θT(50) of all populations studied (Figs. 1 and 2).
Differences in To were obtained in the results of the multifactor analysis, mainly between the northern (Cu and Ma) and southern populations (Ba and Se) (Table 4). This difference in To is common with the results of other studies with different populations of weeds or with T. latifolia (17,22,23). The To for the Swedish populations of T. latifolia was approximately 20 °C(23) or 10–30 °C with alternating temperatures in Italian populations(22). Australian populations of the Typha genus germinate readily at high temperatures and decline when the mean temperature is lower than 20 °C (66).
Table 6 shows the results of different studies on the seed germination of T. latifolia and T. domingensis. There are few studies about the seed germination of T. domingensis. Lorenzen et al. (2000) stated that a To of 30 °C and 25/10 °C occurred in southeastern American populations of T. domingensis at constant and alternating temperatures, respectively. These To values are distinct from those obtained in our study (22.5–25 °C), but there are other studies with To values very similar to those obtained in this work (Table 6). These results showed different To values according to origins that were closely related to climate conditions at each location(17). Populations determine the conditions of germination, such as the temperature of the mother plants(38,67), regardless of whether the seeds were of the same species(24,37).
Table 6
Optimal temperature in T. domingensis and T. latifolia in different populations from various studies. Line 343 page 18
Plant species | Reference | C | A | Seed location |
Typha domingensis | This study | 22.5; 25 ºC | | Spain |
Lorenzen et al. (2000) | 30 ºC | 25/10 ºC | Florida, U.S. |
Royal Botanic Gardens (2002) | 20 ºC | | Wakehurst, England |
Typha latifolia | Sifton H.B (1959) | 30 ºC | 20/30 ºC | Ontario, Canada |
Bonnewell, V. et al (1983) | 35 ºC | | Minnesota, U.S. |
Lombardi, T et al. (1997) | | 20/30 ºC | Pisa, Italy |
Ekstam and Forseby (1999) | 20 ºC | | Linköping, Sweden |
Heinz, S (2011) | 25 ºC | 10/25 ºC | Germany |
Meng, H. et al. (2016) | | 25/15 ºC | Northeast of China. |
C: constant temperature. A: alternating temperature |
In the Typha genus, temperature and amplitude were shown to be factors related to germination (23). The favourable effect of alternating temperatures on the germination response is well known in different weeds(22) because the effect enables a seed to understand when it is buried and to prevent germination. In nature, seeds of the cattail are usually submerged. In this situation, fluctuations in the ambient temperature are rare; therefore, an increase in this fluctuation could indicate that seeds have reached land and germination could be increased. In this study, both thermal factors (level of and fluctuation in temperatures) influenced the final germination of cattail seeds. In the treatments within the same population and photoperiod, there was a greater germination response as the temperature approached To from values close to Tb, causing the existence of significant differences depending on the temperature level (Tables 1 and 2). An increase in the germination response is obtained with higher temperatures until To; above this value germination begins to decrease. The same results occur in other studies with Typha(17,22,23,33) and weeds(27,29).
The use of different amplitudes of temperature in weeds is related to the loss of dormancy in weeds(29,69) or crops such as lentil(30). In the case of cattail seeds, the loss of dormancy is related to changes in germination responses. Treatments with ΔT=0°C and 15°C had a higher germination response than those with ΔT= 5°C and 10°C (Table 1), so it seems that these last two amplitudes of temperature negatively affect germination. However, in studies with T. latifolia, the germination response in treatments with ΔT=0°C had a lower germination response than that with ΔT˃0°C(17). On the other hand, θT(50) corresponds to treatments of the same population, and ΔT=0°C is lower than treatments with ΔT≥0°C (Figure 4), in contrast to Solanum physalifolium(29) whose thermal time is considerably reduced in an alternating regime (Table 3). These data are consistent with the germination rate (Figure 3) in which treatments with alternating temperatures reach lower values than those corresponding to constant temperatures. According to these results, the best season to germinate T.domingensis would be spring or autumn because these seasons have a regimen temperature of approximately ΔT=15°C under natural conditions.
The thermal time value (Table 3) was substantially lower than that of other weeds, such as different species of Solanum(20,22) or tropical species such as Pennisetum typhoydes(44,70). This indicates a rapid germination response compared with those of other plant species. There were also differences between populations, with Cu being the one with the lowest thermal time, both in ΔT=0 and ΔT>0 treatments. Although Cu and Ma obtained similar germination values (Table 2), θT(50) was the highest in Ma.
Therefore, Cu seems to be the population that presented the most vigour during this process because this population had the fastest germination under the conditions tested. The final germination percentages were very similar in all populations. It would be necessary to carry out new tests to determine whether the development in other stages of plant growth would also be fast in this population.
In comparison to other species of the genus, such as T. angustifolia, T. domingensis is a plant species more adapted to warm temperatures. In Spain, it has been observed that T. domingensis has been colonizing places where T. angustifolia once stood (3). If this capacity is occurs with an increase in temperatures due to climate change, then it is possible to consider that T. domingensis increases its expansion to the detriment of other Typha species such as T. angustifolia.