Effects of Heat Treatment on Seed Germination

Introduction

Seed germination has been found to be influenced by many factors. Some of these include water availability, nutrients, light, incubation, and heat shock (Masamba, 1994). In the natural Western Australian environment, heat shock is most commonly provided by bushfires. Periodic fires result in an open environment providing enhanced moisture, light, and nutrients which are conducive to the survival of germinated seeds (Bell, Plummer, & Taylor, 1993). There are many species of the Western Australian flora, especially in the Leguminosae families, that have a hardened testa in order to suppress germination until fire provides a better chance of seedling survival (Herranz, Ferrandis, & Martinez-Sanchez, 1998).

Heat shock is required in some plant species to fracture the hard seed coat which allows for water imbibition, gas exchange, and releases the embryo from physical restraints (Mucunguzi, & Oryem-Origa, 1996). Short exposures to the high temperatures reached in soil during fires can greatly increase germination percentages of certain species (Bell et al., 1993).

Under laboratory conditions, the heat shock usually provided by fire can be simulated using boiling water. Dry heat or scarification and acid treatments can also increase the percent of seeds germinated (Bell et al., 1993). The aim of the experiment was to examine the effects of different temperature heat treatments on the percent germination of four species of legumes.

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Materials and Methods

Plant Material

Four different commercially obtained plant species were used to examine the effects of different heat pre-treatments on the percent seed germination of set sample sizes. The four species used in the experiment were Kenndia coccinea, Acacia saligna, Hardenbergia, and Acacia pulchella.

Experimental Treatment

A total of 600 seeds were taken from each species and divided into sets of 110. Each set was pre-treated at one of five temperatures. The temperatures were: room temperature (24ËšC), 40ËšC, 60ËšC, 80ËšC, and 100ËšC. The seeds from each treatment were divided into 50 labelled petri dishes, 11 seeds per dish. All of the seeds in a single petri dish underwent the same pre-treatment. The petri dishes were then placed into a dark cupboard for incubation at room temperature and randomized.

Viability Test

In order to assess the viability of the seeds collections used for the germination experiment, a tetrazolium test was carried out on 96 untreated seeds from each species. The testa of each seed was cracked before being tested.

Scoring Germination

The numbers of seeds germinated in each petri dish were recorded at the end of each week for four weeks, along with the species and pre-treatment the seeds underwent. A drop of fungicide was used to kill any moulds that were found growing in the petri dishes during incubation.

Statistical Analysis

The heat treatments of each species were compared using the chi square analysis, allowing for 5% error. The null hypothesis (Ho) for the chi square tests is that the treatments had no effect on the percent of seeds germinated. The alternate hypothesis (Ha) is that the different treatments did have an effect on the percent of seeds germinated.

Results

The chi square analysis compares the total number of germinated seeds between treatments for one species to determine if statistically, we should accept or reject the null hypothesis. Table 1 displays that Kenndia coccinea, Acacia saligna, and Acacia pulchella all have a chi square value greater that the 5% error value. Therefore, we can be 95 % confident that the Ho should be rejected and Ha accepted. Hardenbergia, however, has a chi square value less than the 5% error value, therefore, Ho is accepted.

Table 1 Chi square values and degrees of freedom calculated from the number of germinated seeds of four different plant species after a variety of controlled heat treatments

Species Chi Square value 5% error

Kenndia Coccinea 52.90909 9.49

Acacia saligna 39.84615 9.49

Hardenbergia 6.15444 9.49

Acacia pulchella 38.5 9.49

Data shows that three of the four chi square values are greater than the 5% error value. This indicates that the null hypothesis should be rejected for Kenndia coccinea, Acacia saligna, and Acacia pulchella. Therefore, Ha is accepted for these species.

It is obvious from the graphs in figure 1 that the different heat treatments had little effect on the percent germination of c) Hardenbergia. Significant variations can, however, be seen in the germination of the other three species. Attention should be drawn to the significant increase in germination of d) Acacia pulchella between the 80ËšC treatment and 100ËšC treatment.

a) b)

c) d)

Fig. 1 Percent germination of a) Kenndia Coccinea, b) Acacia saligna, c) Hardenbergia, and d) Acacia pulchella at the end of a four week growth period. Each species had 500 seeds which were divided into five different heat pre-treatments.

Discussion

Heat shock treatments have two primary effects on seeds that cease dormancy. Cracking of the seed coat appears to be most common result of heat shock; however, heat can also be used to denature seed coat inhibitors (Hanley, & Lamont, 2000).

It is obvious from the information displayed in table 1 and figure 1 that temperature has a significant effect on the germination of Kenndia coccinea, Acacia saligna, and Acacia pulchella. In the natural environment, extreme temperatures on the soil surface can be lethal to seeds (Bell et al., 1993). Due to thermal diffusion, seeds below 6 to 8 cm may be too deep to have their seed coats cracked (Hanley, & Lamont, 2000). A. pulchella has developed a relationship with ants to maximise germination. The ants bury the seeds at a depth of approximately 4cm which is the depth where heat penetration and temperature required to break dormancy appears to converge (Hanley, & Lamont, 2000).

A similar heat pre-treatment experiment (Table 2) to the one carried out in this report was carried out by Bell, Plummer, & Taylor (1993). They examined the effects of seed scarification and boiling on the percent germination of native Western Australian legumes. The data indicates that a 300 second heat treatment tends to reduce germination percentages in most of the species listed in table 2. Acacia pulchella is one Western Australian species that shows no significant germination in the percent germination (Bell et al., 1993). This information suggests that A. pulchella evolved in an environment that experiences prolonged burning (Bell et al., 1993).

The results obtained by Bell, Plummer, & Taylor (1993) after examining the effects of no pre-treatment’s, seed scarification, and heat shock on 55 species of native Western Australian legumes.

It is interesting to note that the percent germination graph of A. saligna in figure 1 shows an increase in germination as treatment temperature increased. This result is significant as A. saligna is a coastal habitat species whose seeds have the capacity to survive mild fires, but are unable to endure intense heat (Herranz et al., 1998).

While high temperatures are required to crack the seed coat of many native Western Australian species, germination may also be cued by incubation temperatures that would best support the survival of the seedlings (Bell et al., 1993). It is possible that this factor may have influenced the germination results of K. coccinea, A. saligna, Hardenbergia, and A. pulchella.

In Western Australia, heat is a key requirement for the successful germination of many plant species. Combinations of factors, however, are often required to maximise the chance of germination of any plant species. Due to the diversity of flora in Western Australia, more research is required to determine the optimal environment for commercial or private cultivation of many species.