Introduction:

enzyme claims restated as a hypothesis

 

In the raw vegan/vegetarian community, one may encounter a number of claims about food enzymes, including the following.

1. Enzymes in raw foods are [a]live and are the life force in those foods.
2. Enzymes in raw foods are reportedly “destroyed” (denatured may be more accurate) when food is heated above a certain critical temperature, approximately 48° C/118° F.
3. Foods heated above this critical temperature – even if only for a very brief period - are said to be “cooked” or “dead”.

Many raw fooders, whether they believe the raw food enzyme claims or not, use the temperature of 48° C/118° F (or an even lower value) as a dividing line between raw and cooked foods.   

 

Let’s start with claim #1. The term live enzymes can be immediately dismissed as incorrect because enzymes are merely molecules and lack nearly all the basic characteristics of life forms (see Davison, 2004, for a list of attributes).  The term life force is undefined here, making it difficult to understand the claim. If we interpret the words literally, we can immediately reject the claim, since enzymes are molecules (matter), and a life force is not matter but some kind of energy.

 

However, assuming that those who make the claim are using figurative language, we can restate #1 above – for clarity – as:  the heat-sensitive enzymes in raw foods are a proxy or marker for the (undefined) life force in foods. Since claim #2 asserts that the relevant enzymes are “destroyed” by heat above the critical temperature of 48° C/118° F, it follows that any/all foods heated above that temperature have lost their life force and must be “dead”.

 

The term all used in the preceding sentence must include seeds for the claim to be true. Seeds have a well-known property: seeds that are alive can germinate (sprout) and grow, if they are planted under appropriate conditions. The fact that a growing plant is alive – and presumably has the (undefined) life force - is self-evident. If we focus on seeds, the enzyme claims (1-3 above) can be combined to produce a testable hypothesis:

 

IF the heat-sensitive enzymes in raw foods are a proxy/marker for the (undefined) life force,

THEN seeds heated above the critical temperature of 48° C/118° F must be dead and cannot germinate (sprout).

 

Note that by recognizing that a growing plant is alive and has the (undefined) life force, we avoid the requirement to formally define the term.

 

Relevance of heat tolerance and heat shock/heat stress tests on seeds

The published scientific literature includes many studies that test seeds for heat tolerance or heat stress/shock. Heat tolerance studies are conducted to investigate the effects of heat treatments on seeds, including: a) possible reduction of pathogens (e.g., fungi, bacteria), b) standardize germination rates, and c) break the dormancy of seeds. Heat stress/shock studies typically use very high temperatures, and may be conducted to investigate the effects of fire on plant reproduction. Many of the published heat tolerance/heat stress studies on seeds provide excellent tests for the enzyme hypothesis stated above. The enclosed table summarizes the results from a sample of relevant studies.

 

Table 1.0:  Summary of sample heat treatment & heat shock/heat stress studies on seeds

 

Research paper	Species tested	Temperatures  tested	Exposure time/ treatments	Germination period	Germination rates (%)	Comments
Lee et al., 2002	Rice: Oryza sativa	Unheated, 90° C/194° F, 90° C/194° F	Control,  1 day, 4 days	12 days	86-100%, 22.0-96.5%, 54.3-70.0%	5 varieties in test, 5 varieties in test, 6 varieties tested, only 2 germinated
Fourest et al.,  1990	Barley: Hordeum vulgare	71° C/159.8° F, 75° C/167.0° F, 84° C/183.2° F	11 days	7 days; root sprout>1 cm. to count	92%, 60%, 60%	Rates read from fig.2 in paper (note 2). 25 varieties in test.
Daws et al., 2007	26 Aizoaceae 6 Crassulaceae 5 Cactaceae (desert succulents)	103° C/ 217.4° F	17 hours	Not specified	20-100%, 0-3%, 3-90%	Rates read from fig.1 in paper (note 2). Seeds in study are small: 0.043 – 0.86 mg.
Baker et al., 2005	9 Australian fire ephemerals (see note 3 for definition)	70° C/158° F, 100° C/212° F,	1 hr + smoke water (see note 3 for definition)	84 days	30-80% for 5 species, 8-28% for  4 species	Rates read from fig.4 in paper (note 2).
Bell & Williams, 1998	21 Australian species	100° C/212° F	1 hr, 1.5 hr (in boiling water)	21 days	6 species: 10.7-60%, 5 species: 6.7-36.0%
Weiss & Hammes, 2003	Mung bean: Phaseolus aureus	Unheated, 70° C/158° F, 80° C/176° F	Control, 10 minutes (in hot water)	2 days	99%, 94%, 87.5%	Rates read from fig.2 in paper (note 2).

 

O’Reilly & De Atrip, 2007	Alder: Alnus glutinosa, Birch: Betula pubescens	Unheated, 60° C/140° F	Control,  vs 1-4 hrs	42 days	Alder: 11.5-59.5% vs 29.6-57.4%. Birch: 24.0-27.5% vs 11.0-26.3 %	High proportion of birch seeds are non-viable; birch treatments with 0% germination are excluded
Hanley & Fenner, 1998	6 Mediterranean fire ephemerals (see note 3 for definition)	Unheated, 100-120° C in 10° intervals/   212-248° F	Control, 10 minutes	56 days	Control:   30-90%, 100°: 30-60% for 3 species, 110°: 60% for 1 120°: 30% for 1	Rates read from fig.1 in paper (note 2). The only 2 species to grow after >=110° treatments have small seeds: 0.86-0.97 mg. See note 4.
Sidari et al., 2008	Pine tree: Pinus pinea (Mediterranean region)	Unheated, 80° C/176° F, 110° C/230° F, 140° C/284° F	Control, 20 minutes, 3 minutes, 3 minutes	3 days	100%, 60%, 80%, 60%
De Villalobos et al., 2002	Calden tree: Prosopis caldenia (Argentina)	Unheated, 371° C/699.8° F, 449° C/840.2° F	Control, duration unclear; may be <2 mins at max temp	21 days	5.2%, 11.7%, 6.7%	Study used controlled burns.
Banda et al., 2006	Kiaat tree/Mukua: Pterocarpus angolensis (Africa)	Unheated, 450° C/842° F	Control, 1 minute	2 years	40%, 15%	Rates from paper fig. 3; natural germination rate is ~2%.

 

Notes:

 

1. Three columns: Temperature tested, Exposure time, Germination rates are parallel: multiple rows per paper correspond to multiple temperatures tested.
2. In some studies, the relevant test results were shown only as graphs. For those studies, the numbers were obtained (estimated) by reading the graph.
3. “Fire ephemerals are short-lived plants with seeds that persist in the soil and germinate after a fire or physical disturbance.” Baker et al. 2005, p. 345. Smoke water is water that contains certain chemicals found in wood smoke.  For more information on smoke water, see Flematti et al. (2004).
4. For similar studies, see Hanley (2009) and Hanley et al. (2001).

 

Highlights from the table (note: this section is redundant but is provided for those who want a quick summary): 

• Rice was heated to 90° C/194° F for 4 days and still sprouted, i.e., most seeds were still alive after long exposure to temperatures much higher than 48° C/118° F.
• Barley was heated to 84° C/183.2° F for 11 days and still sprouted.
• Very small seeds were heated to 103° C/217.4° F for 17 hours and still sprouted.
• Seeds of 3 Australian species were boiled in 100° C/212° F water for 90 minutes and still sprouted.
• Seeds of 2 species were exposed to fire, 371-450° C/700-842° F for short periods, and still sprouted.

 

Discussion

 

According to the raw food enzyme claims, the heat-treated seeds described above (and in the table) have lost their enzymes and are “dead”, so consequently they cannot germinate/sprout. The fact that they do germinate provides clear, unequivocal proof that the “heat-sensitive enzymes are the life force” hypothesis is false. It also directly challenges the common raw food belief that foods heated above the temperature of 48° C/118° F are always “dead”. It further raises additional questions as to whether heat-sensitive enzyme content should ever be used as a proxy measure for assessing how “alive” a particular food is, i.e., as a measure of vitality.

 

Enzyme advocates may note the reduced germination rates for heat-treated seeds in the table, and equivocate and say that is proof that heating can reduce the vitality of the seed. The statement is partially valid but also irrelevant; it does not override the fact that the seed studies presented here disprove the enzyme hypothesis. Note also that the table is a summary and excludes details from some of the studies showing that heat treatment in some cases enhanced  seed germination.

 

Other forms of heat-tolerant plant life. Seeds are not the only form of plant life resistant to high temperatures. Pollen shows similar tolerance of high temperatures. Exposure of Petunia pollen to 60° C/140° F for 2 days (also 75° C/167° F for 1 day) did not impair the ability of the pollen to set fruits. Nicotiana pollen exposed to 75°C/167° F for 6-12 hours was able to set seed (Rao et al., 1995). “The grass Dichanthelium lanuginosum can tolerate long-term soil temperatures up to 57°C [134.6° F]” (Daws et al., 2007, p. 265). 

 

Heat-tolerant bacteria. Some species of bacteria – simple single-celled organisms whose bodies are mostly water and who lack cooling mechanisms – show remarkable heat tolerance. “A wide variety of bacteria thrive at temperatures from 70 to 90° C [158-194° F]” (Brock & Boylen, 1973). Heat-tolerant bacteria are found in natural hot water habitats (e.g., hot springs) and in artificial ones as well, including hot-water heaters. Bacteria that grow at temperatures above 100° C/212° F have been found in underwater volcanic thermal vents. Thermophilic (heat-loving) bacteria have thermostable (heat-stable or heat-tolerant) enzymes; an example is the enzyme pullulanase in Clostridium thermohydrosulfuricum, which functions optimally at a temperature of 90° C/194° F (Lowe et al., 1993).

 

Some seeds have heat-tolerant enzymes. The reality is that seeds and seedlings can contain enzymes that are stable above the temperature of 48° C/118° F. The paper by Eglington et al. (1998) is a suggested entry point for those interested in the related scientific literature. Will raw food enzyme advocates change their long-standing claims in response to this information, and assert that the heat-stable enzymes in foods are important? If they do so, is that equivalent to saying that “cooked” foods are as good as raw foods?

 

Do enzyme inhibitors explain seed heat tolerance? Raw food enzyme advocates might try to defend their claims by reminding us that seeds contain enzyme inhibitors, and make a new claim that enzyme inhibitors somehow “protect” seed enzymes from the effects of high temperatures. (Enzyme inhibitors are molecules that decrease an enzyme’s activity.) Any such claims should be rejected, for the following reasons.

1. Dr. Edward Howell, originator of the raw food enzyme theories, asserts that both enzymes and enzyme inhibitors are “destroyed” by heat (Howell, 1985, p. 120).
2. A claim is unproven until credible scientific evidence is presented to verify the claim, i.e., it is easy to make claims, but where is the proof?

 

Why some seeds can withstand extreme heat:  evolutionary adaptation to fire regimes

 

An obvious question that comes to mind here: why did some plant seeds evolve tolerance for heat stress/high temperatures? Fire is an obvious factor, but when temperatures get too high, seeds will die. Many of the studies cited here reported seed mortality for some species/temperature combinations, and a few of the papers calculate the expected fatal temperature for the species being studied. One can argue that living things cannot directly adapt to fire; when the temperature gets high enough, fire will always kill.

 

Fire regimes. Instead, plants have evolved evolutionary strategies for long-term species survival in the context of the fire regime(s) that prevail in their native habitats. The term fire regime refers to the pattern and characteristics of fires that occur over a long period in a habitat.  Relevant characteristics here include fire frequency, periodicity, intensity, depth of burn, and so on (Brown, 2000).

 

Specific adaptations. The morphology (physical structure) and life history pattern of plants can be significantly influenced by fire regimes. Some of the evolutionary adaptations to fire regimes that can impact seeds include:

• Fire-stimulated flowering, i.e., a plant survives a fire and is stimulated to bloom and fruit by heat shock (Bond & Van Wilgen, 1996).
• Seedbank in the soil that is activated by fire, i.e., seeds in soil that are stimulated to germinate after a fire by heat shock (Bond & Van Wilgen, 1996).
• Seed germination stimulated by contact with charred wood in the soil or water filtered through charred wood (Keeley, 1991).
• Seedbank in the tree canopy that is activated by fire, e.g., many Pinus species produce varying percentages of seed-bearing cones that are fused shut with pitch (sap), and which need the heat from a fire to open and release the seeds. (They also produce cones that are not sealed shut, for reproduction that does not depend on fire.)
• Life history pattern to maximize reproduction between fires:  Pinus halapensis grows quickly, matures in a few years, with many young female plants that produce a high percentage of pine cones that are sealed shut with sap (Ne’eman et al., 2004).

 

The adaptive strategy of a plant can determine how much heat the seeds are exposed to, e.g., seeds for plants that flower after a fire may have less exposure to fires and be exposed to less heat than seeds that need heat shock to stimulate germination. In turn, this may influence (over an evolutionary time period) the seed’s ability to tolerate heat. The bottom line is that evolutionary adaptations to fire regimes can be factors that determine the heat tolerance and heat stress response of seeds.

 

Epilogue

 

Many of the studies cited here observed - at sufficiently high temperatures - a decline in the seed germination rate to zero. It seems reasonable to refer to heated seeds with a 0% germination rate as cooked or “dead”.  However, the decline in germination rates occurred over a relatively large temperature range for many seeds, reflecting a “grey zone” between the two extremes of unheated vs. overheated/dead. This suggests that the black-and-white classification process used by many raw fooders: raw vs. cooked/dead based on a single temperature (48° C/118° F) for all foods, is an oversimplification and inaccurate.

 

Here we have discussed seeds and (briefly) pollen, both of which have simple viability tests. Assessing the viability of many fruits and vegetables is not as easy. Recognizing that a single temperature test (48° C/118° F ) is invalid and heat-sensitive enzymes in foods are not a proxy for viability, the obvious question that follows is:  how much heat/duration is required before, say, an apple or a tomato is really “dead” and “cooked”?  The question is simple, but the answer may be complex.  

 

The situation here serves as an example of the complexity of nature, and reminds us that nature is not bound by our simplistic rules. It brings to mind a famous quote from the eminent mathematician & statistician Jerzy Neyman: "Life is complicated, but not uninteresting" (Puri, 1984).  Finally, if the information in this article disturbs you, remember that one does not need to believe inaccurate, over-simplified claims about enzymes in order to follow a raw or predominantly raw diet.

 

 

Thomas E. (Tom) Billings

Berkeley, California, U.S.A.

1 November 2009

 

 

 

References

 

1. Baker KS, Steadman KJ, Plummer JA, Dixon KW, 2005. Seed Dormancy and Germination Responses of Nine Australian Fire Ephemerals. Plant and Soil, 277(1-2): pp. 345-358.
2. Banda T, Schwartz MW, Caro T, 2006. Effects of fire on germination of Pterocarpus angolensis. Forest Ecology and Management, 233(1): pp. 116-120.
3. Bond WJ, Van Wilgen BW, 1991. Fire and Plants. Chapman & Hall, London.
4. Brock TD, Boyle KL, 1973. Presence of Thermophilic Bacteria in Laundry and Domestic Hot-Water Heaters. Applied Microbiology, 25(1): pp. 72–76. URL: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC380738/pdf/applmicro00053-0086.pdf accessed 25 Oct 2009.
5. Brown JK 2000.  Introduction and Fire Regimes (chapter 1) in: Wildland fire in ecosystems: effects of fire on flora. USDA Forest Service technical report RMRS-GTR-42-vol. 2.  URL: http://www.fs.fed.us/rm/pubs/rmrs_gtr042_2.pdf accessed 21 Oct 2009.
6. Davison PG, 2004. How to Define Life. The University of North Alabama. URL: http://www.una.edu/faculty/pgdavison/BI%20101/Overview%20Fall%202004.htm  accessed 30 Oct 2009.
7. Daws MI, Kabadajica A, Mangera K, Krannera I, 2007. Extreme thermo-tolerance in seeds of desert succulents is related to maximum annual temperature. South African Journal of Botany, 73(2): pp. 262-265.
8. De Villalobosa AE, Peláez DV, Boo RM, Mayor MD, Elia OR, 2001. Influences of temperature and water stress on germination and establishment of Prosopis caldenia Burk. Journal of Arid Environments, 49(2): pp. 321-328.
9. Eglinton JK, Langridge P, Evans DE, 1998.  Thermostability variation in alleles of barley beta-amylase. Journal of Cereal Science, 28(3): pp. 301-309.
10. Flematti, G.R., Ghisalberti, R.L., Dixon, K.W. & Trengrove, R.D. (2004) A Compound from Smoke that Promotes Seed Germination. Science, 305 (5686), 977
11. Fourest E, Rehms LD, Sands DC, Bjarko M, Lund RE, 1990.  Eradication of Xanthomonas campestris pv. translucens from Barley Seed with Dry Heat Treatments. Plant Disease, 74(10): pp. 816-818.
12. Hanley ME, Fenner M, 1998. Pre-germination temperature and the survivorship and onward growth of Mediterranean fire-following plant species. Acta Oecologica, 19(2): pp. 181-187.
13. Hanley ME, Fenner M, Ne’eman G, 2001. Pregermination heat shock and seedling growth of fire-following Fabaceae from four Mediterranean-climate regions. Acta Oecologica, 22(5-6): pp. 315-320.
14. Hanley, M.E, 2009. Thermal shock and germination in North-West European Genisteae: implications for heathland management and invasive weed control using fire. Applied Vegetation Science, 12(3): pp.385-390.
15. Howell E, 1985. Enzyme Nutrition, the Food Enzyme Concept. Aver Publishing, New Jersey.
16. Keeley JE, 1991. Seed germination and life history syndromes in the California chaparral. The Botanical Review, 57(2): pp. 81-116.
17. Lee SY, Lee JH, Kwon TO, 2002. Varietal differences in seed germination and seedling vigor of Korean rice varieties following dry heat treatment.  Seed Science and Technology, 30: pp. 311-321.
18. Lowe SE, Jain MK, Zeikus JG, 1993. Biology, ecology, and biotechnological applications of anaerobic bacteria adapted to environmental stresses in temperature, pH, salinity, or substrates. Microbiological Reviews, 57(2): pp. 451–509. URL:  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC372919/pdf/microrev00025-0167.pdf accessed 25 Oct 2009.
19. Ne’eman G, Shirrinka Goubitz S, Nathan R, 2004. Reproductive traits of Pinus halepensis in the light of fire – a critical review. Plant Ecology, 171: pp. 69–79.
20. O’Reilly C, De Atrip, N, 2007. Seed moisture content during chilling and heat stress effects after chilling on the germination of common alder and downy birch seeds. Silva Fennica, 41(2): 235–246. URL:  http://www.metla.fi/silvafennica/full/sf41/sf412235.pdf  accessed 21 Oct 2009.
21. Puri PS, 1984. Jerzy Neyman (1894-1981) -- An Appreciation. Technical report 84-42, Purdue University Department of Statistics. URL:  http://www.stat.purdue.edu/research/technical_reports/pdfs/1984/tr84-42.pdf accessed 21 Oct 2009.
22. Rao GU, Shivanna KV, Sawhney VK, 1995. High-temperature tolerance of Petunia and Nicotiana pollen. Current Science (Bangalore), 69(4): pp. 351-355.
23. Sidari M, Mallamaci C, Muscolo A, ,2008. Drought, salinity and heat differently affect seed germination of Pinus pinea. Journal of Forest Research, 13(5): pp. 326-330.
24. Weiss A, Hammes WP, 2003. Thermal seed treatment to improve the food safety status of sprouts. Journal of Applied Botany – Angewandte Botanik, 77: pp. 152-155.
25. Williams DS, Bell DT, 1998. Tolerance of Thermal Shock in Seeds. Australian Journal of Botany, 46(2): pp. 221-233.

 

 

Web supplement

 

Relevance of heat tolerance and heat shock/heat stress tests on seeds:

additional information on heat tolerance tests

 

Lee et al. (2002) tested heat treatments on rice and found:

• The highest temperature treatments produced almost no abnormal seedlings.
• Percentage germination increased (vs. control) at 2 days for the 70-80° C/158-176° F treatments.
• The 75° C/167° F treatment reduced total germination overall but increased the germination rate at 2 days. That is the meaning of the phrase “standardize germination rates” used above.

The authors suggest that dry heat treatment promoted germination by a) causing cracks in the seed hulls, b) the treatment promoted faster water absorption during the early stages of germination.

 

The heat treatments in Fourest et al. (1990) were tests to determine if heat treatments would reduce incidence of barley pathogens: bacterial black chaff and leaf streak (both caused by Xanthomonas campestris pv. Translucens).

 

Weiss & Hammes (2003) studied the use of heat treatments to reduce the incidence of Salmonella bacteria in mung beans used for sprouting. The objective was to enhance the safety of commercially-produced raw mung bean sprouts.

 

Discussion

 

Do seed hulls explain heat tolerance?  Perhaps raw enzyme advocates will claim that the seeds in the studies cited here were protected from heat by their hulls? If they do, solid evidence is required for such claims to be credible.  Before making the claim, raw enzyme advocates would have to first read full text of all the studies cited here and validate/document which seeds tested had hulls vs. those with no hulls. Significant secondary research might be required as not all the studies specify which seeds have hulls/do not have hulls. Lacking this prerequisite validation/documentation, the claim is merely unsubstantiated speculation.

 

Other problems with the claim include:

• some of the seeds tested had no hulls, e.g., Lee et al. (2002) tested both hulled and unhulled rice,
• some of the seeds studied were very small so any seed hulls would be very thin and provide only very limited protection from heat, e.g., Daws et al. (2007) where small seeds were exposed to 103° C/217.4° F for 17 hours.

 

Confusion regarding critical temperature for food enzymes. Howell is not clear/possibly contradictory regarding the temperature where food enzymes are “damaged”. Quotes and page numbers below are from Howell (1985):

• “If water is hot enough to feel uncomfortable to the hand, it will injure enzymes in food” (p. 5).
• “[H]ow ultrasensitive enzymes are to heat” (p. 121).

The preceding quotes, in general, suggest a temperature close to 48° C/118° F. Contrast that with the quotes:

• “At 160° F [71° C], the enzymes wear out in about half an hour and can no longer do any work” (p. 22).
• “Official chemistry will tell you that 160° F [71° C] denatures (changes the nature of) the protein in enzymes” (p. 23).

 

Howell reports that enzymes do more work as the temperature range approaches 71° C/160° F, but that they are “used up faster” (p. 23). That in turn suggests that slightly heated foods, i.e., foods that are slightly warm/not hot, might be preferred over unheated foods by those who believe that enzymes are nutritionally important.

 

However, the table in this paper includes multiple tests of seeds heated to temperatures above 71° C/160° F for hours or days, and the seeds still sprouted. That suggests that a single temperature test for all foods/seeds of 71° C/160° F is also an inaccurate test for the (undefined) life force/vitality.

 

Why some seeds can withstand extreme heat:  evolutionary adaptation to fire regimes

 

Soil surface temperatures can be a factor. Fire is not the only heat source that impacts seeds/plants.  “Soil surface temperatures in desert conditions have been reported in excess of 80° C [176° F]” (Daws et al. 2007, p. 264). In that study the seed size is small, which means – in order for the seeds to germinate – the seeds are buried very shallow in the soil, i.e., only a few millimeters. In turn, that means the seeds studied would be exposed to very high soil temperatures for long periods, in the wild. It also means the (buried) seeds would be exposed to extremely high temperatures when the ground cover burns in a fire.

 

Other factors mentioned in some of the studies as being relevant to seed heat tolerance, include:

• moisture level of the seeds (lower moisture level was correlated with higher heat tolerance in some tests),
• heat shock proteins in the seeds.

 

Terminology: The scientific literature refers to pine cones that are sealed shut with sap using the adjective serotinous. The term comes from Latin and means (literally) late-arriving. In the botanical context, it can be interpreted as describing “late-developing” plant parts. Some pine cones open up as they grow and release their seeds at maturity. The serotinous cones are late-developing in comparison because they are sealed shut by sap, and don’t open (to release seeds) until the proper environmental trigger – fire – is present.

 

A personal note

 

Over the years I have read numerous scientific/academic review papers, many of which included table(s) that summarize the results of the studies reviewed. This is the first time I have created a similar table, and can report that producing the summary table for this paper required a very large effort.

 

The reasons why a large effort is required to produce summary tables (for use in a review paper) include:

 

• some source studies may be poorly written and/or poorly organized,
• some studies may report data in graphic rather than tabular form,
• a source study may use a complex statistical model that – because of the extra parameters and specific analysis reported – makes interpretation more difficult for use in the summary table, and
• crafting a summary table requires great attention to detail.

 

 

 

The following subsections are intended for a technical audience, and document –in detail– the sources for the information in this paper.

 

Data sources: germination rates in the table

 

The specific sources (for seed germination rates) from the cited papers, as used in the table in this paper are:

 

• Lee et al., 2002: temperature rows 1 & 2, table 1; last temperature, table 4.
• Fourest et al., 1990: figure 2.
• Daws et al., 2007: figure 1.
• Baker et al., 2005: figure 4.
• Bell & Williams, 1998: table 3.
• Weiss & Hammes, 2003: figure 2.
• O’Reilly & De Atrip, 2007: table 5.
• Hanley & Fenner, 1998: figure 1.
• Sidari et al., 2008: table 2.
• De Villalobos et al., 2002: tables 3 & 4.
• Banda et al., 2006: figure 3.

 

Validation: papers that reports seeds whose germination rate was enhanced by heat shock or heat treatment

 

Some of the papers cited here report higher germination rates for select seed heat treatments vs. controls. However, some of the papers did not report results of tests for statistically significant differences and/or the differences observed were not statistically significant at the p=0.05 level.

 

Papers cited here that report higher germination/seedling survival rates for at least some heat treatments (vs. control) and with statistically significant differences, include:

 

• Lee et al., 2002; see tables 1, 3, 4.
• Baker et al., 2005; see figure 4.
• Bell & Williams, 1998; see table 3.
• O’Reilly & De Atrip, 2007; see table 5.
• Hanley & Fenner, 1998; see discussion of Pinus brutia.
• Hanley et al., 2001; see table II.
• Hanley, 2009; see table 2.