A Technique for Determining Quantitative ...

Ann. Bot. 50, 459-463, 1982

A Technique for Determining Quantitative Expressions of Dormancy in Seeds D. D. RICHTER and G. L. SWITZER* Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A. Accepted: 29 December 1981

A method of quantifying the heterogeneity of dormancy within seed populations is described. The method compares germination responses of unstratified and stratified seeds of Pinus taeda L. (loblolly pine). Mathematical functions were fitted to cumulative germination curves of unstratified and stratified seeds and functions were integrated to determine areas beneath respective curves. The area between respective curves was calculated by difference and was considered to be a measure of dormancy within seed lots. The method may be used to help identify various controls of seed dormancy. Key words: seed germination, stratification, germination curves, Pinus taeda, loblolly pine, dormancy. INTRODUCTION

Seed dormancy varies among and within plant species and is assumed to increase the survival potential of a species. Despite the importance of germination patterns to theoretical and applied ecology (Heydecker, 1966; Levins, 1969), variations in dormancy within populations of seeds have not been adequately quantified (Nichols and Heydecker, 1968; Goodchild and Walker, 1971; Janssen, 1973). This report presents a quantitative technique that expresses seed dormancy as the degree of germination response to dormancy-modifying conditions (Amen, 1968; Nikoleava, 1969). The approach recognizes the physiological heterogeneity of undomesticated plants (Koller, 1964) and is used: (1) to describe the heterogenous character of dormancy within seed populations and (2) to quantify total amounts of dormancy within seed populations from different sources. METHODS

Germination experiments were conducted with seeds of Pinus taeda L. (loblolly pine), a species whose dormancy has been attributed to seed coat constraints (Barnett, 1976). Seed lots were from four sources (north-east, central, south-west, and south-east Mississippi), and each lot represented composite samples from many natural stands within each source. Seeds were subjected to pre-germination stratification treatments (moist conditions at 4 °C) for 0, 15, 30, 60, 120 and 135 days to determine the duration of stratification required to reduce dormancy to minimum levels in this species. Four replicates of 50 seeds were germinated for each stratification period and seed source. Germination, scored when geotropism was evident in emerging radicles, was observed daily for a 30 day period in a water curtain germinator with alternating temperatures of 30 and 20 °C and with 8 and 16 h light and dark periods respectively. Mathematical functions werefittedto real, cumulative germination data of unstratified seed and seed stratified for 120 days, because stratification for 135 days resulted in germination responses that were not distinguishable from responses to 120-day treatments. Polynomial regression equations werefittedto germination data (Goodchild and Walker, * School of Forest Resources, Mississippi State University, Mississippi State, Mississippi 39762, U.S.A. 0305-7364/82/100459 + 05 $03.00/0

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ABSTRACT

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Richter and Switzer—Dormancy in Seeds

1971) of unstratified seed, because these curves were characterized by a positive skew, as noted by Heydecker (1966) and Campbell and Sorensen (1979). Thus, rate of germination of unstratified seeds was greater in the first 50 per cent of the germinants than in the final 50 per cent. Sigmoid functions were fitted to germination data of seed stratified for 120 days, because these data conformed to a cumulative normal distribution, as noted by Janssen (1973). The generalized sigmoid function is f(x) = A/[l+b.exp(-cx)],

RESULTS AND DISCUSSION

Increasingdurations of stratification rapidly diminished seed dormancy, while germination response to lengthy stratification approached a maximum in all seed lots (Fig. 1). Such extended stratification caused germination responses to be remarkably similar among the four seed sources (Fig. 2). Differences were found in the heterogeneity of dormancy among seed from different sources (Fig. 2). For example, 90 per cent of the seed from the south-western source had fewer than 9 days of dormancy (as measured by difference in time between germination curves), compared with 75 per cent of the seed from the south-eastern source and 57 per cent from both the north-eastern and central sources. Thus, south-western seed was less dormant than seed from other sources in all fractions of the lots. However, source differences were most obvious in the final 50 per cent of the germinants, largely because germination rates of unstratified seed exhibited a variable positive skew among the four sources (Fig. 3). This variation may be important to both theoretical and practical aspects of seed dormancy (Nichols and Heydecker, 1968; Levins, 1969). For example, seed populations with a positively skewed germination pattern have a greater chance of establishment in circumstances with randomly varying environments. However, positively skewed germination patterns are not desirable for crop seeds because rapid and uniform germination is generally necessary for maximum crop production. Determinations of total amounts of dormancy in the four seed lots (measured as the area between germination curves in Fig. 2) revealed differences that were associated with the geographical location of the seed sources. Areas between germination curves for seed from south-west, south-east, north-east and central Mississippi were respectively 637, 775, 975 and 1021 per cent-days. Thus, southern seed exhibited less dormancy than seed from more northern sources, a pattern that suggests an inverse relationship between duration of growing season and intensity of dormancy. Similar conclusions are reported in provenance studies with other tree species (Stearns and Olson, 1958; Wilcox, 1968). The integration technique can quantify germination response to treatments other than stratification that also alleviate dormancy. Furthermore, the approach can provide guide-lines for pre-germination treatments by estimating treatments required to reduce dormancy by proportional amounts (e.g. by 75 per cent; see Fig. 1).


where f(x) is cumulative germination percentage, x is time to germination in days, A is an asymptote of maximum germination percentage (100 per cent), b and c are non-linear regression coefficients estimated by iterative procedures, and exp is the base of the natural logarithm (Daniel and Wood, 1980). Functions were subsequently integrated to determine areas beneath each curve. The area between cumulative germination curves of unstratified seed and seed stratified for 120 days was obtained by difference and was considered to be a measure of total dormancy within a seed lot. To compare dormancy in various fractions of the seed lots, the elapsed time between germination curves of unstratified seed and seed stratified for 120 days was determined by difference for seed at 10 levels of germination (for germinants at 5, 15, 25 to 95 per cent).


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FIG. 1. Cumulative germination curves for Pinus taeda seed from north-east Mississippi following (A) 0, (B) 15, (C) 30, (D) 60 and (E) 120 days of stratification. 100 75

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FIG. 2. Cumulative germination curves for (U) unstratified and (S) stratified Pinus taeda seed from (a) north-east, (b) central, (c) south-west and (d) south-east Mississippi. The eight regression equations had r2 values greater than 0-98.

Seed dormancy may be of survival value and adaptive significance, because it helps maintain a reserve of viable seed in the event of catastrophic environments (Harper, 1977). Catastrophic conditions may be regular or random, and dormancy appears to reduce risks to both conditions. Regular disasters are averted by germination being keyed to stimuli that signal the end of a dangerous season, whereas reproduction in a randomly varying environment appears less risky, given the heterogenous character of dormancy within seed populations. However, the assumption, that the existence of dormancy mechanisms is proof of physiological adaptation, is teleological and lacks supporting evidence in many instances (Harper and McNaughton, 1960). Based on laboratory tests, 17

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Richter and Switzer—Dormancy in Seeds 70 Stratified ssed

FIG. 3. Germination rates for stratified (120 days) and unstratified Pinus taeda seed from north-east ) and south-west (—) Mississippi. (

general inferences can be formulated about environmental conditions that control seed dormancy. But because specific conditions that break seed dormancy in situ are poorly understood, only hypotheses can be made about the survival values of most dormancy mechanisms. Furthermore, evaluation of the genetic and ecological significance of seed dormancy has been hindered by available mathematical treatments of germination response, many of which have been soundly criticized (Heydecker, 1966; Nichols and Heydecker, 1968; Goodchild and Walker, 1971; Janssen, 1973). Techniques described in this report can be used in provenance and control-pollination studies to help identify genotypic, maternal and environmental controls of seed dormancy. ACKNOWLEDGEMENTS

We thank R. B. Reuther, L. E. Hinesley and M. G. Shelton for technical assistance. Partial support for manuscript preparation received from the Office of Health and Environmental Research, U.S. Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation. Publication No. 1906, Environmental Sciences Division, ORNL. LITERATURE CITED AMEN, R. D., 1968. A model of seed dormancy. Bot. Rev. 34, 1-31. BARNETT, J. P., 1976. Delayed germination of southern pine seed related to seed coat constraint. Can. J. For. Res. 6, 504-10. CAMPBELL, R. K. and SORENSEN, F. C , 1979. Anew basis for characterizing germination. J.SeedTech. 4,24-34. DANIEL, C. and WOOD, F. S., 1980. Fitting Equations to Data, 342 pp. John Wiley and Sons, New York. GOODCHILD, N. A. and WALKER, M. G., 1971. A method of measuring seed germination in physiological studies. Ann. Bot. 35, 615-21. HARPER, J. L., 1977. Population Biology of Plants, 892 pp. Academic Press, New York and London. HARPER, J. L. and MCNAUGHTON, I. H., 1960. The inheritance of dormancy in inter- and intraspecific hybrids of Papaver. Heredity 15, 315-20. HEYDECKER, W., 1966. Clarity in recording germination data. Nature, Lond. 210, 743-4. JANSSEN, J. G. M., 1973. A method of recording germination curves. Ann. Bot. 37, 705-8. KOLLER, D., 1964. The survival value of germination-regulating mechanisms in thefield.Herb. Abs. 34, 1-7. LEVINS, R., 1969. Dormancy as an adaptive strategy. In Dormancy and Survival, ed. H. W. Woolhouse, Symp. Soc. exp. Biol. 23, 1-10.


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NICHOLS, M. A. and HEYDECKER, W., 1968. Two approaches to the study of germination data. Proc. Int. Seed Test. Ass. 33, 531-40. NIKOLAEVA, M. G., 1969. Physiology in Deep Dormancy in Seeds. Israel Prog, for Sci. Trans., Jerusalem. STEARNS, F. anU OLSON, J., 1958. Interactions of photoperiod and temperature affecting seed germination in Tsuga canadensis. Am. J. Bot. 45, 53-8. WILCOX, J. R., 1968. Sweetgum seed stratification requirements related to winter climate at seed source. For. Sci. 14, 16-9.


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