83 WOODY PLANT SEEDS PlSc 300 Lab 12 REFERENCE: Text pp. 166 167; 191 195; 211 218. OBJECTIVES: 1. Become familiar with the handling and pre-germination requirements for seeds from some woody tree and shrub species. 2. Observe and compare the effects of dormancy, stratification, and scarification on germination of woody plant seeds. INTRODUCTION: Seeds from woody trees and shrubs grown in temperate zones are borne in a variety of structures that can make collection and handling quite complicated. Acorns and maple samaras are simply swept up. Pods of redbud, honeylocust, black locust and mimosa, and lantern-like structures of the golden rain tree are collected from the tree or ground and flailed, broken or cut open. Conifer cones have to be collected before shattering so the small seeds can be recovered. Crabapples must be cut open and seeds removed with a knife. Small berries from many plants must be macerated before the pulp and empty seeds are separated from good seeds by flotation. Storage procedures also vary widely. Fleshy seeds, such as acorns, chestnuts and maples, have little seed coat protection, are short-lived and usually stored under moist cold conditions. Seeds with hard coats, such as redbud, goldenraintree and mimosa, are long-lived and remain viable for 15 or more years in open storage. Some smaller seeds, such as apple, can be stored dry in open storage for a few years and are considered medium-lived.
84 Most spring and early summer ripening seeds, such as silver and red maples and poplars, are ready to germinate as soon as they drop and are difficult to store. Fall-ripening seeds generally have hard seed coats, a dormant embryo, or both as a natural mechanism to delay germination until a time more suitable than before an approaching winter. Seeds of others species, such as sycamore seeds, simply remain in their fruits on the tree with neither hard coats nor dormant embryos over the winter. Hard coats break down naturally by freezing and thawing and microbial action. Since the hard coat may still be intact by spring, the propagator speeds the process by abrasion or acid soaking. Breaking down hard seed coats is called scarification. Dormant embryos usually require moist-chilling or stratification, before they can germinate. The effective temperature is always around 4.5ºC (40ºF), but the time in stratification can vary widely. Fall planting allows for natural stratification, but problems with weeds, rodents, birds, etc. may develop. Once these pre-germination requirements are completed, conditions must still be favorable before seeds germinate. Different seeds have different optimum, maximum and minimum temperature requirements. Moisture, oxygen and often light can also affect germination. A. Genetic Variation in Moist Chilling Requirement. The chilling requirement of seeds (and buds) is under genetic control. This phenomenon means that in a population of seeds, some individuals will require more and some less chilling than the average. During a 10-week period of this semester, you planted apple seeds, which were subjected to varying stratification periods. Plot your data on the following chart and make some conclusions about dormancy requirements for apple seeds based on your results. On the second sheet, be sure to plot the class data for apple seed germination by plotting data for each individual group
85 on the second graph. Be sure to calculate an overall average from all the data and plot it against your group data (on the first graph). In this way, you can compare your data against those of your classmates. Number of germinated seeds Group Data 20 18 16 14 12 10 8 6 4 2 0 Stratification (weeks) Conclusions:
86 Class Averages Number of germinated seeds 20 18 16 14 12 10 8 6 4 2 0 Stratification (weeks) Conclusions:
87 B. Effects of Combined Stratification and Scarification. Twelve weeks ago you initiated an experiment to observe the effects of stratification with or without scarification on Gleditsia triacanthos inermis and Cercis canadensis seeds. You concluded this experiment by planting the last batch of seeds a couple weeks ago. Record all your data, including the final data of percent germination from the final planting date in the tables provided. Enter your data on the Class Summary Data Table as well. You should calculate an overall class average for the two species and compare your results with the class averages. Personal/ Group Data: Species Treatment No. of seeds per batch No. of seeds germinated % germination Control 30 min acid 60 min acid 4 wk @ 4.5ºC 10 wk @ 4.5ºC 30 min acid + 4 wk @ 4.5ºC 30 min acid + 10 wk @ 4.5ºC 60 min acid + 4 wk @ 4.5ºC 60 min acid + 10 wk @ 4.5ºC Observations:
88 Personal/ Group Data: Species Treatment No. of seeds per batch No. of seeds germinated % germination Control 30 min acid 60 min acid 4 wk @ 4.5ºC 10 wk @ 4.5ºC 30 min acid + 4 wk @ 4.5ºC 30 min acid + 10 wk @ 4.5ºC 60 min acid + 4 wk @ 4.5ºC 60 min acid + 10 wk @ 4.5ºC Class Summary Data Treatment Control 30 min acid 60 min acid 4 wk @ 4.5ºC 10 wk @ 4.5ºC 30 min acid + 4 wk @ 4.5ºC 30 min acid + 10 wk @ 4.5ºC 60 min acid + 4 wk @ 4.5ºC 60 min acid + 10 wk @ 4.5ºC Average % germination Gleditsia triacanthos inermis Average % germination Cercis siliquastrum
89 C. Your Own Seed Treatment Experiment. Your group began an experiment by using various seed treatments in an effort to improve germination of your selected plant species. What species did you select, what treatments did you use, and did the treatments improve seed germination? For your lab report, you need to: 1. Briefly describe the experimental methods used, and be sure to include enough details so that the instructor knows how you completed the study. 2. Present germination data either in tables, graphs or text. 3. What happened in the germination experiment you designed? Mention expected versus actual results. What worked and what didn't. Describe highlights of your results and discuss the most important results and their implications on seed germination. 4. What can you conclude from your experiment? In other words, what treatment would you recommend to use for meeting dormancy requirements of your selected plant species? D. Apomixis (also pertains to seeds of some herbaceous species). For a few species, asexual propagation (and its genetic uniformity) is possible through seed propagation, resulting in one, or two or more (polyembryony) seedlings emerging per seed. In Citrus for example, one sexual plus one or more apomictic embryos may develop. The propagator rogues out the non-uniform sexual seedlings and is left with a highly uniform batch of asexually propagated plants. Seeds from different kinds of citrus were previously germinated in individual flats. Note that in some cases two or more seedlings emerged from a single seed (only one seedling per seed is
90 possible in the case of sexual seedlings). You can determine if individual seedlings are apomicts or sexual. Scan entire flats of seedlings at once. You should note more uniformity in those flats where you see a greater degree of polyembryony (apomixis). Within those flats, the experienced propagator can pick out the non-uniform seedlings, and be left with a very high percentage of uniform, asexual seedlings. Conclusions (Comment on): a. Polyembryony - b. Uniformity in seedlings (height, color, etc.) in flats with greater and lesser degrees of polyembryony. Note: a discussion of apomixis and polyembryony should be excluded (left out) of your lab report. Questions: 1. Did the apple seeds require a cold treatment (stratification) to germinate? How do you know? Provide evidence for your conclusion.
91 2. Did your apple seed germination follow the same general trends of other groups in the class? If not, why do you think the germination trends differed? Be specific or carefully explain differences. If they were similar, describe how they were similar. 3. Did the germination trends for the Cercis and Gleditsia seeds follow the similar trends to each other? If so, what can you conclude? If not, what do you think caused the differences? 4. Did the Cercis and Gleditsia seeds that lacked an acid treatment germinate readily or germinate after storage time? What do you think caused the results you obtained with seeds that lacked an acid treatment? 5. Did other groups in the class have similar germination results for Cercis and Gleditsia seeds? If not, why do you think the germination trends differed? Be specific or carefully explain differences. If they were similar, describe how they were similar. 6. In general, what can you conclude about cold storage or other treatments used on your INDIVIDUAL group experiment seeds with regard to necessity of the treatment and the length of the storage period? c:\plsc300\lab12 seed germination-12.doc