Seed biology
4.4 Discussion Germ ination requirements
experiment,stem length ofseedlings exposed to 17%sunlight was significantly greater than those grown at 45 and 71 % (Kruskall Wallis ANOVA,p< 0.05) (Fig. 4.8). Number of leaves per seedling was maintained at a relatively constant level in all treatments for the first 12-15 weeks (Fig. 4.8). However, after eight months the average number of leaves per seedling was significantly greater for those seedlings grown at 45 % sunlight (H=8.51, p<0.05) (Fig. 4.8).
There was no significant difference in leaf size of seedlings exposed to the different light treatments, althoughleaves ofseedlings exposed to only 17% sunlight were generally larger than those grown at 71%(H=1.29, p>0.05) (Fig.4.9).
The flowering period forD. odorataoccurs over the months of April to June.In June 1999 the seedlings for this investigation were eight months old and they had no flowers.This is contrary to expectationas weedySenecio species usually reach sexual maturity rapidly and flower within their first year of establishment (Lawrence, 1985;Abbot, 1986; Holm et al., 1997). Further studies are required to confirm the result obtained for this investigation. Eight months after sowing the seedlings were harvested.Present results revealed no significant difference between treatments in the allocation of biomass to leaves, stems and roots for seedlings exposed to the different lightlevels (leaves;F=I.68, p>0.05; stems,F=0.5,p>0.05;roots,F=I.20,p>0.05)(Fig.
4.10).Maximum biomass allocation was to stems, followed by that to roots and leaves.
4.4 Discussion
40
30
E
E ....c 20
Clc
-
~~ro10
. _.... •... . .
71% 45%
sunlight(%)
17%
Fig. 4.9. Length of the longest leaves (mm) ofD. odorata seedlings following eightmonths exposure to one of three light levels. Error bars represent standarderror .
80 1J71%
~ .45%
~c 0 17%
.Q 60
1§
,g.m :z
ro 40E0 :0
20
leaves stems
plant part
roots
Fig. 4.10. Dryweight allocation (%) to roots, stems and leaves of D. odorata seedlings following eight months exposure to one of three light levels. Error bars represent standard error.
Clearly light stimulates the germination ofD. odorata seeds. Light is one of the major factors regulating seed germination and light requirements are frequently associated with small seeds (Harper et al., 1970; Mayer and Poljakoff-Mayber 1989;Bell, 1993; Plummer and Bell, 1995;
Rokich and Bell,1995).Plummer and Bell (1995) documented a light requirement for germination in several small seeded Australian Asteraceae.Several other authors have also reported a light stimulation withinthe Asteraceae (Popay and Roberts, 1970;Mott, 1972; Atwater, 1980; Grime et al.,1981; Willis and Groves, 1991). Lack ofgermination ofburiedD.odorata seeds may also indicate a light requirement. Light-promoted seed germination appears to be an adaptation to enhance the chanceof establishment where germination ofdeeply buried seeds would prove fatal.
Physiologically active light fluxdensities rarely penetrate more than a few millimeters into the soil (plummer and Bell, 1995). Small seeds contain limited reserves and it is advantageous for germination to occur where photosynthesis can quickly take over from stored carbohydrates (Pons, 1992;Plummer and Bell, 1995). Plummer and Bell (1995) found that Asteraceae seeds weighingless than 0.5 mg required light, whereas the response was mixed among species with heavier seed.
Low temperaturesclearlyhave a beneficial effect on the germination of seeds ofD. odorata when not exposed to light. Treatment with alternating temperatures or low temperature incubation has long been known to increase germination in darkness oflight requiring seeds (Popay and Roberts, 1970; Bewley and Black, 1994). Grand Rapids lettuce, for example, is generally dormant in darkness above23 QC,below this value seeds germinate without illumination (Bewley and Black, 1994).The achenes ofD.odorata exposed to 30 QC germinated easily when moved to a lower temperature.This phenomenon is known as thermoinhibition (Horowitz and Taylorson, 1982).
Thermoinhibition refers to seeds where germination fails at high temperature but proceeds upon subsequent transfer to an optimal temperature,thermoinhibition thus prevents germination when conditions are unfavourable.
Itmay be unwise to assume that the ability offreshly collected seed to germinate in the laboratory is a reliable indication that under field conditions germination occurs soon after fruit release. In the field, the germination of freshly dispersed seed may be prevented by limiting factors not operating in the laboratory tests (Grime et al., 1981). Available literature suggests that the germination of achenes of many members of the Asteraceae coincides with the onset of wet conditions (GrimeetaI.,1981;HolmetaI., 1997). In southern Africa the small wind-dispersed
fruits ofD. odorata are set and dispersed in the dry winter months of June and July when average daily temperatures range between 5 and 20 QC (CCWR, 1999). Although the achenes ofD. odorata have no stratification requirement, the cold dry winter conditions may prevent germination in the field. Germination of achenes may coincide with the onset of the spring rains and warmer temperatures oflate September. Seedling germination, establishment and growth in the field will need to be examined to confirm these preliminary observations.
Seedlingestablishment
Competitive capacity of plants is governed, in part, by the efficiency with which they intercept light (Keeley and Thullen, 1978).In relation to light requirements for plant establishment and growth Grime (1966) made a distinction between shade avoiding and shade tolerant species.The observations made in this investigation indicate that the seeds ofD. odorata germinate at both high and low light intensities, although establishment in lower light conditions (17 and 45%) appears to be favoured.D. odorata is therefore likely to be a shade tolerant species.Maximum leafproduction occurred at 45%sunlight,a factor confirmed by other observations made for this project (Chapter 5). In its native environmentD. odorata frequents forests and forest margins where variation in light intensity is typical. Furthermore, in areas whereD. odorata creates a mat- like groundcover, extensive die back of the adult plants over winter may expose seeds to an environment of varied and changing light intensity throughout the day; an ability to tolerate a range oflight intensities may thus confer a competitive advantage to the seedlings ofD.odorata.