Seed biology
4.1 Introduction
Chapter 4-
dispersal. Hooks and spines frequently aid dispersal by animals and man (Heywoodet al., 1977;
Heywood, 1993). Most of the weedy species of this family mature early and produce large numbers of single-seeded, dry fruits often with unspecialised germination requirements (popay and Roberts, 1970;Lawrence, 1985;Holm et a!., 1997). Among the weedy members of the Asteraceae listed by Holmet al. (1997) many (62 %) are annuals,reproducing principally by wind dispersed achenes, and some are perennials relying heavily on sexual as opposed to vegetative reproduction. For example, Senecio vulgaris L. (an annual) and Chondrilla juncea L. (a perennial), both serious economic weeds, are described as"prolific seeders" with rapid seedling establishment (Holm et a!., 1997).Despite a mild dormancy in a small.portionof the seeds of these species, most are capable of immediate germination. Seeds ofC.juncea andS. vulgaris germinate in both the light and dark and the optimum temperature for germination is 25 0C (popay and Roberts, 1970;Holmet a!., 1997). Lawrence (1985) analysed characteristics of32 Senecio species in Australia and found that 29 species reproduced sexually,producing over 10 000 small achenes (0.1-0.6mg)per plant. Few species showedspecific germination requirements.
General biology ofweed seeds and seedlings
The germination process involves the inception of rapid metabolic activity within the seed, resulting in perceptible growth of the embryo. It is usuallyassociated with the uptake of water and oxygen, use of stored food and normally, release of carbon dioxide (Klingmanet a!., 1982;
Zimdahl, 1993).For many investigators radicle emergence has been one ofthe best visible criteria identifying the initial stage ofgermination (King, 1966;Evetts and Burnside, 1972;Robocker, 1977; Grime et a!., 1981; Smreciu et al., 1988). For a seed to germinate it must have an environmentfavourable to the germination process,factors promoting germination may include specific temperature,light, moisture and pH requirements (King, 1966; Klingman et al., 1982).
Generally, weeds have no special requirements for germination (Baker, 1965;Stephens, 1982;
Zimdahl,1993). However, in a few species dormancy mechanisms do occur (Zimdahl,1993). Five environmental factors affect seed dormancy: temperature, light, moisture, oxygen and the presence ofinhibitors (including allelopathic effects). Otherfactors directly related to the seed and its dormancy include impermeable seed/fruit coats, immature embryos and an after-ripening period (King, 1966; Grime et a!., 1981;Klingman et al., 1982;Zimdahl, 1993; Cousens and Mortimer, 1995).
While there are great differences in the rate of seedling growth among the various weed species, the most competitive usually have rapid growth and early, extensive root development compared to the plants with which they are competing (King, 1966; Zimdahl, 1993; Cousens and Mortimer, 1995, Holmet a!., 1997). They grow tall quickly or gain competitive advantage by twining up larger plants. Furthermore, they are frequently tolerant of shade in that their highest carbon dioxide assimilation is not in full sunlight (Zimdahl, 1993). Certain weed seedlings have large expansive type foliar cotyledons that, through early photosynthetic function enable young seedlings to become established quickly.This is true for the Asteraceae (King, 1966). Similarly, the rate of new leafproduction is important, as it affects surface area available for photosynthetic production.
The present study
In southern Africa, Hawaii and AustraliaD.odorata reproduces sexually by seed and vegetatively by stolons. However, in California, where this vine is a serious weed, no viable seed is produced and the plant reproduces only by vegetative means (Balciunas, pers. comm.). In a study conducted by the USDA Agricultural Research Service on the germination requirements of the seeds produced by D. odorata in California, for the entire testing procedure, only one seed germinated (USDA, 1998). The reason for the sterility of the populations in California is at present unknown (USDA, 1998). The contrasting modes of reproductionofD. odorata in exotic infestations raises interesting questions as to the nature of the spread of this vine. Sexual reproduction is usually an important method of propagation for weeds of the Asteraceae; they generally mature early and produce copious amounts ofeasily dispersed single-seeded fruits which germinate quickly or remain buried beneath the soil until germination requirements are met.
Frequently, it is because of the large amount of seed produced that eradication and control of these weeds is difficult. However, ifD. odorata is a serious invader in California despite its lack of viable seed then the importance of the seed biology of this vine in the process of invasion needs investigation.
Besides the investigation conducted by the USDA Agricultural Service (California) into seed germination, no studies have been conducted into the seed biology of the fruits ofD. odorata.
Without an understanding of the seed biology ofD. odorata in southern Africa, Hawaii and Australia no predictions about the importance of sexual reproduction in the spread ofthis species
in·exotic locations can be made. Furthermore, if this species does spread through sexually reproduced seed in Hawaii and Australia,knowledge of the seed biology may be importantin the development and implementation of management strategies in those areas.
The present investigationwas conducted to obtain a better understanding of the seed biology of D. odorata in southern Africa.It is a baseline study, the results of which may be used as a comparative reference for studies made on the seed biology of this species in exotic locations.
A series oflaboratory experimentswere conducted to determine the germination characteristics of the seeds. Anexamination of the effect of light on seedling development was also made. Although the difficulties in relatinglaboratory experiments to field conditions have been pointed out (Koller and Roth, 1964;Popay and Roberts, 1970), such analyses are an essential beginning to an understanding of the behaviour of seeds/fruits in response to the complex and fluctuating conditions in the field (popay and Roberts, 1970).