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Germination physiology
intact seed of these species may need a longer period of cold stratification, or culture
conditions at 5°C. There is a period of about two months with average daily minimum
temperatures of 5°C in Nieuwoudtville where R. sabulosa is found (Figure 2.20).
Although R. diversiformis, R. flava, R. leipoldtii and R. minutiflora all had in vitro
germination percentages higher than 57% at 15°C, seeds of other species showed
very low germination. R. camerooniana was the only species of which neither the
seeds germinated nor the embryos responded. Low viability of R. camerooniana
seeds was indicated by TTC. However, all species endemic to South Africa showed
positive responses to either seed or embryo germination treatments.
The results of the temperature experiment for R. rosea confirms that of EDDY &
SMITH (1975) who found that R. rosea seeds has a clear germination optimum in the
temperature range of 9.5 to 13°C. These results suggest that invasive individuals of
R. rosea could be eradicated (by mechanical or chemical control) when temperatures
are low. EDDY & SMITH (1975) reported that pre-chilling of seeds or KNO3 treatment
does not increase germination of this species. However, in the present study KNO3
significantly increased germination over the control. This result was confirmed where
seed germination was low in Hoagland’s nutrient solution without K in comparison to
the control (HS 50%). These findings suggest that the use of fertilizers containing
high potassium should be avoided to reduce R. rosea invasion which will be higher in
agricultural areas.
It appears that seeds of R. diversiformis, R. flava, R. leipoldtii, R. minutiflora, R.
monadelpha, and R. sabulosa all exhibit non-deep endogenous morphophysiological
dormancy, as excised embryos showed a growth response and the causes could
include a physiological germination inhibiting mechanism or an underdeveloped
embryo (BASKIN & BASKIN, 1998). The percentage germination of R. rosea seeds
on filter paper moistened with KNO3 was significantly higher than the control, this
response to potassium nitrate is a characteristic of non-deep morphophysiological
dormancy (COPELAND, 1976). The causes of non-deep physiological dormancy
include the physical barrier created by covering structures, the resulting low oxygen
supply to the embryo, inhibitors within the covering structures and/or
physical/chemical changes in the covering structures (BASKIN & BASKIN, 1998).
COPELAND (1976) states that the dormancy of such seeds can often be broken by a
cold stratification treatment. This happened in the case of R. flava. However, this was
not tested for R. leipoldtii and R. minutiflora due to limitations in seed availability. The
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