DECLARATION 2 PUBLICATIONS
6.1 INTRODUCTION
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Growth responses of grass seedlings to smoke-water and smoke- derived butenolide in the presence or absence of NPK nutrients
“I think science has enjoyed an extraordinary success because it has such a limited and narrow realm in which to focus its efforts“
Ken Jenkins (1940~)
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TAINTON, 1984) accompanied by a significant decline in basal cover (EVERSON and TAINTON, 1984; FYNN, 2004; SNYMAN, 1998; FYNN et al., 2005). In other words, the short native species require frequent exposure to fire to remain competitive (EVERSON and TAINTON, 1984; MORGAN and LUNT, 1999). Though much is known of how trends in species composition alter in response to fire and fire-related factors (EVERSON and TAINTON, 1984; GIBSON and HULBERT, 1987; FYNN, 2004), understanding of the underlying mechanisms responsible for these responses is still incomplete. Although widely debated, previous studies on grassland fire suggest that the strong fire-species relationship syndrome is the result of fire-induced temporary increases on light availability (KNAPP and SEASTEDT, 1986; GHEBREHIWOT et al., 2006); lowered productivity (O’CONNOR, 1997;
FYNN, 2004); competitive release (O’CONNOR, 1997; MORGAN and LUNT, 1999;
FYNN, 2004) nutrient enrichments (FYNN et al., 2005) and smoke (BAXTER et al., 1994). Hence, quantifying the role of these factors in single or in combination is crucial for understanding the dynamics of such fire-climax ecosystems (TAINTON, 1999). More importantly, an understanding of the constraints on seedling establishment is essential for the ecology and successful management of grasslands.
In the mesic and semi-arid grasslands of South Africa (Figure 1.1), the existence of competitive displacement trends among species in response to fire- induced nutrient gradients such as nitrogen (N), potassium (P) and phosphorus (K) has been reported (SAMUEL and HART, 1992; FYNN et al., 2003; FYNN et al., 2005). Fire causes severe losses of large amounts of carbon (C), nitrogen (N) and sulfur (S) present in the herbage through volatilization and erosion of ash, which subsequently results in a significant loss of organic matter content from the soil surface (HAYES and SEASTEDT, 1989; KAUFFMAN et al., 1993; COOK, 1994;
OJIMA et al., 1994; BLAIR, 1997; TURNER et al., 1997; FYNN et al. 2003). As a result N becomes more deficient to plants due to a decrease in the rate of its mineralization. Post-fire semi-arid grasslands are also typified by rapid leaching of potassium (K) and phosphorus (P), and as a consequence such elements become less available to plants thereby limiting growth (ALLEN, 1964). CHAPMAN (1967) reported that controlled burning of lowland heath vegetation eliminated 95% N, 26%
P and 21% K from the burnt site. This indicates that the species which dominate
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post-fire habitats thrive more on nutrient-poor soils. Therefore, soil nutrient gradients between burnt and unburnt grasslands may play a significant role in competitive displacement of the species involved (GRIME, 1979; TILMAN, 1988;
GHEBREHIWOT et al., 2006).
On the other hand, more recent studies on the role of fire and fire-related factors on germination and regeneration ecology of grasslands indicate that plant- derived smoke influences both the germination and grassland species growth differently (VAN STADEN et al., 2000; GHEBREHIWOT et al., 2009). Further, certain native grass species that dominate frequently burnt South African semi-arid grasslands are significantly more responsive to smoke treatments than others. Such a difference among species would have an inevitable impact on species composition (BAXTER et al., 1994; see also Chapter 3). Currently, plant-derived smoke treatments, whether in the form of aerosol or liquid have shown stimulating effects on seed germination and seedling growth (BROWN and VAN STADEN, 1997; VAN STADEN et al., 2000; BROWN and BOTHA, 2004; SPARG et al., 2005;
KULKARNI et al., 2007; COMMANDER et al., 2009; NELSON et al., 2009;
DAYAMBA et al., 2010). Smoke treatment has also shown positive effects on germination and seedling vigour of different grass species along gradients of temperature and water potential (see Chapter 4). A compound isolated from burnt cellulose (FLEMATTI et al., 2004) and plant-derived smoke (VAN STADEN et al., 2004) known as butenolide (3-methyl-2H-furo[2,3-c]pyran-2-one) is responsible for enhancing seed germination of many plant species. There is a general suggestion that butenolide (recently termed as KAR1) behaves in a manner similar to that of other plant hormones (SENARATNA et al., 1999; VAN STADEN et al., 2000;
GARDNER et al., 2001; CHIWOCHA et al., 2009; NELSON et al., 2009). Several studies on smoke have shown that aqueous smoke solutions exhibit hormone-like responses in many species and interact with gibberellins, cytokinins, abscisic acid and ethylene in photoblastic and thermodormant seeds (VAN STADEN et al., 2000;
JAIN et al., 2008; NELSON et al., 2009). Hence, the effect of plant-derived smoke as a new factor influencing plant species growth (VAN STADEN et al., 2000;
Chapter 3), and its interaction with major soil macronutrients (NPK) in influencing seedling growth has become a crucial subject that needs to be understood (TAINTON and MENTIS, 1984; MATERECHERA et al., 1998).
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In southern Africa, annual burning is being commonly practiced around large- scale commercial and semi-subsistence grassland farming. South African grass species are poorly represented in smoke-stimulation research and there is very little or no information on the response of species to plant-derived smoke and its interaction with soil nutrients (ZACHARIAS et al., 1988; BAXTER et al., 1994). This study sought to gain greater insight into the role of plant-derived smoke and its interactive effect with major soil nutrients (NPK) in influencing seedling growth. The specific objective was to determine the effect of plant-derived smoke-water and a smoke-isolated butenolide compound, at different concentrations, on various parameters of seedling growth of three grass species from mesic and semi-arid grasslands in South Africa.