The effect of light-limitation on spinescent structural defence and its implications on resistance to herbivory in the shade
Bachelor Thesis
2013
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University of Cape Town
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Abstract
Plants can resist herbivore pressure through structural or chemical defence or both. The ultimate
goal of defence against herbivory is to reduce the amount of damage to biomass, but more
specifically to protect against damage to meristematic tissue. The defences employed depend on the
type of herbivory experienced, which is contingent on the herbivore and its mouthparts. This
investigation was concerned with structural defence presented by spines. This type of defence
protects against mammalian herbivores such as browsers. Spiny species do not dominate in low-light
deep forest environments. Therefore the aim of this study was to determine the constraints on
spines as a defence strategy under shaded conditions by assessing the effect of reduced light on
spine efficiency. Spine efficiency was defined as the amount of defence afforded the plant given the
resources available. Spines require carbon to be built and need to be arranged properly in order to
present an adequate defence. Thus two non-mutually exclusive hypotheses were proposed: Light
limitation reduces the ability of spines to present an adequate defence against browsers due to the
architectural strategy employed and/or its influence on carbon gain. The spinescent plant chosen for
study was Carissa macrocarpa (Ecklon) A.DC. Light condition of plants was determined using
hemispherical photography. Spine efficiency of sun and shade plants was determined using a bite
test and was evaluated using architectural and physiological analyses. Architectural analysis involved
identifying levels of organisation within the plant across ontogeny and indentifying sun and shade
growth strategies. Physiological analysis involved determining carbon gain of sun and shade
individuals using gas-exchange measurements, as well as the measurement of biomass allocation by
harvesting and oven drying different plant parts. Results showed that biomass allocation patterns of
C.macrocarpa did not change in sun and shade but total biomass increased from shade to sunlit
conditions. Architectural analysis revealed that in the sun the plant adopted a stout dense structure
with high spine efficiency, while in the shade it was more elongated with lower spine efficiency.
Therefore C.macrocarpa adapts to the light environment by adopting either the shade or the sun
architectural strategy. The way in which this works is that light affects carbon gain, which either
increases or decreases biomass and in turn leads the plant to adopt the sun or shade architectural
strategy. The architectural strategy affects spine efficiency such that plants in the sun have higher
spine efficiency than plants in the shade. Thus, spinescent plants do not do well in light limited
environments because they are architecturally constrained to elongate in such conditions. This
constraint would put them at higher risk of browser damage than plants in light-sufficient
conditions, ultimately decreasing their fitness. If the patterns observed in C. macrocarpa prove to be
general, then it helps to explain why spiny plants are more commonly found in open, sunlit
environments than in deep shade.
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Reference:
Adams, I. 2013. The effect of light-limitation on spinescent structural defence and its implications on resistance to herbivory in the shade. University of Cape Town.