Sexual dimorphism in the genus Leucadendron: Morphology and plant hydraulics

The genus, Leucadendron, of the Cape Proteaceae family, is made up of over 70 dioecious species that vary in their degree of sexual dimorphism. Males are generally more highly ramified (branched) with smaller leaves compared to corresponding females.  It has been hypothesised that sexual dimorphism in Leucadendrons is linked to serotiny (a fire-adapted reproductive strategy), where highly serotinous females may incur extra resource costs in order to keep their transpiring cones alive between fires. This hypothesis predicts that the female morphology might be associated with more efficient resource acquisition compared to males in order to support their greater resource requirements. Another hypothesis suggests that selection for greater floral display in males has lead to a higher degree of ramification as male cones are borne terminally on branches. This highly branched morphology may be associated with subsequent physiological costs.  The idea that different male and female morphologies might be associated with different physiological costs or benefits was tested in this experiment with a focus on plant hydraulics. Hydraulic supply is known to affect photosynthetic capacity and maximum assimilation rate.  Using a specially designed vacuum chamber, leaf-specific and xylem-specific hydraulic conductance was measured in males and females of the highly dimorphic Leucadendron rubrum and non/marginally dimorphic Leucadendron daphnoides. Using microscopic imagery, xylem anatomy was analysed in an attempt to explain the hydraulic conductance results.  Male and female L. rubrum individuals were highly dimorphic and differed significantly in their leaf size and degree of ramification. Males had smaller leaves and were more highly ramified (branched) than females. L. daphnoides males and females were only marginally dimorphic and had similar leaf size and degree of ramification.  The highly dimorphic morphologies of males and females in L. rubrum were associated with significant hydraulic conductance differences. Females were found to have significantly higher hydraulic conductance (leaf-specific & xylem specific) than males. The morphologically similar L. daphnoides males and females had similar hydraulic conductance values.  Higher conductivity in L. rubrum females was associated with larger average vessel size. Average vessel size in L. daphnoides males and females was similar.  Hydraulic conductance differences between males and females appeared to be associated with a high degree of sexual dimorphism. This empirical evidence suggests that in highly dimorphic species such as L. rubrum, highly ramified males may be physiologically disadvantaged compared to females. This can be used to support a number of current hypotheses that attempt to explain the reasons for sexual dimorphism.