Redressing the roles of anthocyanin pigments in vegetative and reproductive organs
Anthocyanin pigments are common in both reproductive and vegetative organs in plants, yet their functional significance is not entirely understood. While communicative functions have received considerable attention in reproductive organs and the role of anthocyanic colouration in frugivore and pollinator attraction is well understood, it has also been suggested that anthocyanins provide a communicative function in vegetative organs i.e. it may be that anthocyanic colouration in leaves deters herbivores by signalling a plant’s defensive investment. Conversely, there is evidence that anthocyanins in vegetative organs perform a number of physiological functions such as photoprotection and mitigation of various environmental stressors. While these physiological roles have received considerable attention in leaves, little is known about the applicability of these functions to anthocyanins in reproductive organs. There is evidently a gap in anthocyanin research; no study has provided unequivocal support for a communicative function for anthocyanins in vegetative organs and no study has shown that anthocyanins perform a physiological function in the reproductive organs in any species other than domesticated crop plants. To address this imbalance in anthocyanin research my thesis tested for a signalling role in vegetative organs, and then investigated a physiological role for anthocyanins in reproductive organs. In chapter two, I hypothesised that for Pseudowintera colorata, red (anthocyanic) leaf margins reduce leaf herbivory by signalling to herbivorous insects the presence of increased chemical defences. Using a natural population of P. colorata, I showed that leaves with the wider red margins contained higher concentrations of anthocyanins and polygodial, a sesquiterpene dialdehyde with known anti-feedant properties, and incurred less natural herbivory. Additionally, laboratory feeding trials involving a natural P. colorata herbivore, Ctenopseustis obliquana larvae, showed a preference for green-margined leaves over red, but only when feeding trials were conducted under light regimes which allowed discrimination of leaf colour. Collectively, my data show that red leaf margins provide a reliable and effective visual signal of chemical defence in P. colorata. Moreover, C. obliquana larvae apparently perceive and respond to the colour of leaf margins, rather than to olfactory cues. My study is therefore the first to provide direct support for a communicative function for anthocyanins in vegetative organs. In peduncles, rays and pedicels, the sterile components of an inflorescence, anthocyanin accumulation has exclusively been considered an adaptation to promote frugivore visitation; however, anthocyanins may instead be produced to mitigate light stress. In chapter three, I tested the requirements of a physiological function, that anthocyanins provide photoprotection for Sambucus nigra peduncles which turn red prior to fruit maturation. I found that accumulation of red pigmentation required exposure to full sunlight and that anthocyanins significantly reduced the quantity of green light that would normally reach chlorenchyma in the peduncle. Under saturating white light, red peduncles maintained higher quantum efficiencies of photosystem II compared to green peduncles, and red portions of peduncle recovered from photoinactivation more quickly than did green portions. My data are, therefore, the first to show that anthocyanins perform a physiological function in the reproductive organs of a naturalised species. In chapter four, I hypothesised that anthocyanin accumulation in senescing Sambucus canadensis peduncles prolongs senescence and enhances nitrogen resorption. Red peduncles possessed several traits indicative of a prolonged senescence; their rates of chlorophyll and xanthophyll decline were lower, while tensile strength and elasticity were higher than for green peduncles. Red peduncles were also less susceptible to photoinactivation than the green ones at the later stages of senescence. However, manipulating green peduncles with light filters possessing transmittance properties comparable to an anthocyanic tissue layer did not increase peduncle longevity or nitrogen resorption. I concluded that like senescing leaves, red peduncles display many characteristics indicative of a prolonged senescence, but I am unable to attribute this benefit to the presence of anthocyanins. This thesis provides a significant contribution to our understanding of the role of anthocyanins in plants in two ways: it is the first to directly demonstrate that anthocyanins perform a communicative function in vegetative organs, and is the first to show for a naturalised (non-cultivar) species, that anthocyanins perform a physiological function in reproductive organs.