Redressing the roles of anthocyanin pigments in vegetative and reproductive organs

<p>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.</p>

Redressing the roles of anthocyanin pigments in vegetative and reproductive organs

<p>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.</p>

Seedling size and nutrient availability in the fall determine nitrogen resorption and storage compound allocation in Quercus variabilis

Aims Soil fertility and resorption of leaf compounds in the fall can influence resource buildup in plants. However, whether intraspecific differences in seedling size can affect nutrient reserve buildup is unknown. This study examined the effects of seedling size and fall fertilization on the uptake and resorption of nitrogen (N), as well as the allocation of non-structural carbohydrates (NSC) and N in cultivated Quercus variabilis Blume. Methods After the formation of terminal buds (T1), seedlings were stratified into small (shoot height < 30 cm) and large seedlings. During the hardening period, seedlings were treated with three different rates of 15N-enriched fertilizer (0, 12, or 24 mg N seedling− 1) and monitored until leaf fall (T2). Results Small seedlings had lower N resorption efficiency and resorbed proportionally less N than large seedlings. Fall fertilization notably improved N and NSC reserves, without reducing N resorption efficiency. Large seedlings allocated proportionally less N to leaves than small seedlings although both sizes seedlings absorbed similar amounts of N from fall fertilization. The priority perennial organ for NSC allocation was roots, while N allocation was dependent on the phenological growth stage of the seedling. Roots were prioritized during the rapid growth phase, while stems were prioritized during the hardening period. Conclusions Under same fertilizer regime during the growth phase, large seedlings tends to have lower N concentration and have higher resorption efficiency compare to small seedlings, fall fertilization can increase N storage in large seedlings and NSC levels in both seedling sizes, without affecting growth.

Nitrogen resorption efficiency in autumn leaves correlated with chlorophyll resorption, not with anthocyanin production

A prominent hypothesis for the adaptive value of anthocyanin production in the autumn leaves of some species of trees is that anthocyanins protect leaves from photooxidative stress at low temperatures, allowing a better resorption of nutrients, in particular, nitrogen, before leaf fall. While there is evidence that anthocyanins enable photoprotection, it is not clear whether this translates to improved nitrogen resorption and how this can explain inter-specific variation in autumn colours. A recent comparative analysis showed no correlation between temperature and anthocyanin production across species but did not analyse nitrogen content and nitrogen resorption efficiency. Here we provide this comparison by comparing the nitrogen content of mature and senescent leaves and their autumn colours in 55 species of trees. We find no correlation between the presence of anthocyanins and the efficiency of nitrogen resorption. We find, instead, that nitrogen resorption is more efficient in species with yellow autumn colours, pointing to chlorophyll resorption, rather than anthocyanin synthesis, as the main determinant of nitrogen resorption efficiency. Hence our results do not corroborate the photoprotection hypothesis as an explanation for the evolution of autumn colours.

The C:N:P Stoichiometry of Planted and Natural Larix principis-rupprechtii Stands along Altitudinal Gradients on the Loess Plateau, China

Carbon:nitrogen:phosphorus (C:N:P) stoichiometry plays a critical role in nutrient cycling, biodiversity, and ecosystem functionality. However, our understanding of the responses of C:N:P stoichiometry to elevation and forest management remains elusive. Here we sampled 18 Larix principis-rupprechtii sites along altitudinal gradients (1700-2300 m) on Guandishan Mountain in the Loess Plateau, China. We determined the leaf, litter, and soil C N P contents and C:N:P stoichiometric ratios, as well as nutrient resorption efficiency (NuRE), and diameter at breast height (DBH) increments in both planted and natural stands, and then tested the impacts of elevation and stand origin on these parameters’ management. We found different C:N:P stoichiometry between natural and planted forests. The results revealed that: soil C, N, and N:P ratios, litter C:P and N:P ratios, leaf C:N and N:P ratios increased significantly; however, soil C:N ratios, litter P, leaf N and P, nitrogen resorption efficiency (NRE), and DBH increments decreased significantly with elevation in the planted forests. Soil C,N and N:P ratios, litter C, as well as C:N and C:P ratios increased significantly with elevation in natural forests. The soil N, P and N:P ratios, litter C:P and N:P ratios, leaf C, C:P and N:P ratios, nitrogen resorption efficiency (NRE), phosphorus resorption efficiency (PRE), and DBH increments were, on average, higher in the planted, rather than natural forests. Our results indicated that there was an enhancing P-limitation in both the planted and natural forests, and the plantations were more restricted by P. Moreover, compared to natural forests, plantations converged toward a higher conservative N- and P-use strategy by enhancing resorption efficiencies of internal nutrient cycling and a higher annual growth rate.