Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood

Why the leaves of many woody species accumulate anthocyanins prior to being shed has long puzzled biologists because it is unclear what effects anthocyanins may have on leaf function. Here, we provide evidence for red-osier dogwood (Cornus stolonifera) that anthocyanins form a pigment layer in the palisade mesophyll layer that decreases light capture by chloroplasts. Measurements of leaf absorbance demonstrated that red-senescing leaves absorbed more light of blue-green to orange wavelengths (495–644 nm) compared with yellow-senescing leaves. Using chlorophyll a fluorescence measurements, we observed that maximum photosystem II (PSII) photon yield of red-senescing leaves recovered from a high-light stress treatment, whereas yellow-senescing leaves failed to recover after 6 h of dark adaptation, which suggests photo-oxidative damage. Because no differences were observed in light response curves of effective PSII photon yield for red- and yellow-senescing leaves, differences between red- and yellow-senescing cannot be explained by differences in the capacities for photochemical and non-photochemical light energy dissipation. A role of anthocyanins as screening pigments was explored further by measuring the responses PSII photon yield to blue light, which is preferentially absorbed by anthocyanins, versus red light, which is poorly absorbed. We found that dark-adapted PSII photon yield of red-senescing leaves recovered rapidly following illumination with blue light. However, red light induced a similar, prolonged decrease in PSII photon yield in both red- and yellow-senescing leaves. We suggest that optical masking of chlorophyll by anthocyanins reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves.

Pigment dynamics and autumn leaf senescence in a New England deciduous forest, eastern USA

The leaves of woody plants at Harvard Forest in Central Massachusetts, USA, changed color during senescence; 70% (62/89) of the woody species examined anatomically contained anthocyanins during senescence. Anthocyanins were not present in summer green leaves, and appeared primarily in the vacuoles of palisade parenchyma cells. Yellow coloration was a result of the unmasking of xanthophyll pigments in senescing chloroplasts. In nine red-senescing species, anthocyanins were not detectable in mature leaves, and were synthesized de novo in senescence, with less than 20 m g cm - 2 of chlorophyll remaining. Xanthophyll concentrations declined in relation to chlorophyll to the same extent in both yellow- and red-leaved taxa. Declines in the maximum photosystem II quantum yield of leaves collected prior to dawn were only slightly less in the red-senescing species, indicating no long-term protective activity. Red-leaved species had significantly greater mass/area and lower chlorophyll a / b ratios during senescence. Nitrogen tissue concentrations in mature and senescent leaves negatively correlated to anthocyanin concentrations in senescent leaves, weak evidence for more efficient nitrogen resorption in anthocyanic species. Shading retarded both chlorophyll loss and anthocyanin production in Cornus alternifolia , Acer rubrum , Acer saccharum , Quercus rubra and Viburnum alnifolium . It promoted chlorophyll loss in yellow-senescing Fagus grandifolia . A reduced red : far-red ratio did not affect this process. Anthocyanins did not increase leaf temperatures in Q. rubra and Vaccinium corymbosum on cold and sunny days. The timing of leaf-fall was remarkably constant from year to year, and the order of senescence of individual species was consistent.

Catholic student movements in Latin America: Cuba and Brazil, 1920s to 1960s

This dissertation examines the ideological development of the Catholic University Student (JUC) movements in Cuba and Brazil during the Cold War and their organizational predecessors and intellectual influences in interwar Europe. Transnational Catholicism prioritized the attempt to influence youth and in particular, university students, within the context of Catholic nations within Atlantic civilization in the middle of the twentieth century. This dissertation argues that the Catholic university movements achieved a relatively high level of social and political influence in a number of countries in Latin America and that the experience of the Catholic student activists led them to experience ideological conflict and in some cases, rupture, with the conservative ideology of the Catholic hierarchy. Catholic student movements flourished after World War II in the context of an emerging youth culture. The proliferation of student organizations became part of the ideological battlefield of the Cold War. Catholic university students also played key roles in the Cuban Revolution (1957-1959) and in the attempted political and social reforms in Brazil under President João Goulart (1961-1964). ^ The JUC, under the guidance of the Church hierarchy, attempted to avoid aligning itself with either ideological camp in the Cold War, but rather to chart a Third Way between materialistic capitalism and atheistic socialism. Thousands of students in over 70 nations were intensively trained to think critically about pressing social issues. This paper will to place the Catholic Student movement in Cuba in the larger context of transnational Catholic university movements using archival evidence, newspaper accounts and secondary sources. Despite the hierarchy's attempt to utilize students as a tool of influence, the actual lived experience of students equipped them to think critically about social issues, and helped lay a foundation for the progressive student politics of the late 1960s and the rise of liberation theology in the 1970s. ^