5 resultados para Autumn.
em Digital Commons at Florida International University
Resumo:
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.
Resumo:
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.
Resumo:
Anthocyanins are synthesized during leaf senescence in certain plants across virtually all biomes, but are most spectacular in the autumn foliage of temperate deciduous forests. The patterns of color production in senescing foliage depend at least partly upon species composition and their phenology. Both ecological and physiological explanations have been raised to explain why plants produce this pigment just before leaf fall. Physiological explanations, as photoprotection, predict that cyanic leaves would be better able to resorb nitrogen during the process of chlorophyll degradation. Ecological explanations predict better dispersal of propagules advertised by association with the brilliantly colored leaves (plausible for only a minority of species), or warning against egg-laying activity of herbivorous insects, as aphids. These hypotheses make predictions that we now can test, to help us understand this old mystery - and majestic phenomenon.
Resumo:
Leaf colour change is commonly observed in temperate deciduous forests in autumn. This is not simply a side effect of leaf senescence, and, in the past decade, several hypotheses have emerged to explain the evolution of autumn colours. Yet a lack of crosstalk between plant physiologists and evolutionary ecologists has resulted in slow progress, and so the adaptive value of this colour change remains a mystery. Here we provide an interdisciplinary summary of the current body of knowledge on autumn colours, and discuss unresolved issues and future avenues of research that might help reveal the evolutionary meaning of this spectacle of nature.
Resumo:
This study documents the 1996 and 1997 autumn migration seasons at Grassy Key for 16 species of raptors (hawks, eagles, and falcons). My results indicate the Florida Keys are a major raptor migration flyway (over 26,000 sightings). I identified factors influencing watch-site location in the Keys. Northbound flights must be included to avoid inflating southbound counts. By removing the "season effect" (natural rise, peak, and wane of raptor numbers during migration), I demonstrate wind has little consistent effect on raptor counts in the Keys. I further demonstrate we do not see more raptors on cold front days than on non-cold front days. However, cold fronts following tropical storms (as in 1996) increase the number of raptors observed for most species. I conducted a nightly roosting survey on Boot Key resulting in near or over 3,000 raptor sightings per season and present a model to predict aerial counts from roosting counts.