|MadSci Network: Botany|
In the fall when leaves senesce, they often produce more anthocyanins, which often function to protect the leaves from high light or ultraviolet light. Recent research described in the first webpage cited describes how anthocyanins protect a leaf in the fall -
"Lee and co-workers think open grown red-osier leaves accumulate anthocyanins in advance of chloroplast senescence so the pigments will be in place to prevent oxidative damage brought on by light later on. Destruction of leaf photosystems liberates lots of free chlorophyll, which if excited, produces reactive oxygen species including free radicals and peroxide. By filtering out blue and green light, anthocyanins prevent chlorophyll excitation, thereby avoiding toxic reactive oxygen.Why is that important? Species such as dogwood recycle much of the nitrogen and other constituents of leaves before they fall, importing them back into the stems and roots. The research team hypothesizes that reactive oxygen species could poison the recovery process. In other words, anthocyanins act as sunscreen to block light induced damage that would upset retrieval of nutrients important for next year's growth."
A similar mechanism might be operating for a nitrogen deficient leaf which is also exporting large amounts of nitrogen. Fall leaf senescence is similar to a leaf nitrogen deficiency because the chlorophyll is no longer being synthesized and mineral nutrients, such as nitrogen, are being exported from the leaf. An older leaf dying from lack of nitrogen is undergoing a similar process. Chlorophyll cannot be synthesized due to lack of nitrogen so the leaf becomes chlorotic, meaning it turns yellow or white as chlorophyll is degraded and the yellow carotenoids are revealed. The nitrogen in the leaf is transported to growing points such as shoot tips, which have a higher demand for nitrogen.
Anthocyanin formation occurs with some other deficiencies, particularly phosphorus and sulfur. Tomato plants often show purpling of their leaf undersides as a phosphorus deficiency symptom. Remember that one thing that may complicate the appearance of leaf anthocyanins in nutrient deficient leaves is whether they are being synthesized in response to a nutrient deficiency or if preexisting anthocyanins are simply becoming visible because of the loss of chlorophyll from the leaves.
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