Thursday, May 7, 2015

Black spots on Hoya leaves : fungi or pigments ?

These spots resemble bacterial or fungi infections ( such as those described here  ) but on this picture they are probably a pigmentation that occured in the outer cell layers located at micro lesions done by insects. The pigments are known as anthocyanins, they occur in all tissues of higher plants, including leaves, stems, roots, flowers, and fruits.
On leaves they may act as a protection against high light stress. They may also act as antioxidants.
 
In http://www.ncbi.nlm.nih.gov/pmc/articles/PMC281624/#ref5 : " ..Foliar anthocyanins arise in a great diversity of plant species across a broad range of environments, often occurring in response to environmental stresses such as nutrient deficiency, drought, and low temperature (Steyn et al., 2002). In many species, anthocyanins are produced at specific physiological stages, appearing
in expanding, mature, or senescing leaves exposed to high light."
 
At the damaged points on the leaves the photosystems have been destroyed and they would liberate lots of free chlorophyll causing toxic free radicals and peroxide when excited by light; so the anthocyanins decrease the amount of light where free chlorophyll has been liberated.

".. 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.. "
 
 
Photograph by : Agnetha Wolgast Widell
 
The color of Anthocyanins changes with pH and they are generally degraded at higher pH; they are pink in acidic solutions, purple in neutral solutions, greenish-yellow in alkaline solutions , and colourless in very alkaline solutions, where the pigment is completely reduced.
 
More on anthocyanins

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