There are a lot of reasons that plant leaves turn yellow and this can be beyond just the most obvious like nutrient deficiency or sickness, and sometimes includes things that we don’t think about at the beginning.
In this vein, consider the pictures below of the yellowing caused in raspberry leaves when they are blocked from a sufficient amount of light. In this case, two leaves were covering another leaf below, and the result, which is visible when those two leaves are pulled aside, was a pattern of yellowing in the same shape as the leaves which were lying upon it. The lack of sunlight causes the chloroplasts which are responsible for the manufacture of green chlorophyll to become less active, eventually degrade with the net result of degreening and a fade to yellow.
Understanding this concept explains as well why the insides of raspberry hedgerowsare often filled with yellow and dying plants. There is little light getting through there, so the leaves lose their purpose and over time senesce. —Story by Mark P. Bolda, UC Ag and Natural Resource
