Why would the tissues of a leaf need to have the ability to resist changes in pH in their natural environment? i. e. what is the source of acid or base is that a leaf might be exposed to under normal circumstances?

The makeup of air/atmosphere,can change the pH.

For example, when the USA relied heavily on coal fired electric generating plants the air became acidic.

This polluted air would drift or be blown around the continent. When the rains came, “acid rain” would overwhelm many trees and other vegetation, harming it.

See pasted in below:

Normal, clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides—produced from power plants and automobiles—the rain becomes much