What does glazing, silvering, or bronzing on the underside of a leaf indicate?

Glazing, silvering, or bronzing on the underside of a leaf is typically associated with injury caused by peroxyacyl nitrates (PANs). These compounds are secondary air pollutants formed from the reaction of volatile organic compounds (VOCs) with nitrogen oxides in the presence of sunlight. When plants are exposed to these compounds, it can lead to oxidative stress, resulting in the characteristic silvering or bronzing of leaves. This injury specifically affects the leaf's ability to photosynthesize and can significantly impact plant health.

In contrast, other forms of leaf injury are associated with different pollutants. Ozone, for example, can cause stippling or mottling, and sulfur dioxide typically results in chlorosis or leaf drop. Hydrogen fluoride may lead to necrosis at the leaf margins rather than the glazing symptoms observed with peroxyacyl nitrates. This distinction helps in identifying the particular pollutant causing the leaf damage.