Frost and hail may damage, reduce or completely destroy the plants' leaf area. This reduction of leaf area limits photosynthetic activity of the plant. Also, plants require temperatures above about 55F for active photosynthesis. At lower temperatures, photosynthetic activity is severely reduce. Under either condition, nitrates absorbed by the roots may not be converted to plant protein and therefore accumulate the stems or stalks.
Light provides the source of energy for both processes.
Consequently, shaded plants may be higher in nitrates than plants grown under adequate light. Under normal growing conditions, the nitrate content usually will be higher in the morning than it is in the mid-afternoon. It also will be higher on cloudy days. For instance, wheat hay harvested after cloudy weather often will be much higher in nitrate content than that harvested during sunny periods. However, this usually is just temporary.