Symptoms
Symptoms of boron deficiency are severely stunted plants and darker green leaf color than normal. Especially butterhead lettuce builds only a rosette and no head, which results in high yield losses.
The youngest leaves are stiff and show necrotic mottling on the margins and between the veins. With severe deficiency, the growing point dies and the youngest folded leaves show brown spots.
The roots are poorly developed and brownish. They look scrubby, because a great many very short, branched lateral roots arise. With severe deficiency root tips die off.
The greenish discoloration of the roots is no symptom of boron deficiency. Seemingly the pots with nutrient solution let some light inside, which caused green color.
Reasons
Boron deficiency (right) compared to optimum supply (left)
Symptoms may be confused with calcium deficiency, which also starts on the youngest leaves. But boron deficiency is much more concentrated on the growing point and in opposite to Ca-deficiency no lesions spread over the leaves.