Due to the passive nature of boron (B) uptake and the nonmobility of B in the plant, expression of symptoms under marginal deficiency conditions is extremely variable, influenced largely by the weather prevailing at particular plant growth stages. The first characteristic sign of B deficiency is a splitting of the newer leaves close to the midrib. This is accompanied by some unusual indentations, also along the length of the leaf, but on the opposite side of the midrib to the splitting. Some new leaves show a loss of chlorophyll in a very narrow strip along the full length of the leaf.

Although not always present, another characteristic symptom of B deficiency is the development of a saw-tooth effect on young leaves, reflecting abnormal cellular development. As the deficiency becomes more severe, an increase in tillering occurs, new shoots have a water-soaked appearance and are paler than the older parts of the plant. Considerable distortion occurs along the margins of these leaves. Finally, new growth becomes necrotic and shoots wither, not unlike extreme copper and calcium deficiencies.

Head sterility is also a characteristic symptom of B deficiency. In some cases, the entire spike is sterile; the anthers do not open and the ovary does not develop. In some years, leaf symptoms described above may be apparent, but no sterility occurs. In other years, severe sterility can be observed without the characteristic leaf symptoms. Reduced 1000-grain weight and grain shrivelling can also result from B deficiency.