Some anatomical and physiological changes inCitrusproduced by boron deficiency

Abstract

does not appear. First page follows. Introduction Recent investigations (Haas(4)) have shown that traces of boron are essential for the normal growth of Citrus, even though comparatively small additional amounts are toxic. Citrus trees make excellent growth when the culture solution contains 0.2 parts per million of boron but show pronounced physiological disturbances when boron is absent. A concentration of boron less than that required for the normal growth of Citrus brought about the following symptoms of decline: Leaves curled along the midrib with the tip of the leaf curling downward; leaves colored a brownish or yellowish green, often with a yellowing along the midrib; midrib or veins conspicuous, corky and split; and a progressive loss of affected leaves in the basipetal direction. In severe cases there is a tendency towards ‘multiple bud’ formation due to new twigs dying when barely visible. This and other symptoms are similar to those found in exanthema. When the bark of the internodes of the twigs, or in severe cases that of the trunk, splits, an amber-colored gum oozes out. Eventually the cracks may widen so that the woody tissue is exposed. In severe cases the apical portion of the branch dies back. The roots become dark brown in color and fail to elongate, and in advanced cases the rootlets decay. Upon the addition of a suitable concentration of boron to the culture solution, the symptoms of decline disappear. A study of these anatomical changes and their effect on the translocation of sugars from the leaves forms the basis of this paper.