Gorsline et al. (106) noted that boron concentration in the whole corn plant decreased during initial growth, remained unchanged during most of the vegetative period, and then decreased after silking.
Gupta (113) found that without added boron, the bottom third of the leaves of alfalfa and red clover contained significantly higher boron than did the upper leaves. In the case of stems the opposite was the case, i.e., the upper third of the stems contained more boron than the bottom third. This trend was similar for the unfertilized and boron-fertilized areas for leaves; however, in the presence of added boron, differences in the boron content in the upper and lower stems were not significant.
The general theory is that boron translocates readily in the xylem, but once in the leaves, it becomes one of the least mobile of the micronutrients. Thus the boron immobility in leaves in terms of localized cyclic movement prevents escape and transport of this element over long distances (114). The results of Shelp (115) have also shown that younger leaves contain less boron than mature leaves; the authors assumed that the boron supply for mature leaves is delivered principally via the xylem.
The fact that boron deficiency exhibits in the younger leaves and not in the older leaves can be explained by the fact that the boron concentration is higher in the older leaves than in the younger leaves, as reported for alfalfa and red clover (113) and for broccoli (115). Since the boron concentration in the upper leaves was easily increased with boron fertilization (113), boron deficiency is controlled without much difficulty using boron applications.
It is suggested that leaves should be sampled to determine the boron status of the plants. Also, it is important to be consistent with the plant sampling technique in the field as well as the plant part sampled.
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