EVOLUTION OF XYLEM RESULTED IN A REQUIREMENT FOR BORON IN THE APICAL MERISTEMS OF VASCULAR PLANTS

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Abstract
Summary: Critical review of the literature regarding the influence of boron on the metabolism of vascular plants shows that the cessation of cell division in the apical meristem is the earliest and most prevalent result of boron deficiency in vascular plants. Furthermore, the literature, in general, can be interpreted to support the view that perturbations in auxin metabolism, increased signification, phenol accumulation and reduced sucrose transport are probably secondary effects of boron deprivation resulting from the decrease or complete cessation of plant growth. It is proposed that the acquisition of a role for boron essential to the normal functioning of the apical meristem was a consequence of the evolution of xylem and the subsequent passive transport of boron in the transpiration stream. This resulted in the accumulation of boron at the transpiration terminus, i.e. the shoot apex of a primitive vascular plant. Thus, it was in the shoot apex that boron, for the first time in the evolution of life, reached a concentration sufficient to influence the metabolism of an organism. In support of this hypothesis, evidence is presented that demonstrates that boron probably was not available in the environment at a concentration great enough to result in its being a primordial essential nutrient. The accumulation of boron in the shoot apex resulted in the acquisition of an essential role for boron in vascular plants, thereby imparting the selective advantage of preventing boron toxicity. In addition, the accumulation of boron in the shoot apex to a concentration sufficient to influence the metabolism of a eucaryotic cell presents the possibility that boron may be required by vascular plants in a metabolic pathway that is not unique to them, but one which they may have in common with non‐vascular plants and, potentially, all living organisms. A possible role for boron in maintaining an adequate supply of one or more species of pyrimidine nucleotide or in facilitating their utilization is proposed.