Boron in fruit crops: plant physiology, deficiency, toxicity, and sources for fertilization

Boron (B) is one of the most complex and important micronutrients in terms of management in agricultural systems. The soil B levels are typically low under normal growing conditions, due to leaching when the soil pH is within the acidic range, wherein B prevails as boric acid (H3BO30), and by the strong adsorption of the borate anion (BOH4−) to iron and aluminum oxyhydroxides, clay minerals, organic matter, and calcium carbonate when the soil has a neutral to alkaline pH. Although mobile in the soil, B is immobile in the phloem of most cultivated species, and since it plays a fundamental role in root formation through the development of meristems, there must be enough B in the soil layers where the roots normally grow, so that there is a growth of roots and, consequently, of the aboveground part of plants. Some paradigms limit the rates of B applied in the field: (1) that, invariably, B would be easily leached out of the soil, and (2) that among micronutrients B has the narrowest range of soil levels between deficiency and toxicity to plants, causing caution among growers about borate fertilization, both due to low efficiency and the risk of toxicity to plants. Thus, B management in different agricultural systems needs to be better understood as there are mismatches and generic knowledge about B, without specifications regarding soils and crops, such as demands and adaptations to deficiencies and toxicity. Understanding the interactions between plant root growth and soil strata with different B levels, root and leaf absorption processes, transport and redistribution in plant tissues, the efficiency of B sources, rates, and timing of B fertilization, via soil and leaf, are necessary for search of greater productivity. As most of the fruit crops are trees and perennial plants, managing information on all those areas is a quite complex task that requires a holistic approach. Although analysis of soil and leaf tissues may usually not present B deficiency, fruit trees, in general, have difficulty transporting B for new flower buds. Boron is especially immobile in most tissues of plants and does not readily move from the different parts of the tree to the buds, where it is necessary for the pollen production, growth of the pollen tube, and other reproductive functions. Under cold climates or other limiting growth conditions, flowers may deteriorate before fecundation is completed. In apple (Malus domestica) trees, the adequate B availability improves Ca translocation in plants as well as the Ca uptake by fruits, whereas its deficiency can cause serious physiological disorders such as the bitter pit, justifying why foliar applications and soil fertilization with B are common in the pomiculture. Based on these concepts, this chapter aims to address the aspects related to the role of B in the physiology of fruit plants, the requirements of some species, considering the problems resulting from a deficiency or excess, and the possible ways to minimize these consequences.