Research progress in micronutrients in pears

Abstract: Pear is an important fruit in China with high economic value. In pear production, the imbal-ance of fertilization is serious, especially in the aspect of micronutrients. Micronutrients are a crucial part of the physiological metabolic network of pear trees, which can affect the growth and development of plants, as well as the yield and quality of pears. Therefore, it is of great significance to explore the mechanism of micronutrients in pears and its application in production. The physiological functions, nu-trient deficiency or toxic symptoms of iron (Fe), boron (B), copper (Cu), manganese (Mn), molybde-num (Mo), zinc (Zn), silicon (Si) and chlorine (Cl) in pear trees and the research progress in micronutri-ents in pear production and basic scientific research in recent years are reviewed in the paper. Fe is an important micronutrient involved in redox reaction in pear, which is closely related to chlorophyll syn-thesis and photosynthesis. Fe deficiency can lead to the decrease of chlorophyll content, resulting in the yellowing between leaf veins, the decrease of yield and even the death of pear trees. The initial Fe deficiency symptoms usually occur in the young leaves. Fe uptake and transport may be related to ferric chelate reductase (FCR), gibberellin, GID1 and DELLA protein genes. B is involved in the physiologi-cal processes in nitrogen assimilation, cytoskeleton polymerization, cell signal transduction and osmo-regulation. It is closely related to the development of reproductive organs and fruits. Lack of B can lead to slow down the growth of pear roots and leaves, causing bark cracking, flowers wilting, pollen abor-tion, lower fruit setting, internal cork, fruit cracking, Jizhua (Brown rot) disease and so on. Excessive B can also cause poisoning symptoms in pear trees. The edge of the leaves will become yellow andscorched with brown spots when the concentration of B is above 300 μmol·L^-1. Cu is involved in many metabolic processes. Cu deficiency mainly affects plant reproductive organs and young leaves, resulting in tissue necrosis, wilting, leaf discoloration and growth retardation, inhibiting root growth and promot-ing ROS production. Excess Cu can also slow down plant growth. The growth of pear seedlings is sig-nificantly inhibited by high concentration of Cu 2+, which exerts a greater impact on the growth and de-velopment of root system. Mn is a key micronutrient in the construction of PSⅡ chloroplast, and Mn deficiency in pear leaves will lead to chlorosis between veins from the edge. Manganese cation diffu-sion facilitator (Mn-CDF) is a key factor for Mn absorption and transportation by pear trees, and it is ex-pressed in the root, stem and leaf tissue of pear seedlings. Zn is involved in synthesis of many enzymes,and plays an important role in DNA transcription, maintaining the integrity of membrane and stress re-sistance. Zn deficiency in pear will lead to“little rosette leaf”disease. Foliar spraying is the most effec-tive way to supplement Zn, which can significantly increase the Zn content in leaves, but the increase of Zn in fruits is lower than that in leaves. The concentration of Zn is higher in young fruits and decreases gradually at the later stage of fruit development, which indicates that the absorption and distribution of Zn in fruit are very important at the early stage of fruit development. Si influences pear quality improve-ment, which can enhance the cell wall of fruit trees, improve plant health and productivity, and play an important role in plant stress resistance and antioxidant metabolism. Si has also a good effect on improv-ing root quality and density. The absorption of Si and Ca has synergistic effect, which can reduce the rate of brown spot disease. Mo is the key element of redox enzymes. Lack of Mo will lead to chlorosis of leaves and appearance of yellow spots on the surface. Plants are more prone to symptoms of Mo defi-ciency in acid soil, so pear orchards on acid red soil in southern areas should pay more attention to sup-plementing foliar Mo fertilizer. Mo and Fe have synergistic absorption effect. Cl is a necessary cofactor to maintain some enzyme activity in pear trees. It participates in PSII and acts as an osmotic solute. As many fertilizers contain chlorine, the chlorine deficiency rarely occurs on pear trees. The purpose of this review is to provide a reference for revealing the mechanism of absorption and transportation of trace elements and developing new and efficient fertilizers for pears.