Enhancing Fruit Quality with Mineral Nutrition Management

• A given effect of a mineral on fruit quality can vary depending on local weather and soil conditions.
• The effects of several minerals depend on the relative concentrations of other elements.
• Total soluble solids, color, polyphenols, pH, titrable acidity, and aroma are affected by mineral nutrition management.

Minerals affect fruit quality’s morphological, organoleptic, physical, and chemical properties. Mineral nutrition management can be adjusted to provide the necessary inputs so that fruits meet market standards for various purposes like fresh consumption, processing, and storage. For any nutrition management to succeed, first, it is necessary to understand how each nutrient affects fruit quality. This article provides insights on fruit quality effects of deficiency and excess of some common macro- and micro-nutrients.

Mineral Nutrition for Plants

Fertilizer addition aims to make up for soil nutrient deficiency to realize a fruit’s genotype potential. The time and rate of application are crucial and have been based on tree requirements, soil fertility, and expected yield. Gradually, growers have started incorporating Mineral Nutrition Management, using parameters like crop nutritional status, plant growth analysis, and soil and tissue nutrient level correlation for more effective nutrient management. Results from long-term analyses are also providing valuable inputs for Mineral Nutrition Management.

Essential minerals are those without which the plant and fruits can’t develop. Another mineral can’t be replaced in plant metabolism.

• The macronutrients are the essential minerals required in large quantities, like nitrogen, potassium, phosphorus, calcium, magnesium, and sulfur.
• The micronutrients are vital minerals needed in small amounts, such as manganese, copper, boron, iron, zinc, chlorine, sodium, molybdenum, and nickel.

The effect of minerals on plant vegetative growth and yield is well known, but each mineral can also impact fruit quality. Often, not just deficiency but also excess availability negatively impacts fruit quality. This article will discuss only the minerals that affect fruit quality.

Nitrogen

Nitrogen is needed more during the initial stages of fruit development. The fruit quality attributes that nitrogen affects are fruit size, color, formation of total soluble solids (TSS), polyphenols, organic acids, anthocyanin, and flavor.

Nitrogen is usually added in abundant quantities as the primary macronutrient for vegetative plant growth. Therefore, more than deficiency, an excess of nitrogen often causes problems in fruit quality development.

• Excess nitrogen encourages vegetative growth and is determinantal to the development of flowers and fruits. Excess nitrogen reduces flowering and produces fewer fruits, for example, grapes.
• Excess nitrogen delays fruit maturation.
• Excessive vegetative growth due to nitrogen will also increase shade, encouraging fungal infection of leaves and fruits in grapes and reducing plant tolerance to pests and diseases in pears.
• Excess nitrogen produces lighter-colored fruits in grapes, pears, citrus, peaches, nectarines, etc., affecting light interception in all parts of a tree or vine due to excessive vegetative growth.
• Excess nitrogen reduces TSS due to more allocation of resources for vegetative growth in grapes and pears.
• As nitrogen increases, anthocyanins and polyphenols increase in fruits like grapes in the berries’ skin. However, too much nitrogen causes the dilution of anthocyanins in fruits.
• Adequate nitrogen is essential for ensuring juice’s chemical composition, for instance, in grapes to make wine. Nitrogen influences the formation of chemicals vital in winemaking, such as TSS, pH, organic acids, total titrable acids, anthocyanin, and polyphenols, to ensure proper taste, color, tonality, longevity, and stability. Low nitrogen in fruits will also negatively affect fermentation.
• Fruits storability also decreases due to excess nitrogen in soils.

Nitrogen effects can be variable and depend on soil type and organic matter content.

Potassium

Like nitrogen, potassium (K) is also used by fruits in the early stages. Potassium is necessary for cell division, and the nutrient uptake remains high till harvest. Being readily absorbed from the soil and highly mobile in the phloem, there are usually adequate amounts of the mineral in a plant and fruit. For example, in grapes, nearly 50 percent of the absorbed potassium is in the berries.

Potassium is crucial for fruit quality and essential to Mineral Nutrition Management. It controls stomatal conductance, photosynthetic rate, enzyme activity, cell division, and protein, sugar, and starch synthesis. Through Mineral Nutrition Management, the quality attributes influenced by potassium can be optimized, including color, total soluble solids, titrable acidity, size, firmness, juiciness, and aroma.

• Potassium affects yield and fruit size by influencing cell division, cell turgor maintenance, and movement of solutes.
• The nutrient has been reported to influence the intensity of skin color of several fruits like citrus, pears, peaches, etc.
• Potassium is responsible for fruit maturation.
• Potassium determines TSS content as it is needed for sugar synthesis and transporting solutes.
• Aroma is also influenced by potassium as it is involved in the synthesis of polyphenols. However, too much potassium can increase pH and lower total titrable acidity, affecting sensory and visual attributes.
• Potassium is essential for winemaking, affecting grapes’ TSS and phenols and regulating PH and acidity. Less acidity reduces the stability of wine by making it prone to microbial spoilage and oxidation.
• Juiciness in fruits like pears and citrus is also affected by potassium.

A proper potassium-to-calcium ratio needs to be maintained; otherwise, fruits may show calcium deficiency symptoms like cork pit in pears.

Phosphorus

Usually, there is enough phosphorus in the soil to meet plant requirements, so fruit development deficiency is unlikely. Phosphorous is found near the skin, and its concentration decreases toward the fruit interior. Phosphorus affects photosynthesis, skin color, and sugar content.

• Phosphorus is necessary for flower development and ensuring fruit set, for example, in citrus.
• Sugar content in fruits is determined by phosphorus, which is involved in enzyme activation of its production, metabolism, and movement.
• Phosphorus foliar sprays enhance skin color and marketability by increasing the anthocyanin concentrations in the skin, for example, in plums, apricots, and peaches.
• Phosphorus is vital for winemaking and should be present in adequate amounts; otherwise, the mineral also triggers the fermentation of must or grape juice to affect the aroma and taste of wine.

Nutrient management must be done carefully when vegetative parts show signs of phosphorus deficiency, as the element accumulates in the soil due to excess applications. In these circumstances, it can lead to zinc deficiency.

Calcium

Calcium transfer to fleshy fruit is lower than to leaves; therefore, even though there may be enough calcium in the soil, fruits will show deficiency disorders that can become a problem.

Calcium accumulation is higher in the initial stages of fruit development and gradually decreases till harvest. In mature fruits, calcium concentration in the peels is nearly four times higher than in the pulp.

• Like phosphorus, calcium affects color by increasing anthocyanins in the skin.
• Calcium is vital for fruit quality during storage, especially for apples and pears.
• Fruit calcium concentrations contribute to fruit firmness.
• A potassium-to-calcium balance with adequate calcium is necessary to prevent pre-harvest disorders like the bitter pit in pears.
• Postharvest fruit storage disorders like internal breakdown and apple cork spot can be prevented by supplying calcium.
• Adequate fruit calcium levels increase tolerance to pathogens. A mineral deficiency makes fruits susceptible to pre- and postharvest fungal attacks such as brown spots, side rot, or superficial scald in pears.

Fruit calcium depends on soil content, genotype, and rootstocks. The fruiting density will also affect calcium levels in fruits. When there are more fruits, there is more calcium in them. When the leaf-to-fruit ratio is higher, the fruit density is less, and the nitrogen-to-calcium ratio is high, fruit calcium levels fall.

Preharvest calcium sprays can be helpful even up to storage. Lime application to soils will increase calcium in tissues.

Postharvest calcium sprays reduce decay due to fungal diseases like Penicillium.

Magnesium

Magnesium uptake by plants is curtailed by potassium, calcium, and ammonium. Therefore, soils rich in calcium will show magnesium deficiency, resulting in physiological disorders and reduced yield.

• Magnesium is crucial in chlorophyll synthesis; as magnesium levels rise, so does chlorophyll.
• Magnesium is involved in sugar formation and carbohydrate metabolism, affecting BRIX.
• Phosphorous uptake by plants is promoted by magnesium.
• Limestone application that increases magnesium levels in soils helps reduce aluminum toxicity.

Figure 1. “Fruit cracking in pears. (A, B) Fruit cracking in the early stage of growth; (C) macro-cracks in the mature stage of growth; (D) micro-cracks on fruit,” Seo et al. 2022. (Image credits: https://doi.org/10.3390/agronomy12102437)

Micronutrients

Though the micronutrients are necessary in trace amounts, several of them affect fruit quality, like boron, zinc, copper, iron, and manganese.

Copper

Copper affects fruit quality when it is present in excess. Applying copper sprays to control diseases leads to the accumulation of the element in the soil and causes toxicity. Soil texture, pH, clay, and hydrous oxide content will influence copper uptake by plants and the extent of toxicity.

Excess copper affects fruit yield and composition. As a result, fruit juices’ composition changes; for example, in grapes, that will subsequently affect wine and must quality. By affecting composition, excess copper causes biochemical and physiological stress.

Zinc

Zinc deficiency can affect fruit quality and yield. However, deficiencies are taken care of by using zinc sprays to control diseases. Any further application is not necessary.

Zinc is needed for auxin and chlorophyll formation, photosynthesis, enzyme activation, and metabolic processes.

• Zinc deficiency leads to fruits that are smaller or deformed and are prone to early maturation.
• Fruits suffering from zinc deficiency are also very sour.

Boron

Boron is one of the most critical micronutrients for fruit quality, especially for pears with a high element requirement.

• Boron deficiency causes a reduction of fruit set and yield. Adequate boron levels promote flowering and fruit set and stable fruit production.
• Fruits can be small and deformed and suffer from physiological disorders like cracking and corking, for example, in pear; see Figure 1.
• Adequate boron levels help in calcium mobility.

The most appropriate application period for boron is the end of summer, especially for the Rosacea family (apples, pears, peaches, strawberries, loquat, quince, plum, and apricot). The mineral is stored in permanent tissues and used the following season for developing fruits.

Iron and Manganese

Though iron is abundant in soils, its uptake is restricted in fruit trees, affecting fruit yield and quality. Iron deficiency in trees depends on soil, climate, environment, and orchard management.

In grafted trees, the rootstocks can play a significant role. For example, in calcareous soils in Italy, pears grafted on “quince” can’t solubilize and absorb iron. Therefore, the grafts are treated with artificial iron chelates and supported by sustainable practices like grass intercropping, organic matter application, soil drainage improvement, etc.

Manganese shows similar chlorosis symptoms, like iron deficiency. Manganese deficiency that reduces fruit yield can occur in alkaline and calcareous soils, even with high organic matter content.

Fruit Producers Need Quality Meters

Growers need a thorough knowledge of the external symptoms of each nutrient’s deficiency and excesses. However, a precise quality meter that can track fruit quality in terms of TSS, titrable acidity, color, and pH can help them monitor the internal chemical composition of fruits as they develop. Felix Instruments Applied Food Science’s F-750 Produce Quality Meter can measure the necessary quality parameters non-destructively in real time to help growers understand how to adjust their mineral nutrition management to improve fruit quality and meet market demands.

Sources

Brunetto, G., Melo, G. W., Toselli, M., Quartieri, M., & Tagliavini, M. (2015). The role of mineral nutrition on yields and fruit quality in grapevine, pear and apple. Revista Brasileira de Fruticultura, 37(4), 1089–1104. https://doi.org/10.1590/0100-2945-103/15

Marcelle, R. (1995). Mineral nutrition and fruit quality. Acta Hortic. 383, 219-226

DOI: 10.17660/ActaHortic.1995.383.22

Patel, G. (2020, March 1). What is plant nutrition. Retrieved from https://vikaspedia.in/agriculture/crop-production/integrated-nutrient-management/what-is-plant-nutrition

Seo, H.-J., Sawant, S.S., & Song, J. (2022). Fruit Cracking in Pears: Its Cause and Management—A Review. Agronomy, 12, 2437. https://doi.org/10.3390/agronomy12102437

Van Dang, L., Phuong Ngoc, N., & Hung, N. N. (2022). Effects of foliar fertilization on nutrient uptake, yield, and fruit quality of pomelo (citrus grandis Osbeck) grown in the Mekong Delta soils. International Journal of Agronomy, 2022, 1–11. https://doi.org/10.1155/2022/7903796 https://www.hindawi.com/journals/jfq/2022/8618854/