Boron Nutrition in Horticultural Crops: Constraint Diagnosis and Their Management

Abstract

Out of 30 elements, 16 horticulture crops require them to thrive. All horticultural crops require boron, one of the necessary elements, to function. Extremely trace amounts of boron are present in soils. The majority of the boron that is readily available in humid areas is primarily contained in organic debris, which is broken down by microorganisms for the benefit of plants. In the tropics this element is leached down in soil due to heavy rainfall. As trace element B plays an important role in the growth and development of plants. Various crops exhibit symptoms of deficiency as well as of toxicity when there is even a slight aberration of available boron in soil. Therefore, it is imperative to study and understand the optimum requirement of B by specific crops. Boron also interacts with other elements and manifests in crop plants in various ways. This chapter attempts to understand some of the roles of boron in horticultural crops (fruits and vegetables) and its management for optimum growth and development in crop plants.

1. Introduction

Among the essential elements, boron plays an important role in all the horticultural crops. The microbial decomposition of organic matter in soil releases the available boron for use in plants particularly in humid region. Most of the different macro and micronutrients become exhausted in soil due to continuous uptake of these elements by the crops, thereby causing deficiency in plants which manifests in nutritional disorders often resulting in low yields. Warington in the year 1923 [1] demonstrated the essentiality of boron (B) for the growth and development of higher plants. Boron is crucial as a vital micronutrient for achieving optimal crop growth, development, yield, and quality. It plays a key role in forming and stabilizing cell walls, promoting lignification, and facilitating xylem differentiation [2]. Moreover, boron is essential for enhancing the protein and enzymatic functions of cell membranes, thereby improving membrane integrity. It’s worth noting that sexual reproduction in plants is more vulnerable to boron deficiency compared to vegetative growth. During the pollination of flowers, boron supports pollen tube growth, ensuring successful seed set and fruit development. Additionally, boron contributes to increased nectar production in flowers, attracting pollinating insects. In the realm of crop production, boron stands out as a critical micronutrient required for the healthy growth of most crops. Furthermore, boron imparts drought tolerance to plants. Among the essential micronutrients, boron is unique as the sole non-metal present in a non-ionic form. Commercially, boron is sourced from minerals such as ulexite, natural boric acid (sassolite), borax (tincal), colemanite, and kernite, with the United States (US) and Turkey serving as the primary and richest sources of this element.

In vascular plants, known as tracheophytes, boron follows a path from the roots through the transpiration stream to the points of active growth, where it collects in the stems and leaves. It has been proposed that the localized accumulations of boron in these growing tissues have contributed to the evolutionary development of a reliance on boron for certain metabolic processes in plant meristems. Once boron accumulates in leaves, its subsequent movement within the plant is limited, and it becomes fixed in the apoplast. What sets boron apart from other nutrients is the considerable variation in its mobility among different plant species. The mobility of boron is facilitated by its interaction with polyols. In species where polyols (such as sorbitol, mannitol, and dulcitol, which are water-soluble sugar alcohols) are the primary products of photosynthesis, boron can move rapidly and effectively through the phloem. Examples of such species include those from the Malus, Prunus, and Pyrus genera (including apple, prune, plum, peach, pear, cherry, almond, and apricot), as well as olive and coffee in fruit plants, and carrot, onion, pea, celery, bean, cauliflower, cabbage, asparagus, wheat, and barley in vegetables and cereals. Typically, boron concentrations are highest at the tips and edges of leaves, but in species where boron is phloem-mobile, the concentration remains consistent throughout the leaves. Once again, it is suggested that boron’s movement through the phloem is limited when boron levels are higher in older leaves. The symptoms of boron toxicity, which manifest at the edges of older leaves, have been interpreted as evidence of boron’s immobility within plants [3]. Conversely, when there is a higher concentration of boron in young leaves and fruits, it serves as an indicator of phloem mobility [4, 5, 6, 7]. Research indicates that maintaining optimal levels of boron enhances the germination of pollen and the growth of pollen tubes in almond trees, leading to improved fruit development and seed growth [8]. Applying boron as a foliar treatment to sour cherry trees just before the leaves fall significantly boosts boron levels in flower buds and enhances fruit set [9]. When boron is applied to trees in the autumn, it translocates from the leaves to the adjacent buds, where elevated levels persist and become evident in the flowers during anthesis. Generally, flowers receive their boron supply from reserves in the wood, which become mobilized during the development of the flowers. Boric acid, being a soluble compound susceptible to leaching by rainfall, frequently results in boron deficiency in regions with high rainfall and humidity. In contrast, soil boron toxicity is less common and tends to occur in arid and semi-arid areas. The concentration and availability of boron in soils are influenced by various factors, including the soil’s parent material, texture, clay mineral composition, pH levels, liming, organic matter content, and its interactions with other elements. Consequently, understanding these factors that affect boron uptake is essential for assessing boron deficiency and toxicity under different environmental conditions.

2. Diagnosis of boron deficiency by soil and plant analysis

As early as 1942, Dregne and Power [10], reported that B availability in the soil is influenced by numerous factors such as soil reaction, soil moisture, active calcium and organic matter content. Boron deficiency in soil is mostly found in acidic soils, irrespective of parent material where excess B is leached out. Soils arising from igneous and metamorphic sandstone rocks are found to be naturally low in B. soils deficient in B are also found in laterite soils which have limited silica and strong Fe and Mn. Although alkaline soils are rich in total B but poor in available B for plants. Similarly, in peat soils and acid sludge and soil with low clay composition B deficiency is evident. In general, it is estimated that the total B concentration is between 20 and 200 mg B/kg, where its available concentrations vary greatly in different soil types. Boron is available to plants in water soluble form in soil. It is naturally present in the soil in the water-soluble form, which is readily available as a nutrient for plants. Movement of boron in soil is through “mass flow” which is absorbed by plants through transpiration stream as in water absorption. Consequently, in dry soils, boron uptake may be reduced or unavailable at a time of maximum B requirements in plants. Certain environmental conditions can also lead to low availability of soil boron. High rainfall can cause leaching of available boron from the root zone which is a major problem, especially in coarse-textured soils and when plants are at or just before the rapid growth of leaves and flower development stages. The other adverse environmental condition is when less rainfall or drought period occurs just prior to, or during flowering and seed set.

The requirements for boron (B) in plant growth exhibit significant variability across different plant species, as well as within the same species at various growth stages [2, 11, 12, 13, 14]. In soil, a boron level below 0.5 mg/kg of water-soluble boron is considered low or deficient. An optimal or medium range of boron falls within the range of 0.5–2 mg/kg, while soils containing more than 2 mg/kg are deemed to have a high or excessive boron content. Consequently, soils containing less than 0.5 mg/kg of hot water-soluble boron are generally incapable of providing sufficient boron to support normal growth and yield in most plant species.

The total boron content in Indian soils has been observed to vary between 7 and 630 mg B/kg [15]. Reports of boron deficiency have primarily emerged from the soils in Indian states such as Assam, Bihar, Meghalaya, West Bengal, Jharkhand, and Odisha. This deficiency is notably prevalent in acid-red and lateritic soils, including high-pH calcareous soils [16]. Light-textured soils, like sandy loam and loamy sands, also tend to lack sufficient boron content due to their excellent drainage properties, which result in effective leaching [17]. Given that boron is a weakly held anion, it can be easily washed out of the soil, rendering acid-sand soils particularly susceptible to boron deficiency [18].

Leaf analysis, supplemented by soil analysis, can be reliable diagnostic tool for analysis of B availability and status in plants. The leaf analysis is derived from four factors:

• A leaf is a primary site for metabolic activities;

• Variations in nutrients are reflected in leaf content;

• The variations are more noticeable at certain developmental stages than others; and

• Nutrient levels in leaves at a particular growth stage are linked to crop productivity.

Boron, which plants absorb in the form of undissociated boric acid (B (OH)₃ or H3BO₃), possesses a strong capacity to create complexes within the plant system. In the case of most crops, a boron content ranging from 15 to 100 mg/kg in plant tissue is considered sufficient for regular growth. Conversely, levels exceeding 200 mg/kg may be excessive, leading to a potentially toxic or inhibitory impact on crop growth and yields. When the concentration of boron in plant tissue falls below a critical threshold, it hinders the optimal growth and yield of the crop. Plants have quite specific demands for boron, and the gap between excess and deficiency is quite narrow. For instance, maintaining a leaf boron concentration within the range of 30–70 μg/g is ideal in mini cucumbers. Deficiency becomes evident when levels drop below 20 μg/g, while toxicity is observed when concentrations exceed 100 μg/g. Notably, boron does not readily migrate from older to newer plant tissues, making the roots to continuously uptake essential for the plant to grow normally. The importance of boron in pollination and in regulation of cells development is well established and its deficiency leads to poor seed set and fruit development. The cost of boron in correcting boron deficiency is justified with improvement in both the growth and yields of crop [19].

3. Interaction of boron with other elements

There has been reports of interactive effect of boron in plants with the other nutrient elements. In general, excessive applications of potassic fertilizers results in high potassium concentration which induces boron deficiency. B has been found to counter the toxic effects of aluminum on root growth of dicotyledonous plants [20]. In limestone soils rich in soil calcium reduces the availability of boron. Conversely, when there are excessive boron levels in soil, plant toxicity due to boron can be prevented by calcium application. Reports on the application of zinc to neutralize toxic effects of boron in some plants and there was subsequent increase in the crop yield is documented [19].

4. Correction of boron deficiency

When plants display deficiency symptoms of Boron, the issue can be easily addressed by administering borax, which is a white crystalline salt that is needed in very small amounts. Applying 5–10 kg/ha of boron can rectify boron deficiency in most plants. Boron can be delivered through both foliar and soil fertilization methods to ensure an adequate supply to plants. Foliar application is most effective when root activity is restricted and boron deficiency symptoms become evident during dry soil conditions in the growing season. Generally, a 0.2% borax solution can be sprayed onto the foliage every 10 days until the deficiency is corrected. For many vegetable crops, Solubor (100 g/100 L of water) can also be utilized as a foliar spray [19]. However, Das et al. [21] noted that soil application of boron outperforms foliar sprays. In calcareous soils, introducing sulfur at a rate of 2 MT/ha helps reduce soil pH to the range of 6.0–7.0 and enhances the boron solubility in the soil solution. To address hidden deficiencies, a spray of 0.2% boric acid or borax during the pre-flowering or flower head formation stages has been shown to increase yields. Typically, the application of boron ranges from 0.25 to 3.0 kg/ha, depending on the specific crop requirements and the chosen application method. Broadcast applications typically necessitate higher rates compared to banded soil applications or foliar sprays. It’s crucial to exercise caution when addressing boron-deficient crops, as boron is highly toxic to most plants at relatively low levels. Additionally, ensuring an even distribution across the soil is essential to prevent crop plants from experiencing toxicity. In general, applying 10 kg of borax (Granubor) per hectare to deficient soil before planting is an effective preventive measure against boron deficiency.

5. Effect of boron in vegetable crops

Boron requirements vary among different plant species, and this variability may be attributed to differences in cell wall composition. Cereals and grasses exhibit lower sensitivity to low levels of available boron compared to legumes and certain vegetable crops. In dicots such as soybean and alfalfa, it’s observed that the deficiency concentration of boron is three to four times higher in younger leaves than in older ones, which indicates the limited mobility of boron through the phloem in these species. Given boron’s immobility within plants, soils with marginal boron levels can lead to crop deficiencies during critical growth phases, namely, during vegetative growth, flowering, and seed development. Consequently, maintaining a consistent boron supply during the growth season is crucial for achieving optimal growth and the seed yield. Experimental evidence demonstrates that boron significantly enhances yield, mobility, and stress tolerance in various crop species (see Table 1). In extreme cases, crops grown in low-boron soils perform well until the flowering stage, at which point there can be substantial yield losses due to floral abortion or failed seed set. The most effective time for boron application is at the beginning of the reproductive phase, considering its immobility within plants, and this timing depends on the photosynthetic efficiency of the specific plants. Some examples of deficiency symptoms are outlined in Table 2.

Salient achievement	Reference
In sunflower, application of B @ 1.5 kg/ ha gave highest seed yield (2.01 t/ha)	[23]
In maize, mustard, onion, sweet potato, and sunflower, the application of B @ 1.5 kg/ha increased the B concentration and uptake.	[24]
In green gram B application increased the dry matter, yield and uptake of B	[25]
In groundnut, soil and foliar application of B had positive effect on growth and yield	[26]
Application of boronated super phosphate recorded significantly higher dry matter and yield of sunflower	[27]
Significant interaction effect of P × S × B was found for seed and oil yield of castor	[28]
Soybean grain yield was found to be statistically at par with either soil application of 20 kg sodium tetraborate (14% B) or two foliar sprays of 0.2% solution of the same salt.	[29]
Foliar application of B @ 0.2% at flowering stage of summer mungbean was found to be optimum for optimum economic yield in West Bengal.	[30]
Tuber number and yield of potato significantly increased with B fertilization.	[31]

Table 1.

Influence of B application on various crop plants [22].

Crop	Symptoms	Reference
Cauliflower	Roughened stems, leafstalks and midribs, poor flowering and development of curds with brown patches called Brown rot/ Browning, deformed and discolored brown heads with an empty space known as “Hollow Heart”	[19]
Cabbage	Distorted leaves and hollow areas in stems, cracked and corky stems, petioles, midribs and small heads.	[19]
Tomato	Stunted growth, dwarfed, leaves are twisted and small with variegated appearance. Yellowing and death of fruits in severe deficiency, fruits may be ridged, show corky patches, ripen unevenly and show cracking	[19]
Sugar beet	Young plants have distorted crown leaves. The leaf are small sized which becomes scorched and ultimately wilts	[19]
Turnip	Brown or gray concentric rings develop inside the root	[19]
Carrot	Leaves show longitudinal splitting, rough and small roots are formed which has a distinct central white core and top starts browning. The fruits also show blackened spots	[19]
Cucumber	Abnormal shoots, stunted apical growing points which eventually die. The new leaves are distorted and appear mottled while older leaves develop yellow border. Cracks in the stem and fruits are often deformed. The skin shows longitudinal and mottled yellow streaks leading to corky mark development on the fruits.	[32]
Muskmelon	Cracked stems and growing points dieback, leaves small, fruits distorted	[33]
Pepper	Stunted or dwarfed, leaves small, twisted and discolored, fruits twisted and show black spot	[32]
Bittergourd	Distortion of new leaves, appearance of broad yellow margins of oldest leaves	—
Okra	Leaves brittle and distorted, irregular lobe development	—
Solanum gilo	Upper leaves small, deformed and twisted, few flowers form, growing point die	—

Table 2.

Visible symptoms of boron deficiency in some vegetable crops.

The issue of hollow stem in cauliflower was effectively addressed by applying Farm Yard Manure at a rate of 7.5 tons/ha as a basal application, coupled with either the use of Boric Acid at 0.3% or Liquid Boron at 1.5 g/l, carried out 30 days after planting. Notably, the application of Liquid Boron was found to be the more cost-effective choice. This boron application significantly resolved these problems and led to an increase in curd yield [34]. In the case of muskmelons, applying boron improved both plant growth and fruit quality. Additionally, it reduced the occurrence of chilling injury in harvested fruit during cold storage at 5°C [33]. An optimal treatment combination consisting of 120 kg N/ha along with 0.6 kg B/ha was identified for achieving the maximum yield of tomatoes [35]. Furthermore, Naz et al. [36] reported that applying 2 kg B/ha resulted in the highest number of flower clusters per plant, fruit set percentage, total yield, fruit weight retention, and total soluble solids for the Rio Grand and Rio Fique tomato cultivars.

In a four-year study assessing the direct and residual effects of applied boron (B) in French bean-cauliflower cropping sequences, it was observed that applying 8 kg/ha of B led to reduced crop yields in eight crops spanning 4 years. When 2 kg/ha of B was applied, it enhanced French bean yields in soils with low-available B, whereas high-available B soils initially reduced yields in the first two crops but improved yields in the third and fourth crops. However, in both French beans and cabbage, elevated levels of B in plant tissue resulted in toxicity. The hot water-soluble B (HWS-B) content at harvest for each crop indicated a rapid decrease in the availability of applied B in these soils. Applying high levels of B-containing fertilizer to these soils led to the accumulation of B in toxic concentrations, resulting in lower vegetable yields. For French beans, which are low accumulators of B and more sensitive crops, it is advisable to rely on residual B rather than applying B-containing fertilizer directly to the soil in any vegetable cropping system, especially in red soils [37].

Solanki et al. [38] found that the application of boron at rates of up to 1 kg B/ha led to a significant increase in the yields and dry matter production of vegetable crops, including carrot, cauliflower, and onion. However, when the boron level was raised to 2 kg B/ha, yields tended to decline. The extent of this response varied from one crop to another and followed a descending order of magnitude: carrot > cauliflower > onion. Additionally, the application of boron resulted in an improvement in both the content and yield of protein in vegetable crops compared to the control. Over time, the introduction of boron into the soil led to a progressive increase in its concentration and uptake by vegetable crops. The highest removal of boron was observed in cauliflower curds, while the lowest was noted in carrot roots. The percentage of apparent boron recovery was influenced by its levels, with the highest recovery occurring at the 1 kg B/ha level. However, as boron levels increased, boron use efficiency decreased, with the lowest efficiency recorded at 1.5 kg B/ha.

6. Effect of boron on fruit crops

Boron has been identified as a crucial element influencing reproductive processes, impacting anther development, pollen germination, and pollen tube growth. Apple trees, scientifically known as Malus domestica Borkh., are particularly recognized for their high demand for boron [14]. The initial visual indicators of boron deficiency are evident in poor fruit set, which subsequently leads to reduced yields. This deficiency is critical because boron plays a pivotal role in reproductive growth [39]. In apple trees suffering from boron deficiency, the fruits tend to be undersized, corky, distorted, and prone to cracking and russeting. They exhibit yellow skin and fail to develop a healthy red coloration [40]. Additionally, apple fruits may have diminished concentrations of soluble solids and acids when boron levels are insufficient [41]. Stone fruits are likewise affected by boron deficiency. For instance, cherry shoots that lack adequate boron growth initially but subsequently undergo necrosis at the tips and ultimately die. In B-deficient plants, some buds may remain closed during springtime, while others wither and perish. The presence of cracking, deformation, shriveling, both internal and external browning, as well as corking around the pit and in the flesh, serves as clear indicators of boron deficiency in cherry fruit [42]. Similar responses to low-boron soils are also observed in nut crops grown in temperate regions.

Boron stands as an indispensable mineral for the growth and proper functioning of citrus plants; however, it is frequently deficient in many types of soil. Pronounced deficiency, ranging from 39 to 68%, is notably prevalent in red and lateritic soils, as well as in leached acidic soils within the hot semi-arid ecoregion. It is also observed in soils derived from alluvium within the hot sub-humid ecoregion, brown and red hill soils in the warm, humid ecoregion, and highly calcareous soils within the hot sub-humid ecoregion. Interestingly, these are the specific climate and soil combinations that yield abundant high-quality citrus crops in countries like Brazil, China, and Japan. Ruchal et al. [43] reported an increased fruit set in mandarin trees in response to higher micronutrient concentrations in their application. This could possibly be attributed to the enhanced translocation of essential nutrients and hormones to the ovary tissue, thereby stimulating fruit formation. Another potential factor contributing to this effect could be the improved availability of microelements, which enhances photosynthesis, reduces fruit drop, and results in improvements in fruit size and quality. Assam lemon (Citrus limon (L.) Burm.), an indigenous lemon cultivar of Assam, is extensively grown on the warm southern slopes of the Himalayas in northeastern India. This particular lemon variety is characterized by its ability to bear fruits in multiple flushes throughout the year, necessitating adequate nutrition to achieve optimal yields with high-quality fruits. Sheikh et al. [44] noted that the treatment involving ZnSO4 (0.2%), FeSO4 (0.2%), Borax (0.2%), and CuSO4 (0.2%) led to improved lemon fruit yields and quality. This treatment also resulted in the highest number of fruits per plant (73), yield per plant (11.5 kg), fruit fresh weight (158 g), fruit length (9.60 cm), fruit diameter (5.80 cm), juice content (152 mL/fruit), TSS (6.40°B), ascorbic acid content (49.10 mg/100 g), total sugar (6.30%), reducing sugar (3.90%), non-reducing sugar (2.40%), and the lowest titratable acidity (3.13%).

Guava trees that were treated with boric acid at concentrations of 0.1% and 0.2% exhibited notable improvements in various aspects of growth and fruit development. These improvements included increased extension of the terminal shoot, greater leaf count, enhanced leaf area per shoot, and accelerated fruit ripening, with reductions of 7 and 11 days, respectively. Additionally, the yield increased by 82 and 73%, respectively, with the higher concentration producing a slightly lower yield increment. Furthermore, the fruit size saw an increase when treated with 0.1% boric acid [45]. Pre-flowering applications of boric acid at concentrations of 0.1, 0.2, 0.3, or 0.4% on Allahabad Safeda guava resulted in significant enhancements in growth, flowering, and fruiting processes [46]. For Guava cv. L-49, the largest fruits, measuring 6.68 × 7.12 cm and weighing 125.8 g, were obtained with the application of 3.0% urea and 0.3% boric acid [47]. Similarly, spraying borax at a concentration of 0.2% effectively increased the size of Sardar guava fruits, their weight (95.25 g), and the yield (63.49 kg/tree) [48]. Combining the spray of borax at 0.2% with urea at 2% in three applications (pre-flowering, fruit setting, and 3 weeks after fruit setting) for Allahabad Safeda guava resulted in fruits measuring 4.84 cm in length, 5.00 cm in width, weighing 72.67 g, and yielding 19.08 kg per tree. Alternatively, foliar applications of borax at 0.2% alone recorded a higher fruit weight of 80.67 g and a yield of 20.17 kg per plant [49]. Yadav [50] observed that the best yield of high-quality fruits (67.7 kg per tree), the highest number of fruits per tree (686), and the largest fruit volume (107.5 cc) were achieved with foliar application of a combination of urea (3.0%), borax (0.15%), and NAA (10 ppm) in guava trees. Furthermore, a pre-harvest spray of borax with concentrations ranging from 0.2 to 1.2% applied twice in October resulted in improved guava fruit quality in the Sardar cultivar, particularly in terms of size and weight. In another experiment involving foliar application of H3BO3 at concentrations of 0.3, 0.5, and 1.0% on guava cultivar L-49 during the winter season [51], fruit weight and yield both increased, with the highest values recorded at 1.0% B, reaching 141.0 g and 73.0 kg/tree, respectively.

The most effective method for reducing fruit drop in rose-scented litchi, as reported by Ref. [52], was the application of borax. Through foliar sprays of borax at concentrations of 0.5 and 1.0%, fruit drop was reduced to 75–76%, compared to the 92.4% fruit drop observed in the control group. In the case of litchi, Haq and Rab [53] found that foliar application of CaCl2 and borax led to significant increases in the average fruit skin calcium content (from 4.79 mg/100 g DW in the control to the highest 8.88 mg/100 g DW with CaCl2 3% + boron 1.5% treatment), boron content (from 0.109 mg/100 g DW in the control to 0.247 mg/100 g DW with the same treatment), and skin strength (from 2.43 kg/cm2 in the control to 3.01 kg/cm2 with the same treatment). In addition, ion leakage (from 35.17% in the control to 16.17%) and fruit cracking (from 25.40% in the control to 11.14%) were reduced with the CaCl2 3% + boron 1.5% treatment. Boron plays a role in sugar movement and promotes the formation of fruit buds in plants. In the context of litchi cultivation, the use of borax at a concentration of 0.4% resulted in minimal fruit drop (69.45%), reduced fruit cracking (4.63%), and lighter seed weight (2.30 g). Furthermore, this treatment led to maximum fruit retention (30.55%), fruit set (62.50%), larger fruit dimensions (4.50 cm in length and 3.96 cm in width), greater fruit weight (24.85 g), higher pulp weight (20.73 g), an elevated fruit pulp-to-seed ratio (5.50%), increased fruit yield (120.85 kg per plant), and superior quality characteristics. These quality attributes included higher total soluble solids (22.55°Brix) and increased total sugar content (18.42%) with a lower percentage of titratable acidity (0.41%). These findings were observed in the plains of central Uttar Pradesh, India [54].

Rana and Sharma [55] discovered that grape berries and clusters exhibited an increase in both weight and volume when grapevines were subjected to boron spraying at concentrations of 0.025 and 0.05%. Furthermore, the application of calcium and boron, either individually or in combination, led to larger fruit size and a higher number of fruits per plant in the ber cv. Dongzao, consequently enhancing the fruit yield per plant [56]. In the case of Indian gooseberry and ber, the introduction of boron also contributed to an elevation in vitamin C content [57, 58]. This improvement in fruit quality may be attributed to the role of boron in facilitating carbohydrate transport within plants. Dhaker et al. [59] reported that the use of a foliar spray containing 0.6% borax significantly increased fruit weight (962.0 g) and fruit yield (21.21 kg) in bael (Aegle marmelos Corr.). Additionally, this concentration resulted in minimal fruit cracking (2.14%) and reduced peel thickness (2.41 mm), while also enhancing various qualitative characteristics of the fruit. Furthermore, the combined effect of organic manure (50 kg per tree) and foliar application of 0.6% borax significantly increased tree height (49.20 cm), stem girth (2.75 cm), fruit weight (980.0 g), and fruit yield (36.34 q/ha) compared to control plants.

Sotomayor et al. [60] reported that kiwifruits derived from shoots with boron-treated leaves exhibited a 14.1% increase in weight compared to the control, while fruits from boron-treated flowers were 17% heavier than those from untreated flowers. Additionally, significant differences in fruit length were observed between treated and control plants. In the case of papaya cv. Shahi, foliar application of 1.0 kg/ha of boron resulted in the highest fruit yield (49.01 t/ha) [61]. Boron also had a positive impact on pineapple fruit quality [62], where its application at a concentration of 2.0 mg/kg of B was deemed beneficial for enhancing fruit weight, total soluble solids (TSS), the TSS-to-acidity ratio, vitamin C content, and the concentration of aroma volatiles. As a result, the use of boron fertilizer is recommended for pineapple cultivation.

Mangoes exhibited leaf boron (B) deficiency levels ranging from 20 to 49 ppm, with sufficiency levels falling within the 50 to 100 ppm range [63]. Under the agro-climatic conditions of Doon Valley, Uttarakhand, India, an emerging physiological anomaly known as internal necrosis affects developing mango fruits due to boron deficiency, often resulting in fruit cracking. Notably, ‘Dashehari’ proved to be highly sensitive to both disorders, while ‘Chausa’ displayed the greatest tolerance. The analysis of nutrients in the leaves and fruit-bearing branches revealed that internal necrosis was primarily caused by boron deficiency. ‘Dashehari’ exhibited elevated leaf nitrogen levels, which may have contributed to the low levels of leaf boron and subsequently led to the occurrence of internal necrosis disorder. The most effective solution was found to be foliar sprays of boron (in the form of disodium octaborate tetrahydrate) at a concentration of 0.10%, resulting in substantial increases in boron levels in both leaves and fruits (149.64 and 120.14% increases, respectively) of cv. ‘Dashehari’. Furthermore, it was observed that the internal necrosis disorder exacerbated fruit cracking in ‘Dashehari’. The study also highlighted that foliar application was more effective than soil application in terms of increasing yield while reducing internal necrosis and fruit cracking disorders [64]. In mango cv. SB Chausa, a combined application of KNO₃ (1.0%) and boric acid (0.2%) led to enhanced fruit set (38–42%), increased fruit retention (56–88%), higher fruit weight, and improved yield per plant [65]. Application of Agricol at a rate of 5 grams on two sides of the plant canopy, specifically on the N-E and S-W aspects, resulted in the highest fruit yield, with each tree producing 46.2 and 45.92 kilograms of fruit [66]. Additionally, when Disodium Octaborate Tetrahydrate (DOT) was applied at a rate of 5 grams, it led to the highest number of hermaphrodite flowers per panicle (208.55 and 207.71), the best sex ratio per panicle (0.66 and 0.61), and the lowest fruit drop (51.23 and 50.50). On the other hand, the application of Agricol at 5 grams significantly improved fruit quality parameters and pulp weight (153.23 and 152.04). Maximum fruit dimensions (9.38 and 9.17 centimeters in length and width) were achieved with the use of Chemibor-P, while the application of DOT at 10 grams on the north-east and south-west canopy of the plant resulted in the highest levels of fruit bioactive compounds such as vitamin C (65.31 mg/100 g pulp and 65.64 mg/100 g pulp) and beta-carotene (3330.55 μg/100 g and 3315.18 μg/100 g). When boron was applied to mango cv. Amrapali through soil from various mineral sources, it had a clearly positive impact on the promotion of reproductive growth compared to control plants. The application of boron at two stages, namely pre-flowering and the pea stage of fruit development, influenced mango flowering and fruiting by increasing the number of flowers, reducing fruit drop, and ultimately resulting in higher mango yields. These findings underscore the role of boron in the reproductive physiology of plants, including processes like pollen tube elongation, pollen germination, sugar transport, and carbohydrate synthesis [67].

7. Boron uptake and management

Plant growth and development depends on several factors including nutrient uptake capacity and distribution to other growing parts of the plant [68]. In a single growing season crop plants may face either deficiency or toxicity [69]. This is due to very narrow range of B deficiency and toxicity in soils and plants [70] therefore, it is important to apply optimum B fertilizer for supply of B in deficient soils for normal growth, yield and quality of produce. In gladiolus, B at 0.3% showed marginal increase in flower duration while greater concentration proved toxic and lesser concentration was found to be deficient [71]. The frequency of B application would depend on doses and nature of the crop. The best response was obtained with basal application of B on crops. When boron deficiency arises, it can be addressed by applying foliar sprays containing 2.0–2.5 grams per liter of either boric acid or solubor [15]. To ensure the soil solution maintains optimal boron concentrations for maximum production, it is essential to employ environmentally friendly and advantageous techniques. These approaches not only boost boron absorption and its distribution to various plant components but also enhance soil fertility and crop yield. Some important approaches to enhance the B acquisition are as follows:

7.1 Grafting

Soils experiencing boron deficiency can be addressed by either incorporating boron directly into the soil or by applying boron-containing fertilizers through foliar methods. However, this approach can raise the overall cost of crop cultivation and potentially lead to boron toxicity issues due to the narrow margin between deficiency and toxicity. Therefore, an environmentally friendly and suitable alternative is the utilization of specific rootstocks tailored for different crops [72, 73]. These rootstocks possess the capability to efficiently absorb substantial amounts of boron from the soil and transport it to the upper parts of the plant, ensuring proper physiological functioning. Extensive research demonstrates that various rootstocks significantly improve the nutritional status of plants across different crops, primarily due to their effective water and mineral absorption abilities from the soil solution, surpassing those of self-rooted plants [74, 75, 76, 77]. Furthermore, rootstocks enhance the resilience of scion cultivars to both boron deficiency [78] and toxicity [79]. The intricate physiological interactions between the scion and rootstock and their impact on mineral acquisition have been thoroughly explored in various plant species. For instance, in the case of citrus trees, Carrizo citrange (Citrus sinensis Osb. × Poncirus trifoliata [L.] Raf.) and red tangerine (C. tangerina) have been identified as highly effective rootstocks with strong genetic traits for boron uptake and transportation to the upper canopy under conditions of limited boron availability. Trifoliate orange (P. trifoliata [L.] Raf.) exhibits moderate efficiency in this regard, while sour orange (C. aurantium L.) and fragrant citrus (C. medica) have been deemed inefficient rootstocks for boron uptake and transport [72]. In studies by Liu et al. [80, 81], the impact of boron on Carrizo citrange (C. sinensis Osb. × P. trifoliata [L.] Raf.) and trifoliate orange (P. trifoliata [L.] Raf.) rootstocks grafted onto orange plants was examined. Their findings indicated a notable increase in boron absorption and newly absorbed boron concentration in the lower and upper leaves of Carrizo citrange grafted plants when compared to those grafted onto trifoliate orange rootstocks. In the case of pistachio trees (Pistacia vera cv. Kerman), P. atlantica rootstock demonstrated a high capacity for boron absorption and uptake of other nutrients from the soil solution, resulting in significantly higher concentrations (1.2–2.4 times more) in the leaves [82].

The distribution of ions in various plant parts can significantly differ based on their availability in the soil solution. El-Motaium et al. [79] noted a strong connection between the rootstock and boron uptake in pear plants, resulting in an increase in boron concentration of up to 50–80% in leaves and 100–300% in stems. However, the use of Prunus rootstock did not yield a notable increase in boron content in the root tissues. In the case of Newhall orange (C. sinensis Osb.), Sheng et al. [83] observed that when exposed to limited boron supply, boron content decreased in leaf (23–53%) and scion (40–65%) tissues but increased in rootstock parts (35–60%) when grafted onto Carrizo citrange (C. sinensis Osb. × P. trifoliata [L.] Raf.) as compared to trifoliate orange (P. trifoliata [L.] Raf.). Wang et al. [84] conducted a study on boron absorption patterns in four citrus rootstock-scion combinations and found that, under inadequate boron supply, the maximum boron concentration was observed in the buds and leaves of C. sinensis [L.] Osb. cv. Fengjie-72 when grafted onto Carrizo citrange and trifoliate orange plants. Notably, the boron accumulation in the Newhall scion grafted onto Carrizo citrange was higher (24%) compared to other combinations. Furthermore, a higher proportion of available boron (36%) was detected in the leaves of Carrizo citrange when compared to plants grafted onto trifoliate orange.

7.2 Biostimulators

Biostimulants are substances, distinct from fertilizers, soil conditioners, or pesticides, that have the capacity to impact various metabolic processes in plants, such as cell division, respiration, photosynthesis, and ion absorption, even when applied in small quantities [85]. These biostimulators can be employed to augment mineral uptake in plants while requiring minimal inputs. In recent times, biostimulants have played a pivotal role in altering plant physiology and optimizing plant growth [86]. They engage with the plant’s signaling pathways to mitigate adverse plant responses during stressful conditions, ultimately promoting optimal plant development. Crops treated with biostimulants exhibit greater resilience to challenging environmental circumstances and demonstrate enhanced efficiency in ion absorption when faced with limited ion availability, primarily due to improved antioxidant production [87]. Humic substances (HS), a type of organic biostimulant, are renowned for their ability to enhance soil structure and root architecture by increasing the activity of root H + -ATPase. Consequently, they find widespread application in ion acquisition, with the effectiveness depending on factors such as concentration, plant species, and environmental conditions [88]. Field trials involving the application of biostimulants to the soil of Vicia faba cv. Giza beans demonstrated enhanced soil structure and ion uptake compared to untreated controls [89]. Conversely, the use of composted sewage sludge containing HS resulted in improved growth and yield of Capsicum annuum L. cv. Piquillo [90]. Additionally, these benefits were associated with increased availability of micronutrients in the substrate and enhanced microbial activity within plants. This microbial activity aids in reducing ion leaching by lowering soil pH through the production of organic acids like citrate, oxalate, and malate. HS forms complexes with micronutrients, and the plant’s plasma membrane generates a proton motive force that facilitates the active and passive transport of ions through the symplastic pathway, thereby increasing the availability of trace elements to plants [91].

7.3 Mycorrhizal fungi (MF)

In rough lemon (Citrus jambhiri Lush), the application of boron through foliar spraying and soil amendment, coupled with Glomus fasciculatum inoculation, not only led to an increase in total boron accumulation in the leaves by 11–18% but also resulted in enhanced exudation of root sugars and amino acids when compared to plants that were not inoculated [92]. The presence of arbuscular mycorrhizal fungi (AMF) in the soil can impact boron concentration in plants. Research findings on the effect of AMF inoculation vary, with some studies reporting reduced boron acquisition in shoots of MF-inoculated plants [93], others showing no significant effect [94], and yet others indicating enhanced boron acquisition [95]. However, the precise role of mycorrhizal activity in relation to boron remains unproven and necessitates further investigation. The vascular structure found in higher plants underlines the importance of boron in lignification [96]. While passive uptake of boric acid seems to occur in plants, the mechanism behind mycorrhizal boron uptake is not yet fully understood. The critical role of sugar alcohols such as sucrose, sorbitol, and mannitol in the remobilization of boron within plant tissues is well-documented [4, 97]. According to Lewis’s hypothesis [96], sucrose, due to its low affinity for boron in vascular plants, is primarily responsible for boron mobilization. In contrast, fungal carbohydrates, especially mannitol, exhibit a high affinity for forming complexes with boron, leading to limited boron mobility from the fungal symbiotic partner to the host plant. However, certain mycelia have been observed to facilitate the mobility of the mannitol-boron complex, allowing for the continuous uptake and long-distance transport of boron in plants [6]. While it is established that the application of AMF can aid in boron acquisition and transport within plants, there are still specific aspects of this process that require further investigation.

7.4 Plant-growth-promoting rhizobacteria (PGPR)

Rhizobacteria, also referred to as plant-growth-promoting rhizobacteria (PGPR), are beneficial and actively root-colonizing microorganisms that establish a symbiotic relationship with plant roots. They play crucial roles in various agricultural aspects, including nitrogen fixation [98], improving tolerance to salinity and drought [99], producing enzymes that combat pathogenic microorganisms, solubilizing nutrients, and generating phytohormones like IAA, cytokinins, and gibberellins, which stimulate root growth [100]. This root proliferation, in turn, enhances water and nutrient uptake by plants.

In the case of lentils (Lens culinaris Medik), the inoculation of PGPR not only increased nitrogen (N) uptake (2.26–2.95%) and phosphorus (P) uptake (0.52–0.82%) in the roots, stems, and grains but also improved plant growth parameters such as root and shoot length, as well as their fresh and dry weights. Additionally, the application of PGPR resulted in higher levels of phytohormones (IAA, GA₃) and increased macro- and micronutrient concentrations in crops like Raphanus sativus and Musa spp. [101, 102]. In potatoes (Solanum tuberosum L.), the P content saw a 43.1% improvement with the introduction of the Bacillus cereus P31 strain, while the Achromobacter xylosoxidans strain P35 increased N and K content by up to 50.5 and 48.3%, respectively [103].

Numerous studies support the role of bacteria in absorbing excessive boron levels from soil solutions. These studies involve B-tolerant bacterial strains belonging to genera such as Bacillus, Gracilibacillus, Lysinibacillus, Boronitolerans, Variovorax, Pseudomonas, Mycobacterium, and Rhodococcus, which are capable of absorbing toxic levels of boron from the soil [104, 105]. Cheke et al. [106] reported that the availability of micronutrients (Fe, Mn, Cu, Zn, and B) was higher in the rhizosphere soil compared to non-rhizosphere soil, indicating that the tree rhizosphere has an impact on the availability of trace elements in the soil. In a study conducted in Nagaland, India, rhizosphere soils collected from healthy Khasi mandarin (Citrus reticulata Blanco) plants in an orchard displayed a higher bacterial population compared to the fungal population in the rhizosphere of four high-yielding plants [107]. However, additional research is necessary to identify efficient boron-capturing bacteria that can enhance boron availability for crops under conditions of limited boron supply.

7.5 Nanotechnology

Nanotechnology presents an innovative strategy applicable in agriculture for managing both biotic and abiotic stress, detecting diseases, and improving nutrient absorption [108, 109]. This cutting-edge technology is essential to address the challenges of limited nutrient and water resources while aiming to increase the production of high-quality agricultural crops. Nanotechnology enhances plant production and nutrient utilization efficiency by requiring fewer resources compared to traditional methods. Nanoparticles (NPs) have unique physicochemical properties that positively impact plant metabolism, leading to increased crop yield and nutritional value [110]. To illustrate, the use of copper NPs in watermelon cultivation resulted in improved plant growth and development compared to the control group [111]. Baruah and Dutta [112] demonstrated that hydrogels and zeolites have the capacity to absorb environmental contaminants and enhance soil water retention. Chitosan NPs have proven effective in reducing fertilizer consumption, contributing to a reduction in environmental pollution [108]. While the application of CeO₂ and ZnO NPs did not lead to an increase in macronutrient concentrations in Cucumis sativus fruit, they did elevate micronutrient levels [113]. The use of nano-titanium dioxide (TiO₂) boosted chlorophyll synthesis and photosynthetic activity by enhancing ion uptake efficiency in spinach [114]. Consequently, adopting nanotechnology approaches is a promising strategy for enhancing boron uptake and utilization efficiency in plants, potentially reducing the need for boron fertilizer in crop cultivation.

8. Conclusions

Boron (B) is a vital trace element essential for the proper physiological functioning of higher plants. B deficiency represents a nutritional disorder that has detrimental effects on plant metabolism and growth. The ability of crops to efficiently utilize B resources can vary significantly, and it is crop-specific. Thus, from an agricultural perspective, there is a necessity to identify key cultivars among agronomic and horticultural crops, as well as different conditions that enable optimal utilization of available B resources, especially those that thrive under B-deficient conditions. The intricate relationship between rootstock and scion requires further in-depth studies to identify exceptional root systems, particularly those indigenous to specific regions that exhibit tolerance to both B deficiency and toxicity. Grafting and arbuscular mycorrhizal fungi (AMF) inoculation have been shown to enhance various aspects of plant physiology and nutrition, with several studies highlighting their crucial role in B uptake. Additionally, there is potential for molecular-level investigations into the role of B in plants, which could pave the way for novel strategies to enhance B stress tolerance in crops. Nanotechnology is an emerging agricultural technique designed to address plant nutrition-related challenges. The combination of these techniques has the potential to improve B uptake. Research has demonstrated that the simultaneous use of grafting and copper nanoparticles (NPs) can enhance the growth and development of watermelon by increasing ion uptake. In certain plant species, the combined inoculation of mycorrhizal fungi (MF) and plant-growth-promoting rhizobacteria (PGPR) has improved growth by augmenting water and macronutrient levels. Therefore, these existing techniques should be harnessed and further refined, considering crop-specific and location-specific factors, to achieve better outcomes and enhance B uptake and utilization in plants.