Retranslocation of boron in broccoli and lupin during early reproductive growth

The objective of the present study was to determine if boron (B) retranslocation depends on plant-B status and external-B supply. The stable ¹⁰B isotope was supplied to the root system of broccoli ( Brassica oleracea var. italica Plenck cv. Commander) and lupin ( Lupinus albus L. cv. Ultra) plants to provide a quantitative picture of B distribution during early reproductive development. Regardless of the B regime (i.e. continuous supply with luxury, sufficient or deficient B; transfer at inflorescence emergence from either a luxury- or sufficient-B supply to a deficient one) and whether ¹⁰B was acquired before or during inflorescence development, a significant proportion of the B recovered in broccoli florets and lupin fruit was ¹⁰B enriched. B acquired during inflorescence development was an important source of B for reproductive structures, but the relative importance of B acquired before and after inflorescence emergence appeared to be species dependent. The occurrence of B retranslocation was not dependent upon the induction of B deficiency. The concentrations of B in phloem exudates (0.38 to 0.03 mM) were 4- to 23-fold those in xylem sap, and more similar to the concentrations in the reproductive structures (0.86 to 0.07 mM) than those in source leaves (2.4 to 0.19 mM). The decreasing acropetal gradient of tissue-B concentrations with luxury-B supply declined dramatically or was reversed in plants grown with sufficient or deficient B. The data are consistent with B being a phloem-mobile element, and suggest that newly acquired B is particularly important during the early reproductive growth of plants.     

Evidence of phloem boron transport in response to interrupted boron supply in white lupin (Lupinus albus L. cv. Kiev Mutant) at the reproductive stage

The present study investigates whether previously acquired boron(B) in mature leaves in white lupin can be retranslocated intothe rapidly growing young reproductive organs, in response toshort-term (3 d) interrupted B supply. In a preliminary experimentwith white lupin in soil culture, B concentrations in phloemexudates remained at 300–500 µM, which were substantiallyhigher than those in the xylem sap (10–30 µM). Thehigh ratios of B concentrations in phloem exudates to thosein the xylem sap were close to values published for potassiumin lupin plants. To differentiate ‘old’ B in theshoot from ‘new’ B in the root, an experiment wascarried out in which the plants were first supplied with 20µM ¹¹B (99.34% by weight) in nutrient solution for 48d after germination (DAG) until early flowering and then transferredinto either 0.2 µM or 20 µM ¹⁰B (99.47% by weight)for 3 d. Regardless of the ¹⁰B treatments, significant levelsof ¹¹B were found in the phloem exudates (200–300 µMin 20 µM ¹⁰B and 430 µM in 0.2 µM ¹⁰B treatment)and xylem sap over the three days even without ¹¹B supply tothe root. In response to the 0.2 µM ¹⁰B treatment, thetranslocation of previously acquired ¹¹B in the young (the uppermostthree leaves), matured, and old leaves was enhanced, coincidingwith the rise of ¹¹B in the xylem sap (to >15 µM) andphloem exudates (430 µM). The evidence supports the hypothesisthat previously acquired B in the shoot was recirculated tothe root via the phloem, transferred into the xylem in the root,and transported in the xylem to the shoot. In addition, somepreviously acquired ¹¹B in the leaves may have been translocatedinto the rapidly growing inflorescence. Phloem B transport resultedin the continued net increment of ¹¹B in the flowers over 3d without ¹¹B supply. However, it is still uncertain whetherthe amount of B available for recirculation is adequate to supportreproductive growth until seed maturation. Key words: ¹⁰B, ¹¹B, B recirculation, Lupinus albus L., phloem exudate, xylem sap Received 9 October 2007; Revised 28 November 2007 Accepted 30 November 2007     

Effects of salinity and varying boron concentrations on boron uptake and growth of wheat

Summary Two sandculture experiments were conducted with wheat (Triticum aestivum) to determine the effects of (1) osmotic potential (Ψ_π) and (2) fluctuating boron (B) concentrations on B availability (toxicity), shoot growth and leaf concentrations of B of wheat. The first experiment consisted of growing wheat to the spike emergence stage in sandcultures irrigated with a complete nutrient solution containing 1.0, 7.5, and 15.0 mg Bl⁻¹ and having Ψ_π values of −0.02, −0.07, −0.12, and −0.17 MPa produced by CaCl₂−NaCl additions. Statistically, shoot weight was independently influenced by the B and Ψ_π treatments but not by their interaction. Only the B treatment had a significant effect on leaf boron concentrations; the B x Ψ_π interaction was nonsignificant with respect to leaf B concentrations. The second experiment was designed to determine if growth and B uptake of wheat responds to the time integrated mean (TIM) concentration of B. This experiment consisted of four fixed-B concentrations and four fluctuating-B concentrations designed to produce two TIM concentrations (3.9 and 7.4 mg Bl⁻¹) approached low to high and vice versa. With respect to shoot weight, there was no statistical difference among treatments having the same TIM concentration during the 10 week experiment. However, shoot B concentrations differed greatly; they were higher when the B concentration was progressively increased over the 10 week period. Leaf B concentrations (Y leaf at flowering), while not as high as the shoot B concentrations, were also higher under the treatment of increasing B concentration, indicating B uptake rates are higher for mature plants than for seedlings.     

Effects of liming and boron fertilization on boron uptake of Picea abies

The effects of liming on concentrations of boron and other elements in Norway spruce [Picea abies (L) Karst.] needles and in the mor humus layer were studied in long-term field experiments with and without B fertilizer on podzolic soils in Finland. Liming (2000+4000 kg ha^-1 last applied 12 years before sampling) decreased needle B concentrations in the four youngest needle age classes from 6–10 mg kg^-1 to 5 mg kg^-1. In boron fertilized plots the corresponding concentrations were 23–35 mg kg^-1 in control plots and 21–29 mg kg^-1 in limed plots. Both liming and B fertilizer decreased the Mn concentrations of needles. In the humus layer, total B concentration was increased by both lime and B fertilizer, and Ca and Mg concentrations and pH were still considerably higher in the limed plots than controls. Liming decreased the organic matter concentration in humus layer, whilst B fertilizer increased it.The results about B uptake were confirmed in a pot experiment, in which additionally the roles of increased soil pH and increased soil Ca concentration were separated by means of comparing the effects of CaCO₃ and CaSO₄. Two-year-old bare-rooted Norway spruce seedlings were grown in mor humus during the extension growth of the new shoot. The two doses of lime increased the pH of soil from 4.1 to 5.6 to 6.1, and correspondingly decreased the B concentrations in new needles from 22 to 12 to 9 mg kg^-1. However, CaSO₄ did not affect the pH of the soil or needle B concentrations. Hence the liming effect on boron availability in these soils appeared to be caused by the increased pH rather than increased calcium concentration.     

不同供硼水平对绿豆植株形态及其叶片生长特征的影响

利用水培以绿豆为材料,研究不同供硼水平对绿豆植株形态和叶片生长特征的影响。结果表明缺硼抑制绿豆生长,但对根的影响较对冠的影响更大,表现在缺硼导致冠根比增大;缺硼明显抑制叶面积;降低特定叶面积（SLA）,这可能是由于缺硼影响细胞伸展的缘故,造成叶片密度增加,缺硼也提高叶片重量比（LWR)并导致叶脉间失绿,说明缺硼叶片可能过量碳水化合物积累,引起叶绿素降解,与适量供硼比较,过量供硼也影响绿豆的生长,但对冠根比没有影响,表明过量供硼对根和冠具有相同的抑制作用,硼中毒导致成熟叶片脱落,从而影响叶面积,但对特定叶面积（SLA）和叶片重量比（LWR）没有影响。     

NaCl and Na2SO4 alter responses of jack pine (Pinus banksiana) seedlings to boron

Using sand culture, we examined the responses of 6-month-old jack pine (Pinus banksiana Lamb.) seedlings to boron and salinity (sodium chloride and sodium sulfate) treatments. During 4 weeks of treatments, 60 mM NaCl and 60 mM Na₂SO₄ significantly decreased survival, new shoot length, number of new roots, shoot to root dry weight ratio and transpiration rates. When applied in absence of the salts, B had little effect on the measured variables. However, when applied together with salts, B decreased seedling survival, increased needle injury and altered tissue elemental concentrations in jack pine seedlings. In 2 mM B treatment, B concentration was higher in the shoots than in the roots. However, when 2 mM B was present in NaCl and Na₂SO₄ treatments, shoot boron concentration declined and greater proportion of B accumulated in the roots. This shift corresponded to a decline in transpiration rates. In plants treated with NaCl, Na accumulated primarily in the shoots, while in Na₂SO₄-treated plants Na accumulated mostly in the roots. Based on the electrolyte leakage and needle necrosis data, Cl^– appears to be the major factor contributing to seedling injury and B aggravates the injurious effects of NaCl. We suggest that Cl^– may contribute to Na and B toxicity in jack pine by altering cell membrane permeability leading to increased Na concentration in the shoots.     

Effects of boron fertilization on `Conference' pear tree vigor, nutrition, and fruit yield and storability

The aim of the study was to examine the response of pear (Pyrus communis L.) trees to soil and foliar applications of boron (B). The experiment was carried out during 2000–2001 in a commercial orchard in Central Poland on mature `Conference' pear trees grafted on Pyrus communis var. caucasica seedlings planted at a spacing of 4 × 2.5 m on a sandy loam soil with a low hot water-extractable B status. Annually, foliar sprays with B were applied. (i) before full bloom (at green and white bud stage, and when 1–5% of flowers was at full bloom), (ii) after flowering (at petal fall, and 7 and 14 days after the end of flowering), or (iii) postharvest in fall (approximately 6 weeks before leaf fall). Spray treatments involved application of B at a rate of 0.2 kg ha^–1 in spring or 0.8 kg ha^–1 in fall. Additionally, other trees were supplied with soil-applied B at the bud break stage at a rate of 2 kg ha^–1. Trees untreated with B served as the control. The results revealed that foliar applications of B before full bloom or after harvest increased fruit set and fruit yield. Tree vigor, mean fruit weight, firmness, soluble solids concentration and titratable acidity of fruits at harvest were not affected by B treatments. Foliar B sprays before full bloom or after harvest increased B concentrations in flowers, and both leaves and fruitlets at 40 days after flowering. Only the foliar treatments after flowering and soil fertilization with B increased the content of this microelement in fruit and leaves at 80 and 120 days after full bloom. Foliar B application before full bloom or after harvest increased calcium (Ca) in fruitlets at 40 days after full bloom, in fruit, and in leaves at 80 and 120 days after full bloom. Nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) in plant tissues were not affected by B fertilization. After storage, and also after the ripening period, fruits from the trees sprayed with B before full bloom or after harvest had higher firmness and titratable acidity than those from the control trees. After the ripening period, fruits from the trees sprayed with B before full bloom or after harvest had lower membrane permeability and were less sensitive to internal browning than the control fruits. These findings indicate that prebloom and postharvest B sprays are successful in increasing pear tree yielding and in improving fruit storability under the conditions of low B availability in the soil.     

Boron accumulation and partitioning in wheat cultivars with contrasting tolerance to boron deficiency

Two pot experiments at the Plant Environment Laboratory (PEL), Reading, UK investigated sterility, boron (B) accumulation and B partitioning of wheat cultivars grown with limited B in the growing medium. The first experiment evaluated nine cultivars of spring wheat with diverse field responses to low available soil B, supplied with or without 20 μM B. A second experiment examined the response of a susceptible (SW-41) and a tolerant (Fang-60) cultivar to B-deficiency. These cultivars were supplied with either 20 μM B from sowing to flag leaf emergence and no added B thereafter, or 20 μM B from sowing to maturity. When B was not supplied in the nutrient solution, the number of grains ranged from 4 per ear (cv. BL-1135) to 32 per ear (cv. BL-1249) and sterility of competent florets ranged from 39% to 93%. Boron concentration in the flag leaf at anthesis did not differ greatly when the growing medium contained limited B, but differences between cultivars were evident when B was unlimited. Tolerance of B-deficiency was not related to the B concentration in the flag leaf. Some cultivars produced viable pollen and set grains while others failed to do so at similar B concentrations in the flag leaf. The two contrasting cultivars did not differ much in their pattern of B partitioning when B supply was restricted from flag leaf emergence onwards. Similarly, little evidence was found that the tolerant cultivars translocated B from their leaves, roots or stems when the supply in the growing medium was restricted. The proportion of total B partitioned in different organs was the same irrespective of B supply and cultivar. On average, leaves contained 68% of the total B content in the whole plant compared to 16% in the roots, 10% in the ears and only 6% in the stems. Tolerant or susceptible cultivars of wheat could not be distinguished based on the B concentration and B content of the flag leaf. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of boron on somatic embryogenesis of Daucus carota

Although the micronutrient boron (B) apparently exerts no influences on the induction of somatic embryogenesis of cultured carrot petiole explants, strong influences on the development of somatic embryos were observed (0–8 mg l⁻¹ B). At lower B concentrations the development of roots is promoted with simultaneous retardation of shoot development and at higher B concentrations shoot development is favored at the expenses of the development of the root system. Parallel to this, the ratios of concentrations of endogenous indole-3-acetic acid to total cytokinins (with one exception) changes continuously from 4 (zero B) to 0.22 (8 mg l⁻¹ B). Analogies to morphogenetic reactions following the application of various ratios of auxins/cytokinins to the nutrient medium of cultured tissue (Skoog and Miller, 1957 and others) are suggested. An increase in the B concentration in the nutrient medium results generally in a reduced concentration of endogenous abscisic acid.     

Effects of liming and boron fertilization on mycorrhizas of Picea abies

Effects of liming and B fertilization on Norway spruce [Picea abies (L.) Karst.] mycorrhizas were studied in factorial field experiments. The lime was applied twice, about 30 years and 12 years before sampling (2000 and 4000 kg ha^-1 dolomite). B was applied at the rate of 1.5 kg B ha^-1 two years before sampling.Boron fertilization doubled the number of root tips in the top 10 mm of the humus layer. The proportion of dead short root tips was increased from 10 % in control plots to 29 % in the limed plots. Numbers of dead root tips were increased when both lime and B were applied. The % of mycorrhizas with external mycelium was slightly increased and the % of Piloderma croceum Erikss. and Hjortst. was decreased by lime. In conclusion, adverse effects of lime on mycorrhizas were found, which were ameliorated by B fertilization, but lime-induced B deficiency alone was not the only reason for the effects of lime on root mortality.     

Phloem transport of sulfur in Ricinus

Mature leaves of Ricinus communis fed with ³⁵SO ₄ ^2- in the light export labeled sulfate and reduced sulfur compounds by phloem transport. Only 1–2% of the absorbed radiosulfur is exported to the stem within 2–3 h, roughly 12% of ³⁵S recovered was in reduced form. The composition of phloem translocate moving down the stem toward the root was determined from phloem exudate: 20–40% of the ³⁵S moved in the form of organic sulfur compounds, however, the bulk of sulfur was transported as inorganic sulfate. The most important organic sulfur compound translocated was glutathione, carrying about 70% of the label present in the organic fraction. In addition, methionine and cysteine were involved in phloem sulfur transport and accounted for roughly 10%. Primarily, the reduced forms of both, glutathione and cysteine are prsent in the siever tubes.Abbreviations CySH cysteine - GSH glutathione - GSSG glutathione disulfide - NEM N-ethylmaleimide - CyS-SCy cystine     

Boron Mobility and Nutrition in Broccoli (Brassica oleracea var. italica

Broccoli (Brassica oleracea var. italica cv Premium Crop) plantswere germinated in soil, transferred to vermiculite three weekslater and grown in the glasshouse, then either supplied continuouslywith boron levels ranging from 0.0 (deficient) to 12.5 (toxic)mg l^–1 of nutrient solution or transferred from 2.5 to0.0 mg B l^–1 at the initiation of inflorescence development.At commercial maturity the concentrations of various inorganicand organic solutes in phloem exudates and xylem saps, as wellas plant characteristics and elemental composition of the variousplant parts, were determined. Under deficient B levels leaf midrib and stem corkiness wereevident, together with signs of stem pith breakdown, symptomswhich resemble the initiation of the hollow stem disorder. Thexylem sap B concentration declined by about 50 % when B wasnot supplied or was removed after a period of adequate supply;the phloem concentration was unaffected. Also, the decreasingB concentration gradients from mature transpiring tissues toyoung developing sinks disappeared. Therefore, it is concludedthat when B is deficient, it is retranslocated from source leavesin the phloem stream supplying the developing leaves and inflorescence.The data also suggested that at toxic levels B undergoes extensivelateral transfer, probably from xylem to xylem, thereby enhancingthe B concentration of developing sinks. The B regime influenced dry-matter partitioning, retranslocationof some elements, and the synthesis and distribution of aminoacids and sugars, reflecting the general nature of B involvementin plant processes. Brassica oleracea var., italica, broccoli, phloem, mobility, retranslocation, boron nutrition, transport fluids, concentration gradients     

Redistribution of boron in leaves reduces boron toxicity

High soil boron (B) concentrations lead to the accumulation of B in leaves, causing the development of necrotic regions in leaf tips and margins, gradually extending back along the leaf. Plants vary considerably in their tolerance to B toxicity, and it was recently discovered that one of the tolerance mechanisms involved extrusion of B from the root. Expression of a gene encoding a root B efflux transporter was shown to be much higher in tolerant cultivars. In our current research we have shown that the same gene is also upregulated in leaves. However, unlike in the root, the increased activity of the B efflux transporter in the leaves cannot reduce the tissue B concentration. Instead, we have shown that in tolerant cultivars, these transporters redistribute B from the intracellular phase where it is toxic, into the apoplast which is much less sensitive to B. These results provide an explanation of why different cultivars with the same leaf B concentrations can show markedly different toxicity symptoms. We have also shown that rain can remove a large proportion of leaf B, leading to significant improvements of growth of both leaves and roots.Key words: Bor genes, boron tolerance, boron toxicity, efflux pumping, leaf necrosis, membrane transportB-toxic soils are widespread throughout agricultural areas of the world where they cause significant and often substantial reductions in crop quality and yield. The mechanism by which B is toxic to plants is not well understood¹ but toxicity symptoms include reduced root growth which affects uptake of water and nutrients, and the development of necrotic patches on leaves which impairs photosynthesis. Tolerance to B toxicity has been recognized in a number of crops, notably in cereals. In most cases, tolerance is achieved by reduced uptake of B into the root, which then leads to reduced uptake into the shoot. Genetic studies established that in barley, a locus associated with reduced tissue B occurred on chromosome 4 and that this locus could be transferred to other barley cultivars with desirable agronomic traits.²Hayes and Reid³ made a careful study of the characteristics of B uptake in a highly tolerant landrace barley cultivar Sahara, and found that although B was highly permeable, the root B concentration in this cultivar could be maintained at only half that in the external medium, whereas in sensitive cultivars, B was the same in both intracellular and extracellular phases. It was concluded that tolerant cultivars must have a membrane active transporter that exports B from the root. A B exporter, AtBor1 had previously been discovered in Arabidopsis where it was involved in B loading into the xylem⁴ but it was later found to be degraded under high B conditions⁵ and therefore would not be useful in B tolerance.However, other Bor1 homologues were subsequently discovered in Arabidopsis and in rice. Based on homology with rice, Reid⁶ cloned genes from barley and from wheat (HvBor2 and Tabor2 respectively) which were shown to be strongly upregulated in roots of tolerant cultivars, and virtually undetectable in sensitive cultivars. Thus, a simple mechanism to explain tolerance was established; efflux of B from the root reduced the intracellular concentration of B in the root cells, thereby reducing toxicity and improving root growth. At the same time, the lower root content meant that less B was transferred to the shoot, resulting in lower shoot toxicity.Yet there remained several unanswered questions regarding B toxicity. Firstly, it was commonly observed that toxicity symptoms were not reliably correlated with leaf B concentration, and that often after rain, toxicity symptoms became less severe. Nable et al.⁷ had investigated the effect of rain on shoot B concentrations and concluded that although rain did reduce the B concentration in leaves, it did not affect growth and yield. Secondly, field trials with cultivars in which the B tolerance traits were expressed, did not show the improvements in growth and yield that could be observed in glasshouse trials.^8,9Our recent work¹⁰ has provided new insights into these phenomena. Sensitive and tolerant cultivars of both wheat and barley were grown in varying levels of B. Then, ignoring the level of B in the growth solution, leaves of the different cultivars that displayed the same degree of leaf necrosis were selected. This revealed that in the tolerant cultivars, necrosis began to appear at leaf B levels that were two-to five-fold higher than in sensitive cultivars. Since no internal tolerance mechanism had been reported, it was hypothesised that in the tolerant cultivars, internal toxicity was reduced by pumping B from the cytoplasm into the cell wall where B is much less toxic. To prove this hypothesis three types of experiment were conducted. Firstly protoplasts were isolated from leaves of tolerant and sensitive cultivars of barley, and it was shown that when incubated in the same concentration of B, the tolerant cultivar was able to reduce the intracellular B concentration to approximately half that of the sensitive cultivar. Secondly, it was reasoned that if more B was accumulated in the apoplast of the tolerant cultivar, then it should be more quickly released by washing of the leaf; this was confirmed. Thirdly, it was shown that the same efflux transporters that were responsible for B export from the root were also highly expressed in leaves of tolerant cultivars of wheat and barley. The combination of these three experiments provided compelling evidence that redistribution of B in the leaf was a significant factor in B tolerance.The elution experiment also highlighted the fact that because B is highly soluble and has high membrane permeability, it can easily be washed from leaves. Obviously in the field B could be removed from leaves by rain, but no positive effect of this on growth had been quantified. In our experiments, we simulated the average rainfall during the early growing season in a high B region of Southern Australia by spraying plants with calibrated amounts of water for 16 d. At high B concentrations, rain reduced leaf B by around 50% while simultaneously improving growth of shoots by up to 90%. Rather surprisingly, the rain treatment, which had no significant effect on root B concentrations, caused a two-fold increase in root growth, presumably by improving the supply of photosynthate from the shoot.This study has enabled an evaluation of the importance of three main factors in determining the severity of B toxicity; two genetically determined processes, efflux pumping of B in roots and leaves, coupled with the variable leaching of B from leaves by rain (Fig. 1). The results also provide an explanation for the poor correlations observed between toxicity and shoot B concentrations in cereals.^7,11Open in a separate windowFigure 1Summary of processes contributing to reduced B toxicity in wheat and barley. The intensity of shading indicates the level of B in different regions of the plant. Boron (B) enters the leaf via the xylem and continues to accumulate as the leaf grows. When plants are grown in high concentrations of B, the older parts of the leaf become necrotic first while the younger basal tissues continue to expand. In tolerant cultivars, B efflux transporters in leaves pump B from the cytoplasm where it is toxic into the cell walls where it can be tolerated at high concentrations. Sensitive cultivars have a very low capacity for B efflux and therefore retain much higher concentrations inside the cell than in tolerant cultivars. rain can remove large amounts of B from leaves, thereby alleviating toxicity. In roots of tolerant cultivars, the same B efflux transporters that occur in leaves are used to pump B from the cells into the external medium. This reduces the toxicity to roots and limits the amount of B entering the xylem and reaching the leaves.     

Manipulation of in Vivo Sorbitol Production Alters Boron Uptake and Transport in Tobacco

Recent evidence that some species can retranslocate boron as complexes with sugar alcohols in the phloem suggests a possible mechanism for enhancing boron efficiency. We investigated the relationship between sugar alcohol (sorbitol) content, boron uptake and distribution, and translocation of foliar-applied, isotopically enriched ¹⁰B in three lines of tobacco (Nicotiana tabacum) plants differing in sorbitol production. In tobacco line S11, transformed with sorbitol-6-phosphate dehydrogenase, the production of sorbitol was accompanied by an increase in the concentration of boron in plant tissues and an increased uptake of boron compared with either tobacco line A4, transformed with antisense orientation of sorbitol-6-phosphate dehydrogenase, or wild-type tobacco (line SR1, zero-sorbitol producer). Foliar application of ¹⁰B to mature leaves was translocated to the meristematic tissues only in line S11. These results demonstrate that the concentration of the boron-complexing sugar alcohol in the plant tissue has a significant effect on boron uptake and distribution in plants, whereas the translocation of the foliar-applied ¹⁰B from the mature leaves to the meristematic tissues verifies that boron is mobile in sorbitol-producing plants (S11) as we reported previously. This suggests that selection or transgenic generation of cultivars with an increased sugar alcohol content can result in increased boron uptake, with no apparent negative effects on short-term growth.     

Boron distribution and mobility in navel orange grafted on citrange and trifoliate orange

Background and Aims

In China, boron (B) deficiency is frequently observed in citrus orchards, and is responsible for considerable loss of productivity and quality. A better understanding of B distribution and remobilization within orange plants is important for developing programs in rational fertilization and effective mitigation of B-deficiency. In the present study (i) the distribution of newly absorbed B and (ii) the translocation of foliar-applied B in ‘Newhall’ navel orange grafted on citrange and trifoliate orange was investigated.

Methods

¹⁰B was applied in the nutrient solution or sprayed on the lower-old leaves of both grafted plants for 35?days.

Results

In the ¹⁰B uptake experiment, citrange-grafted plants showed higher newly acquired total B content and B concentration in both lower-old and upper-old leaves than those in trifoliate-orange-grafted plants. The newly absorbed B in the new leaves was much higher than that in the lower-old leaves and the upper-old leaves in both grafted plants. Foliar application of ¹⁰B to the lower-old leaves resulted in B translocation to the upper-old leaves and the new leaves with preference mainly to the new leaves in both citrange and trifoliate orange when root B supply was relatively low. However, ¹⁰B sprayed to the lower-old leaves not only did not increase the abundance percentage of ¹⁰B in the root, but also reduced B concentration and the total B content in the root.

Conclusions

The results suggest that foliar-applied B can be translocated within both grafted plants, which might also depress B uptake from root medium with low B supply. Rootstock can affect the B distribution in old leaves in navel orange, and newly absorbed B was preferentially transported to the new leaves.     

Effects of boron toxicity on root and leaf anatomy in two Citrus species differing in boron tolerance

Key message

Typical toxic symptom only occurred in B-toxic C. grandis leaves. B-toxicity induced PCD of C. grandis leaf phloem tissue. The lower leaf free B might contribute to the higher B-tolerance of C. sinensis.

Abstract

Seedlings of ‘Xuegan’ (Citrus sinensis) and ‘Sour pummelo’ (Citrus grandis) differing in boron (B)-tolerance were irrigated with nutrient solution containing 10 (control) or 400 (B-toxic) μM H₃BO₃ for 15 weeks. Thereafter, the effects of B-toxicity on leaf photosynthesis, chlorophyll, plant B absorption and distribution, root and leaf anatomy were investigated to elucidate the possible B-tolerant mechanisms of Citrus plants. Typical toxic symptom only occurred in B-toxic C. grandis leaves. Similarly, B-toxicity only affected C. grandis photosynthesis and chlorophyll. Although total B concentration in B-toxic roots and leaves was similar between the two species, leaves from B-toxic C. grandis plant middle had higher free B and lower bound B as compared with those from C. sinensis. Effects of B-toxicity on leaf structure were mainly limited to the mesophyll cells and the phloem of leaf veins. Although irregular cell wall thickening was observed in leaf cortex cells and phloem tissue of B-toxic C. grandis and C. sinensis leaves, exocytosis only occurred in the companion cells and the parenchyma cells of B-toxic C. sinensis leaf phloem. Also, B-toxicity induced cell death of phloem tissue through autophagy in C. grandis leaf veins. B-toxicity caused death of root epidermal cells of the two Citrus species. B-toxicity restrained degradation of middle lamella, but did not alter ultrastructure of Golgi apparatus and mitochondria in root elongating zone cells. In conclusion, C. sinensis was more tolerant to B-toxicity than C. grandis. The lower leaf free B and higher bound B might contribute to the higher B-tolerance of C. sinensis.     

Comparison of boron uptake and translocation in two vetiver genotypes and evaluation of boron removal efficiency of vetiver floating islands

The objectives of this study were to identify genotypic differences in the uptake and translocation of boron (B) in vetiver (Chrysopogon zizanioides L.), a promising species in B phytoremediation, and to determine the efficiency of vetiver floating island system in B phytoremediation. Changes in plant biomass and B uptake and translocation were determined in two vetiver genotypes, Sierra and Sunshine, cultured hydroponically in a nutrient solution and exposed to six different B concentrations for 7 d and 14 d. The efficiency of B removal by Sierra (a high B accumulative genotype) grown in floating islands was also determined. Shoot B concentration differed significantly between the two genotypes but differences in root concentration were not significant. Root to shoot B transfer and B uptake ability were higher in Sierra than in Sunshine. The uptake and translocation of B was affected by B concentration and time of exposure. Sierra plants grown in floating islands were more efficient in B removal at lower B concentrations. Most of the B removed accumulated in the middle-upper sections of old leaves. Sierra is more suitable for the removal of B from wastewaters than Sunshine, especially in vetiver floating island system.     

The importance of sampling immature leaves for the diagnosis of boron deficiency in oilseed rape (Brassica napus cv. Eureka)

Plant analysis can diagnose boron (B) deficiency when the standards used have been properly developed by establishing that a close relationship exists between B concentration in a plant part and its physiological function. The purpose of the present study was to demonstrate the importance of choosing the growing immature leaves for B deficiency diagnosis and for establishing critical B concentrations for the diagnosis of B deficiency in oilseed rape (Brassica napus). In Experiment 1, the plants were subject to seven levels of B supply using programmed nutrient addition, for the estimation of critical B concentrations in plant parts for shoot growth. In Experiment 2, the plants were treated with two levels of B supply in solution: 10 (+B) and 0 (-B) M B, for the estimation of functional B requirements for leaf elongation. The results showed that critical B concentrations varied amongst the plant parts sampled and decreased with leaf age. As B taken up by roots is largely phloem-immobile, B concentrations in mature leaves are physiologically irrelevant to plant B status at the time of sampling, giving rise to a significant over- or underestimation of the B requirement for plant growth. By contrast, a growing, immature leaf, in this case the youngest open leaf (YOL), was the most reliable plant part for B deficiency diagnosis. Critical B concentrations developed from both methods were comparable-i.e. 10–14 mg B kg^–1 dry matter in the YOL at vegetative growth stages up to stem elongation.     

Boron Mobility in Two Coniferous Species

In contrast to earlier beliefs, it is now known that boron (B)can be retranslocated complexed with sugar alcohols in someplant species. Conifers had been thought not to translocatesugar alcohols in the phloem. However, 1 d after applying¹⁰Benriched boric acid to shoots of Scots pine and Norway spruceseedlings, we found increases in both the amount and proportionof¹⁰B in the root systems in both species. We conclude thatB is translocated in the phloem from shoots to roots in spruceand pine, and therefore it is possible that these species retranslocateB. Copyright 2000 Annals of Botany Company Scots pine, Pinus sylvestris, Norway spruce, Picea abies, conifers, boron retranslocation, roots, stable isotopes, sugar alcohols, boron complexes, mineral nutrition, forest trees     

Transpiration rate affects the mobility of foliar-applied boron in Ricinus communis L. cv. Impala

In plant species not containing polyols, boron (B) is regarded as practically phloem immobile. This has been explained by the high membrane permeability of boric acid (BA) resulting in a rapid efflux out of the phloem and re-transport into the leaf in the xylem. The present study investigated how the xylem flow rate affects the phloem mobility of foliar-applied BA in Ricinus communis L. cv. Impala. Xylem flow rates were varied by exposure of the canopy to different levels of relative humidity (RH). In seedlings with severed hypocotyls, i.e. without xylem flow, B was highly mobile. In intact seedlings and plants, the degree of mobility and the within-plant distribution of B were strongly RH-dependent. At RH of 70% or above, up to 16–24% of the B was translocated to other plant parts, whereas at lower RH no significant movement of B was detected. Only at an intermediate RH (70–80%), did leaf-applied B accumulate in roots. At 100% RH, B transport in the xylem was significantly increased, suggesting that the build up of root pressure induced the recycling of phloem delivered B from roots to shoots. These results indicate that in R. communis phloem B mobility is not constant, but strongly affected by transpiration rates.