Plant Soil (2010) 330:281–289 DOI 10.1007/s11104-009-0199-3

REGULAR ARTICLE

Boron mobility in peanut (Arachis hypogaea L.) Sawika Konsaeng & Bernard Dell & Benjavan Rerkasem

Received: 27 July 2009 / Accepted: 12 October 2009 / Published online: 27 October 2009 # Springer Science + Business Media B.V. 2009

Abstract In most plant families, boron (B) is phloem immobile. For plants such as peanut which bury their fruit, the mechanism for B delivery and the B source for fruit and seed growth remains enigmatic. Therefore, this study aimed to establish evidence of B retranslocation in peanut and to identify its importance in plant development. In a sand culture experiment, the increase in B contents in new organs after B withdrawal and the corresponding decline in B contents in older organs was evidence of B redistribution. In a foliar 10B experiment, the 10B abundance of treated-leaves decreased and 10B was detected in leaves and flowers formed after the application of foliar B. Application of 10B to the roots for a period also provided evidence for the retranslocation of 10 B accumulated during the first growth period. The 10 B abundance in older plant parts declined and 10B appeared in new organs (flowers, pegs, leaves) that had developed after the 10B supply had been replaced by 11B. In the fourth experiment, foliar application of Responsible Editor: Richard W. Bell. S. Konsaeng (*) : B. Rerkasem Department of Plant Science and Natural Resources, Faculty of Agriculture, Chiang Mai University, Chiang Mai 50200, Thailand e-mail: [email protected] B. Dell Research Institute for Sustainable Ecosystems, Murdoch University, Perth, WA 6150, Australia

B reduced hollow heart, a symptom of B deficiency in seeds, in cv. TAG 24 from 39 to 8% and in Tainan 9 from 63 to 18%. These experiments all provide evidence for B retranslocation in peanut, but further work on the relative importance of the xylem and phloem pathways for B loading into the fruit is needed. Keywords Boron (B) . B mobility . Retranslocation . B-isotopes . Arachis hypogaea L.

Introduction For many years, since it became known that B is transported largely via the xylem (Raven 1980), a question has remained unanswered namely how do the buried peanut fruit and seed obtain their B. In an early study, Campbell et al. (1975) found that peanut fruit developed and accumulated B while buried in a B-free medium. They concluded that B must have been supplied via the phloem from the mother plant that was growing in soil with sufficient B. The idea of B mobility in the phloem received a boost with the confirmation that B can indeed be recycled via the phloem in some plants. The formation of cis-diols, such as B-polyol complexes, was proposed as the reason for B being transported in the phloem of some plants (Raven 1980) and this was later established to be the mechanism of B mobility within certain species of plants (Brown and Hu 1996). Phloem B mobility

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has usually been demonstrated in polyol-rich species such as almond, apple and nectarine (Brown and Hu 1996), celery and peach (Hu et al. 1997), or even in transgenic tobacco which had the sorbitol synthesis gene inserted into its genome (Brown et al. 1999; Bellaloui et al. 1999). However, other observations indicated that B can also be phloem mobile in polyolpoor species, for example, in deciduous trees (Lehto et al. 2004b) and white lupin (Huang et al. 2008). In the mean time, reports have continued to be published on peanut seed yield increase from foliar B fertilizer (Hobbs 1974; Nasef et al. 2006). However, in spite of the above advances, the supply route for B into the subterranean fruit of peanut remains unanswered. Therefore, this study aimed to determine if B is retranslocated from older leaves of peanut under conditions of low external B supply and what effect this recycled B has on vegetative growth and reproductive development. Four experiments were undertaken. The first examined B distribution and redistribution in peanut following withdrawal of B supply to the root. The second and the third were tracer experiments using 10B to confirm B remobilization. The fourth experiment looked at variation of B mobility between two peanut cultivars. Results of these studies should either confirm or refute the efficacy of foliar B fertilizer in peanut as well as establish whether peanut can remobilize B from its shoot to supply the buried fruit and seed.

Materials and methods Experiment 1: Boron distribution and redistribution in peanut This study was conducted to determine if B is retranslocated from the old leaves of peanut under conditions of B deficiency and withdrawal of external B supply. The experiment was set up as a randomized complete block design, with 4 replicates. Peanut (Arachis hypogaea L. cv. Tainan 9) was grown in sand culture in free-draining earthenware pots (30 cm in diameter and 30 cm deep) lined with plastic, to which a commercial Rhizobium inoculum had been applied. There were 4 plants per pot. A basal nutrient solution (pH 7) containing (µM): KNO3, 5000 (for first 7 days only); CaCl2.2H2O, 1000; KH2PO4, 500;

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MgSO4.7H2O, 250; K2SO4, 250; C6H5Fe.H2O, 10; MnSO4.H2O, 1; ZnSO4.7H2O, 0.5; CuSO4.5H2O, 0.2; CoSO4.7H2O, 0.1; NaMoO4.2H2O, 0.1, [modified from Broughton and Dilworth (1971)], with different B treatments was applied to each pot, one liter per pot twice daily, after germination. The B treatments consisted of 10 μM B throughout (BA/ BA: adequate), B withdrawn at flowering (BA/BDF), B withdrawn at pod set (BA/BDP) and nil B added to the nutrient solution throughout (BD/BD: deficient). Plants were harvested at flowering (H1: 25 days after germination), pod set (H2: 33 days after flowering), seed filling (H3: 50 days after flowering, 17 days after pod set) and seed maturity (H4: 78 days after flowering, 45 days after pod set). The number of nodes and flowers were counted at H1. The number of pegs, normal or hollow heart seed (Harris and Brolman 1966), were quantified at H2–4. Leaf blades were harvested acropetally as L1-L8, L9L16, L17-L21 and L22-L26. Samples were ovendried at 70°C for 48 h, ground to pass a 1-mm mesh, extracted by dry-ashing and the B concentration determined by the azomethine-H method (Lohse 1982). Experiment 2: Boron remobilization - foliar application of 10B This experiment aimed to follow B mobility from foliar-treated leaves. Peanut (Tainan 9) was grown in sand culture in earthenware pots with basal nutrients and Rhizobium inoculation as for experiment 1. The experiment was arranged in randomized complete block design with 4 B treatments and 4 replications. With 2 plants per pot, the peanut was grown with 10 μM B (BA: B-adequate) or without added B (BD: Bdeficient) until stage V6 (the 6th leaf was fully expanded). Half of the plants in each treatment were given foliar B at V6 by immersing leaf 1 to leaf 4 (from the base) for 20 s in 20 mM boric acid containing 95% enriched 10B (National Institute of Standards and Technology, Department of Commerce, USA). The B treatments were: BA + F 10B, BA + NF 10 B, BD + F 10B and BD + NF 10B, F designating foliar 10B application and NF designating nil foliar B. To prevent B contamination a plastic sheet was used to cover the sand during foliar application. Plants were harvested before 10B application (harvest 1), 1 day after 10B application (harvest 2) and at flower-

Plant Soil (2010) 330:281–289

ing (13 days after 10B treatment, harvest 3). Plants were separated into the main stem, lateral branches and roots. At harvests 1 and 2, leaf blades + petioles of leaf 1–4 from the main stems were sampled. Additionally, there were leaf 8–11 and flowers at harvest 3. Samples were dried and ground as in experiment 1, and digested in nitric acid in a microwave oven (CEM Mars5, CEM Corp., USA) (Huang et al. 2004). Beryllium (Be) was added as an internal standard and the 10B: 11B isotope ratio was determined with an inductively coupled plasma-mass spectrometer (ICP-MS, Elan 6000, Perkin-Elmer Sciex). Experiment 3: Boron remobilization—10B applied to the roots This experiment in solution culture aimed to follow the transport of B taken up by roots. The basal nutrients were similar to those used in the sand culture in experiments 1 and 2. Stock solution of each element was prepared with B-free water (Triple Deionized Water). The B-free water was also used for the preparation of the solution culture. Peanut seeds (cv. TAG 24) were germinated in paper towels moistened with B-free water. The 1-week old seedlings were transplanted to basal solution culture without B for 2 weeks. The seedlings were then transferred to basal solution culture containing 10B (prepared from boric acid 99.52 atom % 10B enriched, Eagle Ep Picher, Boron Department, USA) at either 10 µM (10BA: 10B-adequate) or 0.1 µM (10BD: 10Bdeficient) until V5 (the 5th leaf is fully expanded (10 days after starting the treatment), when harvest 1 (H1) was taken. After H1, the remaining plants were transferred into solution culture with 11B (prepared by boric acid 99.27 atom% 11B enriched, Eagle Ep Picher, Boron Department, USA), and the B treatments were imposed. Half of the plants in the 10BA treatment were transferred to 0.1 µM 11B (10BA → 11 BD). The remainder of the 10BA and all the 10BD plants were supplied with 11B at their original B concentrations (10BA → 11BA and 10BD → 11BD) as before the transfer. Plants were harvested 10 days after transfer [H2; flowering (R1)] and 31 days after transfer [H3; pod filling (R5)]. Plants were subdivided into old leaves, new leaves, flowers (only at harvest 2) and pegs (only at harvest 3). Samples were prepared and analysed as in experiment 2.

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Experiment 4: Comparison of B mobility between two cultivars of peanut This experiment was set up to compare B mobility in two peanut cultivars. Tainan 9 and TAG 24 were grown in sand culture, 2 plants per pot, with Rhizobium and basal nutrients as in experiments 1 and 2. The plants were supplied with nutrient solution without added B (BD, deficient) after germination until R3 (peg formation) when each cultivar were given foliar B treatments of foliar B (F) and nil foliar B (NF), 4 replicate pots each. The foliar B treatment was applied by painting 20 mM B as boric acid (H3BO3) solution onto mature leaves everyday for 7 days, while the nil foliar B received painting of DI water at the same time. So, the B treatments were as follows: BD + F (with foliar B) and BD + NF (nil foliar B). A plastic sheet was used to prevent B contamination of the sand during foliar B application. Plants were harvested at the end of foliar B application and at seed maturity (R7- 8). The plants were separated into the main stem, lateral branches, reproductive parts and roots for determination of B contents as described in Experiment 1. Statistical analysis The data were submitted to an analysis of variance (ANOVA). Boron content in plant parts and the percentage of B isotope abundance are given as means ± standard error.

Results Experiment 1: Boron distribution and redistribution in peanut Symptoms of B deficiency did not appear in BD/BD plants until day 42 (peg formation). The youngest mature leaves (YFEL and YFEL + 1) of these plants formed water-soaked areas where the B concentration was less than 9 mg B kg DW−1. The BA/BDF peanut also developed this B deficiency symptom after withdrawal of B for 26 days, but only in the YFEL. The BA/BA plants produced seed that was almost all normal; while plants not supplied with B (BD/BD)

284

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produced seed with 85.4% hollow heart, the internal cavity abnormality typical of B deficiency in peanut (Harris and Brolman 1966). Withdrawing B at flowering caused 53.4% hollow heart, but later B withdrawal at pod set resulted in only 10.3% of the seed being defective. The number of pods and seeds in BD/BD plants were also only a fraction on those in BA/BA. Withdrawing B at flowering halved the number of mature pods and depressed seed number by 60%. However, B withdrawal at pod set actually increased the number of mature pods by almost 50% but had no effect on seed number (Table 1). Evidence of phloem-boron mobility in peanut was found in the decline of the B content of leaf blades of old leaves, L1-8 from the base (Table 2). This happened in B-adequate (BA/BA) plants, which lost 75% of their leaf blade B content, and in B-deficient (BD/BD) plants, which lost 66.9% B from the leaf blades of their 8 oldest leaves by seed maturity. When B was withdrawn from the nutrient solution, the B decline in old leaflets was detected in the harvests following B withdrawal, both at flowering and pod set. At the same time, B continued to accumulate in new leaflets that had subsequently developed following B withdrawal and in the seed at maturity (Table 3).

Table 1 The effect of B withdrawal on pod number, seed number and % hollow heart in seed of peanut cv. Tainan 9 at harvest 4 (seed maturity: 28 days after flowering, 45 days after pod set), Experiment 1 Treatmenta

Mature pods plant−1

Seed number plant−1

Hollow heartb (%)

BA/BA

20.3 c

37.7 c

0.5 a

BA/BDP

29.0 d

39.6 c

10.3 b

BA/BDF

9.0 b

15.0 b

53.4 c

BD/BD

4.3 a

4.3 a

85.4 d

Analysis of variance P (Treatment) LSD0.05c

< 0.01

< 0.01

4.0

7.5

< 0.01 0.04

a

Treatment: BA/BA—supplied with 10 µM B (B-adequate) throughout; BA/BDp—B withdrawal at pod set; BA/BDF—B withdrawal at flowering; BD/BD—supplied with 0 µM B (Bdeficient) throughout

b

Arcsin transformation before analysis of variance

c

Different letters indicate significant difference at P<0.05

Experiment 2: Boron remobilization—foliar application of 10B The translocation of B is presented as changes in percent atom abundance of 10B (Fig. 1). The percentage of 10B abundance in plant parts before 10B treatment (harvest 1) was similar in all 4 B treatments (18.97±0.10%), and was similar to the natural abundance of B (19.19± 0.19%) that has been previously reported (Brown et al. 1992). The percent abundance of 10B of the various parts of both B-adequate and B-deficient plants that had not been given foliar B all remained in the same range as the natural B abundance throughout the experimental period (Fig. 1a, b). One day after 10B was applied to leaves 1–4, the percent abundance of 10B in blades + petioles of treated leaves increased to 33.4±2.7% in B deficient plants (Fig. 1d) and 28.8±1.5% in B adequate plants (Fig. 1c). Thirteen days after 10B application, percent abundance of 10B declined in leaves 1–4. Leaves 8-11 and flowers produced after the application of foliar 10 B had percent abundance higher than those without foliar B treatments. Experiment 3: Boron remobilization—10B applied to the roots The first B deficiency symptoms, water-soaked areas in the young leaves, appeared in plants grown continuously without B (BD), 5 days before H2. Five days later, roots and shoots were visibly stunted. Less severe foliar symptoms of B deficiency occurred in plants which had been transferred from B-adequate to B-deficient solution (10BA → 11BD), ten days after they appeared in BD plants. At flowering, 30 days after withdrawal of B, plants which had received 10 µM B before withdrawal were able to produce flowers but fewer than in plants which received 10 µM B continuously (data not shown). Moreover, only a very small number of these flowers produced pegs, and none progressed to pods. Plants grown in continuous 0.1 µM B produced some flowers, but they were abnormal and did not develop into pegs or pods. The data on percent abundance of 10B isotope and 10 B content suggest that some of the 10B taken up was remobilized into the new growth in all three B treatments. The % abundance of 10B declined in old leaves and 10B accumulated in new growth such as

Plant Soil (2010) 330:281–289 Table 2 Boron content in older leaf blades (L1-L8) of peanut cv. Tainan 9 at four harvests of Experiment 1 showing decline over time. Values are mean of four replications ± SE

a

Treatment—see Table 1

285 Treatmenta

Stage of growth (days from germination in brackets) Flowering (25) Pod set (58) Boron content (µg plant−1)

Seed filling (75)

Maturity (103)

BA/BA

40.0±2.6

44.4±4.2

42.7±2.4

11.5±1.2

BA/BDp

48.9±3.2

39.9±3.6

26.7±2.2

11.1±1.2

BA/BDF

44.9±3.5

20.5±0.7

21.5±1.8

7.9±0.7

BD/BD

21.5±0.9

18.8±2.3

14.8±2.2

6.9±0.3

new leaves, flowers and pegs that had developed since the transfer to 11B (Fig. 2). The same pattern was also found in 10B content (Fig. 3). The 10B content in flowers of 10BA → 11BD plants was the same as that in 10BA → 11BA plants. The percent of 10 B abundance in the flowers in the B-deficient plants was about twice that in B-adequate plants. An even higher percent of 10B abundance was found in the flowers and pegs that had developed since the withdrawal of B supply. Experiment 4: Comparison of B mobility in two peanut cultivars No effect of B treatment was evident in the two peanut cultivars at H1 and H2. By H3 (83 days after germination), differential response to B between the two cultivars became evident. The two cultivars responded differently to the foliar B application in the number of mature pods/plant and percent hollow heart (P<0.05−0.01) (Table 4). Although it had no effect on the number of mature pods in Tainan 9,

foliar B application partially alleviated the adverse effect of B deficiency on percent hollow heart seed. Without foliar B, TAG24 produced more mature pods and seed that had less incidence of hollow heart than in Tainan 9. Foliar B almost doubled the number of mature pods in TAG24 and depressed hollow heart from 39 to 8%. At 7 days following foliar B treatment, the B content in treated leaves had increased by about 60% in Tainan 9 and almost tripled in TAG 24. By 16 days following the foliar B treatment, B-treated leaves in Tainan 9 had lost about half of the applied B, while TAG 24 had lost all of the applied B and the B content in its treated leaves dropped to the same level as before the foliar B treatment. By contrast, the B content of old leaves of both cultivars in the same position as the treated leaves that did not receive foliar B remained constant through the three harvests (Fig. 4a). At the same time, the B content of the new leaves and seed developed since foliar B treatment was elevated relative to those in the untreated plants of both cultivars (Fig. 4b, c).

Table 3 Boron content in new leaf blades formed at three stages during reproduction and in seeds at seed maturity of peanut cv. Tainan 9, Experiment 1. Values are means of four replications ± SE Treatmenta

Leaf numberb, period of development L9-L16 L17-L21 Flowering—pod set Pod set—seed filling Boron content (µg plant−1)

Seeds at seed maturity L22-L26 Seed filling—maturity

BA/BA

98.9±4.0

44.1±2.5

33.4±1.1

511.3±18.9

BA/BDP

92.7±5.5

15.7±0.9

15.4±1.8

333.3±37.2

BA/BDF

38.8±1.5

18.6±0.8

15.5±0.5

19.5±2.1

BD/BD

28.4±2.3

15.1±0.8

19.6±4.4

3.7±0.2

a

Treatment—see Table 1

b

Leaves numbered acropetally

Plant Soil (2010) 330:281–289

Fig. 1 10B isotope abundance in above ground parts of peanut cv. Tainan 9 plants. Bars are means of four replicates with one standard error. Treatments: a) BA + NF 10B - supplied 10 µM B to rooting medium without foliar 10B; b) BD + NF 10B supplied nil B to rooting medium without foliar 10B; c) BA + F 10B - supplied 10 µM B to rooting medium with foliar 10B; d) BD + F 10 B - supplied nil B to rooting medium with foliar 10 B Plant part: 1) Leaves 1– 4 (blades + petioles); 2) Leaves 8–11 (blades + petioles); 3) Flowers. HarH1- 2 hours vests: before foliar 10B applicaH2—1 day after tion, 10 B application and H3—13 days after 10B application

% Abundance of 10B

286

a)

BA + NF 10B

c)

BA + NF 10B

b)

BA + NF 10B

d)

BA + NF 10B

□

■

Plant part

Discussion All four experiments in this study provided evidence in support of the hypothesis that B was indeed retranslocated from older tissues into newer ones in peanut. The B contents of newer tissues, younger leaves and stem tissues, flowers and buried pods and seed, continued to increase after the external B supply to the roots or as foliar application had been curtailed. This was invariably accompanied by a decline of B in older tissues. Tracer 10B, applied to the roots or leaves, confirmed the movement of B from older leaves into newer tissues. In Experiment 1, the effect of B treatments became evident after the mid reproductive stage. Boron deficiency depressed seed yield and quality. In this study, B deficiency symptoms occurred on leaves with B concentration less than 9 mg B kg DW−1. This is low when compared to a previous report of Bell et al. (1990) which found the critical B concentration for diagnosis of B deficiency was 12 mg B kg DW−1. Boron remobilization was clearly indicated by the decline in B content in older leaves, especially after withdrawal of B supply to the root. However, the remobilized B may not necessarily account for

all the B found in new leaves and seeds that developed after B withdrawal. After B withdrawal at pod set, the peanut plants still accumulated more than 300 μg B plant−1, some nine times the amount of B lost from old leaves. Some authors have suggested that a considerable amount of watersoluble B may be present in the stem and some of the B might be donated to the xylem fluid (Matoh and Ochiai 2005). Thellier and collaborators (1979) suggested a technique for studying B in plants with the use of stable isotopes of B. Later, tracer experiments became widespread with both root (Marentes et al. 1997; Bellaloui and Brown 1998; Matoh and Ochiai 2005) and foliar application (Perica et al. 2001; Lehto et al. 2000, 2004a, 2004b), providing evidence for the movement of B within plants (Brown et al. 1992). In the current study, tracer experiments with the isotope 10B confirmed B retranslocation in peanut. This was shown by the decline in the % abundance of 10 B from where it had accumulated from root uptake or from leaves, to which B had been directly applied, accompanied by detection of 10B in newer leaves, flowers and pegs that had developed subsequently. In addition, further proof of B retranslocation was

Plant Soil (2010) 330:281–289

287

c)10BD → 11BD

Fig. 2 The percent abundance of 10B at 3 harvests in leaves, flowers and pegs of peanut cv. TAG 24 grown hydroponically in 10 B and then transplanted into 11B at V5. Bars are means of three replicates with one standard error. a) 10BA → 11BA - seedlings (1-week-old) grown in basal solution culture containing 10 µM 10 B and transferred into solution culture containing 10 µM 11B; b) 10BA → 11BD—seedlings grown in basal solution culture containing 10 µM 10B and transferred into solution culture with 0.1 µM 11B; c) 10BD → 11BD—seedlings grown in basal solution culture containing 0.1 µM 10B and transferred into H1 – V5 solution culture with 0.1 µM 11B. Harvests: (Vegetative growth, before 11B treatment); H2 – R1 (FlowerH3 – R5 (Pod filling, ing, 10 days after 11B treatment) and 31 days after 11B treatment)

■

□

provided by the decline in 10B contents of old leaves, and the presence of 10B in new leaves, flower and pegs that had developed after transfer to the nutrient solution with 11B. That this happened in plants that were both B adequate and B deficient (in experiment 1 with sand culture with B withdrawal at flowering or pod set and experiment 3 with 10B tracer) suggests that remobilization of B from old to new organs may be part of the growth and development process in peanut. Furthermore, from the root application experiment, the percent abundance of 10B and 10B content of leaves 1–6 decreased and there was 10B in the new growth (new leaves and reproductive organs) of all treatments when plants grew until R1 and R5. The decline in 10B in stems + petioles of old leaves may be a consequence of stored B in these tissues having been reloaded into the xylem (Raven 1980; Shelp et al. 1998). Huang et al. (2001) demonstrated that a substantial amount of B in the basal parts of wheat is

a)10BA → 11BA

content (µg plant-1)

b)10BA → 11BD

transported to reproductive organs, the developing ears, even though concurrent B from the rooting solution is the major source of B. Moreover, plants which were grown in the B withdrawal condition were able to use the retranslocated-B for flower production, but this B was not sufficient to maintain reproductive growth of the wheat ear. On the other hand, supply via the phloem to the buried fruit can be the only explanation for the response to foliar B in the number of mature pods, seed yield and % hollow heart in both peanut cultivars, Tainan 9 and TAG 24. Thus, these results support the early report of phloem B mobility in peanut (Campbell et al. 1975). The results also explain how peanut yield was increased by 60 % after foliar B application at the rate of 0.6 kg B ha−1 (Hobbs 1974). The two peanut cultivars, which are both Spanish type, showed evidence of B retranslocation, in the

b)10BA → 11BD

10B

% Abundance of 10B

a)10BA → 11BA

c)10BD → 11BD

Fig. 3 Content of 10B (µg plant−1) at 3 harvests in leaves, flowers and pegs of peanut cv. TAG 24 grown hydroponically in 10 B and then transferred into 11B at V5. Bars are means of three replicates with one standard error. a) 10BA → 11BA—seedlings (1-week-old) grown in basal solution culture containing 10 µM 10 B and transferred into solution culture containing 10 µM 11B; b) 10BA → 11BD—seedlings grown in basal solution culture containing 10 µM 10B and transferred into solution culture with 0.1 µM 11B; c) 10BD → 11BD—seedlings grown in basal solution culture containing 0.1 µM 10B and transferred into H1- V5 solution culture with 0.1 µM 11B. Harvests: (Vegetative growth, before 11B treatment); H2 – R1 (FlowerH3 – R5 (Pod filling, ing, 10 days after 11B treatment) and 31 days after 11B treatment)

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□

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Table 4 The effect of foliar B application on reproductive growth of 2 peanut cultivars at seed maturity (83 days after germination), Experiment 4

Treatment

Cultivar

Foliar B

Tainan 9

Nil foliar B

Mature pod (number plant−1) 5.8 aba

Seed yield (g plant−1) 6.5

Hollow heart (%) 17.8 b

TAG 24

12.0 c

4.2

7.7 a

Tainan 9

4.5 a

3.6

63.1 d

TAG 24

6.8 b

3.1

38.8 c

Analysis of variance

a

Different letters indicate significant difference at P< 0.05

b

Values in parentheses are LSD (P<0.05)

P (Cultivar)

< 0.01

< 0.05 (1.1)b

< 0.01

P (Treatment)

< 0.01

< 0.01 (1.1)

< 0.01

P (Cultivar x Treatment)

< 0.05 (1.8)

ns

< 0.01 (4.0)

decline of B contents of old leaves and the corresponding increase in B in the new growth, but there were clear differences in the magnitude of these responses between the cultivars. Tainan 9, introduced from Taiwan in 1972, is a medium seeded, high yielding cultivar (Vorasoot et al. 2003), whereas TAG 24 is a semi-dwarf commercial, high yielding cultivar from Australia (Patil et al. 1995; Basu and Nautiyal

B content (µg plant-1)

a) Old leaves

b) New leaves

c) Seed

Tainan 9

TAG 24

Fig. 4 Boron content (µg plant−1) within a) old leaves (leaves 7–11, foliar-treated position), b) new leaves and c) seed of 2 peanut cultivars. Bars are means of three replicates with one standard error. Treatments: BD + F—supplied nil B to rooting medium with foliar B; BD + NF—supplied with nil B to H1- Before foliar rooting medium without foliar B. Harvest: H2- End of foliar B B application (R3: Peg setting); H3- Seed maturity (16 days application (7 days after H1); after the end of foliar B application)

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□

2004). The consistently higher B remobilization in TAG 24 than in Tainan 9 corresponded with greater response to foliar B in TAG 24, which almost doubled pod yield and eliminated four-fifths of the % hollow heart. These cultivar differences could have implications for the effectiveness of foliar B application in overcoming B deficiency problems in the field. The evidence gained from this study strongly indicates that B is retranslocated in peanut regardless of the external supply and cultivar. Since phloem transport is independent of transpiration (Pate et al. 1975) and bidirectional (Pate and Gunning 1972), the movement of 10B to young vegetative tissues and developing pods is likely to have been via the phloem. However, xylem transport remains a possible route for some B translocation to the buried peanut fruit because although the peanut fruit and seed develop underground, transpiration could occur via the pod epidermis (Moctezuma 1999). Moreover, the recycling of B may deliver into the xylem according to the activation of root pressure, especially at high relative humidity as hypothesized by Eichert and Goldbach (2009). Both xylem and phloem tissue occur within the peduncle. More direct evidence of phloem B mobility can be obtained in the future by analysis of phloem and xylem sap in the main stem and peduncles. However, while the actual mechanism of B transport to the underground fruit and seed remains to be explained, these results strongly support the efficacy of foliar B application as a means to overcome B deficiency in peanut and perhaps other plants that bury their fruit. Acknowledgements The first author was awarded a Royal Golden Jubilee PhD. Scholarship supported by the Thailand Research Fund. Support from Thailand’s Commission on

Plant Soil (2010) 330:281–289 Higher Education, including a postdoctoral fellowship to the first author, is acknowledged. We thank Assoc. Prof. Dr. Sansanee Jamjod for valuable advice; Dr. Longbin Huang for seed of TAG 24, and useful suggestions for doing research on boron; Mr. Michael Smirk for assistance with B-isotope analysis by ICP-MS at the University of Western Australia; and Dr. Sittichai Lordkaew for analysis facilities at Chiang Mai University.

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