Abstract

Foliar application may be used to supply boron (B) to a crop when B demands are higher than can be supplied via the soil. While B foliar sprays have been used to correct B deficiency in sunflower (Helianthus annuus L.) in the field, no studies have determined the amount of B taken up by sunflower plant parts via foliar application. A study was conducted in which sunflower plants were grown at constant B concentration in nutrient solution with adequate B (46 µm) or with limited B supply (0·24, 0·40 and 1·72 µm) using Amberlite IRA‐743 resin to control B supply. At the late vegetative stage of growth (25 and 35 d after transplanting), two foliar sprays were applied of soluble sodium tetraborate (20·8 % B) each at 0, 28, 65, 120 and 1200 mm (each spray equivalent to 0, 0·03, 0·07, 0·13 and 1·3 kg B ha–1 in 100 L water ha–1). The highest rate of B foliar fertilization resulted in leaf burn but had no other evident detrimental effect on plant growth. Under B‐deficient conditions, B foliar application increased the vegetative and reproductive dry mass of plants. Foliar application of 28–1200 mm B increased the total dry mass of the most B‐deficient plants by more than three‐fold and that of plants grown initially with 1·72 µm B in solution by 37–49 %. In this latter treatment, the dry mass of the capitulum was similar to that achieved under control conditions, but in no instance was total plant dry mass similar to that of the control. All B foliar spray rates increased the B concentration in various parts of the plant tops, including those that developed after the sprays were applied, but the B concentration in the roots was not increased by B foliar application. The B concentration in the capitulum of the plants sprayed at the highest rate was between 37 and 93 % of that in the control plants. This study showed that B foliar application was of benefit to B‐deficient sunflower plants, increasing the B status of plant tops, including that of the capitulum which developed after the B sprays were applied.

Received: 3 March 2003;; Returned for revision: 13 May 2003. Accepted: 8 July 2003; Published electronically: 21 August 2003

INTRODUCTION

Sunflower is one of the most sensitive crops to low boron (B) supply, developing characteristic B deficiency symptoms on leaves, stems and reproductive parts (Blamey et al., 1979, 1987; Bergmann, 1986; Asad et al., 2002). These deficiency symptoms first become evident on the younger leaves, which develop a bronze colour and become hardened, malformed and necrotic, the stem becomes corky, the capitulum deformed, and poor seed set results (Blamey et al., 1987). Sunflower roots are also sensitive to B deficiency as they stop their growth in <6 h after the removal of B from the growth medium (Dugger, 1983). The B requirement of many plants during reproductive growth is reputedly much higher than during vegetative growth (Gupta, 1993), a finding recently demonstrated in sunflower (Asad et al., 2002).

Fertilizers containing B have increased sunflower production on soils low in B, but problems exist in B uptake with low soil moisture or in applying B when symptoms only become evident during reproductive growth. To overcome these problems, Diggs et al. (1992) applied a foliar spray of soluble sodium borate (15·5 % B) at 1·8–16·1 % (w/v) supplying 1·1–1·4 kg B ha–1 when sunflower plants were between 40 and 50 cm high, followed by a second spray shortly before flowering. This increased seed yield by 20 %.

While the practice of B foliar application appears worthwhile in the field, such studies do not clarify the way in which B is taken up by the plant. The B from such sprays may be absorbed by the leaves or it may be deposited on the soil surface and taken up by the roots. The objective of the present study was to determine if B applied as foliar sprays to B‐deficient sunflower plants is taken up and utilized to improve vegetative and reproductive growth under solution culture conditions in which B supply to the roots is controlled and monitored.

MATERIALS AND METHODS

A solution culture study was conducted to ensure that no contamination occurred in the uptake of B by plant roots. Triple‐deionized (TDI) water was used throughout the study, and was further purified by passing drop‐wise through a B‐specific resin (Amberlite IRA‐743) column. Analytical grade chemicals were used, and the macronutrient stock solutions further purified by bubbling with Amberlite IRA‐743 for 24 h.

Seeds of sunflower (cultivar ‘Hysun 25’) were germinated by wrapping them in paper towels moistened with 1000 µm Ca(NO3)2 and placing them in the dark at 25 °C for 48 h. Ten selected seedlings were transferred to 22‐L pots lined with polyethylene bags containing aerated solutions of (µm) 563 Ca, 310 N, 83 K, 29 Mg, 29 S, 16 Fe, 11 P, 0·2 Mn, 0·1 Zn, 0·03 Cu and 0·01 Mo in a glasshouse with day and night temperatures of approx. 18 and 32 °C. During the experiment, the programmed nutrient addition technique (Asher and Blamey, 1987) was used to frequently add small amounts of all nutrients except B to each pot. The concentration of B was maintained using Amberlite IRA‐743 in acid‐washed cotton bags (see below). The final number of two plants per pot was established by thinning 4 d after transplanting (DAT). Solution pH was adjusted to 6·0 ± 0·2 every second day with analytical grade 4 % H2SO4 or 2 % NaOH.

Four different solution B concentrations were imposed, ensuring a constant supply of B to the plants via the roots. Three treatments were designed such that the plants suffered various degrees of B deficiency, plus a control treatment with plants adequately supplied with B. Amberlite IRA‐743, with a B sorption capacity of 2·16 mg B g–1 wet resin, was cleaned and loaded with 4, 12, 32 and 100 % of its B‐saturation capacity by shaking for 72 h in solutions containing B as described by Asad et al. (1997). Thereafter, the resin was rinsed with TDI water, and 15 g of wet resin was transferred to an acid‐washed cotton bag for each 22 L pot. This ensured that the solutions contained on average 0·24, 0·40 and 1·72 µm B for the first 20 d, at which time the B‐loaded resin was removed from the pots. The resin was retained in the control treatment pots, maintaining on average a concentration of 46 µm B over the entire experimental period. Twenty pots were established initially at 0·24, 0·40 and 1·72 µm B, while four pots were established at 46 µm B.

Five B foliar spray treatments were imposed using highly‐soluble sodium tetraborate (20·8 % B) at 0, 28, 65, 120 and 1200 mm B, which supplied the equivalent of 0, 0·93, 2·2, 4·0 and 40 mg B plant–1 or 0, 0·03, 0·07, 0·13 and 1·3 kg B in 100 L ha–1. The plants in the control treatment were not sprayed with B. Boron was first sprayed at 25 DAT when mild B deficiency symptoms were evident, and plants were at the late vegetative stage of growth (Growth Stage V10) (Schneiter and Miller, 1981). Boron was sprayed from the pair of leaves above the cotyledonary leaves to Leaf 10. A second B spray, which repeated the first application, was given at 35 DAT. There were four replicates of each B foliar spray treatment.

Care was taken not to contaminate the nutrient solutions with B spray by covering the pots with a polyethylene sheet when spraying the plants. A sample of nutrient solution (250 mL) was taken from each pot before and after each B spray, passed through a 2‐mL column of Amberlite IRA‐743 which was then soaked overnight in 10 mL 20 % H2SO4 to extract B for determination by inductively coupled plasma atomic emission spectrometry (ICPAES) (Asad et al., 2000). These analyses showed that no contamination of nutrient solution occurred following the B sprays applied to the plants. Indeed, the B concentration in solution declined slightly from the time of the first to the second spray due to B uptake by the plants.

At harvest (75 DAT), the most advanced plants were at Growth Stage R2–R4 (Schneiter and Miller, 1981). Plants were separated into capitulum, youngest opened leaves (YOL), young leaves, petioles of young leaves, mature leaves, petioles of mature leaves, stems and roots. The dry mass of the plant parts was recorded after drying for 48 h at 70 °C. Plant material (500 mg) was dry ashed at 500 °C for 6 h, and the ash dissolved in 10 mL 1 m HCl prior to B analysis using ICPAES.

The three B solution concentration treatments and five B spray treatments were arranged in a randomized complete block factorial design, and a comparison made with the control treatment in which an adequate amount of B was supplied. The results were statistically analysed by standard analysis of variance techniques (Gagnon et al., 1984). Significant treatment effects were separated with Fisher’s protected LSD test at P ≤ 0·05.

RESULTS

Symptoms of B disorders

As expected from previous studies (Asad et al., 2002), B deficiency symptoms were first observed at 22 DAT in plants grown at ≤0·40 µm B (i.e. with 4 or 12 % B‐saturated resin). These symptoms became more prominent by 30 DAT, and by 60 DAT, the plants sprayed with water only, that had been grown for the first 20 d in nutrient solution with ≤0·40 µm B were dead, with the exception of the stem which remained green.

Plants that were initially B‐deficient but sprayed with 28, 65 and 120 mm B at 25 DAT started to recover from B deficiency approx. 5 d thereafter. These plants, especially where only slightly B‐deficient (i.e. with 0·40 and 1·72 µm B initially in solution), continued to grow well producing healthy young leaves by 40 DAT. The older leaves, however, maintained their deformed appearance as a result of their earlier low B status. The plants produced a well‐developed capitulum where 120 or 1200 mm B sprays were applied. At lower B spray concentrations, however, the reproductive parts were small and deformed.

Leaf burn was evident within 16 h of spraying B‐deficient plants with 1200 mm B solution. This toxic effect was less pronounced on larger plants that had been grown initially with 1·72 µm B in solution. There was initially a grey, sunken area on the leaf surface, followed by a brown necrosis. Despite the leaf burn, there was good recovery of vegetative growth from B deficiency, and the plants produced healthy reproductive heads.

Effects of B foliar sprays on vegetative and reproductive growth

Vegetative and reproductive growth were poor as a result of severe B deficiency in plants grown initially at 0·24 and 0·40 µm B (Tables 1 and 2) and only slightly better with 1·72 µm B initially in solution (Table 3) and which received no B as a foliar spray. All plant parts were affected; total dry mass at the lowest two B levels was only 6–8 % of that achieved in the control treatment, while that at 1·72 µm B initially was 42 % of that achieved in the control treatment. Plants grown with 0·24 and 0·40 µm to which no B was applied as a foliar spray produced no reproductive parts.

The dry mass of a range of vegetative and reproductive parts of sunflower increased with increase in B foliar application in plants that were initially grown with low B supply (Tables 13). Total dry mass of plants grown initially with 0·24 and 0·40 µm B increased three‐ to four‐fold through B foliar sprays, but in all instances compared poorly with that of plants grown in the control treatment (Tables 1 and 2). With 1·72 µm B initially in solution, the total dry mass was approx. 60 % of that achieved in the control treatment (Table 3).

The dry mass of vegetative parts of plant tops showed a similar response pattern to B foliar spray as did the total dry mass. In all cases, there was an increase in dry mass of mature parts (i.e. stems, mature leaf blades and petioles) to which the foliar B was applied. Interestingly, the dry mass of young plant parts increased also; those parts that developed after the B sprays had been applied also increased. The effect of B sprays was especially marked with 0·24 and 0·40 µm B initially in solution (Tables 1 and 2). Only with 1·72 µm B initially present, however, was the dry mass of these parts indicative of normal growth (Table 3). But even in this instance, the dry mass of the vegetative parts was significantly lower than those of the control plant parts.

The dry mass of roots was increased by B foliar sprays, especially under conditions of severe B deficiency (Tables 13). This occurred despite evidence of no increase in solution B concentration through the application of B foliar sprays.

Treatment effects on reproductive development were of special interest in this study given that the aim of B foliar sprays in the field is to increase seed yield and the particular sensitivity of sunflower to low B supply during the reproductive stage (Asad et al., 2002). No capitulum was produced at the lowest two B concentrations initially in solution when the plants were sprayed with water only (Tables 1 and 2). In these treatments, increased B concentration in the foliar sprays increased the dry mass of the capitulum, an effect especially evident with 1200 mm B. In this case, the dry mass of the capitulum was 37 and 78 % of that achieved in the control treatment. The corresponding value with 1·72 µm B initially in solution was 93 %, which did not differ significantly from the control treatment.

Effects of B foliar sprays on B concentration in vegetative and reproductive parts

There was a low B concentration in the vegetative parts of plant tops grown with 0·24 and 0·40 µm B initially in solution which were sprayed with water only (Tables 1 and 2). This was especially evident in the stems, petioles of old and young leaves, and blades of young leaves including the YOL. Increasing the B concentration to 1·72 µm initially in solution resulted in a moderate increase in B concentration in vegetative parts of the plant tops (Table 3).

The application of B sprays resulted in a substantial increase in B concentration in vegetative parts of the plant tops (Tables 13). Not unexpectedly, the most marked effects were evident on the older plant parts which had received the B sprays. There was, however, an increase in B concentration even in the young parts which had not developed by the time that the sprays were applied. Where 1200 mm B sprays had been applied to plants growing in solutions initially containing 0·24 and 0·40 µm B, the B concentration in the YOL (approx. 18 mg kg–1) was not greatly different from that in the YOL of control plants (23 mg kg–1). The same B spray treatment increased the B concentration in the YOL to 33 mg kg–1 where plants were grown in solution initially with 1·72 µm B.

In keeping with the absence of B contamination of the solutions, the B concentration in the roots was not increased by the B foliar sprays (Tables 13). Indeed, the B concentration in the roots declined slightly (though significantly) in the plants suffering the most severe B deficiency (Table 1), possibly as a result of improved root growth. In only one instance (0·40 µm B initially in solution and with a spray of 1200 mm B) was there a significant increase in B concentration in the roots as a result of a B foliar spray (Table 2). Under conditions of low B supply in solution, the B concentration in the roots was consistently lower than that in the control treatment, especially with 0·24 and 0·40 µm B initially in solution.

The B concentration in the capitulum was increased slightly by B foliar sprays (Tables 13). From the most to the least severely B‐deficient plants, the B concentration in the capitulum increased from approx. 11–23 to 31 mg kg–1, values considerably below that in the control plants (namely 63 mg kg–1). These findings indicate that the reproductive parts of plants might be B‐deficient, even where 1200 mm B sprays were used.

Calculation of an approximate mass balance of the B applied as a foliar spray (using the data in Tables 13) showed that an increase in B concentration decreased the percentage of foliar B recovered in (or on) the plant. (It cannot be assumed, of course, that the B concentration measured was present in plant tissues.) On average, 60 % of the B applied as a 28 mm B spray was recovered. This decreased to 50, 30 and 10 % with 65, 120 and 1200 mm B sprays, respectively. The reason for the low recovery of foliar‐applied B is not immediately apparent, but may have been due to the loss of B from the leaf surface as white crystals of sodium borate were visible on the leaf surface following sprays with 1200 mm B. Besides the decreased efficiency of recovery of foliar‐applied B with increasing B concentration, the efficiency of B recovery was decreased in the more severely B‐deficient plants. It is also likely that some B was lost as spray drift during the spraying process.

DISCUSSION AND CONCLUSIONS

The present results have shown that foliar B sprays applied at 25 and 35 DAT to sunflower plants of low B status increased dry matter yield (Tables 13). These findings are similar to those of Diggs et al. (1992) who reported a 20 % increase in sunflower seed yield in a field experiment. In that experiment, however, it is not certain whether the improved growth resulted directly from the foliar‐applied B or via absorption from the soil that was contaminated by the B sprays. The current study, in which no B contamination of the solution occurred, confirms that the improved B status reported by Diggs et al. (1992) resulted, at least in part, directly from the foliar application of B.

To mimic field conditions in which incipient B deficiency symptoms occur at the late vegetative to early reproductive stage (Gupta, 1993; Asad et al., 2002), the B concentration in solution was kept low and constant by the use of Amberlite IRA‐743 B‐specific resin. The resin was then removed, thus ensuring increasingly severe B deficiency without further intervention. This was clearly evident in plants which were sprayed with water only. In contrast, there were various degrees of improved plant appearance and recovery in plant growth (Tables 13) through B foliar sprays. This improved growth was accompanied by increased B concentration in plant parts, including those that developed after the sprays were applied. A number of mechanisms could explain the improved B status and growth of plants during the period after which the B sprays were applied.

First, despite our best efforts, B contamination of the nutrient solution may have occurred. This was not evident immediately following the B sprays but may have occurred later. Again, this is unlikely because (a) the pots were covered except for small areas where the plant stems emerged and to allow aeration, (b) poor root growth with severe B deficiency symptoms was evident throughout the experimental period, and (c) B contamination would have been greatest in those plants growing in low B solutions that were sprayed with water only, given the close proximity of plants in the experiment. This clearly did not occur.

Secondly, there is the possibility that sunflower may translocate B from older to younger plant parts. Several earlier studies have provided suggestions that B may be translocated in the phloem; of particular note is the study of acquisition of B by the developing fruit of peanut (Arachis hypogaea L.) and subterranean clover (Trifolium subterraneum L.) when buried in a B‐deficient white siliceous sand (Campbell et al., 1975). Translocation of B, while not common in higher plants, has been demonstrated to occur in sorbitol‐rich species including apple (Malus domestica B.), nectarine (Prunus persica L.) and almond (Prunus amygdalus B.) by Brown and Hu (1996) and in the mannitol‐rich celery (Apium graveolens L.) by Hu et al. (1997). These species readily translocate B‐polyol complexes in their phloem sap. Further studies are needed to establish whether or not polyols are present in the phloem sap of sunflower and to determine whether B sprayed onto the stem, leaves or buds is responsible for the improved B status and growth of plant parts that developed after the B sprays were applied.

This study was not designed to determine the critical B concentration in sunflower leaves for optimum growth. Nevertheless, the data presented in Tables 13 (plant dry mass and B concentration in the YOL) are in keeping with previous findings on B requirements. Bergmann (1986) reported that 10–13 mg B kg–1 in the upper fully developed leaves of sunflower was associated with severe B deficiency symptoms; in the present study, no capitula were produced by plants with ≤9 mg B kg–1 in the YOL. Furthermore, 95 % of the maximum dry mass of the capitulum was associated with 20 mg B kg–1 in the YOL, a value not greatly different to that of 25 mg B kg–1 associated with 90 % maximum yield (Asad et al., 2002). Maximum dry mass of the capitulum in control plants was achieved with 23 mg B kg–1 in the YOL (Tables 13). These values were, however, lower than the 34 mg B kg–1 in the YOL associated with optimum seed yield (Blamey et al., 1979).

Leaf burn was a problem which resulted from the use of a high concentration of 1200 mm B as a foliar spray. It is noteworthy, however, that the greatest improvement in plant growth occurred with this treatment, and no other detrimental effect of the 1200 mm B foliar spray was evident. Leaf burn was not evident when a similar concentration of B was used in the field (C. A. Diggs, pers. comm.).

The results of this study have shown that improved growth of sunflower is possible through the use of B foliar sprays. These sprays should be applied before severe B deficiency occurs (as with ≤0·40 µm B initially in solution) as these plants appear unable to adequately recover from the morphological or physiological changes resulting from their low B status. There was little recovery of roots from B deficiency (dry mass improved slightly but deficiency symptoms remained) despite the improved B status of plant tops. Furthermore, it was possible to confirm that the improved growth and increased B concentration in young plant parts was not due to B contamination of the root environment. Further studies, including the use of 10B, are required to confirm which mechanism (translocation of B applied to leaves, stem or bud) is responsible for improved B status and plant growth following B foliar application.

ACKNOWLEDGEMENTS

We thank Pacific Seeds Pty Ltd for supplying the sunflower seed used in this experiment and Ms J. Mercer for technical assistance. The senior author is grateful to The University of Queensland for a Postdoctoral Research Fellowship and for support under its Special Research Grants Scheme.

Table 1.

Dry mass of sunflower plant parts and the B concentration in these parts as affected by the application of B foliar sprays at 25 and 35 d after transplanting with 4 % B‐loaded resin in solution

B concn in spray (mmFlower heads YOL Young leaf blades Young leaf petioles Mature leaf blades Mature leaf petioles Stems Roots Total 
Dry weight (g plant–1         
 0 0·0 0·4 1·7 0·3 5·5 1·1 4·6 1·8 15·4 
 28 2·9 0·8 4·3 0·8 13·9 3·1 19·6 6·6 52·0 
 65 7·1 0·9 5·8 3·0 10·1 1·9 16·6 4·6 50·0 
 120 9·0 1·1 6·1 1·2 11·3 2·3 16·1 5·5 52·6 
 1200 13·8 0·4 5·8 1·1 9·2 1·6 16·8 4·4 53·0 
 Control 37·4 1·1 34·7 6·9 32·9 10·6 98·0 49·1 270·7 
          
 LSD (P ≤ 0·05) 2·5 0·8 0·8 1·3 2·3 0·8 2·6 1·1 7·2 
          
B concentration (mg kg–1         
 0 – 3·8 2·8 0·9 6·6 1·0 1·0 14·0 4·8 
 28 0·8 12·1 32·4 0·5 58·1 16·1 14·8 10·0 26·4 
 65 10·0 11·5 55·3 9·9 49·8 50·2 17·7 12·3 28·2 
 120 10·9 13·5 46·6 8·1 59·8 42·6 19·2 10·3 29·8 
 1200 10·9 18·5 32·0 12·5 371·4 76·2 22·0 12·8 85·5 
 Control 63·0 23·2 81·3 28·5 59·5 38·6 18·6 31·7 42·6 
          
 LSD (P ≤ 0·05) 2·1 2·8 6·0 2·7 52·0 7·7 2·7 1·7 7·0 
B concn in spray (mmFlower heads YOL Young leaf blades Young leaf petioles Mature leaf blades Mature leaf petioles Stems Roots Total 
Dry weight (g plant–1         
 0 0·0 0·4 1·7 0·3 5·5 1·1 4·6 1·8 15·4 
 28 2·9 0·8 4·3 0·8 13·9 3·1 19·6 6·6 52·0 
 65 7·1 0·9 5·8 3·0 10·1 1·9 16·6 4·6 50·0 
 120 9·0 1·1 6·1 1·2 11·3 2·3 16·1 5·5 52·6 
 1200 13·8 0·4 5·8 1·1 9·2 1·6 16·8 4·4 53·0 
 Control 37·4 1·1 34·7 6·9 32·9 10·6 98·0 49·1 270·7 
          
 LSD (P ≤ 0·05) 2·5 0·8 0·8 1·3 2·3 0·8 2·6 1·1 7·2 
          
B concentration (mg kg–1         
 0 – 3·8 2·8 0·9 6·6 1·0 1·0 14·0 4·8 
 28 0·8 12·1 32·4 0·5 58·1 16·1 14·8 10·0 26·4 
 65 10·0 11·5 55·3 9·9 49·8 50·2 17·7 12·3 28·2 
 120 10·9 13·5 46·6 8·1 59·8 42·6 19·2 10·3 29·8 
 1200 10·9 18·5 32·0 12·5 371·4 76·2 22·0 12·8 85·5 
 Control 63·0 23·2 81·3 28·5 59·5 38·6 18·6 31·7 42·6 
          
 LSD (P ≤ 0·05) 2·1 2·8 6·0 2·7 52·0 7·7 2·7 1·7 7·0 

In the control treatment, the B concentration was maintained at 46 µm B. The Amberlite IRA‐743 resin was removed from all pots, except the control, at 20 DAT and no B foliar sprays were applied to the plants in the control treatment.

Values are the means of four replications.

Table 2.

Dry mass of sunflower plant parts and the B concentration in these parts as affected by the application of B foliar sprays at 25 and 35 d after transplanting with 12 % B‐loaded resin in solution

B concn in spray (mmFlower heads YOL Young leaf blades Young leaf petioles Mature leaf blades Mature leaf petioles Stems Roots Total 
Dry weight (g plant–1         
 0 0·0 0·4 2·7 0·3 7·2 1·6 6·3 2·6 21·1 
 28 15·3 0·6 6·6 1·8 16·6 4·3 20·4 7·6 73·2 
 65 18·5 0·6 8·2 1·7 16·5 3·8 22·5 8·9 80·8 
 120 25·3 0·5 8·3 1·8 14·1 3·1 20·4 7·7 81·1 
 1200 29·3 0·6 6·2 1·8 10·3 1·9 13·8 5·5 69·2 
 Control 37·4 1·1 34·7 6·9 32·9 10·6 98·0 49·1 270·7 
          
 LSD (P ≤ 0·05) 3·8 0·3 2·4 1·2 3·5 1·2 3·2 1·7 10·0 
          
B concentration (mg kg–1         
 0 – 11·6 2·3 1·3 5·1 0·8 12·0 11·0 7·5 
 28 21·1 14·0 20·3 11·5 48·5 14·6 17·5 10·0 24·3 
 65 24·6 14·4 28·5 10·8 56·1 22·8 20·4 12·3 28·3 
 120 23·7 15·9 31·3 11·4 61·0 50·0 22·4 12·3 30·3 
 1200 22·8 18·1 26·3 24·9 569·0 48·0 26·5 16·0 106·8 
 Control 63·0 23·2 81·3 28·5 59·5 38·6 18·6 31·7 42·6 
          
 LSD (P ≤ 0·05) 2·0 2·3 2·5 4·1 18·5 6·5 2·0 2·3 12·5 
B concn in spray (mmFlower heads YOL Young leaf blades Young leaf petioles Mature leaf blades Mature leaf petioles Stems Roots Total 
Dry weight (g plant–1         
 0 0·0 0·4 2·7 0·3 7·2 1·6 6·3 2·6 21·1 
 28 15·3 0·6 6·6 1·8 16·6 4·3 20·4 7·6 73·2 
 65 18·5 0·6 8·2 1·7 16·5 3·8 22·5 8·9 80·8 
 120 25·3 0·5 8·3 1·8 14·1 3·1 20·4 7·7 81·1 
 1200 29·3 0·6 6·2 1·8 10·3 1·9 13·8 5·5 69·2 
 Control 37·4 1·1 34·7 6·9 32·9 10·6 98·0 49·1 270·7 
          
 LSD (P ≤ 0·05) 3·8 0·3 2·4 1·2 3·5 1·2 3·2 1·7 10·0 
          
B concentration (mg kg–1         
 0 – 11·6 2·3 1·3 5·1 0·8 12·0 11·0 7·5 
 28 21·1 14·0 20·3 11·5 48·5 14·6 17·5 10·0 24·3 
 65 24·6 14·4 28·5 10·8 56·1 22·8 20·4 12·3 28·3 
 120 23·7 15·9 31·3 11·4 61·0 50·0 22·4 12·3 30·3 
 1200 22·8 18·1 26·3 24·9 569·0 48·0 26·5 16·0 106·8 
 Control 63·0 23·2 81·3 28·5 59·5 38·6 18·6 31·7 42·6 
          
 LSD (P ≤ 0·05) 2·0 2·3 2·5 4·1 18·5 6·5 2·0 2·3 12·5 

In the control treatment, the B concentration was maintained at 46 µm B. The Amberlite IRA‐743 resin was removed from all pots, except the control, at 20 DAT and no B foliar sprays were applied to the plants in the control treatment.

Values are the means of four replications.

Table 3.

Dry mass of sunflower plant parts and the B concentration in these parts as affected by the application of B foliar sprays at 25 and 35 d after transplanting with 32 % B‐loaded resin in solution

B concn in spray (mmFlower heads YOL Young leaf blades Young leaf petioles Mature leaf blades Mature leaf petioles Stems Roots Total 
Dry weight (g plant–1         
 0 3·7 0·8 16·0 2·5 23·8 5·0 42·8 19·8 114·4 
 28 27·1 1·3 20·0 3·0 22·3 5·6 56·2 21·3 156·8 
 65 30·2 1·0 20·5 2·3 26·6 6·2 58·2 27·4 172·4 
 120 34·9 1·3 25·0 4·3 28·6 6·1 58·1 23·1 181·3 
 1200 34·9 1·2 23·7 3·0 23·0 6·4 57·6 20·1 169·9 
 Control 37·4 1·1 34·7 6·9 32·9 10·6 98·0 49·1 270·7 
          
 LSD (P ≤ 0·05) 4·1 0·5 3·2 1·3 5·3 1·2 10·0 4·0 15·3 
          
B concentration (mg kg–1         
 0 25·1 12·9 10·1 1·3 25·7 15·2 13·6 23·5 17·3 
 28 27·1 15·4 9·9 4·2 58·7 17·4 19·1 20·0 24·9 
 65 31·6 15·8 8·7 3·8 54·0 30·1 21·9 22·3 27·2 
 120 31·7 17·2 10·6 4·8 67·5 34·1 23·5 22·3 30·1 
 1200 30·8 32·7 29·1 11·1 409·5 60·2 26·3 26·0 80·5 
 Control 63·0 23·2 81·3 28·5 59·5 38·6 18·6 31·7 42·6 
          
 LSD (P ≤ 0·05) 1·2 2·3 3·4 1·7 27·0 4·1 2·7 3·0 5·1 
B concn in spray (mmFlower heads YOL Young leaf blades Young leaf petioles Mature leaf blades Mature leaf petioles Stems Roots Total 
Dry weight (g plant–1         
 0 3·7 0·8 16·0 2·5 23·8 5·0 42·8 19·8 114·4 
 28 27·1 1·3 20·0 3·0 22·3 5·6 56·2 21·3 156·8 
 65 30·2 1·0 20·5 2·3 26·6 6·2 58·2 27·4 172·4 
 120 34·9 1·3 25·0 4·3 28·6 6·1 58·1 23·1 181·3 
 1200 34·9 1·2 23·7 3·0 23·0 6·4 57·6 20·1 169·9 
 Control 37·4 1·1 34·7 6·9 32·9 10·6 98·0 49·1 270·7 
          
 LSD (P ≤ 0·05) 4·1 0·5 3·2 1·3 5·3 1·2 10·0 4·0 15·3 
          
B concentration (mg kg–1         
 0 25·1 12·9 10·1 1·3 25·7 15·2 13·6 23·5 17·3 
 28 27·1 15·4 9·9 4·2 58·7 17·4 19·1 20·0 24·9 
 65 31·6 15·8 8·7 3·8 54·0 30·1 21·9 22·3 27·2 
 120 31·7 17·2 10·6 4·8 67·5 34·1 23·5 22·3 30·1 
 1200 30·8 32·7 29·1 11·1 409·5 60·2 26·3 26·0 80·5 
 Control 63·0 23·2 81·3 28·5 59·5 38·6 18·6 31·7 42·6 
          
 LSD (P ≤ 0·05) 1·2 2·3 3·4 1·7 27·0 4·1 2·7 3·0 5·1 

In the control treatment, the B concentration was maintained at 46 µm B. The Amberlite IRA‐743 resin was removed from all pots, except the control, at 20 DAT and no B foliar sprays were applied to the plants in the control treatment.

Values are the means of four replications.

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Author notes

1School of Land and Food Sciences, University of Queensland, Brisbane, Queensland 4072, Australia

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