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Journal of Animal &Plant Sciences, 2014. Vol.21, Issue 2: 3313-3325 Publication date 30/4/2014, http://www.m.elewa.org/JAPS; ISSN 2071-7024

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Effect of boron toxicity stress on seed germination, root elongation and early seedling development of watermelon Citrullus lanatus

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Mohamed Farag and Zhang Ming Fang Laboratory of Genetics Resources & Functional Improvement for Horticulture Plant, Department of Horticulture, Zhejiang University, Hangzhou, 310029, P. R. China. E-mail: [email protected] , [email protected] Keywords: Watermelon, boron toxicity, seed germination, root elongation, root development,

ABSTRACT The objective of this study is to investigate the effect of boron stress (0, 10, 25, 50 & 100 mg/l) on germination behaviour, root elongation and early seedling stage emergence of 3 watermelon cultivars; Nabite, Zhemi2 and XiYu. Boron stress did not affect germination percentage, uniformity and time to germinate 50% seeds, but enhanced mean germination time and germination index. Absence of Boron increased germination energy, coefficient velocity and germination performance index. Zhemi2 and XiYu cultivars showed positive and negative responses to germination behaviour, respectively. Abnormality of seedlings increased along with increasing in Boron toxicity up to 25 mg/l B. Moreover, 10 mg/l B increased root elongation, total root length, surface area, tips number, project area, root dry weight, and shoot dry weight while highest boron toxicity decreased all of these parameters. Zhemi2 & Nabite cultivars were selected as the tolerant and sensitive cultivars to B toxicity stress.

INTRODUCTION Boron (B) is vital microelement for the plant with concentration that causes toxicity or deficiency disorders to many crops (Kelling, 2010). B toxicity is a critical limited factor for plant growth development and crop yield production in arid and semiarid regions worldwide (Roessner et al., 2006). Although of such great agronomic risk, scientific information is locking on B toxicity mechanism and different functions in plant, and how to use such toxicity to benefit the plant. One of remain gaps of B toxicity is its effects on watermelon crop which consider as one of the 20 significant agriculture products worldwide and China being the first producer of watermelon on the world (http://faostat.fao.org). Commonly, B toxicity

symptoms occur due to low rainfall, uncontrolled irrigation and fertilization (Roessner et al., 2006). These symptoms include Chlorotisis or necrotisis of the marginal region of mature leaves (Ozturk et al., 2010), Disruption of cell wall development, metabolic disruption by binding to the ribose moieties of NADPH, NADH and ATP, and inhibition of cell division and elongation (Reid et al., 2004) as well as osmotic imbalances, photo oxidative damage, membrane leakiness, lipid peroxidation (Eraslan et al., 2007), and inhibition of seed germination and seedling emergence (Bonilla et al., 2004), and root elongation (Reid et al., 2004). Few studies have highlighted B effects on seed germination and root growth development in early seedling stage that mostly


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are sensitive to stresses compared to other stages of plant life. B can be used to increase seed germination when seeds germinate in B solutions. For instance, germination of Themeda triandra Forsk seeds in B solutions stimulates the germination parameters, break seed dormancy, increases α-amylase activity in embryo and endosperm and enhances RNA level (Cresswell and Nelson, 1973), suggesting possible function of B in RNA metabolism (Albert, 1965). On the other hand, B is vital for shoot and root growth development because it involves in cell wall and plasma membrane structure and function (Brown et al., 2002), cell elongation, cell division (Chol et al., 2007) and development of apical root meristems (Blevins and Lukaszewski, 1998) as well as ameliorating of root elongation. B also has a role in nucleic

acid, carbohydrate, protein and indole acetic acidmetabolisms, synthesis of cell wall, integrity and function of membrane, and phenol metabolism (Goldbach et al., 2001). Therefore, B toxicity negatively affected root development. However, no studies carried on B toxicity disorders on watermelon growth development in early seedling stage. Therefore, the aim of present study was to investigate whether B toxicity can use to increase watermelon seed germination, seedling growth and root system development. Moreover, selective of B tolerant and sensitive watermelon cultivars for future studies. To the best of our knowledge, the present report is the first available report that focuses on the effect of B toxicity in seed germination, root elongation and early seedlings emergence of watermelon crop cultivars.

MATERIAL AND METHODS Three experiments were carried out at the Laboratory of Genetics Resources & Functional Improvement for Horticulture Plant, Department of Horticulture, Zhejiang University on 2012. Watermelon seeds of 3 commercial cultivars (produced by Chinese commercial seed companies) were used based

on their different growth and fruit size; small (2-3 Kg); Nabite, middle (5-6 Kg); Zhemi 2 and big (7-10 Kg); XiYu. B solutions were made up with distilled water. Boric acid (H3BO3) was used to prepare different B concentrations (0.0 mg/l; 0B, 10 mg/l; 10B, 25 mg/l; 25B, 50 mg/l; 50B and 100 mg/l; 100B) (Table 1).

TABLE 1: Chemical contents of modified Hoagland medium. Hoagland contents Concentration

(mg/l) Hoagland contents Concentration

(mg/l) NH4NO3 80 CuSO4.5H2O 0.08 KNO3 506 EDTA-Na2fe 30 Ca(NO3)2.4H2O 945 (NH4)Mo7O24.H2O 0.02 MgSO4.7H2O 493 KH2PO4 136 MnSO4.H2O 2.13 H3BO3 0, 57.2, 143, 285.95

& 572 ZnSO4.7H2O 0.22 Seed germination experiment: Watermelon seeds were surface sterilized with 70% (v/v) ethanol for 1 min and 10% (v/v) sodium hypochlorite for 20 min then soaked for 4 h in sterile distilled water. Fifty (50) seeds per replicate were germinated in between paper, in germinator under temperature of 25 ̊ ̊C ±1 and moisture of 85% in dark, according to ISTA

rules 1999 (ISTA, 1999). Eight replicates were used for each treatment. B was added according to table 1. The pH was kept constant between 6.5–6.7. Afterward, following germination parameters were recorded and calculated: Germination percentage (GP) was measured according to ISTA formula (ISTA, 1999):


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Mean germination time in days (MGT) was calculated according to the equation of (Ellis and Roberts., 1981):

Where, n is the number of germinated seeds on day D. D is the number of days counted from the beginning of germination.

Germination index (GI) was calculated according to (AOSA, 1983) using the following formula:

Germination energy (GE) was recorded on the 4th day after planting as the percentage of germinated seeds on 4th day after planting relative to the total number of seeds test. Coefficient velocity (CV) was calculated according (Edwards and Sundstrom, 2002):

Where, MGT is mean time to germination in days. Germination performance index (GPI) was calculated according to (Pill and Fieldhouse, 1982):

Where, GP is germination percentage and MGT is mean time to germination in days. Time to reach 50% germination (T50) days required to 50% germination.

Uniformity of germination (Uniformity): It was calculated as the time between 75 % and 25 % of germination, respectively, (T75 – T25). Seedling abnormality: Twenty Five (25) seeds X 4 replicates were seeded in third top in between paper for 15 days under three different B concentrations; 0, 10 and 25 mg/l, 25 ºC ±1 and 85% of Moisture, then seedling abnormality was calculated. Seedlings were classified as normal if they had at least 2 cotyledon leaves with green colour and an active growing point on the main stem and root, or abnormal if the entire epicotyl is missing, leaves colour are white and had no active point of root growth in apical meristem. Abnormal seedlings percentage was calculated as follows:

Root elongation: Seeds were seeded in between paper then putted in boxes supplied with B solutions (Table 1) at a temperature of

25 ̊C̊ ±1, 85% of Moisture (ISTA, 1999) for 16 hours in light and 8 hours in dark. 100 seeds for

each treatment were used. After 6 days of seeding, 10 seedlings of each treatment were randomly selected and then root length was measured and seedlings photos were taken. (Fig. 3B).


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Seedling emergence: Seeds were germinated in between paper for 6 days then seedlings were then transplanted to plastic boxes that contained modified Hoagland solution medium with different B concentrations (Table 1) for 21 days under the above mentioned conditions. 10 seedlings of each treatment were randomly selected, roots and shoots were separated, and the roots were then putted in the light scanner in plastic boxes containing distilled water. Root samples were scanned with an epson digital scanner (Expression 10000XL 1.0, Epson Inc. Japan) and analyzed with the WinRhizo Pro (S) v. 2009a software provided by (Regent Instruments Inc., Canada) to obtain the following parameters; total root length, surface area, project area, root volume, root diameter,

and number of tips. 10 Shoots and roots were dried in oven at 80 ºC for 24 h to determine shoot dry weight and root dry weight. Statistical analysis: Data was arranged and statistically analyzed, by Costat program, as a factorial experiment in a complete randomized block design. Standard error was calculated and analysis of variance (ANOVA) was performed on the data to determine significance difference between treatment means. The treatments means were compared using the Duncan Multiple Range test (Duncan, 1955). All the experiments were repeated at least 2 times and therefore 8 replicates were used with seed germination parameters and 10 replicates for root elongation and root growth development parameters.

RESULTS Germination Behaviour: B toxicity did not affect GP of watermelon cultivars (Figure 1A). On contrary, B toxicity enhanced MGT significantly compare to control (0 B); 50 B

followed by 100 B showed the shortest time of MGT in comparison to control (Figure 1B). Moreover, cultivars showed different response in this regard (Figure 1B). B toxicity levels were


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found to be less effective to MGT values with XiYu cultivar as compared to Nabite (Figure

1B).

B toxicity also has no effect to T50 (Table 2) while watermelon cultivars showed different responses of T50. The shortest time for T50

was obtained by Zhemi2 followed by Nabite and XiYu with significant differences between their means as show in (Table 3). Under


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different B toxicity conditions, XiYu cultivar took the longest time to reach T50 as compared

to Nabite that took the shortest time to reach T50 (Table 3).

TABLE 2: Effect of boron toxicity on germination behaviour; germination index, uniformity of germination, time to 50% germination, germination performance index, coefficient of velocity and root elongation on watermelon cultivars. B Levels (mg/l)

Germination Index

Uniformity of germination

(d)

Time to 50%

germination (d)

Germination performance index

Coefficient of velocity

Root elongation (cm)

0 18.5 ±0.54 ab 1 ±0.04 a 2 ±0.17 a 36.7 ±1.32 a 39.4 ±1.50 a 9.42 ±0.54 b 10 18.2 ±0.72 b 1 ±0.05 a 2.1 ±0.17 a 33.86 ±1.45 bc 36.3 ±1.61 bc 11.09 ±0.72 a 25 18.8 ±0.72 ab 1 ±0.04 a 2 ±0.17 a 35.7 ±1.55 ab 38.6 ±1.81 ab 8.62 ± 0.72 c

50 18.44 ±1.16 ab

1 ± 0 a 2.1 ±0.18 a 32.3 ±1.47 c 34.44 ±1.56 c 4.95 ±1.16 d

100 19 ±1.28 a 1 ±0.08 a 2.1 ±0.18 a 34.1 ±1.55 bc 35.93 ±1.73 c 1.60 ±1.28 e The values are means of eight replicates ± standard error. Different letters in each column represent significant difference at 0.05 level based on Duncan’s multiple range tests. TABLE 3 : Response of watermelon cultivars to boron toxicity on germination behaviour; germination index, uniformity of germination, time to 50% germination, germination performance index and coefficient of velocity. Watermelon Cultivars

Germination index

Uniformity of

germination (d)

Time to 50% germination

(d)

Germination performance index

Coefficient of velocity

Root elongation (cm)

Nabite 19.4 ±0.15 b 1 ±0 b 2.3 ±0.07 b 37.5 ±0.47 b 39.9 ±0.59 b 5.75 ±0.44 c Zhemi2 20.7 ±0.15 a 1.02 ±0.02 ab 1.02 ±0.02 c 39.7 ±0.84 a 42.9 ±0.96 a 7.63 ±0.51 b XiYu 15.6 ±0.17 c 1.12 ±0.06 a 3 ±0 a 26.3 ±0.66 c 27.9 ±0.74 c 8.02 ±0.54 a The values are means of eight replicates ± standard error. Different letters in each column represent significant difference at 0.05 level based on Duncan’s multiple range tests.

Additionally, highest B toxicity level increased GI as compared to the lower B toxicity (Table 2). Zhemi 2 showed the highest value followed by Nabite and XiYu (Table 3). The most effective treatment was recorded with Zhemi 2 under 100B while the lowest was obtained with XiYu cultivar under other toxicity levels (the data are not shown). Significant differences of GE were observed between cultivars; Zhemi2 was followed by Nabite then XiYu (Figure 1C). Furthermore, GE of XiYu decreased under B toxicity as compared to Nabite and Zhemi2 (Figure 3C). Extreme of B toxicity stress decreased GE compare to lower toxicity 10B that increased GE significantly (Figure 1C). 50B

with Nabite cultivar showed the highest value while the lowest value was obtained with 100B of XiYu cultivar (Figure 1C). GPI and CV values were similar to each other; XiYu showed the lowest GPI and CV values (Table 3) while absence of B showed the highest GPI and CV values (Table 2). 0B showed the highest GPI and CV values with Zhemi2 while 100B showed the lowest GPI and CV values with XiYui cultivar (the data are not shown). Generally, no significant differences were observed in regard of B, cultivars (Table 2 and 3) or interaction (the data are not shown) of uniformity.


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Seedlings growth and root development: Figure 2 and 3, and table 4 and 5 showed that lower B toxicity level 10B increased seedling growth and root development significantly compare to other levels. Root elongation was affected significantly by B toxicity. 10 B was the best treatment to increase root elongation compare to other toxicity levels (Figure 3A and

Table 2). XiYu cultivar appeared to be more tolerant to B stress in this regard while Nabite was the most sensitive cultivar to B stress (Figure 3A and Table 3). Significant differences between cultivars were obtained. The longest root length recorded with 10B with Zhemi 2 and XiYu while the lowest obtained with 100B regardless cultivars (Figure 3A).

TABLE 4: Effect of boron toxicity on root system parameters; project area, surface area, root diameter, root volume and number of tips of watermelon cultivars. B Levels (mg/l)

Project area (cm2)

Surface area (cm2)

Root diameter (mm)

Root volume (cm3)

Number of tips

0 54.7 ±5.56 b 160.1 ±17.12 b 3.7 ±0.20 b 21.1 ±2.71 a 702 ±104.26 b 10 60.1 ±4.83 a 182.7 ±14.17 a 2 ±0.22 d 11.44 ±2.02 b 1586 ±90.65 a 25 25.7 ±4.34 c 80.5 ±12.17 c 3 ±0.31 c 12.1 ± 2.57 b 466 ±66.66 c 50 21.9 ±1.87 d 73.3 ±6.02 c 4.6 ±0.13 a 8.7 ±0.93 c 69 ± 11.64 d 100 5.2 ±0.32 e 15.2 ±1.00 d 3 ±0.15 c 1.2 ±0.14 d 21 ±2.54 d The values are means of ten replicates ± standard error. Different letters in each column represent significant difference at 0.05 level based on Duncan’s multiple range tests.


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TABLE 5: Response of watermelon cultivars root system to boron toxicity; project area, surface area, root diameter, root volume and number of tips. Watermelon Cultivars

Project area (cm2)

Surface area (cm2)

Root diameter (mm)

Root volume (cm3)

Number of tips

Nabite 15.6 ±1.08 c 45.2 ±3.20 c 3.2 ±0.20 b 3.7 ±0.26 c 334.6 ±71.73 c Zhemi2 54.8 ±4.72 a 160.8 ±13.50 a 3.9 ±0.15 a 21.4 ±2.03 a 878.3 ±121.81 a XiYu 30.8 ±3.74 b 101 ±11.89 b 2.7 ±0.22 c 7.6 ±1.24 b 494.4 ±70.61 b The values are means of ten replicates ± standard error. Different letters in each column represent significant difference at 0.05 level based on Duncan’s multiple range tests. After 21 days of germination, the roots were analyzed. The data showed that high B toxicity levels have critical harmful effects on watermelon root system and its development. Further, low B toxicity 10B had promotional effects as compared to B absence. For instance, 10B increased total root length significantly (Figure 2C and 3B), surface area, number of tips, project area (Table 4), root dry weight (Figure 2B) and SDW (Figure 2A) while 50B showed the best values to increase root diameter (Table 4). Indeed, cultivars were different in their response to B toxicity and significant differences were obtained among their means under all the studied parameters (Table 5 and Figure 2C). It is noteworthy that Zhemi2 cultivar was more tolerant to B toxicity than XiYu, and Nabite which showed the sensitive behaviour in this regard (Table 5 and Figure 2C). Only exception recorded with root diameter science we found that Nabite cultivar was moderate between Zhemi2 and Xinong8 (Table 5). 10B was the optimum concentration for the most studied parameters with Zhemi2

as compared to the others; 10 B X Zhemi2 recorded the highest values with total root length (Figure 2C), surface area, number of tips and project area (the data are not shown). Absence of B was more efficient to affect root volume and project area (Table 4). However, highest toxic level decreased all the parameters with all cultivars (Table 5 and Figure 3B). For abnormality seedlings, 3 B levels were only used; 0B, 10B and 25B. It was clear that seedling abnormality percentage increased along with B toxicity increasing (Figure 2D). These results were clearer with Nabite cultivar than XiYu that showed less tolerance in this regard (the data are not shown). Finally, morphological symptoms of B toxicity in early seedling stage were observed clearly under 25, 50 and 100 B; chlorosis (yellowing) of the leaf tip, progressing the leaf margin and into the blade and necrosis of the chlorotic tissue occurs and developed along with the time (data not presented). Moreover, the length of shoots was shorter (the data are not presented).

DISCUSSION Although B toxicity is a critical biotic stress that affects agriculture crop production worldwide, the different B toxicity effects and mechanism in different crops remain unknown. Watermelon is one of the most important agriculture products in the world areas that suffer from B toxicity. This present study, tried to clarify B toxicity morphological effects on early stage of watermelon and select the most tolerant and sensitive cultivar of watermelon for future studies on China which consider

main producer of watermelon. It was found that B toxicity unaffected GP in watermelon cultivars (Figure 1A), suggesting that watermelon seeds are tolerant to the B toxicity stress during the germination stage. These results agree with those of (Roundy, 1985) who found that seed germination of tall wheatgrass and basin wildrye was unaffected by B up to 200 ppm. Similar to GP result, B stress did not affect uniformity and T50 (Table 2). The response of watermelon cultivars are different


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in such way that significant differences were observed among cultivars means with MGT (Figure 1B), GE (Figure 1C), T50, GI, CV, GPI and Uniformity (Table 3). These results prove that no relation between tolerance to B toxicity and cultivar size in seed germination stage since YiYu (big cultivar size) was the most affected cultivar by B toxicity during germination stage. No significant differences were obtained neither in regard of MGT and GE between Nabite and Zhemi2 cultivars (Figure 1B and 1C) nor between XiYu and Zhemi2 in regard of uniformity (Table 3). XiYu had possessed the longest time in regard of MGT (Figure 1B), T50 and uniformity (Table 3) and response negatively to GI (Table 3), GE (Figure 1C), CV & GPI (Table 3). Zhemi2 followed by Nabite showed the most positive response to germination parameters; MGT, GE (figure 1B and 1C), T50, GI, CV and GPI (Table 3). B toxicity negatively affected MGT of XiYu cultivar (Figure 1B), GE (Figure 1C), T50, GI, CV and GPI (Table 3). Moreover, under highest toxicity stress, the lowest values of GE (Figure 1C), CV and GPI were obtained with XiYu cultivar (Table 3). B toxicity affected Zhemi2 positively especially with MGT and T50; all B toxicity levels increased MGT and T50 as compared to control (0B) (Figure 1B and Table 3), 100 B increased GI (Table 3) while the absence of B toxicity was more effective for CV and GPI (Table 3). The previous reports of specific effects of B toxicity on seed germination are not accompanied by similar response among different crops as result of B toxicity. For example, seed germination was decreased by 15 and 20 mm of B toxicity in maize (Ismail, 2003), and inhibited by 93 µm in pea seeds (Bonilla et al., 2004). The inhibition reached up to 96% with 200 ppm B and 100% with 400 ppm B in cotton (Sulochana, 1952). In addition, (Bañuelos et al., 1999) noted that increasing of B toxicity resulted in low GP for six corn, carrot and tomato genotypes. On contrary, B toxicity had no effect on germination percentage in wheat (sheikh and Khanum, 1976), green gram, barley and sesame

(Khudairi, 1961). GP was not affected in tall wheatgrass and basin wildrye under 200 ppm B, (Roundy, 1985). In the present study, B was found to have no effect on GP of different watermelon cultivars (Figure 1A). Likewise, no effect of B toxicity was observed on GP between two grasses (Roundy, 1985), seven bread cultivars (Paull et al., 1988) or nine durum wheat genotypes (Yau and Saxena, 1997). The conflicts between the results of our study and the previous papers may be attributed to the genetic characteristics of the tested material and/or the differences in the experimental procedures (Jensen, 1951). Although B toxicity did not affect GP, B toxicity was found to stimulate the germination processes because it enhanced MGT (Figure 1B) and GI (Table 2) as compared to absence of B. This may be attributed to increasing RNA levels, and the α-amylase activity in the embryo and endosperm tissue followed by a decrease in starch and sugars levels as well as an increase in sucrose by B toxicity (Cresswell and Nelson, 1973). The possible implication of B in RNA metabolism was also recorded by (Albert, 1965). On the another development stage, results in Tables 4 and 5, and Figures 2 and 3 showed that low B toxicity had a promotional effects for watermelon seedling growth and root development contrary to high B toxicity. In the earliest stage of root growth, this study found that low toxicity increased root elongation rapidly during the first 6 days of germination then increased slightly up to 21 days (Tables 4 and 5, and Figure 3A and 3B). Similar results were obtained previously by (Albert and Wilson, 1961). Moreover, it was clear that high B toxicity levels inhibit root elongation (Figure 3A, and Table 4 and 5). Rapid inhibition of root elongation is one of main symptoms of B toxicity since it is well known that root apex is the critical site for B toxicity sensing and expressing (Reid et al., 2004). Moreover, recent manuscripts showed that B cross-links the pectic polysaccharide and the network, therefore physiological and biochemical traits


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of plant cell walls affected (O'Neill et al., 2004). These results is in agreement of those of (Bonilla et al., 2004). The typical effects of B toxicity, as its capacity of complexing intracellular pyridine nucleotide coenzymes (Loomis and Durst, 1992), or its adverse effects on cell division (Chol et al., 2007) could explain those results. One of key parameters that determine B toxicity is dry weight. It was found that low B toxicity increased dry matter of watermelon as compared to other B toxicity levels (Figure 2A and 2B). These findings agree with those of (Sakya et al., 2002) who mentioned that increasing B toxicity from 0.03 to 8.35 uM led to increase in dry weight by 11 fold correspondingly. However, dry weight of both shoots and roots decreased with increasing B toxicity starting from 25B in all cultivars (Figure 2A and 2B). Similar results were recorded with cotton (Sulochana, 1952), upland rice and corn (Fageria and Moreira, 2011). The reduction in total root length can be used as a valuable selection criteria for B tolerance (Brdar-Jokanović et al., 2010). In present work, it was noted that considerable differences in the root parameters among watermelon cultivars especially with the total root length and Zhemi2 cultivar recorded the longest total root length among watermelon cultivars, thus we selected it as the most tolerant cultivar to B toxicity while Nabite was the most sensitive cultivar (Table 4 and 5, and Figure 2C). These results indicated that watermelon cultivars respond to the same B level in different ways, as reported in rice by (Fageria and Moreira, 2011) and wheat by (Hossain et al., 2004). The ability of B tolerance in plants commonly due to the ability of plant to accumulate less B content when it expose to B toxicity stress in agriculture medium. For example, It was found that the sensitive cultivar of wheat to B deficiency accumulates high B content in roots less than the tolerant one, due to the capacity to accumulate B in sensitive cultivar (Sakya et al., 2002). Moreover, (Fageria and Moreira, 2011) found a positive linear correlation between the total root length and B

content of roots among the cultivars. Low B toxicity not only increased total root length but also all of other root parameters. Absence of B was the second best value after 10B while extreme B toxicity decreased these parameters significantly, suggesting that B deficiency is less sufficient for roots than low B toxicity but it is more sufficient than other B toxic levels (Table 4 and 5, and Figure 3B). The data in (Figure 2B, 2C, 3A and 3B, and Table 4 and 5) showed that high toxic levels of B suppressed the root emergence (Farooq et al., 2011) and caused harm effects of root system; notably decreasing and inhibition of development of total root length (Ismail, 2003), surface area, root volume, number of tips, project area (Table 4), shoot dry weight and root dry weight (Figure 2A and 2B). The previous findings can be assigned to that B content of the roots increased with increasing B concentration (Hossain et al., 2004) therefore caused inhibition of root elongation (Bonilla et al., 2004) and missing of lateral roots that eventually reduce the plant growth because of inability of the plant to up take enough requirements of nutrients and water (Bañuelos et al., 1999). Essential of B roles in roots could explain those results. B is involved in division and elongation of cell (Gupta et al., 1993), apical meristems development (Blevins and Lukaszewski, 1998), maintenance of cell wall structure (Redondo et al., 2003), influencing RNA formation (Albert, 1965), stability of the pectin fraction in cell walls (Kobayashi et al., 1999), and recovering of root growth under other stress conditions such as Ni (Yadav et al., 2009) and Al toxicity (Lenoble et al., 1996). In another physiological pathway, (Cervilla et al., 2009) recorded that high B toxicity also exhibit increases in malondialdehyde contents and hydrogen peroxide that cause membrane peroxidation and oxidative stress. Therefore, extreme of B toxicity cause significant reduction on shoot and root growth. Finally we found that increasing of B toxicity increased seedling abnormality (Figure 2D). These results are supported by those of (Rerkasem et al., 1997)).


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CONCLUSION This study results provided novel findings: (1) B toxicity unaffected GP of watermelon, but it induces other parameters of germination especially under 10B suggesting that watermelon is tolerant crop to wide range of B stress during seed germination stage compare to other crops, (2) Low B toxicity increased the shoot and root growth and development compared to the other B concentrations, Therefore It could be used to enhance plant growth and development in early seedling stage,

but not in long term treatment. In addition, this fact open the door to study possibility of grow short age leaf crops under B toxic environment, (3) Zhemi2 is the most tolerant watermelon cultivar to B toxicity, thus we strongly recommend it to be cultivated in arid and semi-arid regions that suffer of B toxicity in China and also it can be a good starting material for breeding B tolerant watermelon genotypes. On other hand Nabite cultivar is appropriate to grow in B deficiency areas of the world.

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