Excessive boron fertilization-induced toxicity is related to boron transport in field-grown pomelo trees

Abstract

Boron (B) is an essential micronutrient for plant growth and development; however, the process of B toxicity in citrus production is still poorly understood. We proposed a hypothesis that B toxicity in citrus trees is related to the characteristics of B transport from soil to leaf or fruit. For this, a field experiment was conducted for two treatments, control (B free or without B) and B fertilizer treatment (100 g Na₂B₄O₇·10H₂O plant^-1), to investigate the effects on plant growth, nutrient uptake, fruit yield and quality, and B transport in 10-year-old pomelo trees [Citrus grandis (L.) Osbeck cv. Guanximiyou]. Our results showed that excess B fertilization directly led to B toxicity in pomelo trees by dramatically increasing soil total B and water-soluble B contents. B toxicity induced interveinal chlorosis in leaves and decreased leaf biomass and function, resulting in a decreased 45.3% fruit yield by reducing 30.6% fruit load and 21.4% single fruit weight. Also, B toxicity induced changes in mineral elements between leaf positions and fruit parts, in which the concentrations of B, potassium, and magnesium were increased while those of nitrogen and iron were decreased. Under B toxicity conditions, fruit quality parameters of total soluble solids (TSS), TSS/titratable acidity (TA), total soluble sugar, sucrose, pH, vitamin C, and total phenol contents decreased, which were regulated by the lower carbohydrate production in new leaves and the lower transport capacity in old leaves. Moreover, B toxicity significantly increased the transfer factor and bio-concentration factor of B in pomelo plants, with higher levels in leaf organs than in fruit organs. Taken together, excess B fertilization-induced B toxicity in pomelo trees, with induced growth inhibition and nutrient disorder, results in reduced fruit yield and quality, which are related to B transport from soil to organs. The findings of this study highlight the understanding of B toxicity in citrus plants and strengthen B management in pomelo production for high yield and high quality.

Keywords: B biochemical cycle; B toxicity; citrus plants; fruit yield and quality; mineral profiles.

Figure 1

Effects of excess B fertilization on concentrations of soil total B (A) and soil water-soluble B (B) in soil depths of 0–20 and 20–40 cm. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–d) in the different soil depths.

Figure 2

Field symptoms of the control and B toxicity in pomelo trees.

Figure 3

Effects of B toxicity on leaf growth variables of relative chlorophyll content [soil plant analysis development (SPAD) A], single leaf fresh weight (B), single leaf dry weight (C), leaf water content (D), single leaf area (E), and specific leaf weight (SLW; F) in different leaf positions of new leaves with fruit (NL+F), old leaves with fruit (OL+F), new leaves without fruit (NL−F), and old leaves without fruit (OL−F) in pomelo trees. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–g) in the different leaf positions.

Figure 4

Effects of B toxicity on the concentrations of B (A), N (B), P (C), K (D), Ca (E), Mg (F), Fe (G), Mn (H), and Cu (I) in different leaf positions of new leaves with fruit (NL+F), old leaves with fruit (OL+F), new leaves without fruit (NL−F), and old leaves without fruit (OL−F) in pomelo trees. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–g) in the different leaf positions.

Figure 5

Effects of B toxicity on the concentrations of B (A), N (B), P (C), K (D), Ca (E), Mg (F), Fe (G), Mn (H), and Cu (I) in different fruit parts of pulp and peel in pomelo trees. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–d) in the different fruit parts.

Figure 6

Effects of B toxicity on the content of total soluble sugar (A), starch (B), and non-structural carbohydrate (NSC; total soluble sugar + starch; C) in different leaf positions of new leaves with fruit (NL+F), old leaves with fruit (OL+F), new leaves without fruit (NL−F), and old leaves without fruit (OL−F) in pomelo trees. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–f) in the different leaf positions.

Figure 7

Effects of B toxicity on fruit quality characteristics of total soluble solids (TSS; A), titratable acidity (TA; B), TSS/TA (C), total soluble sugar (D), sucrose (E), sucrose percent (sucrose/total soluble sugar × 100; F), pH (G), vitamin C (Vc; H), and total phenol (I) in pomelo trees. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus t-test, and statistical significance (*p< 0.05) is indicated in the different treatments.

Figure 8

Principal component analysis (PCA) of measured parameters for leaf (A), fruit (B), and leaf + fruit (C) of both control and B toxicity in pomelo trees.

Figure 9

The transfer factors (TFs) of B from soil total B to organ (A), from soil water-soluble B to organ (B), and from organ to organ (C) in pomelo trees between control and B toxicity. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–j) in the different organs.

Figure 10

The bio-concentration factors (BCFs) of B from soil to organ in pomelo trees under B toxicity conditions. The error bars indicate standard deviation, statistical analysis was carried out by ANOVA plus least significant difference (LSD) test, and statistical significance (p< 0.05) is indicated with different lowercase letters (a–e) in the different organs.

Figure 11

The structural equation model (A) and a conceptual diagram (B) showing the effects of excess B fertilization on plant growth, fruit yield, and quality in pomelo trees. Black arrows represent significant positive or negative pathways between variables. Gray dashed arrows represent insignificant associations between variables. Bold numbers beside arrows indicate the standard path coefficients. Arrow width is proportional to the strength of the relationship. R ² represents the proportion of variance explained for each dependent variable in the model.