Phosphorus application reduces aluminum toxicity in two Eucalyptus clones by increasing its accumulation in roots and decreasing its content in leaves

Abstract

Under acidic conditions, aluminum (Al) toxicity is an important factor limiting plant productivity; however, the application of phosphorus (P) might alleviate the toxic effects of Al. In this study, seedlings of two vegetatively propagated Eucalyptus clones, E. grandis × E. urophylla 'G9' and E. grandis × E. urophylla 'DH32-29'were subjected to six treatments (two levels of Al stress and three levels of P). Under excessive Al stress, root Al content was higher, whereas shoot and leaf Al contents were lower with P application than those without P application. Further, Al accumulation was higher in the roots, but lower in the shoots and leaves of G9 than in those of DH32-29. The secretion of organic acids was higher under Al stress than under no Al stress. Further, under Al stress, the roots of G9 secreted more organic acids than those of DH32-29. With an increase in P supply, Al-induced secretion of organic acids from roots decreased. Under Al stress, some enzymes, including PEPC, CS, and IDH, played important roles in organic acid biosynthesis and degradation. Thus, our results indicate that P can reduce Al toxicity via the fixation of elemental Al in roots and restriction of its transport to stems and leaves, although P application cannot promote the secretion of organic acid anions. Further, the higher Al-resistance of G9 might be attributed to the higher Al accumulation in and organic acid anion secretion from roots and the lower levels of Al in leaves.

Fig 1

Root dry weight (DW), shoot DW, and root /shoot DW ratio of DH32-29 (A, C, E, G) and G9 (B, D, F, H) seedlings subjected to different aluminum (Al) and phosphorus (P) treatments. Bars represent means ± standard deviations (n = 3). Differences among the six treatments were analyzed using 2 (Al) × 3 (P) analysis of variance. Different letters above the bars indicate a significant difference at P < 0.05.

Fig 2

Aluminum (Al) content in the root, stem, and leaf of DH32-29 (A, C, E) and G9 (B, D, F) seedlings subjected to different Al and P treatments, respectively. Bars represent means ± standard deviations (n = 3). Differences among the six treatments were analyzed using 2 (Al) × 3 (P) analysis of variance. Different letters above the bars indicate a significant difference at P < 0.05.

Fig 3

Phosphorus (P) content in the root, stem, and leaf of DH32-29 (A, C, E) and G9 (B, D, F) seedlings subjected to different Al and P treatments. Bars represent means ± standard deviations (n = 3). Differences among the six treatments were analyzed using 2 (Al) × 3 (P) analysis of variance. Different letters above the bars indicate a significant difference at P < 0.05.

Fig 4

Secretion of malate, oxalate, and citrate from the roots of DH32-29 (A, C, E) and G9 (B, D, F) seedlings treated with different levels of Al and P. Bars represent means ± standard deviations (n = 3). Differences among the six treatments were analyzed using 2 (Al) × 3 (P) analysis of variance. Different letters above the bars indicate a significant difference at P < 0.05.

Fig 5

Root enzyme activities in DH32-29 (A, C, E, G, I, K) and G9 (B, D, F, H, J, L) seedlings treated with different levels of Al and P. Bars represent means ± standard deviations (n = 3). Differences among the six treatments were analyzed using 2 (Al) × 3 (P) analysis of variance. Different letters above the bars indicate a significant difference at P < 0.05.