The effects of aluminium on plant growth in a temperate and deciduous aluminium accumulating species

Abstract

Aluminium (Al) is a phytotoxic element affecting the growth and yield of many crop plants, especially in the tropics. Yet, some plants are able to accumulate high levels of Al. The monogeneric family Symplocaceae represents an Al accumulating family including many tropical and evergreen species with high Al levels in their above ground plant tissues. It is unclear, however, whether Al accumulation also characterises temperate species of Symplocos, and whether or not the uptake has a beneficial growth effect. Here, we investigate if the temperate, deciduous species Symplocos paniculata is able to accumulate Al by growing seedlings and saplings in a hydroponic setup at pH 4 with and without Al. Pyrocatechol-violet (PCV) and aluminon staining was performed to visualize Al accumulation in various plant tissues. Both seedlings and saplings accumulate Al in their tissues if available. Mean Al levels in leaves were 4107 (±1474 mg kg^-1) and 4290 (±4025 mg kg^-1) for the seedlings and saplings, respectively. The saplings treated without Al showed a high mortality rate unlike the Al accumulating ones. The seedlings, however, showed no difference in growth and vitality between the two treatments. The saplings treated with Al showed new twig, leaf and root development, resulting in a considerable biomass increase. PCV and aluminon staining indicated the presence of Al in leaf, wood and bark tissue of the plants. S. paniculata shares the capacity to accumulate Al with its tropical sister species and is suggested to be a facultative accumulator. Whether or not Al has a beneficial effect remains unclear, due to developmental differences between seedlings and saplings. Al is suggested to be transported via the xylem transport system into the leaves, which show the highest Al levels. Radial transport via ray parenchyma to bark tissue is also likely given the high Al concentrations in the bark tissue.

Keywords: Accumulation; Symplocaceae; Symplocos paniculata; aluminium; aluminon; hydroponics; pyrocatechol-violet.

Figure 1

The roots of Symplocos paniculata saplings after growing for 17 weeks in hydroponics. The +Al plants grew new white roots, while these were clearly absent in –Al plants. In the +Al treatment, 1 mM AlCl₃ was added to the standard nutrient solution (pH 4), but no Al was available to the –Al plants. Scale bar = 2 cm.

Figure 2

(A) Boxplot showing the aluminium concentration in saplings (n = 16) and seedlings (n = 20) of Symplocos paniculata at the entire plant level. All plants were grown in hydroponics for a minimum of 2 months. The +Al condition had a 1 mM AlCl₃ solution added to the standard nutrient solution, while no Al was available to the –Al plants. (B) Relative change in biomass defined as the difference between the total biomass before and after the hydroponic experiment. (C) Root–shoot ratio of the two experiments. n_saplings = 8 specimens per treatment, n_seedlings = 10 specimens per treatment. Different letters indicate significant in a pairwise comparison of each group (P < 0.05; A, B = Wilcoxon rank-sum test; C = Welch’s t-test).

Figure 3

Root morphological features of the Symplocos paniculata seedlings, analysed with WinRhizo 2012. The +Al condition had a 1 mM AlCl₃ solution added to the standard nutrient solution, which was adjusted to pH 4, but no Al was available to the –Al plants. Shown is the sum of the root length (A), the calculated surface area of the roots (B), the number of root tips (C) and the number of forks (i.e. when a root segment bifurcates, D). None of the features showed a significant difference between the two treatments (Wilkoxon rank-sum test), although +Al levels were typically higher and more variable.

Figure 4

Comparison of Aluminium concentrations in various organs of Symplocos paniculata saplings and seedlings. All plants were grown in a hydroponic solution for a minimum of two months. The +Al condition had a 1 mM AlCl₃ solution added to the standard nutrient solution with a pH adjusted to 4, and no Al was available to the –Al plants. New roots were only formed in the +Al sapling plants and not in the seedlings.

Figure 5

Leaf and wood cross sections of Symplocos paniculata saplings stained with aluminon or pyrocatechol-violet (PCV) after treatment with 1 mM AlCl₃ (+Al treatment). (A) Stem cross section stained with aluminon. (B) Leaf cross section stained with aluminon. (C) Stem cross section stained with aluminon showing the presence of Al in the ray parenchyma (white asterisk). (D) Leaf cross section stained with PCV. (E) Tangential stem wood section of the –Al treatment stained with PCV. (F) Tangential stem wood section of the +Al treatment stained with PCV. (G) Leaf cross section of the –Al treatment after staining with aluminon. UE = upper epidermis, LE = lower epidermis, PP = palisade parenchyma, SP = spongy parenchyma, VB = vascular bundle. Scale bar = 100 µm.