Morphological, Anatomical, and Physiological Characteristics of Heteroblastic Acacia melanoxylon Grown under Weak Light

Abstract

Acacia melanoxylon is a fast-growing macrophanerophyte with strong adaptability whose leaf enables heteromorphic development. Light is one of the essential environmental factors that induces the development of the heteroblastic leaf of A. melanoxylon, but its mechanism is unclear. In this study, the seedlings of A. melanoxylon clones were treated with weak light (shading net with 40% of regular light transmittance) and normal light (control) conditions for 90 d and a follow-up observation. The results show that the seedlings' growth and biomass accumulation were inhibited under weak light. After 60 days of treatment, phyllodes were raised under the control condition while the remaining compound was raised under weak light. The balance of root, stem, and leaf biomass changed to 15:11:74 under weak light, while it was 40:15:45 under control conditions. After comparing the anatomical structures of the compound leaves and phyllode, they were shown to have their own strategies for staying hydrated, while phyllodes were more able to control water loss and adapt to intense light. The compound leaves exhibited elevated levels of K, Cu, Ca, and Mg, increased antioxidant enzyme activity and proline content, and higher concentrations of chlorophyll a, carotenoids, ABA, CTK, and GA. However, they displayed a relatively limited photosynthetic capacity. Phyllodes exhibited higher levels of Fe, cellulose, lignin, IAA content, and high photosynthetic capacity with a higher maximum net photosynthetic rate, light compensation point, dark respiration rate, and water use efficiency. The comparative analysis of compound leaves and phyllodes provides a basis for understanding the diverse survival strategies that heteroblastic plants employ to adapt to environmental changes.

Keywords: Acacia melanoxylon; heteroblasty; phenotypic plasticity; photosynthesis; phyllodes; stress.

Figure 1

The phenotype and growth characteristics of A. melanoxylon seedlings under weak light treatment. (a) The phenotype of A. melanoxylon seedlings throughout weak light treatment, the seedlings on the left of the photo are the seedlings in the control, and the right one shows the seedlings under weak light treatment; (b) Height, (c) ground diameter, and (d) number of leaves of the seedlings under weak light and the control treatment, WL, the weak light treatment; CL, the compound leaves; P, the phyllodes; T0, T30, T60, and T90 mean the seedlings treated for 0 d, 30 d, 60 d, and 90 d, respectively; different lowercase letters in each column indicate significant differences between treatments (p < 0.05).

Figure 2

The anatomical structure of A. melanoxylon heteroblastic leaves. (a), Phyllode (red region represents sample range); (b), phyllode transverse sections; (c), phyllode adaxial surface; (d), phyllode stomata; (e), compound leaf (red region represents sample range); (f), compound leaf transverse sections; (g), compound leaf adaxial surface; (h), compound leaf stomata. Abbreviations: c, cuticle; cp, central parenchyma; cvb, central vascular bundle; e, epidermal cell; f, fiber; lp, lignified parenchyma; mvb, marginal vascular bundle; ph, phloem; pm, palisade mesophyll; s, stomata; svb, small vascular bundle; x, xylem.

Figure 3

The content of nutrient elements, lignin, cellulose, and hemicellulose in A. melanoxylon heteroblastic leaves. (a) Nitrogen, (b) phosphorus, (c) potassium, (d) iron, (e) cuprum, (f) zinc, (g) calcium, (h) magnesium, (i) boron, (j) hemicellulose, (k) cellulose, and (l) lignin contents in A. melanoxylon heteroblastic leaves under weak light and the control treatment. *, significant differences at p-value < 0.05 (n = 3), **, significant differences at p-value < 0.01 (n = 3).

Figure 4

Differences in antioxidant enzyme activities and the content of osmotic regulatory substances in A. melanoxylon heteroblastic leaves. (a) SOD, (b) POD, and (c) CAT activities, and (d) Pro content in A. melanoxylon heteroblastic leaves under weak light and the control treatment. **, significant differences at p-value < 0.01 (n = 5).

Figure 5

Light response curve and photosynthetic pigment content of A. melanoxylon heteroblastic leaves. (a) Light response curve and (b) chlorophyll a, (c) chlorophyll b, (d) total chlorophyll, and (e) carotenoid content in A. melanoxylon heteroblastic leaves under weak light and the control treatment. **, significant differences at p-value < 0.01 (n = 6).

Figure 6

Endogenous hormone levels in A. melanoxylon heteroblastic leaves. (a) IAA, (b) ABA, (c) CTK, and (d) GA content in A. melanoxylon heteroblastic leaves under weak light and the control treatment. **, significant differences at p-value < 0.01 (n = 5).

Figure 7

Correlation analysis among the comparison indexes of A. melanoxylon heteroblastic leaves. *, significant correlation at p-value < 0.05; **, significant correlation at p-value < 0.01. T, treatment; H, height; GD, ground diameter; NL, number of leaves; RB, root biomass; SB, stem biomass; LB, leaf biomass; TB, total biomass; SW, stomatal width; SAL, stomatal aperture length; SAW, stomatal aperture width; SD, stomatal density; CE, cellulose content; LN, lignin content.

Figure 8

Differences in the development and physiology of compound leaves and phyllodes in A. melanoxylon. The orange fonts represent significantly or extremely significantly higher values that were in compound leaves, while the blue fonts represent significantly or extremely significantly lower values in compound leaves. H, plant height; GD, ground diameter; PN, phyllodes number; CLN, compound leaves number; RB, root biomass; SB, stem biomass; LB, leaf biomass; TB, total biomass; R/S, the ratio of root to stem; CL/T, the ratio of compound leaves to total biomass; P/T, the ratio of phyllodes to total biomass; C, cuticle; EWL, epidermal waxy layer; PM, palisade mesophyll; DVT, developed vascular tissue; FCs, fiber caps; SW, stomatal width; SAL, stomatal aperture length; SAW, stomatal aperture width; SD, stomatal density; P′_max, the maximum net photosynthetic rate; LCP, light compensation point; R_d, dark respiration rate; Ci, intercellular CO₂ concentration; WUE, water use efficiency.