Optimal allocation of resources in response to shading and neighbours in the heteroblastic species, Acacia implexa

Abstract

Background and aims: Heteroblasty is an encompassing term referring to ontogenetic changes in the plant shoot. A shaded environment is known to affect the process of heteroblastic development; however, it is not known whether crowded or high density growing conditions can also alter heteroblasty. Compound leaves of the shade-intolerant Acacia implexa allocate less biomass per unit photosynthetic area than transitional leaves or phyllodes and it is hypothesized that this trait will convey an advantage in a crowded environment. Compound leaves also have larger photosynthetic capture area - a trait known to be advantageous in shade. This studied tested the hypothesis that more compound leaves will be developed under shade and crowded environments. Furthermore, this species should undergo optimal allocation of biomass to shoots and roots given shaded and crowded environments.

Methods: A full factorial design of irradiance (high and low) and density levels (high, medium and low) on three populations sourced from varying rainfall regions (high, medium and low) was established under controlled glasshouse conditions. Traits measured include the number of nodes expressing a compound leaf, biomass allocation to shoots and roots, and growth traits. Key Results A higher number of nodes expressed a compound leaf under low irradiance and in high density treatments; however, there were no significant interactions across treatments. Phenotypes strongly associated with the shade avoidance syndrome were developed under low irradiance; however, this was not observed under high density. There was no significant difference in relative growth rates across light treatments, but growth was significantly slower in a crowded environment. Conclusions Heteroblastic development in Acacia can be altered by shade and crowded environments. In this experiment, light was clearly the most limiting factor to growth in a shaded environment; however, in a crowded environment there were additional limiting resources to growth.

Fig. 1.

An example of heteroblastic development in Acacia implexa. Compound leaves, transitional leaves and phyllodes indicated with an arrow. The plant was grown in full sunlight and the photograph was taken on the 86th day of growth.

Fig. 2.

Compound leaf nodes represents the total number of nodes that developed a compound leaf before the onset of transitional leaves. Displayed are means (± s.e.) for low (L), medium (M) and high (H) levels for each variable. *** P < 0·001; **** P < 0·0001. Complete statistical results are presented in Supplementary Data Table S3 (available online).

Fig. 3.

Compound leaf nodes and the interaction of shade and density treatments. Displayed are means (± s.e.) for low and high light, as indicated. Comparisons were made by Tukey's HSD; **** P < 0·0001.

Fig. 4.

Means (± s.e.) of whole-plant traits across experimental treatments: height-to-diameter ratio (HtoD, square-root transformed); average internode length (sqare-root transformed); stem mass ratio (StMR); root mass ratio (RMR); leaf mass ratio (LMR); leaf area ratio (LAR); net assimilation rate (NAR); and specific leaf area (SLA). * P < 0·05; ** P < 0·01; *** P < 0·001; **** P < 0·0001. Units of measurement are given in Table 2. Complete ANOVA results can be found in Supplementary Data Table S4 (available online).

Fig. 5.

Grouping of population, light and density treatments following discriminant function analysis for whole-plant traits. Displayed are group centroids with circles indicating 95 % CIs for whole-plant and leaf-level traits (overlapping circles indicate no difference between groups). Variance explained by first and second discriminant functions are displayed in brackets. (A) Hr-Ll, Mr-Ll, Lr-Ll, high rainfall, medium rainfall, low rainfall population by low quality light treatments; and Hr-Hl, Mr-Hl, Lr-Hl, high rainfall, medium rainfall, low rainfall population by high quality light treatments. (B) Hr-Hd, Mr-Hd, Lr-Hd, high rainfall, medium rainfall, low rainfall population by high density treatments; Hr-Md, Mr-Md, Lr-Md, high rainfall, medium rainfall, low rainfall population by medium density treatments; and Hr-Ld, Mr-Ld, Lr-Ld, high rainfall, medium rainfall, low rainfall population by low density treatments. (C) Hd-Ll, Md-Ll, Ld-Ll, low quality light by high, medium and low density treatments; and Hd-Hl, Md-Hl, Ld-Hl, high quality light by high, medium and low density treatments. (D–F) Factor loadings of whole-plant traits corresponding to DFA of treatments across first and second discriminant functions. See Table 2 for abbreviations.

Fig. 6.

Mean (± s.e.) of growth traits across experimental treatments. * P < 0·05; ** P < 0·01; *** P < 0·001; **** P < 0·0001. Complete ANOVA results can be found in Supplementary Data Table S5 (available online).