Comparative Insights into Photosynthetic, Biochemical, and Ultrastructural Mechanisms in Hibiscus and Pelargonium Plants

Abstract

Understanding photosynthetic mechanisms in different plant species is crucial for advancing agricultural productivity and ecological restoration. This study presents a detailed physiological and ultrastructural comparison of photosynthetic mechanisms between Hibiscus (Hibiscus rosa-sinensis L.) and Pelargonium (Pelargonium zonale (L.) L'Hér. Ex Aiton) plants. The data collection encompassed daily photosynthetic profiles, responses to light and CO₂, leaf optical properties, fluorescence data (OJIP transients), biochemical analyses, and anatomical observations. The findings reveal distinct morphological, optical, and biochemical adaptations between the two species. These adaptations were associated with differences in photochemical (A_MAX, E, C_i, iWUE, and α) and carboxylative parameters (VC_MAX, ΓCO₂, g_s, g_m, Cc, and AJ_MAX), along with variations in fluorescence and concentrations of chlorophylls and carotenoids. Such factors modulate the efficiency of photosynthesis. Energy dissipation mechanisms, including thermal and fluorescence pathways (ΦPSII, ETR, NPQ), and JIP test-derived metrics highlighted differences in electron transport, particularly between PSII and PSI. At the ultrastructural level, Hibiscus exhibited optimised cellular and chloroplast architecture, characterised by increased chloroplast density and robust grana structures. In contrast, Pelargonium displayed suboptimal photosynthetic parameters, possibly due to reduced thylakoid counts and a higher proportion of mitochondria. In conclusion, while Hibiscus appears primed for efficient photosynthesis and energy storage, Pelargonium may prioritise alternative cellular functions, engaging in a metabolic trade-off.

Keywords: biochemical compounds; chlorophyll a fluorescence; chloroplasts; gas exchange analyser; horticulturae; hyperspectroscopy; microscopies; mitochondria; plant breeding.

Figure 2

Spectral analysis of leaves (in vivo) and pigments (in vitro) in Hibiscus and Pelargonium plants. (A) Reflectance factor (Ref) from 350 to 2500 nm. (B) Transmittance factor (Trans) from 350 to 2500 nm. (C) Absorbance factor (Abs) from 350 to 2500 nm. (D) Spectral analysis of chloroplast and extrachloroplast pigments from 350 to 750 nm, with specific peaks for chlorophylls (green arrow) and flavonoids (pink arrow). The solid lines represent the adaxial surface, and the dashed lines represent the abaxial surface. The arrows highlight peaks for chlorophyll and flavonoid concentrations. Blue arrows denote water-specific spectral signatures. Peak shifts indicate variations due to pigments such as chlorophylls, carotenoids, and phenolic compounds. (n = 100).

Figure 2

Spectral analysis of leaves (in vivo) and pigments (in vitro) in Hibiscus and Pelargonium plants. (A) Reflectance factor (Ref) from 350 to 2500 nm. (B) Transmittance factor (Trans) from 350 to 2500 nm. (C) Absorbance factor (Abs) from 350 to 2500 nm. (D) Spectral analysis of chloroplast and extrachloroplast pigments from 350 to 750 nm, with specific peaks for chlorophylls (green arrow) and flavonoids (pink arrow). The solid lines represent the adaxial surface, and the dashed lines represent the abaxial surface. The arrows highlight peaks for chlorophyll and flavonoid concentrations. Blue arrows denote water-specific spectral signatures. Peak shifts indicate variations due to pigments such as chlorophylls, carotenoids, and phenolic compounds. (n = 100).

Figure 3

Concentrations of compounds in Hibiscus and Pelargonium plants. (A) Chlorophyll a (g m⁻²). (B) Chlorophyll b (g m⁻²). (C) Total chlorophyll (a+b) (g m⁻²). (D) Carotenoids (g m⁻²). (E) Chl a/b ratio. (F) Car/Chl a+b ratio. (G) Flavonoids (nmol cm⁻²). (H) Phenolic compounds (mL cm⁻²). (I) Chlorophyll a (mg g⁻¹). (J) Chlorophyll b (mg g⁻¹). (K) Total chlorophyll (a+b) (mg g⁻¹). (L) Carotenoids (mg g⁻¹). (M) Flavonoids (μmol g⁻¹). (N) Radical scavenging (% of antioxidant activity). (O) Lignin (mg g⁻¹). (P) Cellulose (nmol mg⁻¹). Asterisks over bars indicate statistically significant differences in the t-test (p < 0.01). Mean ± SE (n = 100).

Figure 4

Daily curves between 6 and 20 h were evaluated over three days for Hibiscus and Pelargonium plants. (A–C) Net assimilation rate (μmol CO₂ m⁻² s⁻¹). (D–F) Internal CO₂ concentration (μmol CO₂ mol⁻¹). (G–H) Net transpiration rate (mmol H₂O m⁻² s⁻¹). (J–M) Stomatal conductance (mol H₂O m⁻² s⁻¹). Black bars indicate darkness, and yellow bars indicate light environments. Mean ± SE (n = 20).

Figure 5

Response curves for Hibiscus and Pelargonium plants. (A) Net photosynthetic light (A-PPFD) response. (B) Net photosynthetic CO₂ (A−C_i) responses. (C) Stomatal conductance (g_s) and transpiration rate (E). (D) Intrinsic water use efficiency (iWUE) response curves. The red arrow indicates the inflection point of 426 μmol mol⁻¹ CO₂ for decreased C_i in leaves. Mean ± SE (n = 10).

Figure 6

Fluorescence response curves obtained simultaneously with the photosynthetic response to light in Hibiscus and Pelargonium plants. (A) Effective quantum yield of PSII (Fv’/Fm’). The inset shown in the bar graph indicates the maximum quantum yield of PSII (Fv/Fm) in dark−adapted leaves. (B) Operational efficiency of photosystem II (ΦPSII). The inset shows the electron transport rate (ETR). (C) Nonphotochemical quenching (NPQ). (D) Photochemical dissipation quenching (qP) and nonphotochemical dissipation quenching (qN). Asterisks over the bars indicate statistically significant differences according to the t-test (p < 0.01). “ns” denotes no statistical significance. Mean ± SE (n = 10).

Figure 7

Chlorophyll a fluorescence kinetic parameters derived from the JIP test in Hibiscus and Pelargonium plants. (A) Chlorophyll a fluorescence induction kinetics using normalised data. (B) Pipeline leaves display phenomenological energy flow through the excited cross-sections (CSs) of leaves. Yellow arrow—ABS/CS, absorption flow by approximate CS; green arrow—TR/CS, energy flow trapped by CS; red arrow—ET/CS, electron transport flow by CS; blue arrow—DI/CS, energy flow dissipated by CS; circles inscribed in squares—RC/CS indicate the % of active/inactive reaction centres. The white circles inscribed in squares represent reduced (active) QA reaction centres, the black circles represent non-reducing (inactive) QA reaction centres, and 100% of the active reaction centres responded with the highest average numbers observed in relation to Hibiscus. Arrow sizes indicate changes in the energy flow to Hibiscus plants. (C) ΨEO. (D) ΨRO. (E) ΦPO. (F) ΦPO. (G) ΦRO. (H) ΦDO. (I) δRO. (J) ρRO. (K) KN. (L) KP. (M) SFI_ABS. (N) PI_ABS. Different asterisks inside the arrows indicate significance, as determined by a t-test (p < 0.01). Mean ± SE (n = 100).

Figure 1

Representative of Hibiscus (Hibiscus rosa-sinensis L.) and Pelargonium (Pelargonium zonale (L.) L’Hér. Ex Aiton) plants. Hibiscus leaves exhibit a waxy surface and large size, while Pelargonium leaves are smaller, lobed, and covered with trichomes.

Figure 8

Representative images of optical microscopy (OM) in top–bottom and anatomical analyses of Hibiscus (first and second columns) and Pelargonium (third and fourth columns) plants. (A–D) Cross-sections. (E–H) Historesin cross-sections under false colour. (I–L) Details of the leaf thickness and cells. (M–P) Structures present in cellular tissues. Green arrows indicate chloroplasts, red arrows indicate diffuse crystals, and yellow arrows indicate dense cytoplasmic content. Accumulative and secretory structures of the adaxial epidermis are highlighted. Scale bars = 200 µm and 50 µm, left to right, respectively.

Figure 9

Representative scanning electron microscopy (SEM) images of adaxial and abaxial surfaces of Hibiscus and Pelargonium plants. (A,E,I,M) Adaxial surface of the Hibiscus. (B,F,J,N) Abaxial surface of the Hibiscus. (C,G,K,O) Adaxial surface of Pelargonium. (D,H,L,P) Abaxial surface of Pelargonium. Scale bars = 250 μm (A–D), 150 μm (E–H), and 50 μm (I–P), top to bottom, respectively.

Figure 10

Representative transmission electron microscopy (TEM) images of chloroplasts in Hibiscus and Pelargonium plants. (A,B,E,F,I,J,M,N,Q,R) Hibiscus. (C,D,G,H,K,L,O,P,S,T) Pelargonium plants. Scale bar = 4 μm (A–D), 1 μm (E–P) and 600 nm (Q–T).

Figure 11

Representative transmission electron microscopy (TEM) images of mesophyll cells in the leaves. (A,B,E,F,I,J) Hibiscus. (C,D,G,H,K,L) Pelargonium plants. Scale bar = 4 μm (A–D), 1 μm (E–P) and 600 nm (Q–T).

Figure 12

Multivariate analysis of Hibiscus and Pelargonium plants. The 2D PCA biplot of principal component analysis (PCA) displayed two dimensions (Dim1 and Dim2) and the contribution of the 20 most important variables to explain the formed clusters. See the abbreviation in Section 4.

Figure 13

Comparative scheme of Hibiscus and Pelargonium plants. It highlights the superior photosynthetic efficiency of Hibiscus, emphasising its enhanced cellular structure, including higher chloroplast density, which contributes to improved photosynthesis and energy storage. In contrast, Pelargonium exhibits cellular adjustments, including changes in thylakoid count and a higher proportion of mitochondria, suggesting resource allocation to alternative cellular functions. Detailed insets and labels elucidate the distinct morphological, biochemical, and photosynthetic adaptations between the two species. Thicker lines indicate more efficient electron flow in the electron transport chain. Elements of the figure were created using Biorender.com (accessed on 5 October 2024).