Radiation quality matters: morphological and biochemical responses of Brassica rapa microgreens to X-rays, C-ions, and Fe-ions

Abstract

Radiation type and dose distinctly modulate microgreens development, revealing trait-specific thresholds where X-rays induce hormesis, carbon ions delay differentiation, and iron ions enhance biochemical balance with moderate anatomical disruption. As space exploration progresses and controlled-environment agriculture becomes increasingly relevant under extreme conditions, understanding how ionizing radiation affects plant development is crucial. Ionizing radiation poses a major constraint in space cultivation systems, also playing a role in terrestrial stress scenarios. Despite growing interest in radiation biology, few studies have systematically compared plant responses to different radiation types with distinct linear energy transfer (LET). In this study, seeds of Brassica rapa L. were exposed to increasing doses of X-rays (low-LET), carbon ions, and iron ions (high-LET). Seed germination, morpho-anatomical, and biochemical traits of plants were assessed up to the microgreens stage. Plant responses were both dose- and radiation-specific. Specifically, X-rays triggered a hormetic response at low doses (1 Gy), with a decline in several analyzed traits at higher doses. Carbon ions increased leaf expansion but reduced the content of pigments, proteins, and the structural investment, suggesting a delayed tissue differentiation and low-cost acclimation mechanism under stress. Iron ions promoted a coordinated upregulation of biochemical defenses and moderate anatomical changes. Overall, radiation quality induced distinct acclimation strategies in B. rapa, influencing the balance between growth, structural integrity, and defense mechanisms, highlighting its notable radioresistance. Moreover, identifying trait-specific thresholds and response patterns suggests that different radiation types could be selectively applied to modulate specific functions (e.g., biomass or antioxidants promotion, anatomical adjustments) based on desired outcomes. These findings provide valuable insights into how different ionizing radiation types impact plant responses, addressing a critical gap in space-oriented research and guiding strategies to optimize plant growth in extraterrestrial environments.

Keywords: Extreme environments; Ionizing radiation; Leaf traits; Phytochemical countermeasures; Radiobiology; Stress response.

Fig. 1

Light microscopy images of leaf lamina cross sections of B. rapa microgreens (a–f, h–m, o–t) grown from non-irradiated seeds (a, h, o, control, 0 Gy) and seeds irradiated with increasing doses of X-rays (b, 0.3 Gy; c, 1 Gy; d, 10 Gy; e, 20 Gy; f, 30 Gy), C-ions (i, 0.3 Gy; j, 1 Gy; k, 10 Gy; l, 20 Gy; m, 25 Gy), and Fe-ions (p, 0.3 Gy; q, 1 Gy; r, 10 Gy; s, 20 Gy; t, 25 Gy). Images are all at the same magnification. Bar = 100 μm. Quantification of intercellular space area (%) in the spongy parenchyma (g, n, u) for each irradiation treatment: g, X-rays; n, C-ions; and u, Fe-ions. Values are means ± SE (n = 15); different letters indicate statistically significant differences (P < 0.05)

Fig. 2

Radiation effect on the leaf anatomical traits (adaxial epidermis thickness, palisade parenchyma thickness, spongy parenchyma thickness, abaxial epidermis thickness, total leaf lamina thickness) in leaves of B. rapa microgreens from the control (0) and seeds irradiated with increasing doses of X-rays (a), C-ions (b), and Fe-ions (c). Mean values and standard errors are shown (n = 45). Different letters correspond to significantly different values between doses within each trait; capital letters refer to the significance of total lamina thickness

Fig. 3

Correlations between leaf lamina thickness and FW biomass (a) and leaf lamina thickness and stomatal frequency on the adaxial (b, red) and abaxial (b, blue) surface in the case of microgreens from C-ions irradiated seeds. R² values and equations of the fitting linear regressions are shown

Fig. 4

Correlation between polyphenols content and FW biomass in the case of microgreens from C-ions irradiated seeds. R² value and equation of the fitting linear regression are shown

Fig. 5

Principal component analysis (PCA) loading plot and scores of morphological (FW fresh weight; DW dry weight; LA leaf area; HL hypocotyl length) anatomical (UET upper epidermis thickness; PT palisade parenchyma thickness; ST spongy parenchyma thickness; LET lower epidermis thickness; TT total leaf lamina thickness; SUE stomata density upper epidermis; SLE stomata density lower epidermis) and biochemical data (AC antioxidant capacity; CHL chlorophyll; CAR carotenoids; POLY polyphenols; PROT soluble proteins; ASA ascorbic acid) in B. rapa microgreens from the control (0) and seedlings irradiated with increasing doses (0.3, 1, 10, 20, 30 Gy, for X-rays; 0.3, 1, 10, 20, 25 Gy) of C-ions and Fe-ions