Morphological and Photosynthetic Pigment Screening of Four Microgreens Species Exposed to Heavy Ions

Abstract

Numerous challenges are posed by the extra-terrestrial environment for space farming and various technological growth systems are being developed to allow for microgreens' cultivation in space. Microgreens, with their unique nutrient profiles, may well integrate the diet of crew members, being a natural substitute for chemical food supplements. However, the space radiation environment may alter plant properties, and there is still a knowledge gap concerning the effects of various types of radiation on plants and specifically on the application of efficient and rapid methods for selecting new species for space farming, based on their radio-resistance. Thus, the hypotheses behind this study were to explore the following: (i) the pattern (if any) of radio-sensitivity/resistance; and (ii) if the morphological parameters in relation with pigment content may be a feasible way to perform a screening of radiation responses among species. To perform this, we irradiated dry seeds of basil, rocket, radish, and cress with iron (⁵⁶Fe; 1550 MeV/(g/cm²)) and carbon (¹²C; 290 MeV/u, 13 keV/µm) heavy ions at the doses of 0.3, 1, 10, 20, and 25 Gy to investigate the growth responses of microgreens to acute radiation exposure in terms of morphological traits and photosynthetic pigment content. Results indicate that the microgreens' reaction to ionizing radiation is highly species-specific and that radiation is often sensed by microgreens as a mild stress, stimulating the same morphological and biochemical acclimation pathways usually activated by other mild environmental stresses, alongside the occurrence of eustress phenomena. Over extended periods, this stimulus could foster adaptive changes, enabling plants to thrive in space.

Keywords: carbon ion; ionizing radiation; iron ion; microgreens; photosynthetic pigment content; space biology.

Figure 1

Germination % of (a) basil (O. basilicum), (b) rocket (E. vesicaria), (c) radish (R. raphanistrum), and (d) cress (L. sativum) seeds irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Mean values and corresponding standard errors are reported; different letters indicate significantly different values according to Duncan’s test (p < 0.05).

Figure 2

Fresh and dry biomass (g/seedling) of (a,b) basil (O. basilicum), (c,d) rocket (E. vesicaria), (e,f) radish (R. raphanistrum), and (g,h) cress (L. sativum) microgreens whose seeds were irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Mean values and corresponding standard errors are reported; different letters indicate significantly different values according to Duncan’s test (p < 0.05).

Figure 3

Chlorophyll a (a), chlorophyll b (b), chlorophyll a + b (c), and carotenoids (d) content of basil (O. basilicum) microgreens irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Mean values and corresponding standard errors are reported; different letters indicate significantly different values according to Duncan’s test (p < 0.05).

Figure 4

Chlorophyll a (a), chlorophyll b (b), chlorophyll a + b (c), and carotenoids (d) content of rocket (E. vesicaria) microgreens irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Mean values and corresponding standard errors are reported; different letters indicate significantly different values according to Duncan’s test (p < 0.05).

Figure 5

Chlorophyll a (a), chlorophyll b (b), chlorophyll a + b (c), and carotenoids (d) content of radish (R. raphanistrum) microgreens irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Mean values and corresponding standard errors are reported; different letters indicate significantly different values according to Duncan’s test (p < 0.05).

Figure 6

Chlorophyll a (a), chlorophyll b (b), chlorophyll a + b (c), and carotenoids (d) content of cress (L. sativum) microgreens irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Mean values and corresponding standard errors are reported; different letters indicate significantly different values according to Duncan’s test (p < 0.05).

Figure 7

Hierarchical cluster analysis (HCA) of morphological traits and pigment content of basil (Ba), rocket (Ro), radish (Ra), and cress (Cr) microgreens whose seeds were irradiated with Fe and C ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL.

Figure 8

Spearman’s rank correlation coefficients between pairs of biometric traits (germination %, G; fresh weight, FW; dry weight, DW; microgreen area, MA) and pigment content (chlorophyll a, Chl.a; chlorophyll b, Chl.b; chlorophyll a + b, Chl.a.b.; carotenoids, Car.) in basil (O. basilicum) microgreens irradiated with Fe (a) and C (b) ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Positive and negative correlations are shown; *; **, and *** are significant at p < 0.05, 0.01, and 0.001, respectively.

Figure 9

Spearman’s rank correlation coefficients between pairs of biometric traits (germination, G; fresh weight, FW; dry weight, DW; microgreen area, MA) and pigment content (chlorophyll a, Chl.a; chlorophyll b, Chl.b; chlorophyll a + b, Chl.a.b.; carotenoids, Car.) in rocket (E. vesicaria) microgreens irradiated with Fe (a) and C (b) ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Positive and negative correlations are shown; *; **, and *** are significant at p < 0.05, 0.01, and 0.001, respectively.

Figure 10

Spearman’s rank correlation coefficients between pairs of biometric traits (germination, G; fresh weight, FW; dry weight, DW; microgreen area, MA) and pigment content (chlorophyll a, Chl.a; chlorophyll b, Chl.b; chlorophyll a + b, Chl.a.b.; carotenoids, Car.) in radish (R. raphanistrum) microgreens irradiated with Fe (a) and C (b) ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Positive and negative correlations are shown; *; **, and *** are significant at p < 0.05, 0.01, and 0.001, respectively.

Figure 11

Spearman’s rank correlation coefficients between pairs of biometric traits (germination, G; fresh weight, FW; dry weight, DW; microgreen area, MA) and pigment content (chlorophyll a, Chl.a; chlorophyll b, Chl.b; chlorophyll a + b, Chl.a.b.; carotenoids, Car.) in cress (L. sativum) microgreens irradiated with Fe (a) and C (b) ions at the doses of 0.3, 1, 10, 20, 25 Gy, and CTRL. Positive and negative correlations are shown; *; **, and *** are significant at p < 0.05, 0.01, and 0.001, respectively.