An Oxygen Delivery Polymer Enhances Seed Germination in a Martian-like Environment

Abstract

Critical to the success of establishing a sustainable human presence on Mars is the ability to economically grow crop plants. Several environmental factors make it difficult to fully rely on local resources for agriculture. These include nutrient sparse regolith, low and fluctuating temperatures, a high amount of ultraviolet radiation, and water trapped locally in the form of ice or metal oxides. While the 96% CO₂ martian atmosphere is ideal to support photosynthesis, high CO₂ concentrations inhibit germination. An added difficulty is the fact that a vast majority of crop plants require oxygen for germination. Here, we report the production of a polymer-based oxygen delivery system that supports the germination and growth of cress seeds (Lepidium sativum) in a martian regolith simulant under a martian atmosphere at 101 kPa. The oxygen-donating system is based on a low-density lightly cross-linked polyacrylate that is foamed and converted into a dry powder. It is lightweight, added in low amounts to regolith simulant, and efficiently donates enough oxygen throughout the volume of hydrated regolith simulant to fully support seed germination and plant growth. Germination rates, plant development, and plant mass are nearly identical for L. sativum grown in 100% CO₂ in the presence of the oxygen-donating lightly cross-linked polyacrylate compared with plants grown in air. The polymer system also serves to protect root structures and better anchors plants in the regolith simulant.

Keywords: Biological life support; Colonization; Extraterrestrial crops; Plant growth..

FIG. 1.

Chemical structure of the lightly cross-linked polyacrylate material. Shown is the structure of the polymer repeating unit and the nature of the siloxy cross-link.

FIG. 2.

Physical structure of the oxygen-donating lightly cross-linked polyacrylate material. Shown are SEM images of the powdered material (A) as well as the bulk powder in the unhydrated (B) and fully hydrated (C) state. SEM, scanning electron micrograph.

FIG. 3.

The oxygen-donating lightly cross-linked polyacrylate material acts as a growth matrix to anchor root structure. A 12-day Lepidium sativum plant was gently pulled from a regolith only growth container (A) or a growth container containing a mixture of regolith and of oxygen-donating lightly cross-linked polyacrylate at a ratio of 50:1 (B) and gently shaken to remove any regolith that is loosely associated with plant root structures.

FIG. 4.

Oxygen release profile. (A) Oxygen release kinetics from 1 g of the oxygen-donating lightly cross-linked polyacrylate into an air space of 10 cm³ when wet, measured as percent oxygen by the in situ probe (closed circles). Open circles are control measurements in the absence of the oxygen-donating lightly cross-linked polyacrylate. (B) Distribution of oxygen content in a regolith simulant growth container without (−) or with (+) the oxygen-donating lightly cross-linked polyacrylate. Thirteen random sample locations were probed throughout the regolith container volume.

FIG. 5.

Growth chamber configuration and typical planting results. Lepidium sativum seeds were planted in JSC Mars-1A regolith simulant in single containers, which were placed into a vacuum bell jar. Growth conditions were regolith simulant in air (A), regolith simulant supplemented with lightly cross-linked polyacrylate (50:1) in 100% CO₂ (B), and regolith simulant supplemented with oxygen-generating lightly cross-linked polyacrylate (50:1) in 100% CO₂ (C). Percent germination is 16/25 (A), 0/25 (B), and 25/25 (C). Atmosphere is controlled via the gas port that can be seen on the right side of the bell jar. The yellow size bar is 4.0 cm.

FIG. 6.

Germination and growth kinetics for Lepidium sativum in regolith simulant under various atmospheric conditions. Closed circles, regolith simulant in air; open circles, regolith simulant with lightly cross-linked polyacrylate (50:1) in air; open squares, regolith simulant with lightly cross-linked polyacrylate (50:1) in CO₂; closed squares, regolith simulant with oxygen-donating lightly cross-linked polyacrylate (1:50) in CO₂. (I) Plumule formation; (II) cotyledon pair emergence; (III) third leaf appearance. Note that the y-axes have different ranges for data visualization clarity. Also note in (III), the open squares and closed circles lines are coincident.

FIG. 7.

Plant mass as a function of growth condition. Total mass (top panel) of L. sativum plants, mass of plant structures above the regolith simulant surface (middle panel), and mass of plant structures below the regolith simulant surface (bottom panel). All L. sativum plants were measured post 12 days growth. Growth conditions were as follows: Regolith simulant alone in air (I) where A, B, and C refer to the three container sizes, Regolith simulant with lightly cross-linked polyacrylate in air (II), Regolith simulant with lightly cross-linked polyacrylate in CO₂ (III), and Regolith simulant with oxygen donating lightly cross-linked polyacrylate in CO₂ (IV). Note that the y-axes have different ranges for data visualization clarity.

FIG. 8.

Optimal oxygen level to support germination. Optimal concentration of oxygen-donating lightly cross-linked polyacrylate added to JSC Mars-1A regolith simulant. Plotted is the percent germination of Lepidium sativum seeds in 25 g regolith simulant with various amounts of oxygen-donating lightly cross-linked polyacrylate added. Growth was in a 99.5% CO₂ atmosphere at 101 kPa.

FIG. 9.

Total L. sativum plant mass post 24-day growth. Twenty plants were grown in air/regolith simulant/lightly crosslinked polyacrylate (experimental condition II-A), or in CO₂/regolith simulant/oxygen donating lightly crosslinked polyacrylate (experimental condition IV-A).

FIG. 10.

Evolution of oxygen into a growth chamber. Oxygen concentration was measured every 2 days over the course of a 24-day period with an in situ oxygen probe. Evolved oxygen from regolith simulant in the absence of Lepidium sativum seeds/plants (open circles) and in the presence of L. sativum seeds/plants (closed circles) is shown.