Document Type

Unpublished Paper

Publisher

Utah State University

Publication Date

2026

First Page

1

Last Page

13

Abstract

The Utah Reusable Root Module (URRM) is a zero-discharge plant growth system under development for operation in microgravity aboard the International Space Station. Qualification of the URRM requires defining expected crop performance under representative operating conditions to evaluate nominal biological behavior. This study presents experimental results documenting the growth of mizuna (Brassica rapa var. japonica) and Outredgeous lettuce (Lactuca sativa L.) under URRM-representative conditions defined by the use of a silicone rubber fringe top cover for root-zone containment. 

Ground-based experiments were conducted using the Greenhouse System, URRM Simulator – Replicate 1, and URRM Simulator – Replicate 2, all filled with peat-based growth media maintained at approximately 60% volumetric water content (θᵥ = 0.60 cm³ cm⁻³) and fertigated with the same nutrient solution formulated for URRM operations. All growth metrics reported here correspond to initial planting cycles established without residual root material at the planting surface. Thus, the results do not reflect potential effects associated with sequential replanting in previously occupied positions. Crop establishment and growth were evaluated through emergence, early leaf development, canopy diameter, plant height, fresh shoot biomass, and variability metrics across developmental stages. Emergence and early seedling development under URRM-representative conditions were comparable to uncovered controls, indicating that the silicone rubber fringe containment approach did not impede plant establishment. Canopy diameter proved to be a practical, non-destructive, and easily measurable indicator of early development through approximately 12 days after seeding (DAS). 

Following establishment, plant height and fresh shoot biomass were used to characterize continued vegetative growth. From 10 to 24 DAS, plant height increased steadily in both crops, with the most rapid increase occurring between 17 and 24 DAS; thereafter, lettuce exhibited a reduced rate of vertical growth consistent with a sigmoidal pattern. Fresh shoot biomass accumulated rapidly in both crops; however, mizuna reached harvest-relevant biomass by 24 DAS, whereas lettuce required approximately one additional week (28–31 DAS) to reach comparable final biomass under conservative light conditions (300–500 µmol m⁻² s⁻¹ PPFD). These results provide a data-informed reference describing expected mizuna and lettuce growth under URRM-representative conditions and support evaluation of system performance during qualification testing and early operations.

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