Leaf yield and mineral content of mizuna in response to cut-and-come-again harvest, substrate particle size, and fertilizer formulation in a simulated spaceflight environment.

The Veggie plant-growth unit deployed onboard the International Space Station (ISS) grows leafy vegetables to supplement crew diets. "Cut-and-come-again" harvests are tested to maximize vegetative yield while minimizing crew time. Water, oxygen, and fertilizer delivery to roots of leafy greens growing in microgravity have become the center of attention for Veggie. Current Veggie technology wicks water into particulate root substrates incorporating controlled-release fertilizer (CRF). Mizuna mustard (Brassica rapa) was grown under ISS-like environments in ground-based Veggie-analogue units comparing crop response to combinations of two different substrate particle sizes, two different fertilizer formulations, and three leaf-harvest times from each plant. Fine-particle porous ceramic substrate (Profile©) was compared with a 40:60 mix of fine-particle porous ceramic Profile© + coarse porous ceramic Turface© substrate. Identical 18-6-8 (NPK) CRF formulations consisting of [50% fast-release (T70) + 50% intermediate-release (T100) prills] vs. [50% fast-release (T70) + 50% slow-release (T180) prills] were incorporated into each substrate, and leaf tissues were harvested from each treatment combination at 28, 48, and 56 days after sowing. The combination of T100 CRF in 100% Profile© substrate gave the highest fresh mass (FM) and leaf area (LA) across harvests, whereas T180 CRF in 40% Profile© gave the lowest. Dry-mass (DM) yields varied with effects on leaf area. Tissue nitrogen (N) and potassium (K) specific contents declined across harvests for all treatment combinations but tended to be highest for T100 CRF/100% Profile©, and lowest for T180 CRF/40% Profile©. These major macronutrients were taken up faster by roots growing in small particle-size substrate incorporating intermediate-rate CRF, but also were depleted faster from the same treatment combination, suggesting it may not continue to be the best combination for additional harvests. Micronutrients did not decline in tissue specific content across treatment combinations, but manganese (Mn) accumulated in leaf tissue across treatments and apparently comes mainly from the ceramic substrate, regardless of particle size. Substrate leachate analysis following final harvest indicated that pH remained in the range for nominal availability of mineral nutrients for root uptake, but electro-conductivity measurements suggested depletion of fertilizer salts from root zones, especially from the treatment combination supporting the highest yields and major macronutrient contents. Although 100% Profile© was the better growth substrate for mizuna in combination with intermediate-rate CRF and three cut-and-come-again harvests in ground-based studies, mixed-particle-size substrates may be a better choice for plant growth under microgravity conditions, where capillary forces predominant and tend to cause saturation of a fine medium with water. Since there were no statistically significant interactions between substrate and fertilizer in this study, our ground-based findings for CRF choice should translate to the best substrate choice for microgravity, but if NASA wants to consider additional cut-and-come-again harvests from the same mizuna plants, more complex CRF formulations likely will have to be investigated.