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.

Comparison of two controlled-release fertilizer formulations for cut-and-come-again harvest yield and mineral content of Lactuca sativa L. cv. Outredgeous grown under International Space Station environmental conditions.

Red Romaine leaf lettuce (Lactuca sativa L. cv. Outredgeous) was grown in ground-based analogues of the Veggie plant-growth units used to grow salad vegetables for astronauts on the International Space Station (ISS). Plants were grown for 56 days with three "cut-and-come again" leaf harvests from the same plants. Six Biomass-Production-Systems-for-Education (BPSe) units were used to grow 'Outredgeous' ('OR') lettuce in a walk-in growth chamber under temperature, humidity, and LED-lighting conditions similar to those occurring in Veggie on ISS. Because of the ISS micro-gravity environment, both Veggie and ground-based BPSe units utilize one-way capillary wicking of water into an arcillite clay root substrate. In the present study, two different controlled-release fertilizer (CRF) formulations incorporated into the arcillite were compared for effects on 'OR' growth rate, overall yield, and mineral content of leaves harvested from the same plants 28, 48, and 56 days after planting. Both CRF treatments had a rapid-releasing T70 component that kept growth rate equivalent over the first two harvests. Growth rate for both CRF treatments increased from the first to the second harvest, but then declined from the second to the third harvest, more so for the slower-releasing T180 CRF than for the moderately-releasing T100 CRF. Tissue content of the macro-nutrients N, P, and K declined at each harvest for both CRFs, while content of the micro-nutrients B, Zn, and Mn increased. Although pH did not go out of the nominal range for availability of mineral nutrients to roots over the cropping cycle, and electrical-conductivity of rootzone salts was neither excessive nor depleted, tissue macronutrient depletion and micro-nutrient accumulation may have contributed to yield declines. Although either CRF formulation can support adequate yield of 'OR' lettuce over a 56-day period, the moderately-releasing T100 formulation tends to give slightly higher yield, especially during the last growth increment, and with non-deficient mineral content.