Bruce Bugbee

Future International Space Station (ISS) mini-gardens must provide fresh produce repeatedly while maintaining water and nutrient conditions within a small, reusable root-zone volume. This study evaluated the operational performance of staggered planting and selective harvesting over four months in a ground-based Utah Reusable Root Module (URRM) simulator operated as a zero-discharge system. Five independent root modules (RMs), providing a total planted area of approximately 0.40 m², were planted with leafy greens, bonsai basil, and dwarf tomato. The system produced 4,299 g of fresh edible biomass, with produce available on 77 of the 100 days within the harvest period. Daily water use remained below approximately 2 L d⁻¹ for most of the trial, and peak demand occurred at different times among RMs. RMs containing established tomato plants behind newly planted leafy greens showed lower water-use peaks and biomass production, suggesting incomplete use of module capacity. Lettuce performed well during the first cycle but was less consistent in subsequent cycles, whereas mizuna and kale provided more consistent production under reused-media conditions. Bulk electrical conductivity (bulk EC) declined during periods of rapid crop growth, indicating that nutrient depletion, rather than salinity accumulation, was the main root-zone risk under zero-discharge operation. These results indicate that ISS mini-garden operations can distribute harvests and water demand over time, but crop calendars should coordinate planting dates, crop architecture, harvest timing, residue management, reused-media constraints, and root-zone nutrient monitoring across independently managed modules.

Elevating nutrient input is thought to increase yield and cannabinoid concentration of medical cannabis, but increased legalization has heightened awareness of the environmental impact of overfertilization. Elevated levels of phosphorus (P) are of particular concern. Here we report the effects of increasing P above levels adequate for other crops (15, 30, 45, 60, or 90 mg per L) and the interactive effects of elevated P with elevated nutrient solution concentration (electrical conductivity; 2 and 4 mS per cm). We used closed-system hydroponics to continuously quantify rootzone nutrient concentrations. The concentration of P in leaf tissue doubled and flower P concentration increased 70% when the P input increased from 15 to 90 mg per L but there was no difference in yield or quality among treatments. Doubling nutrient input from 2 to 4 mS per cm increased nutrient accumulation in solution but did not significantly increase yield or quality. Reducing P in the refill solution from 90 to 15 mg per L reduced P in solution at harvest from 300 to less than 0.1 mg per L. Despite the low steady-state concentration of P in solution in the 15 mg per L treatment, there was no difference in yield or quality among treatments, regardless of the concentration of other elements. Despite the high nutrient concentrations in the rootzone solution there was no leaf necrosis or other visual effects among treatments. These data indicate cannabis tolerates high nutrient concentrations, but neither excessive P nor excessive fertilization improves yield or quality.

Mass balance principles are a cornerstone of efficient fertilizer use and can be utilized to optimize plant nutrition without discarding or leaching solution. Here, we describe the maintenance of closed hydroponic and soilless substrate systems based on mass balance. Water removed by transpiration is restored with solution that replaces the nutrients that were taken up with the water. The concentration of nutrients in this refill/irrigation solution is determined by multiplying the optimal concentration of each nutrient in plant tissue by the water-use efficiency (WUE; ratio of dry mass to water transpired). Optimal leaf nutrient concentrations are well established, but WUE in controlled environments varies widely and is less well characterized. Elevated CO₂ increases photosynthesis and demand for nutrients, but partially closes stomata and reduces transpiration; so high CO₂ dramatically increases WUE. The concentration of the refill/irrigation solution must be adjusted to account for a two-fold range of WUE, from 3 g L⁻¹ in ambient CO₂ in lower humidity, to 6 g L⁻¹ in elevated CO₂ in higher humidity. WUE and nutrient requirements vary during the vegetative and reproductive stages of growth, and adjustment of the solution over the lifecycle can be beneficial. Measurement of solution electrical conductivity (EC) is helpful, but if the solution is appropriate, low EC usually means healthy plants and active nutrient uptake. The ammonium to nitrate ratio is critical to pH management. We have applied these principles across multiple species and environments to achieve long-term, steady-state nutrient concentrations with no discharge or leaching of solution.

Cherry orchards are transitioning to high-density plantings and dwarfing rootstocks to maximize production, but the response of these rootstocks to drought stress is poorly characterized. We used a 16-container, automated lysimeter system to apply repeated water stress to ungrafted Krymsk^® 5 and 6 rootstocks during two growing cycles. Drought stress was imposed by withholding irrigation until the daily transpiration rate of each tree was 25% and 30% of the unstressed rate during the first trial and second trial, respectively. After this point was reached, the root-zone water status was restored to field capacity. Whole-tree transpiration measurements were supplemented with leaf-level gas-exchange measurements. Krymsk^® 6 had a higher rate of photosynthesis, more vigorous vegetative growth and less conservative stomatal regulation during incipient drought than Krymsk^® 5. At harvest, carbon partitioning to roots was greater in Krymsk^® 6 than Krymsk^® 5. The conservative rate of water use in Krymsk^® 5 could be a function of greater stomatal control or reduced carbon partitioning to roots, which thereby limited transpiration rates. Further studies are needed to confirm that these results are applicable to trees grown using a common grafted scion under field conditions.

The photon flux in the green wavelength region is relatively enriched in shade and the photon flux in the blue region is selectively filtered. In sole source lighting environments, increasing the fraction of blue typically decreases stem elongation and leaf expansion, and smaller leaves reduce photon capture and yield. Photons in the green region reverse these blue reductions through the photoreceptor cryptochrome in Arabidopsis thaliana, but studies in other species have not consistently shown the benefits of photons in the green region on leaf expansion and growth. Spectral effects can interact with total photon flux. Here, we report the effect of the fraction of photons in the blue (10 to 30%) and green (0 to 50%) regions at photosynthetic photon flux densities of 200 and 500 µmol m⁻² s⁻¹ in lettuce, cucumber and tomato. As expected, increasing the fraction of photons in the blue region consistently decreased leaf area and dry mass. By contrast, large changes in the fraction of photons in the green region had minimal effects on leaf area and dry mass in lettuce and cucumber. Photons in the green region were more potent at a lower fraction of photons in the blue region. Photons in the green region increased stem and petiole length in cucumber and tomato, which is a classic shade avoidance response. These results suggest that high-light crop species might respond to the fraction of photons in the green region with either shade tolerance (leaf expansion) or shade avoidance (stem elongation).

Light-emitting diodes (LEDs) have enabled a historic increase in the conversion of electric energy to photons, but this is approaching a physical limit. The the

An automated method of determining tree water status would enable tree fruit growers, foresters and arborists to reduce water consumption, reduce orchard maintenance costs and improve fruit quality. Automated measurements could also be used to irrigate based on need rather than on fixed schedules. Numerous automated approaches have been studied; all are difficult to implement. Electromagnetic sensors that measure volumetric water content can be inserted in tree trunks to determine relative changes in tree water status. We performed automated measurements of dielectric permittivity using four commercially available electromagnetic sensors in fruit tree trunks over the 2016 growing season. These sensors accurately measure the ratio of air and water in soils, but tree trunks have minimal air-filled porosity. The sensors do respond, however, to bound and unbound water and the relative change in the output of the sensors thus provides an indication of this ratio. Sapwood is the hydro-dynamically responsive component of trunk anatomy and is nearest the bark. Sensor response improved when the waveguides were exposed to a greater percentage of sapwood. Irrigation-induced increases of approximately 0.5 MPa in stem water potential were associated with 0.5 unit increases in dielectric permittivity. Electromagnetic sensors respond to bound water in trees and thus have the potential to indicate tree water status, especially when the sensor rods are in contact with sapwood. Sensor modifications and/or innovative installation techniques could enable automated measurements of tree water status that could be used to precision irrigate trees.

Native American tribes have been cultivating peaches [ Prunus persica (L.) Batsch] since their introduction to North America in the 1600s. In the American Southwest, peach orchards derived from centuries of seed selections have been maintained in relative isolation from commercial cultivars. These Native American peach selections may be better adapted to the arid climate of the Intermountain West. We compared physiological robustness during water stress of seedling peaches from a 60-year-old orchard maintained by Navajo farmers in southwestern Utah to the commercial peach rootstock Lovell. Six replicate trees of each rootstock were subjected to eight cycles of controlled drought on an automated lysimeter system, which monitored transpiration rate continuously. Trees were selected for uniform size and transpiration rate at the start of the study. During the drought cycles, individual trees were watered when their transpiration rate decreased to less than 250 g of water per day, ≈20% of their well-watered daily transpiration rate. After the first cycle of drought, the transpiration rate of the Navajo trees was greater than the Lovell trees, so they depleted their root-zone water more rapidly and experienced greater water stress. Despite greater stress, the Navajo selection had greater leaf area and dry weight at harvest. Because the root system was confined, these results indicate that the Navajo selection may have greater resilience when experiencing drought, independent of the depth and distribution of the root system. However, this study was not able to determine whether physiological resilience during drought was a result of canopy or root characteristics. Field studies are needed to determine whether root distribution or depth also contribute to drought tolerance in the Navajo selection.

Canopy stomatal conductance is a key physiological factor controlling transpiration from plant canopies, but it is extremely difficult to determine in field environments. The objective of this study was to develop a radiometric method for calculating canopy stomatal conductance for two plant species—wheat and soybean from direct measurements of bulk surface conductance to water vapor and the canopy aerodynamic conductance in controlled-environment chambers. The chamber provides constant net radiation, temperature, humidity, and ventilation rate to the plant canopy. In this method, stepwise changes in chamber CO₂ alter canopy temperature, latent heat, and sensible heat fluxes simultaneously. Sensible heat and the radiometric canopy-to-air temperature difference are computed from direct measurements of net radiation, canopy transpiration, photosynthesis, radiometric temperature, and air temperature. The canopy aerodynamic conductance to the transfer of water vapor is then determined from a plot of sensible heat versus radiometric canopy-to-air temperature difference. Finally, canopy stomatal conductance is calculated from canopy surface and aerodynamic conductances. The canopy aerodynamic conductance was 5.5 mol m⁻² s⁻¹ in wheat and 2.5 mol m⁻² s⁻¹ in soybean canopies. At 400 umol mol⁻¹ of CO₂ and 86 kPa atmospheric pressure, canopy stomatal conductances were 2.1 mol m⁻² s⁻¹ for wheat and 1.1 mol m⁻² s⁻¹ for soybean, comparable to canopy stomatal conductances reported in field studies. This method measures canopy aerodynamic conductance in controlled-environment chambers where the log-wind profile approximation does not apply and provides an improved technique for measuring canopy-level responses of canopy stomatal conductance and the decoupling coefficient. The method was used to determine the response of canopy stomatal conductance to increased CO₂ concentration and to determine the sensitivity of canopy transpiration to changes in canopy stomatal conductance. These responses are useful for improving the prediction of ecosystem-level water fluxes in response to climatic variables.