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In addition to photosynthesis, light is a critical mediator of secondary metabolism in plants, signaling the production of potentially health-promoting phytochemicals and regulating the emission of volatile organic compounds (VOCs) that can alter the sensory perception of a tomato. Light-emitting diodes (LEDs) are a viable way to test the effects of individual wavebands of light and are being quickly adopted by the greenhouse tomato industry. However, studies characterizing the effects of specific wavelengths of light or supplemental lighting on phytochemical content in general are lacking. We hypothesized that enriching the amount of supplemental blue and/or red light that tomatoes receive would positively affect the amount of carotenoids and phenolic compounds that accumulate in tomato fruits through cryptochrome and/or phytochrome-dependent signaling pathways. To test this hypothesis, we compared the chemical and sensory characteristics of tomatoes grown with overhead high-pressure sodium (OH-HPS) lamps to those grown with intracanopy (IC)-LEDs emitting different ratios of red, blue, and far red light. Tomatoes were profiled for total soluble solids, titratable acidity, ascorbic acid content, pH, total phenolics, and prominent flavonoids and carotenoids. Our studies indicated that greenhouse tomato fruit quality was only marginally affected by supplemental light treatments. Moreover, consumer sensory panel data indicated that tomatoes grown under different lighting treatments were comparable across the lighting treatments tested. Our research suggests that the dynamic light environment inherent to greenhouse production systems may nullify the effects of wavelengths of light used in our studies on specific aspects of fruit secondary metabolism.

Implementing nutrient management strategies in soilless culture, which improve water use efficiency (WUE) and limit the loss of eutrophying elements without affecting crop performance, is a priority for the floriculture industry. The aim of the current research was to assess the effect of two nutrient management strategies, based on electrical conductivity (EC) or nitrate-nitrogen (N-NO₃ ⁻) concentration control on plant growth, ornamental quality, plant–water relations, mineral composition, and WUE of greenhouse Hippeastrum grown in semiclosed soilless system. The recirculating nutrient solution was discharged whenever a threshold EC value of 3.0 dS·m⁻¹ was reached (EC-based strategy), or when N-NO₃ ⁻ concentration decreased below the limit of 1.0 mol·m⁻³ (nitrate-based strategy). There were no significant differences in terms of plant growth parameters, stomatal resistance, leaf water relations, and macronutrient composition in plant tissues between the two nutrient management strategies. In the EC- and the nitrate-based strategies, the recirculating nutrient solution was flushed 10 and 5 times, respectively. The water loss (W _L) and the total water use (W _use) in the EC-based strategy were significantly higher by 261.1% and 61.5%, respectively, compared with the N-NO₃ ⁻-based strategy. In contrast with the EC-based strategy, the adoption of the N-NO₃ ⁻-based strategy significantly minimized the nitrate, phosphate, and potassium emissions to the environment. The effective WUE of the system (WUE_S) recorded in the N-NO₃ ⁻-based strategy was higher by 55.9% compared with the one recorded with the EC-based strategy.

Abstract

A hydroponic experiment was conducted to determine the relationship between mycorrhizal dependency (MD) of cowpea [Vigna unguiculata (L.) Walp.] cultivars and their root morphology. Seeds of 19 cowpea cultivars with known MD levels were inoculated with Glomus fasciculatum and Bradyrhizobium in seedling trays. Twelve-day-old seedlings were transferred to a hydroponic culture system, where they were grown for 5 weeks. Leaf area, length of taproot, total root length, root weight, root abundance, average length of fine roots, number of nodules formed on lateral roots, and total nodule weight differed among cultivars. Less than 5% of the root length was colonized by mycorrhizal fungus in all cultivars. Average length of fine roots was negatively correlated with MD of cowpea cultivars; however, only 27% of the variability in MD was explained by this variable. Therefore, root morphology did not appear to determine MD in cowpea.

Abstract

Defruited cucumber (Cucumis sativus L.) plants were grown hydroponically for 28 days in containers with 4.5 liters of capacity, in which constant solution depths of 1, 5, 50, and 170 mm were maintained. The plants grown in the 1- and 5-mm-deep solutions grew more slowly than those in the deeper solutions. Both root and shoot growth were reduced at the shallow depths, but shoot growth was affected more than root growth. Thus, the shoot : root ratios were considerably smaller in the shallower than in the deeper solutions. The root systems in the shallower solutions, initially, were relatively more branched than in the deeper solutions. The shallow solutions caused the plants to allocate a higher proportion of their photosynthetic resources to the root at the expense of leaf growth. In the shallow solutions, a progressively higher proportion of this root growth became exposed above the solution, and, therefore, could not contribute to the absorption of water and nutrients. Control of solution depth may be a useful tool for controlling the vigor of the shoots of cucumber and the data presented may explain why growth problems have been experienced with this crop, particularly where a very thin film of nutrient is used, as in nutrient film technique.

Two greenhouse cucumber (Cucumis sativus) cultivars with differing fruit types [European (`Bologna') and Beit-alpha (`Sarig')] were grown during two seasons in a perlite medium in black plastic nursery containers in a passively ventilated greenhouse in northern Florida to evaluate fruiting responses to nitrogen (N) fertilization over the range of 75 to 375 mg·L^–1. Fruit production, consisting mostly of fancy fruits, increased quadratically with N concentration in the nutrient solution, leveling off above 225 mg·L^–1 for both cucumber cultivars. Fruit length and diameter were not affected by N concentration in the nutrient solution. Leaf N concentration, averaged over three sampling dates, increased linearly with N concentration in the nutrient solution from 46 g·kg^–1 with 75 mg·L^–1 N to 50 g·kg^–1 with 375 mg·L^–1 N. Fruit firmness decreased with increasing N concentration and there was little difference in firmness between the two cultivars. Firmness was similar across three measurement dates during the spring harvest season, but increased during the season in the fall. Fruit color responses to N concentration were dependent on the specific combination of experiment, sampling date, and cultivar. For most combinations of experiment, sampling date, and cultivar, cucumber epidermal color was greener (higher hue angle) with increased N concentration. The color was darkest (lowest L^* value) and most intense (highest chroma value) with intermediate to higher N concentrations.

Abstract

Three rose cultivars, Ilona, Mercedes, Sonia, on Rosa multiflora rootstock were grown in a nutrient film technique (NFT) system for 2 years, with root-zone warming (RZW) to 25°C compared with ambient temperature roots. In the 1st season the night air temperatures were 18°, 12°, and no heating (9°); in the 2nd season, 18°, 14°, and 10°. Harvested flowers were graded according to stem length. In the 1st winter seasons RZW increased the proportion of long stemmed roses and increased the total yield, especially in ‘Ilona’. In the 2nd winter season, RZW again increased the proportion of long stemmed roses in ‘Ilona’ but increased the total number of blooms more in the other cultivars. The effects of RZW persisted into the summer period. Prevailing wholesale prices were used to calculate probable gross returns based on yields. Since RZW tended to give longer stemmed roses and more blooms than did ambient conditions, this treatment enhanced returns more than that of the increased air temperature treatments. RZW increased probable returns over the ambient for ‘Ilona’, ‘Mercedes’, and ‘Sonia’ by 49%, 69%, and 78%, respectively.

Cloudy days cause an abrupt reduction in daily photosynthetic photon flux (PPF), but we have a poor understanding of how plants acclimate to this change. We used a unique 10-chamber, steady-state, gas-exchange system to continuously measure daily photosynthesis and night respiration of populations of a starch accumulator [tomato (Lycopersicon esculentum Mill. cv. Micro-Tina)] and a sucrose accumulator [lettuce (Lactuca sativa L. cv. Grand Rapids)] over 42 days. All measurements were done at elevated CO₂ (1200 μmol·mol^-1) to avoid any CO₂ limitations and included both shoots and roots. We integrated photosynthesis and respiration measurements separately to determine daily net carbon gain and carbon use efficiency (CUE) as the ratio of daily net C gain to total day-time C fixed over the 42-day period. After 16 to 20 days of growth in constant PPF, plants in some chambers were subjected to an abrupt PPF reduction to simulate shade or a series of cloudy days. The immediate effect and the long term acclimation rate were assessed from canopy quantum yield and carbon use efficiency. The effect of shade on carbon use efficiency and acclimation was much slower than predicted by widely used growth models. It took 12 days for tomato populations to recover their original CUE and lettuce CUE never completely acclimated. Tomatoes, the starch accumulator, acclimated to low light more rapidly than lettuce, the sucrose accumulator. Plant growth models should be modified to include the photosynthesis/respiration imbalance and resulting inefficiency of carbon gain associated with changing PPF conditions on cloudy days.

Recycling wastewater containing soaps and detergents for plant growth is highly desirable when fresh water is limited. This is especially true during times of drought and is imperative in some specialized situations such as a regenerative space habitat. To regenerate food, water, and air, the National Aeronautics and Space Administration's Controlled Ecological Life Support System (CELSS) must recycle wastewater commonly known as gray water. The anionic surfactant Igepon is the principal ingredient of many detergent formulations and soaps and is a prime candidate for use in a space habitat. To determine if gray water would have phytotoxic effects on crops grown in a CELSS, `Waldmann's Green' lettuce (Lactuca sativa L.) was grown in nutrient solutions containing varying concentrations of Igepon TC-42. Igepon concentrations of 250 mg·L^-1 or higher in nutrient solutions resulted in phytotoxic effects in lettuce. Thus, the toxic threshold of Igepon is <250 mg·L^-1. Toxicity symptoms include browning of the roots within 4 hours of exposure to Igepon followed by suppression of root dry mass within 24 hours. Plant death never resulted from exposure to Igepon used in these experiments, although roots were killed. The phytotoxic effect of Igepon was not persistent; plants initially displaying acute toxicity show clear signs of recovery within 3 days of initial exposure. Further, when fresh plants were exposed to these same nutrient solutions 3 days or more following initial Igepon addition, no phytotoxic effect was observed. The elimination of the phytotoxicity was associated with a decrease in fatty acid components in the nutrient solution associated with Igepon. The degradation of phytotoxicity appears to be associated with microbes present on the surface of the roots and not directly due to any plant process or instability of the surfactant.

Rapid, synchronized, and high percentage of germination is required for commercial spinach (Spinacia oleracea L.) production using hydroponic techniques. Seed treatments examined to improve seed germination were: 1) decoating; 2) leaching in water; and 3) soaking seeds for 4 hours in 0.5% NaOCl, leaching for 15 hours in water, and sowing in 0.3% H₂O₂ (this treatment will be referred to as NaOCl/H₂O₂). Germination studies were conducted on four cultivars at a constant 18 °C (optimal) or 30 °C (inhibitory). At 18 °C, germination rate (T₅₀) was maximized by both hydration treatments, but uniformity of germination (T_sd) was greatest for decoated seeds; final germination was ≥89% for all treatments. At 30 °C, decoating resulted in greatest uniformity of germination. The NaOCl/H₂O₂ treatment resulted in highest germination (94%) at the high temperature, whereas decoating was least effective (69%). Reduced germination of decoated seeds was attributed to atypical germinants. Cultivars differed in response to the treatments at both temperatures. Component analysis of the NaOCl/H₂O₂ treatment was studied with two slow-to-germinate cultivars. Treatment with H₂O₂, with or without NaOCl, improved the rate, uniformity, and percentage of germination of seeds of both cultivars, but NaOCl alone did not. Pericarp removal or pericarp removal plus NaOCl/H₂O₂ treatments reduced variability in germination time and enhanced speed of germination at 30 °C, but decoating produced a higher percentage of atypical seedlings than did other treatments. Therefore, the NaOCl/H₂O₂ treatment is recommended for growers who are unable to maintain cool germination temperatures and/or cannot afford the costs associated with cooling. If growers can maintain a germination temperature of ≈18 °C, decoated seeds are preferable, based on the high uniformity of germination.

An increase in nutrient solution concentration to produce high-quality fruit vegetables, such as tomatoes, may reduce growth and yield. One reason might be inhibition of photosynthesis, but results of photosynthesis studies in the literature are inconsistent. In this study, we investigated growth and photosynthesis of whole `Celebrity' and `Counter' tomato [Lycopersicon esculentum (L.) Mill.] plants in response to nutrient solution concentration, measured as electrical conductivity (EC). The effects of two levels of photosynthetic photon flux density (PPF = 400 or 625 μmol·m^-2·s^-1) on plant response to nutrient solution EC in a range between 1.25 to 8.75 dS·m^-1 in a series of four experiments in gas exchange chambers placed in larger growth chambers were examined. Increasing PPF enhanced tomato growth and photosynthesis but increasing EC diminished them. Reduction of dry weight was 1.9% to 7.3%, while plant photosynthesis was reduced between 1.7% and 4.5% for each 1 dS·m^-1. Increasing EC did not decrease dry matter content and leaf photosynthesis. Mean plant dry matter content ranged between 70 and 95 g·kg^-1, and net leaf photosynthesis on the last measurement day was between 7.5 and 11.3 μmol·m^-2·s^-1, depending on experiment. The decrease in whole plant photosynthesis with an increase in EC was caused by decreased leaf area but not by a decrease in leaf photosynthesis.