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Daniel I. Leskovar

Irrigation methods, rates, timing, and frequency may influence the physical and chemical properties of the growing media thereby affecting root initiation, elongation, branching, development and dry matter partitioning between roots and shoots.

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Daniel I. Leskovar

Pepper cv. `Jupiter' plants were field-grown from containerized transplants produced with either overhead (SPl) or sub-flotation (SP2) irrigation, or from direct seeding, in 3 years. Shoot and root growth were measured at frequent intervals. At planting, SPl transplants had larger basal root length and numbers than SP2 transplants. At the end of the growth period, basal, lateral, and taproot dry weights accounted for 81, 15, and 4% of the total for transplants, and 25, 57, and 18% of the total for direct-seeded plants. The coordination of growth (linear logarithm relationship) between root and shoot, changed after fruit set only in transplants. Over all seasons, transplants exhibited significantly higher yields than direct-seeded pepper plants.

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Smiljana Goreta and Daniel I. Leskovar

Areas with mild climate conditions are suitable for growing winter spinach (Spinacia oleracea L.). Successful production depends on choosing slow-bolting cultivars resistant to major diseases in each area. Field experiments with a total of 18 cultivars were conducted during 8 years in the Winter Garden region of Texas, an area known for its high white rust (Albugo occidentalis G.W. Wils) inoculum. Spinach cultivars differed widely in their resistance to both white rust and bolting, and the incidence of both traits was more severe as the season progressed. White rust infection increased linearly with average monthly minimal air temperature. Cultivars Fidalgo, Springfield, and Springer were slow bolting and are suitable for areas with no white rust incidence, while cvs. ASR-318, DMC 66-09, Fall Green, Samish, and San Juan were more white rust resistant.

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Shinsuke Agehara and Daniel I. Leskovar

Height control is important to produce compact vegetable transplants that are suitable for shipping and transplanting. Although abscisic acid (ABA) inhibits stem elongation, it can also induce other growth modifications. To optimize its application timing for effective height control, we examined age-dependent sensitivity of various growth variables to ABA in diploid ‘Summer Flavor 800’ and triploid ‘Summer Sweet 5244’ watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai]. Seedlings were sprayed once with 1.9 mm ABA at 25, 18, or 11 days before transplanting (DBT) or twice with 0.95 mm ABA at 25 and 18 DBT. The application rate was 0.55 mg ABA per plant with a spray volume of 0.61 L·m−2 (1.1 mL/plant). Only the single-spray treatment at 25 DBT (cotyledon stage) suppressed plant height by inhibiting petiole elongation. This effect was similar in both cultivars with 13% to 14% reductions at the transplanting stage compared with the untreated control. Undesirable growth modifications were also induced by ABA. In both cultivars, all ABA treatments caused 16% to 23% shoot biomass reductions mainly by inhibiting leaf expansion. Additionally, ABA treatments reduced stem diameter and root biomass in ‘Summer Flavor 800’. The double-spray treatment had similar growth-modulating effects as the single-spray treatments, except that it induced cotyledon abscission in ‘Summer Flavor 800’. These results suggest that although ABA applied at the cotyledon stage can reduce watermelon transplant height, the benefit is limited because of overall growth reductions, which can occur regardless of application timing. On the other hand, in triploid ‘Summer Sweet 5244’, moderate shoot growth delay by ABA may be of value as a growth-holding strategy when transplanting is delayed because of inclement weather at the time of field establishment. Importantly, field evaluations demonstrated that the growth modulation by ABA is only transient with no negative impact on marketable yield and fruit quality.

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Shinsuke Agehara and Daniel I. Leskovar

Vegetable transplants grown in commercial high-density trays can quickly outgrow the optimal size for shipping and transplanting, limiting transplant performance, and marketing flexibility for commercial nurseries. Abscisic acid (ABA) and uniconazole can suppress shoot growth by inducing stress-adaptive responses and inhibiting gibberellin synthesis, respectively. We evaluated the effectiveness of the two growth regulators in prolonging marketability of ‘Florida 91’ and ‘Mariana’ tomato (Solanum lycopersicum L.) transplants at commercial nursery greenhouses in Texas and Florida. Spray treatments in the Texas experiment were 0 and 3.8 mm ABA at 7, 5, 3, or 1 days before maturity (DBM), and those in the Florida experiment were no spray control, 3.8 mm ABA at 7, 5, 3, or 1 DBM, and 34 μm uniconazole at 4 DBM. Both ABA and uniconazole showed minimal cultivar-specific effects. Different growth modifications were induced by ABA and uniconazole. First, suppression of stem elongation by ABA was reversible by 7 days after maturity (DAM), whereas that by uniconazole lasted for 20 days or until 16 DAM with up to 15% suppression in stem elongation. Second, only ABA inhibited leaf expansion and shoot dry matter accumulation. The primary growth-modulating effect of uniconazole was limited to height control, which is beneficial for producing compact transplants, rather than as a growth holding strategy. By contrast, the overall growth suppression by ABA is desirable for prolonging transplant marketability. Importantly, the magnitude of this growth suppression was moderate (up to 22% shoot biomass reduction at 8 DAM) and transient, followed by a rapid recovery. Furthermore, ABA caused relatively smaller inhibition in root growth, allowing sufficient root development and increasing the root-to-shoot ratio at 0 to 8 DAM. The growth suppression by ABA was maximal when it was applied at 7 to 5 DBM, indicating the age-dependent sensitivity of tomato seedlings to exogenous ABA. Although leaf chlorosis was induced by ABA in a similar age-dependent manner, it was transient and reversible by 7 DAM. These results suggest that ABA application 7 to 5 DBM is an effective growth holding strategy for tomato transplants.

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Desire Djidonou and Daniel I. Leskovar

The effect of different nitrogen (N) concentrations on growth changes, leaf N concentration and accumulation patterns, N nutrition index (NNI), fresh weight yield, and N use efficiency (NUE) was determined for lettuce grown over three consecutive seasons (fall, winter, and spring) in a recirculating hydroponic system, under unheated and naturally lit hoop house in Uvalde, TX. The lettuce cultivars Buttercrunch, Dragoon, and Sparx were grown at six N concentrations, initially 100, 150, 200, 250, 300, and 400 mg·L−1 using a nutrient film technique (NFT). Leaf number, accumulated dry weight (DW) and N, and leaf area index (LAI) followed a logistic trend over time, characterized by a slow increase during early growth followed by a linear increase to a maximum. By contrast, plant total N concentrations were the highest at early stage and decreased slightly over time. Effect of season and cultivar on these growth traits was more pronounced than that of the N concentrations. Averaged across cultivar and N concentrations, DW in spring was 73% and 34% greater than that in fall and winter, respectively. At each sampling date, there were linear, quadratic, or cubic effects of N concentrations on each of these variables. The cultivar Sparx was the most productive, with 63% and 32% higher fresh weight yield in fall, 145% and 114% in spring, than ‘Buttercrunch’ and ‘Dragoon’, respectively. Increasing nutrient solution N concentrations from 100 to 400 mg·L−1 increased the yield from 5.9 to 6.7 kg·m−2 in fall, 8.1 to 10.7 kg·m−2 in winter, and 10.3 to 12.6 kg·m−2 in spring. The NUE was the highest at the lowest N concentration (100 mg·L−1) and decreased with increasing N concentrations. The NNI during mid- to late-growth stages was near or greater than one, even at the lowest N. These results demonstrated that N concentrations of 100–150 mg·L−1 maximized the growth and yield of hydroponically grown lettuce.

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Shinsuke Agehara and Daniel I. Leskovar

Excess transpiration relative to water uptake often causes water stress in transplanted vegetable seedlings. Abscisic acid (ABA) can limit transpirational water loss by inducing stomatal closure and inhibiting leaf expansion. We examined the concentration effect of exogenous ABA on growth and physiology of muskmelon (Cucumis melo L.) seedlings during water stress and rehydration. Plants were treated with seven concentrations of ABA (0, 0.24, 0.47, 0.95, 1.89, 3.78, and 7.57 mm) and subjected to 4-day water withholding. Application of ABA improved the maintenance of leaf water potential and relative water content, while reducing electrolyte leakage. These effects were linear or exponential to ABA concentration and maximized at 7.57 mm. Gas-exchange measurements provided evidence that such stress control is attributed to ABA-induced stomatal closure. First, net CO2 assimilation rate and stomatal conductance initially decreased with increasing ABA concentration by up to 95% and 70%, respectively. A follow-up study (≤1.89 mm ABA) confirmed this result with or without water stress and further revealed a close positive correlation between intercellular CO2 concentration and net CO2 assimilation rate 1 day after treatment (r 2 > 0.83). In contrast, ABA did not affect leaf elongation, indicating that stress alleviation was not mediated by leaf area adjustment. After 18 days of post-stress daily irrigation, dry matter accumulation showed a quadratic concentration-response, increasing up to 1.89 mm by 38% and 44% in shoot and roots, respectively, followed by 16% to 18% decreases at >1.89 mm ABA. These results suggest that excess levels of ABA delay post-stress growth, despite the positive effect on the maintenance of water status and membrane integrity. Another negative side effect was chlorosis, which accelerated linearly with increasing ABA concentration, although it was reversible upon re-watering. The optimal application rate of ABA should minimize these negative effects, while keeping plant water stress to an acceptable level.

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Daniel I. Leskovar and Giovanni Piccinni

Restrictions placed on water usage for farmers have prompted the development of irrigation management projects aiming at water savings of economically important crops. The objective of this work was to determine yield, water use efficiency, and leaf quality responses to deficit irrigation rates of processing spinach (Spinacea oleracea L.) cultivars. Three irrigation treatments were imposed with a center pivot system, 100%, 75%, and 50% crop evapotranspiration rates (ETc). Commercial cultivars used were `DMC 09', `ASR 157', and `ACX 3665'. Leaf quality was significantly affected by deficit irrigation rate and cultivar. Leaf yellowness was highest at 50% ETc, and was more evident for `ACX 3665'. The percent excess stem (>10 cm) was higher at 100% ETc. This response was greater in `ACX 3665' than in `ASR 157' and `DMC 09'. Marketable yields were significantly higher for `ASR 157' at either 100% or 75% ETc rates, compared to `DMC 09' and `ACX 3665'. High water use efficiency was also measured at 75% ETc for `ASR 157'. Minimum canopy temperature differences were detected among the irrigation treatments. This work demonstrated that it is possible to reach a 25% water savings in one season, without reducing yields when using vigorous cultivars.

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Virgil Esensce and Daniel I. Leskovar

Spinach (Spinacia oleracea, L. cv. `Ark88-354'. `Fall Green', `Cascade') seeds of varying sensitivities to high temperatures during imbibition and germination were subjected to constant 18, 30 and 36°C for 96 hours during imbibition. Those cultivars less sensitive to high temperatures (`Ark88-354' and `Fall Green') imbibed water more rapidly at higher temperatures and had greater initial levels of raffinose and sucrose than the sensitive cultivar `Cascade'. Glucose levels were initially zero in all cultivars and increased slightly with time. Germination was more rapid at 18°C and 30°C in `Ark88-354' and `Fall Green' than with `Cascade'; the latter also failed 10 germinate at the higher temperature. Raffinose and sucrose have been implicated in membrane stabilization during desiccation and extreme low temperatures. They may serve a similar role during imbibition and germination of spinach at high temperatures, reducing secondary thermodormancy.