Search Results

You are looking at 1 - 10 of 75 items for

  • Author or Editor: Daniel Leskovar x
  • Refine by Access: All x
Clear All Modify Search
Full access

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.

Free access

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.

Free access

Daniel Leskovar and Yahia Othman

Effective nutrition and irrigation are important nursery strategies to produce high-quality seedlings able to withstand heat and drought stress in the field. The objectives of this study were 2-fold, first to identify the influence of two nitrogen (N) levels (75 and 150 mg·L−1) and two fertigation (FR) methods, overhead (OH) and flotation (FL) of artichoke (Cynara cardunculus cv. Green Globe Improved) transplants on root/shoot growth and leaf physiology during the nursery period. A repeated greenhouse experiment was conducted and morphophysiological measurements were determined at 4 and 7 weeks after seeding (WAS). The second objective was to determine the impact of the nursery treatments (FR method and N level) on the subsequent crop growth and yield under three field irrigation methods [surface drip, subsurface drip, and overhead-linear system (OH-L)]. Field measurements were conducted at 50 and 150 days after field transplanting (DAT) during Fall–Winter 2015. Transplants fertilized with 75 mg·L−1 N (low N) had improved root components as compared to those with 150 mg·L−1 N (high N), especially at 4 WAS. The low N transplants had higher root surface area, root length, root branching, thinner root diameter, and less shoot area than the high N transplants. Wilting for low N transplants was 13.5% less than that for high N at 5 DAT, with a total yield similar or slightly higher than those of high N. Although growth of OH and FL transplants was statistically similar at transplanting, those irrigated with OH (greenhouse) had a 10% higher yield than FL irrigated transplants, regardless of the field irrigation method evaluated. Overall, low N level (75 mg·L−1 N) applied with OH irrigation in the nursery positively improved the transplant root system and transplant quality of artichoke seedlings.

Free access

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.

Free access

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.

Free access

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.

Free access

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.

Full access

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.

Free access

Daniel Leskovar and Yahia A. Othman

The impact of nitrogen application on the growth of olive trees has been well studied. However, little is known about the role of levels and forms of N on the development of roots and physiological traits during establishment of young trees. The objective of this 2-year study was to evaluate the influence of N source and level on shoot morphology (tree height, stem diameter, and branch number) and physiology [leaf area and fresh weight, photosynthesis (Pn), transpiration (E), and stomatal conductance (g s)], root components (length, diameter, surface area, and fork number) and N content of young olive (Olea europaea cv. Arbequina) cuttings. Three-month-old olive cuttings were planted in 15-L pots filled with a growing substrate composed of peatmoss + bark + sand (2:1:1/by volume) and placed in a screen house. Two N levels, 2.8 and 5.6 g/tree, and control (0 N) and four N sources, calcium nitrate (CN, 12%N, 17%Ca+2), ammonium nitrate (AN, 35%N), urea (46%N), and the slow-release Osmocote (OSC, 18%N, 6%P, 12%K) were evaluated. Effects of low– (2.8 g/tree) and high N (5.6 g/tree) levels on shoot components (plant height, diameter, branch number, leaf area, and fresh weight) and gas exchange (Pn, g s and E) were similar implying that the low rate of N was adequate for the establishment of young olive cuttings. Nitrogen sources, particularly AN and CN had significant effects on shoot and root morphology, physiology, and leaf and root N concentration. In fact, AN and CN were the best fertilizer sources for olive transplants in term of root and shoot growth. Overall, 2.8 g/tree N level and AN or CN sources were the best treatments for newly established olive ‘Arbequina’ trees. Root components treated with high N rate (5.6 g/tree N) using the slow-release fertilizer (OSC) was similar to those treated with the low AN rate (2.8 g/tree N). Therefore, for nursery containerized olive trees of ‘Arbequina’ or other cultivars with comparable growth rates, we recommend to apply the CN or AN source at 2.8 g/tree N or the controlled-released fertilizer OSC at 5.6 g/tree N.