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J.M.S. Scholberg, L.R. Parsons, and T.A. Wheaton

The interactive effects of irrigation rate and nitrogen concentration of the irrigation water on the growth of seedlings of two citrus rootstocks were studied. Four-month old seedlings of Swingle citrumelo [Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] and Volkamer lemon (C. volkameriana Ten. & Pasq.) were grown for ≈10 months in square citripots filled with a Candler fine sand. Plants were irrigated at 0.5, 0.75 or 1.0 times the evapotranspiration rate. Irrigation was applied using water containing 0, 7, 21, or 63 ppm nitrogen. Plant growth increased with irrigation rate and nitrogen concentration. Evapotranspiration rates, as determined from weight losses of reference plants, increased with nitrogen rate. Overall plant growth and weekly evaporation rates were greater with Volkamer than with Swingle. Leaf senescence of Swingle was more pronounced at low irrigation rates and/or low nitrogen concentrations than it was with Volkamer. Increasing nitrogen concentration of the irrigation water during the winter months reduced leaf senescence of both Swingle and Volkamer seedlings, and also promoted continuous growth in Volkamer. Leaf growth of Swingle ceased during the winter months, regardless of the nitrogen concentration of the irrigation water.

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J.M.S. Scholberg, L.R. Parsons, and T.A. Wheaton

Improving our understanding of processes that control and limit nitrogen uptake by citrus can provide a scientific basis for enhancing nitrogen fertilizer use efficiency. Nitrogen uptake dynamics of two rootstock seedlings will be compared to those of young budded trees. Three-month old Swingle citrumelo [Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] and Volkamer lemon (C. volkameriana Ten. & Pasq.) trees were planted in PVC columns filled with a Candler fine sand. Field experiments were conducted using 4-year-old `Hamlin' orange trees [Citrus sinensis (L.) Osb.] grafted on `Carrizo' [C. sinensis × Poncirus trifoliata (L.) Raf.] or on Swingle citrumelo. Trees were either grown in solution culture using 120-L PVC containers or in 900-L PVC tubs filled with a Candler fine sand. Additional trees were planted in the field during Spring 1998. Two lateral roots per tree were trained to grow in slanted, partly burried, 20-L PVC columns filled with a Candler fine sand. Nitrogen uptake from the soil was determined by comparing the residual N extracted by intensive leaching from planted units with that of non-planted (reference) units. With the application of dilute N solutions (7 mg N/L), plants reduced N concentrations to near-zero N concentrations within days. Applying N at higher concentrations (70 or 210 mg N/L) resulted in higher initial uptake rates, increased residual soil N levels, and reduced nitrogen uptake efficiency. Contributions of passive uptake to total nitrogen uptake ranged from less than 5% at soil solution concentrations around 3 ppm N to 20% to 30% at concentrations of 60 ppm N.

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Tim D. Davis, S.W. George, A. Upadhyaya, and J.M. Parsons

Factors affecting the greenhouse propagation of firebush (Hamelia patens) by leafy stem cuttings during winter were studied. Without bottom heat (BH), mid-day rooting medium temperature was 22 ± 3 C. About half of the auxin-treated cuttings without BH rooted. Maintaining the rooting medium at 29-39 C increased rooting for auxin-treated cuttings to 96-100% and increased root length and visual rating scores several-fold. Rooting percentage, root length, and visual ratings were consistently high in perlite and low in peat. Stem-tip cuttings and sub-terminal stem segment cuttings with basal stem diameters of 3-5 mm rooted slightly better than stem segment cuttings with basal diameters of 6-8 mm. Stem-tip cuttings not treated with auxin but with BH had rooting percentages of 81-86%. Treatment of stem-tip cuttings with auxin generally yielded 90% rooting or above. Despite this, plants grown from auxin-treated cuttings were indistinguishable from plants grown from non-treated cuttings 2 months after the rooting period. Of the variables studied, BH had the most dramatic effect on rooting of firebush cuttings during winter months.

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Tim D. Davis, Daksha Sankhla, N. Sankhla, A. Upadhyaya, J.M. Parsons, and S.W. George

Seeds of Aquilegia chrysantha Gray were germinated under a variety of temperature regimes. Germination was nearly 90% under a day/night cycle of 25/20C, but was reduced to ≤ 40% under constant 25C or a 25/10C day/night cycle. With days between 25 and 29C (night = 20C), germination percentage dropped gradually to ≈ 60% with increasing temperature. With days >29C, germination declined dramatically such that no germination occurred at 31C. Neither kinetin (4.6 to 46 μm) nor ethephon (6.9 to 207 μm) was able to reverse the inhibitory effects of 33C days. Our results indicate that germination of A. chrysantha seed is sensitive to temperature and that germination ≈ 75% can be obtained under a 25 to 27C day/20C night regime. Chemical names used: 2-chloroethylphosphonic acid (ethephon); 6-furfurylaminopurine (kinetin).

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L. Brandenberger, M. Baker, D. Bender, F. Dainello, R. Earhart, J. Parsons, R. Roberts, N. Roe, L. Stein, M. Valdez, K. White, and R. Wiedenfeld

During the past several years, watermelon trials have been performed in the state, but not as a coordinated effort. Extensive planning in 1997 led to the establishment of a statewide watermelon trial during the 1998 growing season. The trial was performed in five major production areas of the state including: The Winter Garden (Carrizo Springs); South Plains (Lubbock); East Texas (Overton); Cross Timbers (Stephenville); and the Lower Rio Grande Valley (Weslaco). Twenty seedless and 25 seeded hybrids were evaluated at each location. Drip irrigation with black plastic mulch on free-standing soil beds was used to grow entries in each area trial and yield data was recorded in a similar manner for each site. Results were reported in a statewide extension newsletter. Future plans include a continuation of the trial in the hope that multiple-year data will provide a basis for valid variety recommendations for watermelon producers in all areas of the state.