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Thomas E. Marler and Leah E. Willis

`Mauritius' lychee (Litchi chinensis Sonn.) trees were planted in root observation chambers in July 1990 to determine the pattern of root and stem extension growth during 12 months. Root and stem lengths were measured at intervals ranging from 7 to 18 days from Aug. 1990 until Aug. 1991. During each period of active canopy growth, up to six stem tips were tagged and measured. Root growth was determined by measuring tracings of the extension of each root in a visible plane of the glass wall of the observation chambers. Stem growth was cyclic, with distinct periods of rapid extension followed by periods with no extension. In contrast, root growth was fairly continuous with only three periods of no visible root extension. Mean absolute extension rates were higher for stems than for roots. There were no consistent relationships between the timing of root and stem extension growth.

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Leah E. Willis and James E. Motes

Six experiments were conducted to determine the effect of priming on spinach seed performance. Performance was determined by percent, uniformity, and speed of germination after 10 days. In Expt. 1, performance at 22 °C was improved for primed seeds compared to unprimed seeds and germination was >90% for both primed and unprimed seeds. In Expt. 2 (incubator @ 40 °C for 16 h/30 °C for 8 h), germination was reduced for both seed treatments and primed seeds had more germination, but less uniformity than unprimed seeds. In Expt. 3 (incubator @ 40 °C for 16 h/30 °C for 8 h), initial temperatures were 40 °C for 16 h, 40 °C for 8 h, or 30 °C for 8 h. There was an interaction between priming and initial temperature for percent germination, indicating that only primed seeds varied in response to initial temperature. Priming improved percent germination but reduced uniformity and did not influence speed of germination. In Expt. 4 (growth chamber @ 40 °C for 16 h/30 °C for 8 h), priming significantly improved percent seedling emergence and speed compared to unprimed seeds but did not influence uniformity. In Expt. 5 (growth chamber @ 40 °C for 16 h/30 °C for 8 h) initial temperatures were 40 °C for 16 h, 40 °C for 8 h, or 30 °C for 8 h. Priming significantly improved seedling emergence and speed and did not affect uniformity of emergence. Seedling emergence was significantly improved for seeds planted at an initial temperature of 40 °C compared to 30 °C.

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Thomas E. Marler and Leah E. Willis

Leaf gas exchange characteristics for 16 species of cycad were determined under field conditions in Miami, Fla. Net CO2 assimilation (ACO2) ranged from 4.9 μmol·m-2·s-1 for Lepidozamia peroffskyana Regel to 10.1 μmol·m-2·s-1 for Zamia furfuracea L. fil. in Aiton. Stomatal conductance to H2O (gs) was more variable, ranging from 85 mmol·m-2·s-1 for Cycas seemannii A. Br. to 335 mmol·m-2·s-1 for Encephalartos hildebrandtii A. Br. & Bouche. Transpiration (E) ranged from 1.7 mmol·m-2·s-1 for Cycas chamberlainii W.H. Brown & Keinholz to 4.8 mmol·m-2·s-1 for Encephalartos hildebrandtii. Highly variable E was more controlling of water-use efficiency than the less-variable ACO2. The difference between air and pinnae temperature ranged from 0.3 to 1.6 °C and was inversely related to mean gs among the species. The values within geographic regions representative of the native habitats of the species were highly variable. For example, two of the African species exhibited the highest and lowest values of water-use efficiency in the survey. Leaf gas exchange for the four largest species with arborescent growth form was less than that for the three small species with subterranean or short bulbous growth form. The diurnal variation in leaf gas exchange for Zamia furfuracea exhibited a two-peaked pattern with a distinct midday depression in ACO2 and gs. The ratio of dark respiration to maximum ACO2 for Zamia furfuracea was 0.04. As a group, the values for ACO2 and gs for these cycads ranked at the lower end of the range for all plants species.

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Fred S. Davies and Leah E. Willis

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Leah E. Willis, Frederick S. Davies, and D.A. Graetz

One-year-old `Hamlin' orange [Citrus sinensis (L.) Osb.] trees on sour orange rootstock (C. aurantium L.) were used to compare various fertigation frequencies and rates with application of granular materials. In Expt. 1, granular fertilizer was applied five times per year or liquid fertilizer was applied five, 10, or 30 times per year at 0.23 kg N/tree per year as an 8N-3.4P-6.6K formulation. In Expt. 2, an additional treatment of granular and liquid material was applied three times per year, but fertilizer rate and formulation were the same as in Expt. 1. Experiment 3 included the same application frequencies as Expt. 1, but with two rates of N (0.11 or 0.06 kg N/tree per year). Soil samples were taken from each treatment 1, 4, and 7 days after fertilization at depths of 0-15, 16-46, and 47-76 cm for nutrient analyses. Trunk diameter, shoot growth, and tree height were similar for all treatments 8 months after planting in Expts. 1 and 2, while trees in Expt. 3 had significantly less growth at the lower rate. Soil NH4-N and NO3-N concentrations for all liquid treatments within 1 week of fertilization were highest for the five times per year treatment at the 0- to 15-cm depth, but nutrient concentrations of all liquid treatments were similar at the other depths. For most dates and depths, NH4-N and NO3-N concentrations were similar for both fertilizer rates.