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Thomas E. Marler and Michael V. Mickelbart

Growth response of containerized carambola (Averrhoa carambola L.) seedlings to GA applied to trunks in lanolin paste were studied under glasshouse conditions. Gibberellic acid at 0, 250, 500, or 750 mg·liter and an untreated control (no lanolin) were used. Internode length and increases in plant height and trunk cross-sectional area (TCA) did not differ for control and 0 mg·liter plants, but mostly increased with concentration of GA. Increase in TCA was determined in a second study with control and treated plants, using 500 mg GA/liter. Mean recommended graftable size (7 mm) was reached in 47 days in plants that were GA treated, and 93 days in control plants, suggesting that GA may be used to shorten nursery time for producing graftable carambola seedlings. Chemical name used: gibberellic acid (GA.,+,).

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Thomas E. Marler and Michael V. Mickelbart

Long-term effects on stomatal conductance of mechanical stress from repeated clamping of a porometer leaf cuvette to laminae of avocado (Persea americana Miller), carambola (Averrhoa carambolu L.), hibiscus (Hibiscus rosa-sinensis L.), mango (Mangifera indica L.), and sugar apple (Annona squamosa L.) plants were determined under glasshouse conditions. Following 10 weeks of applying the mechanical stimulus seven times during every 4th day to mature leaves, stomatal conductance was lower than for untreated leaves of all species except mango. Similarly, following 10.5 weeks of applying the stimulus one time every 4th day to expanding leaves of avocado, carambola, hibiscus, and sugar apple, stomatal conductance was lower than for untreated leaves of the same age in all species except hibiscus. Carambola and sugar apple were more sensitive to the mechanical stress than the other species. Thus, the indirect effect of leaf chamber clamping on gas exchange should be known before any conclusions are formulated regarding environmental, cultural, or genetic effects on gas exchange. Random leaf samples from a canopy instead of measurements on a fixed set of leaves may be more appropriate for repeated determinations of leaf gas exchange on a set of plants.

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Thomas E. Marler and Patrick D. Lawton

Temperature and chlorophyll fluorescence characteristics were determined on leaves of various horticultural species following a dark adaptation period where dark adaptation cuvettes were shielded from or exposed to solar radiation. In one study, temperature of Swietenia mahagoni (L.) Jacq. leaflets within cuvettes increased from ≈36C to ≈50C during a 30-minute exposure to solar radiation. Alternatively, when the leaflets and cuvettes were shielded from solar radiation, leaflet temperature declined to 33C in 10 to 15 minutes. In a second study, 16 horticultural species exhibited a lower variable: maximum fluorescence (Fv: Fm) when cuvettes were exposed to solar radiation during the 30-minute dark adaptation than when cuvettes were shielded. In a third study with S. mahagoni, the influence of self-shielding the cuvettes by wrapping them with white tape, white paper, or aluminum foil on temperature and fluorescence was compared to exposing or shielding the entire leaflet and cuvette. All of the shielding methods reduced leaflet temperature and increased the Fv: Fm ratio compared to leaving cuvettes exposed. These results indicate that heat stress from direct exposure to solar radiation is a potential source of error when interpreting chlorophyll fluorescence measurements on intact leaves. Methods for moderating or minimizing radiation interception during dark adaptation are recommended.

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Thomas E. Marler, Ross Miller and Aubrey Moore

Container-grown Cycas micronesica seedlings were purposefully infested with Aulacaspis yasumatsui and then installed at 0, 75, or 150 cm above the ground to investigate effects of infestation height on predation by Rhyzobius lophanthae. Significantly more scales on elevated seedlings were attacked by the predator. Our results indicate that lower predation at ground level by R. lophanthae may partly explain why the predator is not effectively controlling this armored scale epidemic on Guam. Ephemeral outbreaks of A. yasumatsui documented in quarterly surveys from Sept. 2006 until Aug. 2012 confirm the inadequate biological control. Our results illuminate the importance of fully evaluating the reliance on an alien predator as a solo biological control agent for an alien pest.

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Thomas E. Marler and John H. Lawrence

The leaf nutrient status and stoichiometry of nitrogen (N), phosphorus (P), and potassium (K) were determined for Elaeocarpus joga trees in Guam’s dominant calcareous soils to understand nutrient limitations in limestone soils of Oceania and contribute to global databases on leaf economic spectrum studies. Supplemental N, P, or K was added to soils to determine plant growth and nutrient concentration responses. Leaf and soil quantifications of nutrients enabled multiple trait comparisons. Supplemental N stimulated growth of young cultivated plants without affecting leaf N concentrations. Supplemental K increased leaf K concentration but did not generate a growth response. Supplemental P did not affect growth or leaf P concentration. N:P, N:K, and K:P were most influenced by K additions. Leaf N and P concentrations of mature trees in unmanaged settings were similar to unfertilized young plants in the controlled study, but leaf K concentration was greater in the mature trees. Leaf nutrient relations were not overtly related to soil nutrient relations for mature trees. Results indicate that N and K are the limiting factors in calcareous soils of the Mariana Islands for this endemic tree species, age and size of trees do not greatly influence leaf nutrient content, and leaf stoichiometry is constrained and less variable than soil stoichiometry.

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Michael V. Mickelbart and Thomas E. Marler

Sapodilla [Manilkara zapota (L.) Royen], reportedly tolerant of saline conditions relative to other tropical fruit species, was studied in sand culture under greenhouse conditions to examine the physiology of sapodilla trees exposed to NaCl and to aid in determining the basis for this apparent tolerance. Treatments, consisting of a complete nutrient solution of 1 dS·m–1 (control) or this solution amended to 12 or 20 dS·m–1 with NaCl, were administered from 16 Nov. 1991 until 29 Jan. 1992. Net CO2 assimilation (A) of plants receiving NaCl gradually decreased relative to that of the control plants. At the end of 8 weeks of salinity, A of plants receiving 12 or 20 dS·m–1 was 72% or 31% of control plants, respectively. Substrate NaCl reduced apparent quantum yield, photosynthetic CO2-use efficiency, leaf osmotic potential, and predawn xylem potential of sapodilla leaves. Dark respiration and the variable: maximal chlorophyll fluorescence ratio were not influenced by NaCl. Exposure to NaCl also increased leaf tissue Na+ and Cl concentrations and the Na+: K+ ratio. These results indicate that gas exchange of sapodilla is relatively low for woody evergreen species. Moreover, sapodilla may not be as tolerant of salt stress as previously reported. The responses of sapodilla to root zone NaCl were consistent with other woody perennial glycophyte species. Photochemical efficiency of leaves on plants receiving NaCl was not different from that of leaves on control plants for >8 weeks after NaCl reduced gas exchange.

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Thomas E. Marler and Haluk M. Discekici

Excavation of field-grown `Red Lady' and `Tainung #2' papaya plants was begun 3 months after transplanting to the field to characterize development of the papaya root system. The roots were separated into the taproot system and lateral roots within three size categories: <1, 1 to 5, and >5 mm. Length of the taproot system and the larger lateral roots was measured directly, and that of the smaller roots was determined using the line-intersect method. Mass of the various size categories was measured after drying at 70°C. A typical plant 3 months after field-planting was ≈ 60 cm tall and exhibited a root system radial spread of 34,636 cm2, total root length of 9613 cm, and total dry mass of 17.3 g. The taproot system accounted for >70% of the mass and <5% of the length of the root system. Lateral roots <1 mm in diameter accounted for <10% of the mass and >70% of the length of the root system. A typical plant during the heavy fruit set stage, about 6 months after field planting, was 175 cm tall and exhibited a root system radial spread of 101,736 cm2, total root length of 975 m, and total dry mass of 539 g. The taproot system accounted for ≈38% of the dry mass and <1% of the length of the root system. Lateral roots <1 mm in diameter accounted for ≈5% of the dry mass and 65% of the length of the root system. Plant age influenced root system characteristics more than cultivar, especially the proportional distribution of mass and length among the defined root classes.

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Thomas E. Marler and Louann C. Guzman

Growth and physiological responses of Intsia bijuga trees to flooding were determined in a series of five container experiments to assess the relative tolerance of this species to flooding. The first measurable response to flooding was reduced leaf gas exchange, which began within 5 to 6 days of the onset of flooding. Development of hypertrophied lenticels at the water line and paraheliotropic leaflet movement were evident by 17 days of flooding. Emergence of adventitious roots on the stem above the water line began after about 30 days of flooding. Leaflet abscission was greatly accelerated by flooding. After more than 3 months of flooding, regrowth of roots, stems, and leaves began within two weeks of draining the medium. The data and observations support a relative ranking of moderate flood tolerance for Intsia bijuga.

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Thomas E. Marler and Jonathan H. Crane

Lateral branches arising from the primary bud complex on limbs of containerized `Gefner' atemoya (Annona squamosa L. × A. cherimola Mill.) plants were removed to determine the influence of branch regrowth on the crotch angle. Pruning the lateral branches to a stub (cl cm) was more effective in inducing regrowth and increasing branch angle of the regrowth than stripping lateral branches by hand. Following lateral branch removal, regrowth did not develop from every node along a stem axis. In a second study, the angle of branch regrowth from tagged nodes following pruning of lateral branches was determined. The mean crotch angle of the primary lateral branches was 58°. Regrowth from the second and third supernumerary buds within each node produced branches with an average crotch angle of 72° and 88°, respectively. The largest increase in attachment angle following pruning was obtained at nodes with narrow primary lateral branches and at nodes located closest to the base of a major axis. The increase in branch crotch angle was not correlated with the size of the preceding lateral branch at a node. These results indicate that pruning off lateral branches with narrow crotch angles may be performed during training atemoya plants to produce scaffold limbs from supernumerary buds within the same nodes with desirably wide crotch angles.