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Thomas E. Marler, Michael V. Mickelbart and Roland Quitugua

Leaves of container-grown papaya (Carica papaya L.) plants were inoculated with papaya ringspot virus (PRV) to determine its influence on dark respiration and photosynthesis. Photosynthetic capacity, apparent quantum yield, ratio of variable to maximum fluorescence from dark-adapted leaves, and photosynthetic CO2-use efficiency were reduced by PRV infection. Internal CO2 partial pressure at ambient external CO2 was not affected, but leaf dark respiration was increased by PRV infection. These results suggest that reduced growth, yield, and fruit quality common in PRV-infected papaya plants is caused, at least partially, by reduced photosynthesis and increased respiration.

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Thomas E. Marler, Vivian Lee and Christopher A. Shaw

The concentration of two steryl glucosides and their sterol precursors were measured in gametophyte tissue of Cycas micronesica K.D. Hill seeds before and after two storage treatments. For one study, intact 14-month-old seeds were stored at room temperature for 10 months. For the second study, cleaned (sarcotesta tissue removed) 22-month-old seeds were sown in perlite propagation beds, and ungerminated seeds were harvested after 14 months. Concentration of the steryl glucosides and sterols in the gametophyte tissue did not differ between the fresh seeds and the seeds after storage. The function of these metabolites is not known, but their unexpected stability for up to 14 months of storage indicates they are not metabolized during storage and may be vital during germination and early seedling growth. Implications on human exposure to their neurotoxic effects from gametophyte flour ingestion are discussed.

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Thomas E. Marler, Anders Lindström and Jack B. Fisher

Dimensions of pith, vascular tissue, cortex, live leaf bases, and periderm layers comprising the diameter of the stems of six Cycas species were measured at the standardized stem height where two vascular cylinders existed. The six species represent a range in susceptibility to injuries that occur in routine horticultural operations. We assigned a subjective numeric ranking from 1 for difficult to 10 for easy and then determined if this ranking correlated with any of the dimension characteristics. Pith diameter and cortex width differed among the species with the highly sensitive C. macrocarpa Griff. exhibiting the widest pith and most narrow cortex. Width of tissues peripheral to the vascular tissue (cortex, leaf base, and periderm layers) also differed among the species as did the proportion of total stem diameter occupied by these peripheral tissues. The sensitive C. macrocarpa exhibited the smallest values for these two variables. Simple correlation and multiple regression analyses indicated cortex width, total stem diameter, absolute width of peripheral tissues, and the relative proportion of these peripheral tissues in relation to stem diameter were positively correlated with susceptibility ranking. Of these, the relative proportion of peripheral tissues emerged as the variable with the most significant association with susceptibility ranking. Among these six representative species, the species that tend to be least susceptible to injuries during horticultural operations protect the youngest vascular tissues within a relatively wide zone of peripheral tissue. In contrast, the sensitive species exhibit a narrow zone of protective peripheral tissues.

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Thomas E. Marler, Anders J. Lindström and L. Irene Terry

The extent of Chilades pandava Horsfield herbivory among 85 Cycadaceae species was determined by three evaluators in a common garden setting in Thailand to identify patterns that may improve horticultural and conservation management practices. The significant differences in herbivory damage from this invasive lepidopteran pest ranged 8.7-fold among the species. Phylogenetic sections of this monogeneric cycad family did not correspond to the relative differences among the species, and country of nativity was also not informative for this purpose. We suggest the Cycas L. species that share native habitat with this butterfly or the closely related Theclinesthes onycha Hewitson are among the least damaged taxa when they are comingled with other Cycas species in a common landscape. Grouping the most damaged Cycas species together in a managed landscape may reduce costs associated with plant protection. The inclusion of non-native Cycas plants in gardens nearby native Cycas habitats carries the potential of disrupting the delicate specialist relationship that native butterfly populations have with host Cycas species.

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Thomas E. Marler, Louann C. Guzman and John H. Lawrence

Acacia auriculiformis, A. mangium, and A. koa trees were grown in 5.4-liter containers under conditions of 100%, 44%, or 19% sunlight transmission to determine biomass accumulation and partitioning and phyllode gas-exchange responses to developmental light level. Following ≈100 days of growth, all three species exhibited a linear decrease in relative growth rate and biomass accumulation in response to developmental light level. The influence of reduced developmental light level on growth was similar for the three species, with biomass accumulation under 19% transmission averaging ≈20% of that under full sunlight. In a second study, the diurnal pattern of gas exchange of mature phyllodes was determined. Gas exchange of phyllodes under 19% or 44% transmission depended on photosynthetic photon flux throughout the day. In contrast, gas exchange of phyllodes in 100% transmission was highest in early to midmorning on sunny days. Phyllode gas exchange slowly declined thereafter for A. mangium and A. koa, but rapidly declined then slightly recovered in late afternoon for A. auriculiformis.

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Thomas E. Marler, Vivian Lee and Christopher A. Shaw

Consumption of Cycas micronesica seed tissue has been associated with the amyotrophic lateral sclerosis–parkinsonism dementia complex (ALS–PDC) of the Western Pacific. However, failures to document vital plant and neighborhood descriptors and pronounced variability in toxin concentrations noted within and among studies obfuscate decades of research on this subject. We discuss the theoretical and experimental constraints of plant tissue sampling in relation to human disease research. Comparisons are made between this approach and methods used throughout the history of ALS–PDC research, most notably very recent reports concerning β-methyl-amino-alanine. Methods for studying possible plant neurotoxins need to be standardized and must follow rigorous criteria to be valid in principle. Our discussions reveal why these criteria are essential and highlight the impact that natural variations have on environmental toxin quantification and interpretation. Past research on cycad toxins is deficient on experimental and theoretical grounds, and interpretation of published data is dominated by ambiguities. This area of study as conventionally conceived and carried out needs transforming. We argue that future empirical studies should honor appropriate plant science standards concomitantly with medical science standards. This dual focus will ensure appropriate sampling scheme, sample size, and reporting of background plant and community factors known to influence phenotypic plasticity.

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Thomas E. Marler, Vivian Lee and Christopher A. Shaw

Amyotrophic lateral sclerosis–parkinsonism dementia complex (ALS–PDC) of the Western Pacific has been linked to the consumption of washed Cycas micronesica seed tissue. The search for a causal toxin in the seed tissue has generated decades of research, yet none of the published reports include an adequate description of sampling methods. We set out to design and conduct a study to serve as a model for future research. We used three populations of plants with similar recent plant life history, size, shade, seed load, and co-occurring species to determine intra- and interplant variation of four steryl glucoside variants. Variation was greatest among tissue types within seeds, intermediate among plants, and least among locations within plants. Results demonstrate the need of adhering to appropriate sampling protocols in cycad biochemistry research. Uses of appropriate sampling scheme and sample size are clearly required to avoid artifacts as this important area of research progresses.

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Thomas E. Marler, Ruben dela Cruz and Andrea L. Blas

Four papaya (Carica papaya L.) cultivars were cultured aeroponically or in perlite to determine the magnitude, timing, and root locality of Fe reductase induced by Fe deficiency. Five soybean [Glycine max (L.) Merrill] lines with a known range of Fe-deficiency chlorosis scores were cultured in perlite for comparison. Speed of inducement of Fe reductase activity was determined in plants cultured without Fe for 0 to 17 days. Location of Fe reductase activity was determined by sectioning roots from the tip to 60 to 70 mm proximal to the root tip from plants cultured without Fe for 16 to 19 days. The Fe reductase system was induced in all papaya cultivars after 7 to 11 days without Fe, and activity increased through 17 days. Iron reductase activity in all papaya cultivars was comparable to the most tolerant soybean line. The zone of highest activity was the apical 10 mm of roots. These results indicate that papaya roots are highly efficient in induced Fe reductase activity. The highest activity in root tips underscores the importance of maintaining a healthy, continually growing root system with numerous growing points when culturing papaya in alkaline substrates.

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Thomas E. Marler, Bruce Schaffer and Jonathan H. Crane

Growth and leaf physiology responses of container-grown `Arkin' carambola (Averrhoa carambola L.) trees to long-term exposure of ≈25%, ≈50%, or 100% sunlight were studied in four experiments in Guam and Florida. Shading increased rachis length and leaflet area, and decreased leaflet thickness. Shaded trees also had a more horizontal branch orientation. Shading reduced dark respiration (Rd) and light compensation and saturation points but increased chlorophyll concentration and N-use efficiency. Light-saturated net CO2 assimilation (A) was not affected by developmental light level. Trees in full sun had smaller total leaf area, canopy diameter, and shoot: root ratio and exhibited leaflet movement to avoid direct solar radiation. Also, trees grown in 100% sunlight had a more vertical branch orientation and greater stomatal density than shaded trees. The ratio of variable to maximum fluorescence (Fv/Fm) declined during midday in 100% sunlight trees. This pattern was accompanied by a midday suppression of A in 100% sunlight-grown trees in Guam. `Arkin' carambola trees exposed to ≈25%, ≈50%, or 100% sunlight for up to 39 weeks exhibited physiological and morphological adaptations that resulted in similar growth. These results indicate that carambola efficiently adapts to different developmental light intensities.

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Trenton Hamada, Irene Terry, Robert Roemer and Thomas E. Marler

We have investigated the potential movement on air currents of pollen from Guam’s native cycad, Cycas micronesica, proposed as ambophilic. We measured wind velocities and directions in different cycad habitats that vary in their exposure to trade winds, determined pollen settling velocities, and then modeled the potential horizontal pollen drift distance in each habitat. Similar measurements were performed on several entomophilous Zamia cycads and six zoophilous tropical trees used in horticulture or landscaping. All cycad species’ pollen exhibited relatively slow mean settling velocities (0.73–1.29 cm·s−1) with C. micronesica pollen in the middle of this range. Our models predicted that wind in more open habitats with wind directions east northeast (ENE) to northeast (NE) and velocities >2 m·s−1 could transport C. micronesica pollen, either single grains or clumps, hundreds of meters downwind from the pollen source before falling 1 or 2 m. In forested habitats and at typical heights of cycad cones in the understory, the mean wind velocities ranged from <0.03 m·s−1 to ≈1 m·s−1. In habitats with mean winds ≥0.2 m·s−1, models predicted pollen transport distances of tens of meters from the pollen source. In sheltered habitats with velocities near 0.03 m·s−1, the potential wind transport of pollen was limited to less than a few meters, suggesting that wind would be an ineffective vector in such areas. Pollen grains of all angiosperm species were larger except one, and the species with larger grains had settling velocities 3–26 times faster than that of cycad pollens. Even so, winds in most Guam environments could transport pollen of most angiosperm species over 50 m before falling 1 m. In summary, the results suggested that pollen size, clumping tendencies, and drift of most of these species do not preclude a role for wind in moving pollen in habitats exposed to trade winds, and that other physical and plant characteristics affect their pollination mode. For C. micronesica, these pollen and plant attributes do not preclude entomophily, and insects are likely required in the deep understory where cycads are present.