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Thomas E. Marler and Yasmina Zozor

Leaf gas exchange, chlorophyll fluorescence, water relations, and mineral nutrient relations responses of Annona squamosa seedlings to mild salinity were studied in sand culture in five experiments during 1990, 1991, and 1993. Trees were irrigated with a complete nutrient solution (control) or with this solution amended to 3 or 6 dS·m-1 with sea salt. Inhibition of net CO2 assimilation, stomatal conductance of CO2, and transpiration was apparent within 2 weeks of initiating salinity treatments, and gas exchange continued to decline until day 30 to 35. The diurnal pattern of leaf gas exchange was not altered by increased salinity. Salinity reduced CO2, light energy, and water-use efficiencies. Salinity sometimes reduced the ratio of variable to maximum fluorescence below that of the control, and this response was highly dependent on the ambient light conditions that preceded the measurements. Dark respiration was unaffected by salinity stress. Root zone salinity of 3 dS·m-1 administered for 52 days did not influence foliar sodium concentration or the ratio of sodium to potassium, but increased chloride concentration and decreased nitrogen concentration. The sodium response indicated that some form of exclusion or compartmentation occurred. Salinity reduced osmotic potential of root tissue but did not influence foliar osmotic or predawn xylem potential. These results indicate that A. squamosa is sensitive to salinity stress, and that the responses to salinity are consistent with other salt-sensitive woody perennial species.

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Thomas E. Marler and Nirmala Dongol

Cycas micronesica leaf and strobili expansion patterns were measured in three locations and seasons on Guam and then were fitted with non-linear models to evaluate the use of the parameters for informing management decisions. All growth curves except for microstrobili height conformed to a negative exponential function. Microstrobili height development could not be fitted with any traditional linear or non-linear function, so spline models were used to smooth the effect of elapsed days. Leaf and leaflet expansion patterns were influenced by habitat and season, indicating development of the vegetative organ is plastic. In contrast, the models that described development of megasporophyll length, microstrobili height, and microstrobili diameter were not influenced by habitat or season. Moreover, seed diameter developmental patterns were only minimally influenced by location. These results indicate developmental patterns of the reproductive structures were primarily constitutive. We have demonstrated two empirical approaches to fitting models of Cycas micronesica organ growth and development and that both methods are useful for determining the influence of spatial or temporal factors in the timing of organ development. This approach may be used to inform horticultural or conservation questions of other rare cycad species.

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Thomas E. Marler and L.E. Willis

`Mauritius' lychee (Litchi chinensis Sonn.) trees were planted in root observation chambers in July 1990 to determine the pattern of root and shoot growth during a 12 month period. Root and shoot lengths were measured at intervals ranging from 7-18 days, from Aug. 1990 until Aug. 1991. During each period of active canopy growth, up to six shoots were tagged and measured. Root growth was determined by measuring tracings the extension of each root in a visible plane from the glass wall of the observation chambers. Shoot growth was cyclic, with distinct periods of extension followed by no extension. Alternatively, root growth was fairly continuous with only a few periods of no apparent extension. There were no consistent relationships observed between root and shoot growth patterns.

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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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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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Thomas E. Marler and Patrick D. Lawton

`Smooth Cayenne' pineapple plants were propagated from suckers of uniform size. The plants were grown in containers, and a large percentage of them naturally flowered in synchrony at 13 months. This provided an opportunity to study the accumulation and partitioning of dry matter in fruiting and nonfruiting plants of uniform age. Six plants with or without fruit were harvested when the syncarps developed color. Plants and fruit were separated into crown, syncarp, slips, peduncle, leaves, stems, and roots. Plants without fruit were separated into leaves, stem, and roots. There were no suckers, and the stem was divided at the ground level. Leaves were counted and all tissue was dried to a constant weight. Total dry matter accumulation and the ratio of below- to above-ground dry matter was not different between the two groups of plants. The plants were similar in size and leaf number at the time of flowering, but the number of leaves was fixed at that time in the plants with fruit. As a result, plants that did not flower had about twice as many leaves as the plants with fruit at the termination of the study. Thus, the vegetative growth of continued at a rate similar to that of the reproductive structures of the plants that did flower.

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Thomas E. Marler and Patrick J. Lawton

Leaflets of carambola were restricted to a horizontal position for 3.5-h during late morning and early afternoon on sunny days to determine the influence of natural leaflet movement on temperature and chlorophyll fluorescence. Adaxial temperature of these horizontal leaflets was 5-9 C higher than that of leaflets that were allowed to move in response to high light. Chlorophyll fluorescence was similarly affected. Leaflets that were allowed to move had a higher Fv/Fm than leaflets that were restricted in movement The results indicate that the presence of a pulvinus at the base of each leaflet of carambola leaves allows movement of the leaflet to avoid incident light. This natural leaflet movement under sunny conditions results in a lower temperature and a higher level of photochemical efficiency when compared with leaflets that are exposed to high light due to restricting their movement.

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Thomas E. Marler and Haluk M. Discekici

`Tainung 2' papaya seedlings were transplanted on 30 Jan. 1996 and irrigated with one, two, or three drip irrigation lines per row in one study and with 90°, 180°, or 360° microsprinkler spray patterns in a second study to determine the reproductive and vegetative growth responses to irrigation design. Variable irrigation duration was used to supply a homogeneous amount of water to each plant in the drip irrigation study. The trench profile method was used in the drip irrigation study, and a monolith method was used in the microsprinkler study to determine root distribution at the end of the dry season (30 May to 2 June). All fruits were harvested and weighed on 26 Aug. Roots proliferated underneath the drip lines during the dry season, and root concentration on the profile walls was inversely related to the number of drip lines. Root concentration underneath one drip line was 3.7 times greater, underneath two drip lines was 2.3 times greater, and underneath three drip lines was 1.9 times greater than root concentration in the non-irrigated zones. Roots also proliferated in the wetted zones of the microsprinkler spray patterns. Mean fruit weight and total harvested fruit weight did not differ among the irrigation treatments within each study. The results indicate that papaya roots are highly morphoplastic and proliferate in wetted zones under partial root volume irrigation. One drip line per row supplied ample irrigation coverage under the conditions of this study.

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Thomas E. Marler and Haluk M. Discekici

`Honey Jean #3' sweet corn was planted in one-half of a split-root culture system containing `Tainung 1' or `Known You 1' papaya seedlings to determine if papaya roots could transfer water to the corn seedlings. After the corn seedlings were established, water was withheld from both compartments (2/2) or only the compartment containing the corn seedlings (1/2). Control plants were grown with both halves well-watered. Pre-dawn relative water content (RWC) of corn leaves was measured as an indicator of drought stress. Following 11 days, root competition was relieved in half of the 1/2 plants by cutting the papaya root connection between the half with corn from the rest of the papaya culture system. RWC of 1/2 corn plants was maintained above that of 2/2 plants, but below that of control plants. After relieving root competition, the 1/2 plants in competition with papaya roots maintained higher RWC than the 1/2 plants relieved of competition. Leaf tissue of all corn plants except the control plants was necrotic by 30 days. The results indicate that development of drought stress in corn using this culture system was retarded by watering a portion of the papaya roots not associated with the corn roots. Drought stress was accelerated by relief of competition with papaya, which is evidence that water was being supplied by the papaya roots within the papaya/corn system.

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Thomas E. Marler and Hiphil S. Clemente

Trade winds occur throughout the year and drought occurs seasonally in many papaya (Carica papaya L.) production regions. We conducted four studies with `Known You 1' and `Sunrise' papaya seedlings to determine the combined influence of wind and water deficit on growth. We conducted three additional experiments to determine plant response to wind within a continuous dose range of 0 to 2.5 m·s–1. The main effects of wind and irrigation significantly reduced most response variables, such as dry weight components, leaf area, and height. However, the two factors acted independently of each other for every measure of plant growth. Thus, there was no departure from simple effects of an additive model for each main factor. The relationship between plant growth and wind between 0 and 2.5 m·s–1 could be described by a quadratic model. Results indicate that the influence of wind on plant growth cannot be studied without controlling or quantifying soil moisture among treatment groups. Practically, our results indicate that wind protection of young papaya plants may be warranted more so in the dry season than in the wet season or under sufficient irrigation practices.