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Thomas E. Marler and Louann C. Guzman

Intsia bijuga is an important native tree on Guam, and is being promoted as an urban forestry tree. Container-grown I. bijuga trees were subjected to a drying cycle (50% of mean water loss replaced daily) to determine physiological responses to drought. Early to mid-morning gas exchange began to decline compared to well-watered plants on day 12, and quickly declined thereafter. Net CO2 assimilation (A) was close to zero by day 29 and became negative by day 36. Chlorophyll fluorescence of drought-stressed trees was not different from that of well-watered trees on numerous days of measurement throughout the drying cycle. Unlike leaflets of well-watered trees, leaflets of the stressed trees exhibited heliotropic movement to avoid direct exposure to the sun. The stressed trees did not respond with any change in osmotic potential of leaflets following rehydration. Carbon dioxide-use efficiency of stressed trees was reduced to 66% of well-watered trees. The most profound response following rewatering (day 37) was leaf shedding. All trees shed some leaves, and 33% of the trees shed the entire canopy. On the trees that retained some leaves, A returned to that of the control trees by day 13 of recovery.

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Thomas E. Marler and Haluk M. Discekici

`Known You 1' papaya seedlings were grown in split-root containers and fertilizer was applied to one (1/2) or two (2/2) halves of the root system to determine the influence on transport of assimilates from canopy to roots and transport of nitrogen from fertilized roots to non-fertilized roots and canopy. Following 6 weeks of growth, the plants were bare-rooted and the root system halves and canopy were dried to constant mass at 70°C. Tissue was then analyzed for total nitrogen content. Fertilization increased root mass more than 250% and total plant mass 300% compared with control plants, which received no fertilization during the 6 weeks. Total root or plant mass did not differ between the 1/2 and 2/2 plants. Roots were evenly distributed between the two halves for 2/2 plants, but the fertilized half in the 1/2 plants accounted for 60% of the total root mass. Nitrogen content of roots and canopy were increased by fertilization. Nitrogen content of the non-fertilized roots of 1/2 plants was not different from that of the fertilized roots. These results indicate that fertilizing a portion of the papaya root system increased the sink activity of that portion and that the absorbed nitrogen from that portion is efficiently transported throughout the plant.

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Thomas E. Marler and Haluk M. Discekici

`Red Lady' papaya transplants were planted on a slope with a 30% to 35% grade and grown for 5 months. Excavation was used to determine root distribution on the uphill and downhill sides of the plants. Roots were separated into the taproot system and lateral roots on the uphill and downhill sides. The line intersect method was used to determine length of the lateral roots, and length of the taproot system was measured directly. All roots were dried at 70°C. The taproot system accounted for 2% of the total root length and 66% of the total root mass. Of the 130-m of lateral roots, 71% were located on the downhill side. Similarly, 69% of the dry mass of the lateral root system was located on the downhill side. Primary lateral roots on the uphill side of each plant developed horizontally, but some secondary lateral roots developed against gravity to maintain a portion of the root system close to the surface of the slope. Some of these lateral roots developed at angles of 55° to 60° above the horizontal.

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Thomas E. Marler and Haluk M. Discekici

The influence of partial root volume irrigation on water relations and expansion of roots and leaves of papaya plants was determined using split root containers. In one study, `Tainung #1' and Solo #8 seedling roots were trained into four compartments until well-established, then water was withheld from 0, 1, 2, or 3 quadrants. Mid-morning stomatal conductance and predawn relative leaf water content were not affected by the irrigation treatments. Similarly, relative root water content in the dry quadrants was not different from that in the watered quadrants. In a second study, `Red Lady' seedling roots were trained into four compartments which contained a 13 × 13-cm plexiglass observation window. After the plants were well-established, watering was continued in one of 4 (1:4) or four of four (4:4) quadrants. Leaf midrib and root extension were measured at 06:00 and 18:00 hr each day. Daily growth of roots in the dry quadrants was reduced 25% below that in the watered quadrants, and midrib extension of the 1:4 plants was reduced ≈10% below that of the 4:4 plants. Irrigation treatments did not influence the percentage of growth occurring during the diurnal and nocturnal periods. The dry quadrants of 1:4 plants were almost devoid of fine roots. The number of root tips on the observation windows of the 1:4 plants was reduced 43% in the dry quadrants and increased 22% in the wet quadrant compared with that for the 4:4 plants.

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Thomas E. Marler and Haluk M. Discekici

`Red Lady' and `Tainung #1' papaya plants were grown in nursery trays with cells 5.1 cm in diameter. After 10 weeks, mean height of the `Red Lady' plants was 10.1 cm and that of the `Tainung #1' plants was 9.3 cm. Each of five plants per cultivar was planted between two root observation windows, one at 45 cm and the other at 95 cm. Roots reached the 45-cm observation window in 30 days, when mean height of the `Red Lady' plants was 18.7 cm and that of the `Tainung #1' plants was 13.0 cm. Roots reached the 95-cm observation window in 55 days, when mean height of the `Red Lady' plants was 55.4 cm and that of the `Tainung #1' plants was 40.6 cm. Thus, root extension during these initial 55 days was 17 to 18 mm per day for both cultivars, and stem extension during this period was 8.7 mm·d–1 for `Red Lady' and 5.5 mm per day for `Tainung #1'. Root extension declined for both cultivars to ≈12 mm·d–1 by the initial bloom period, then further declined to ≈4 mm·d–1 during and after the initial fruit set stage. Stem extension increased to about 19 mm·d–1 after the plants were established and remained at this rate until well into the stage of heavy fruit set and growth, when it declined to about 8 mm·d–1. The amount of fruit set influenced root characteristics more than cultivar.

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Robin A. DeMeo and Thomas E. Marler

Two studies were conducted to determine the influence of pH on papaya seed germination and seedling emergence. The germination test was conducted with `Waimanalo' and `Tainung 1' seeds, using a double layer of filter paper disks in plastic petri dishes placed within a growth chamber. Each dish received 40 seeds, and germination was defined as when the radicle was visible. Disks were wetted daily with nutrient solution adjusted to pH of 3, 4, 5, 6, 7, 8, or 9. Germination began on day 5, and the study was terminated on day 23. Solution pH did not influence germination rate or ultimate germination percentage. `Waimanalo' exhibited 58% germination and `Tainung 1' exhibited 64% germination in this test. The seedling emergence study was conducted with `Waimanalo' seeds using sand culture within a growth chamber. Thirty seeds were planted in 10-cm containers, and the sand was irrigated daily with the solutions from the first study. Emergence was defined as when the hypocotyl hook was visible above the sand. Emergence began on day 10, and the study was terminated on day 30. Solution pH did not influence seedling emergence, and mean emergence was 69% in this study. The results indicate that the seed germination and seedling emergence stages of papaya seedling growth are adapted to a wide range of substrate pH.

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Thomas E. Marler and Grace B. Paloma

Container-grown Annona muricata seedlings were bare-rooted and re-potted in sand. Containers were irrigated daily with a complete nutrient solution adjusted to a pH of 3, 4, 5, 6, 7, or 8, and the seedlings were grown for ≈5 months. Numerous growth variables were measured, including canopy volume, increase in mass, and trunk diameter. There were no differences in growth measurements among the pH levels. Moreover, leaf tissue was analyzed for mineral content. Leaf tissue concentration of various minerals did not differ among the pH levels. Annona muricata is known for growing well in a range of soil conditions. These data verify that the species is adapted to a wide range of substrate pH.

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Thomas E. Marler and Ruben dela Cruz

Subsoil from an acid soil series was amended with CaSO4, MgO, or Ca(OH)2 to identify chemical factors that may enhance papaya (Carica papaya L.) root growth in these soils. Root length of `Red Lady' and `Waimanalo' seedlings at two stages of development was increased by the addition of each of the materials. The increase in root length was similar for CaSO4 or MgO amendments, and was greatest for Ca(OH)2 amendment. These amendments increased dry weight of new roots for `Red Lady' and increased root length per unit dry weight in one experiment for `Waimanalo'. The results indicate that both Ca deficiency and Al toxicity may be responsible for limiting papaya root growth in the subsoils of the acid soils of Guam. Correcting these chemical factors should improve rooting depth, thereby increasing the volume of soil from which resources are accessible and lessening the susceptibility to toppling during tropical cyclones.

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Hiphil S. Clemente and Thomas E. Marler

Two studies were conducted with `Known You 1' and `Sunrise' papaya seedlings to determine the combined influence of wind and drought stress on growth. For each study, 4-week-old nursery plants were transplanted into 2.6-L containers and placed in a protected site with rain exclusion provided by polypropylene cover. Industrial fans were used to provide unidirectional wind of ≈2 m/s for 12 hours per day to half of the plants; the remaining half of the plants received no wind. One half of the plants for each cultivar and wind combination were designated as well-watered and received daily irrigation. The remaining half of the plants were designated as drought-stressed and received 25% to 50% of the water applied to the well-watered plants. Plants were grown for 3 weeks under these experimental conditions. There were no interactions between the drought and wind main effects. The reduction in height, trunk cross-sectional area, total plant dry weight, and relative growth rate below that for control plants was similar for drought stress or wind stress. Wind stress reduced growth of `Sunrise' plants more than `Known You 1' plants in both studies. Although the main effects did not interact, the combination of drought and wind stress reduced growth of papaya seedlings more than did either main effect alone. The greatest wind load from trade winds occurs on Guam during the annual dry season. These data indicate that chronic wind stress during the dry season may be more detrimental to growth of papaya seedlings than during the rainy season or under sufficient irrigation practices.