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  • Author or Editor: Bruce Schaffer x
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Net gas exchange and growth were determined for cocoyam [Xanthosoma sagittifolium (L.) Schott] growing in 30%, 50%, and 100% sunlight and fertilized with 0 or 475 mg N/kg nutrient solution. Interactions between N and shade were observed for lamina area per plant, top : corm ratio, corm weight, transpiration (E), stomatal conductance (g,), and lamina N and chlorophyll concentrations. When N treatments were pooled, shade-grown plants (30% and 50% sunlight) had greater lamina areas, lamina and petiole biomass, top: corm (fresh weight) ratios, and corm fresh weights than plants grown in full sunlight. All of these criteria also had higher values for plants that received the N-fertilizer solution (+ N) than for plants that received the N-free solution (- N), when shade treatments were pooled. When N treatments were pooled, 100%-sunlight plants had greater net CO2assimilation (A) rates than shade plants. Water-use efficiency (WUE), A, g., and E for 100%-sunlight-grown plants were higher for + N than for - N plants. For shade plants, however, A and E were similar between N treatments. When N treatments were pooled, shade plants had a greater lamina chlorophyll concentration on a dry-weight basis than 100%-sunlight plants, whereas content on an area basis was similar among shade treatments. Among shade treatments, chlorophyll contents on an area and dry-weight basis were higher for + N than for - N plants. Plants grown in 100% sunlight had higher lamina N concentrations (area and dry-weight bases) than shade plants. The interactions between N and shade showed that cocoyam response to N depends on incident photosynthetic photon fluxes during growth.

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Abstract

Net photosynthesis (Pn), transpiration (Tr), and stomatal conductance for CO2 (gs) were determined at 26° ± 1°C for leaves on intact and excised apple shoots at different vapor pressure gradients (VPG’s). Pn, Tr, and gs of leaves on intact and excised shoots responded similarly to changes in VPG. Pn and gs were not affected directly by VPG. Tr increased as VPG increased since stomatal closure did not counterbalance the increased VPG.

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Abstract

Deblossomed ‘Tribute’ strawberry (Fragaria × ananassa Duch.) plants had increased [14C]-photosynthates in untreated leaves 48 hr after treatment with 14CO2. The summed quantity of radioactivity in the untreated leaves and fruit of fruiting plants approximated that in the untreated leaves of deblossomed plants. There was no effect of deblossoming on the amount of 14C in the crown or roots. Autoradiographs showed that the majority of 14C was in the expanding leaves. Therefore, increased leaf production rates, which often result from deblossoming strawberry plants, may be attributed to an increase in photosynthates partitioned to the expanding leaves.

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The effects of flooding calcareous soil on physiology and growth have been studied for several subtropical and tropical fruit crops including avocado (Persea americana Mill.), mango (Mangifera indica L.), carambola (Averrhoa carambola L.), and several Annona species. In calcareous soils that have a high pH, short-term flooding can actually be beneficial to subtropical and tropical fruit crops by increasing the solubility of particle-bound nutrient elements such as Fe, Mn and Mg due to flooding-induced decreases in soil pH. Additionally, flooding reduces the redox potential in the soil, resulting in Fe being reduced from Fe3+ to Fe2+, which is the cation metabolized by plants. As with other woody perennial crops, one of the early physiological responses of subtropical and tropical fruit trees to flooding is a decrease in stomatal conductance and net CO2 assimilation. If the flooding period is prolonged, lack of O2 (anoxia) in the soil results in a reduction of root and shoot growth, wilting, decreased nutrient uptake and eventual death. The flooding duration required to cause tree mortality varies among species, among cultivars within species, and with environmental conditions, particularly temperature. Several tropical and subtropical fruit crops have anatomical or morphological adaptations to tolerate prolonged flooding, such as development of hypertrophied stem lenticels, adventitious rooting or formation of porous aerenchyma tissue. For grafted trees, flooding-tolerance is conferred by the rootstock and not the scion. Therefore there is a possibility to increase flood tolerance of subtropical and tropical fruit crops by identifying or developing flood-tolerant rootstocks.

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Southern Florida has experienced numerous hurricanes, of which Hurricane Andrew was the most recent. Six years after this storm, nearly one-third of the 8093 ha of tropical fruit that existed in Miami–Dade County before the storm has never been replanted. The damage, reaction, and recovery from the storm varied among fruit species. The effect of heat stress and high light intensity was minimal on avocado, `Tahiti' lime, carambola, mamey sapote, guava, sapodilla, and longan. In contrast, mango trees experienced severe heat stress. Root damage caused by toppling and subsequent re-setting of sugar apple, atemoya, mango, and grafted `Tahiti' lime trees was severe; thus, trees not re-set were less likely to recover than trees left toppled or leaning. The extent and rate of recovery from hurricane-related wind stress also varied among species. Avocado, carambola, guava, and longan refoliated within 3 to 4 weeks after Hurricane Andrew. In contrast, mango, sugar apple, and atemoya trees went through two or more cycles of refoliating and dying back until tree death occurred. Iron and nitrogen deficiencies were common for mango, sugar apple, atemoya, and guava. Other consequences of hurricanes in south Florida include increased weed and vine growth and increased susceptibility to drought stress and insect infestations. Recovery to prehurricane crop production levels has varied among crops. For example, avocado and carambola production is near and exceeds pre-1992 levels, respectively. In contrast, `Tahiti' lime and mango production are about 20% pre-1992 levels. The long-term effect of the most recent hurricane on fruit production in south Florida has been a change in the crop species and/or cultivars planted.

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The influence of floodwater dissolved O2 content on stem lenticel hypertrophy and endogenous ethylene evolution from mango trees, and the influence of exogenous ethylene on mango stem lenticel hypertrophy was examined. In general, floodwater O2 contents of 1-7 ppm resulted in lenticel hypertrophy within about 6 days of flooding, whereas floodwater O2 contents of 15 ppm delayed hypertrophy until about day 9. After 14 days of flooding, there were more than twice the number of hypertrophied lenticels per tree with floodwater O2 contents of 1-7 ppm than with O2 contents of 15 ppm. Ethylene evolution from aerobic stem tissue increased 4- to 8-fold in trees exposed to floodwater with 1-2 ppm O2, increased 2-fold for trees exposed to 6-7 ppm O2, but remained constant with 15 ppm floodwater dissolved O2 content. During a 10-day flooding period, trees in floodwater with 15 ppm dissolved O2 content, and given exogenous ethylene, developed extensive stem lenticel hypertrophy, whereas no hypertrophy developed on stems of trees receiving no exogenous ethylene and maintained in floodwater with 15 ppm O2. These data suggest that ethylene plays a role in promoting stem lenticel hypertrophy in flooded mango trees.

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One-year-old potted `Peach' mango (Mangifera indica L.) trees were flooded at soil temperatures of 15, 22.5 or 30°C. Hypertrophied lenticels were observed after 5-6 days at 30°C and 6-8 days at 22.5°C, but were not observed after 30 days at 15°C. Cells of hypertrophied lenticels were more spherical and randomly arranged than those of nonhypertrophied lenticels, resulting in increased intercellular airspace. Lenticel hypertrophy also occurred on sterns of trees which were kept moist from intermittant misting, and on excised and intact stem sections. Therefore, formation of hypertrophied lenticels in mango occurs independently of root anaerobiosis and is dependent on floodwater temperature.

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`Jonagold'/Mark apple (Malus domestica Borkh.) trees that were chip-budded in Washington and Illinois on 31 Aug. or 21 Sept. 1989 were sampled in Apr. 1990 to determine if magnetic resonance imaging (MRI) could be used to nondestructively examine vascular continuity or discontinuity between the rootstock and scion. Images could be placed into three categories based on signal intensity: 1) the rootstock, bud shield, and the bud or new scion growth had a high signal intensity; 2) the rootstock and the bud shield had a high signal intensity, but the scion had a low signal intensity; and 3) the rootstock had a high signal intensity, but the bud shield and scion had a low signal intensity. High signal intensity was associated with bound water in live tissue and the establishment of vascular continuity between the rootstock and scion. Azosulfamide staining and destructive sectioning confirmed that vascular continuity was established when the rootstock, bud shield, and scion had a high signal intensity in images, whereas budding failure occurred when the bud shield and/or the scion had a low signal intensity. Additional trees that had wilted or weak scion growth were collected from Illinois in June 1990. Parenchyma tissue was found in the scion adjacent to the bud shield that interrupted the vascular tissue. Poor scion growth on trees from the 21 Sept. budding in Washington may be attributed to insufficient growth of rootstock and/or scion tissues at the union in the fall.

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Most tropical fruit trees in southern Florida are grown in calcareous gravelly soil that is mechanically trenched to a depth of about 50 cm (about 20 inches). Fruit trees are often planted at the intersections of perpendicular trenches to provide space for root development. Tree root systems are concentrated in the top 10 to 20 cm (about 4 to 8 inches) of soil. Extreme soil rockiness has made it difficult to obtain consistent and reliable measurements of soil water status and to collect soil samples for constructing soil-water characteristic curves in the laboratory. Multisensor capacitance probes andlow-tension [0 to 40 kPa (centibars) (0 to 5.8 lb/inch2)] tensiometers were installed adjacent to star fruit (Averrhoa carambola L.) and avocado (Persea americana Mill.) trees in trenches to simultaneously measure volumetric soil water content and soil matric potential in situ. Capacitance probes consisted of four sensors centered at depths of 10, 20, 30, and 50 cm (3.9, 7.9, 11.8, and 19.7 inches). Tensiometers were installed at 10- and 30-cm depths, adjacent to the 10- and 30-cm deep capacitance sensors. Measurements obtained with both instruments were used to generate in situ soil-water characteristic curves. Rock fragments were more abundant at 30 cm than at 10 cm (71% to 73% versus 26% to 38% of bulk soil volume, respectively) soil depth, which limited the precision of tensiometers at the greater depth. In situ soil water characteristic curves for the 10-cm soil depth can be used to determine parameters needed for irrigation scheduling by techniques such as the water budget method.

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