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Michael B. Triff, Timothy J. Smalley, Mark Rieger, and David Radcliffe

Isolite is a ceramic-like, porous soil amendment purported to sustain plant growth under reduced irrigation and increase plant survival during drought. The purpose of this greenhouse experiment was to determine the effect of an Isotite-amended soilless container medium on: (1) growth under reduced irrigation frequency and (2) water stress during drought of Impatiens × hybrids `Accent Red'. On 2 June 1993, seedlings were transplanted into 13.2 liter black plastic pots containing a 4:1 composted pine bark:coarse sand (vol.) medium amended with Isolite CG-1 granules at rates of 0%, 10%, 15%, and 20% (vol.). Study I. Seedlings were. irrigated with 500 ml tap water every two days for two weeks followed by a 4 week schedule of 500 ml tap water every 1, 2, 3, 4, or 5 days. In general, growth parameters were explained by irrigation treatment effects and did not differ with Isolite rate. Growth indices ranged from +54% to + 143%, while final visual quality grades ranged from 2.4 to 5.0 (5-point scale), shoot dry weight from 8.7 to 30.7 g, root dry weight from 2.0 to 7.9 g, and leaf area from 0.14 to 0.48 m2. Study II. Seedlings were irrigated with 500 ml tap water every 2 days for rive weeks followed by a two week drought. Plant water status parameters were similar at all rates of Isolite. Leaf expansion rates ranged from + 89% to +98%, white a final mid-day xylem pressure potential of -0.4 MPa and a final visual quality grade of 2.0 were uniform across all treatments. Under these conditions, Isolite did not limit water stress of container-grown Impatiens `Accent Red'.

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Ganesh R. Panta, Mark W. Rieger, and Lisa J. Rowland

The blueberry industry experiences significant losses every year due to environmental factors with a dehydrative component, such as drought and freezing stress. In a recent survey of blueberry research and extension scientists in the United States, lack of cold hardiness and susceptibility to spring frosts were identified as the most important genetic limitations of current cultivars. For these reasons, cloning and characterization of expression of dehydration-responsive genes (dehydrins) have been ongoing in our laboratory. To date, one full-length (2.0 kb bbdhn1 gene) and four partial-length dehydrin cDNAs have been cloned and sequenced. Very high homology at the DNA and protein levels were found among the blueberry dehydrin clones, particularly at the 3' ends. From DNA blots, it appears that blueberry dehydrins are encoded by about three genes with high homology to the full-length 2.0 kb bbdhn1 cDNA clone and a few other less related genes. The 2.0 kb bbdhn1 gene was mapped in a blueberry population segregating for cold hardiness and chilling requirement. Expression studies indicated that dehydrins are induced by cold and drought stress. In general, dehydrins were induced in all organs examined in response to cold stress, including floral buds, leaves, stems, and roots. Under drought conditions, dehydrins were induced primarily in stems and their levels declined in roots. Some of the induced dehydrins were the same for drought and cold stress, whereas others were unique to a given stress. Levels of dehydrin accumulation correlated positively with cold hardiness; however, levels of dehydrins did not correspond precisely to the degree of drought avoidance.

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Mark Rieger, Gerard Krewer, Pam Lewis, Mindy Linton, and Tom McClendon

Sixteen cultivars of citrus (Citrus spp.) and close citrus relatives were planted in Savannah, Georgia to evaluate their potential as fruiting landscape trees in an area that routinely experiences minimum temperatures of 15 to 20 °F (-9.4 to -6.7 °C) during winter. Three to six trees of each cultivar were planted in 1998, and stem dieback and defoliation data were collected in 1999, 2001, and 2002. During the 4 years of the study, air temperatures fell below 32 °F (0.0 °C) 27 to 62 times per season, with absolute minima ranging from 13 to 18 °F (-10.6 to -7.8 °C), depending on year. In general, kumquats (Fortunella spp.), represented by `Meiwa', `Nagami', and `Longevity', were completely killed (or nearly so) in their first year in the field after air temperature minima of 13.5 °F (-10.28 °C). Others experiencing 100% dieback were `Meyer' lemon (Citrus limon × C. reticulata) and `Eustis' limequat (C. aurantifolia × Fortunella japonica), which were tested twice during the study. Kumquat hybrids, including procimequat [(C. aurantifolia × F. japonica) × F. hindsii), `Sinton' citrangequat [(C. sinensis × Poncirus trifoliata) × unknown kumquat], `Mr John's Longevity' citrangequat [(C. sinensis × P. trifoliata) × F. obovat], razzlequat (Eremocitrus glauca × unknown kumquat), and `Nippon' orangequat (C. unshiu × F. crassifolia) survived freezing, but all experienced at least some defoliation and stem dieback. `Owari' satsuma (C. unshiu), `Changsha' mandarin (C. reticulata), nansho daidai (C. taiwanica) and ichang papeda (C. ichangensis) experienced only minor stem dieback but substantial defoliation in most years, except that ichang papeda was substantially damaged in the last year of the study. Seven cultivars produced fruit at least once during their first 4 years: nansho daidai, ichang papeda, `Nippon' orangequat, `Mr John's Longevity' citrangequat, `Owari' satsuma, `Changsha' mandarin, and procimequat. Based on cold hardiness, fruiting, and growth characteristics, `Owari' satsuma, `Changsha' mandarin, `Mr John's Longevity' citrangequat, and `Nippon' orangequat provided the hardiest, most precocious and desirable fruiting landscape trees in this study.

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Riccardo Lo Bianco, Mark Rieger, and She-Jean S. Sung

Sorbitol is the major photosynthetic product in peach [Prunus persica (L.) Batsch.]. In sink tissues, sorbitol is converted to fructose via NAD+-dependent SDH. A new procedure is described that allows rapid, simple quantification of SDH activity in growing tissues. The procedure uses only 0.01 to 5 g of fresh tissue per sample, such that a single shoot tip, a single root tip, or ≈5 g of fruit flesh can be assayed for SDH activity. Storage of samples at 4 or -20 °C overnight resulted in significant loss of enzyme activity. Thus, freshly harvested tissues were ground with sand in buffer at 2 °C in a mortar and pestle, and the homogenate was centrifuged at 3000 g n to remove particulate matter and sand. The supernatant was desalted on a Sephadex G-25 column, and the eluent was assayed for SDH activity immediately. Activity was determined by measuring the production of NADH per minute in the assay mixture using a spectrophotometer (340 nm). Tris buffer at pH 9.0 was the best for extraction of peach SDH. Activity of SDH was strongly inhibited by dithiothreitol (DTT) in the extraction mixture and by DTT, L-cysteine, or SDI-158 in the assay mixture, similar to results reported for SDH from mammalian tissues. Peach SDH has a Km of 37.7 mm for sorbitol and a pH optimum of 9.5, similar to those reported for apple (Malus × domestica Borkh.) SDH. Unlike older protocols for SDH activity in plant tissues, the new procedure features reduced sample size (1/10 to 1/100 of that which was previously used), smaller volumes of buffer, fewer buffer ingredients, greatly reduced time for sample preparation, yet comparable or higher values of SDH specific activity. Following the same procedure, SDH activity was also measured in Prunus fremontii Wats., Prunus ilicifolia (Nutt.) Walp., and Marianna 2624 plum (P. cerasifera Ehrh. × P. munsoniana Wight & Hedr.).

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Richard T. Olsen, John M. Ruter, and Mark W. Rieger

Illiciums, or star-anises, have increased in popularity in the nursery and landscape industries. However, confusion exists as to which taxa are tolerant of high light intensities during production and subsequent establishment in the landscape. We investigated the effect of two light intensity treatments, 45% and 100% full sunlight, on gas-exchange parameters of five Illicium taxa: Illicium anisatum L., I. floridanum Ellis. `Pebblebrook', I. henryi Diels., I. lanceolatum A.C. Sm., and I. parviflorum Michx. Ex. Vent. `Forest Green'. Light-response curves were determined for individual leaves, and mean response parameters calculated. Chlorophyll and total carotenoids were analyzed after extraction in acetone, with total chlorophyll also estimated with a SPAD chlorophyll meter. In general, highest rates of CO2 assimilation (Amax) and lowest rates of dark respiration (Rd) were found in the 45% light treatment for all taxa. Both Illicium anisatum and I. floridanum `Pebblebrook' had substantial reductions in Amax in 100% light, 94% and 81% respectively, compared to plants grown in the 45% light treatment. Illicium henryi failed to survive the 100% light treatment. Illicium lanceolatum and I. parviflorum `Forest Green' were least affected by the 100% light treatment. Severe photooxidative bleaching was noted and confirmed by SPAD and pigment data, although SPAD readings were a poor predictor of total chlorophyll. For taxa of Illicium in our study, photosynthetic gas-exchange parameters and foliage pigment characteristics were improved in the low light treatment, suggesting optimal growth occurs in shaded conditions.

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Riccardo Lo Bianco, Mark Rieger, and She-Jean S. Sung

Terminal portions of `Flordaguard' peach roots [Prunus persica (L.) Batsch] were divided into six segments and the activities of NAD+-dependent sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX), sucrose synthase (SS), soluble acid invertase (AI), and soluble neutral invertase (NI) were measured in each segment 10, 15, and 20 days after seed germination. The same type of experiment was conducted with terminal portions of `Flordaguard' and `Nemaguard' peach shoots except that one of the six segments consisted of the leaflets surrounding the apex. Independent of the age of individual roots, activities of SDH and AI were consistently highest in the meristematic portion and decreased with tissue maturation. In shoots, AI was the most active enzyme in the elongating portion subtending the apex, whereas SDH was primarily associated with meristematic tissues. A positive correlation between SDH and AI activities was found in various developmental zones of roots (r = 0.96) and shoots (r = 0.90). Sorbitol and sucrose contents were low in roots regardless of distance from tip, while sucrose showed a decreasing trend with distance and sorbitol, fructose, and glucose increased with distance from the meristem in shoots. Activity of SDH in internodes, but not apices, correlated with shoot elongation rate of both cultivars, whereas activities of other enzymes did not correlate with shoot elongation rate. We conclude that AI and SDH are the predominant enzymes of carbohydrate catabolism and the best indicators of sink growth and development in vegetative sinks of peach.

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Gerard Krewer, Esendugue Greg Fonsah, Mark Rieger, Richard Wallace, David Linvill, and Ben Mullinix

Bananas are a popular ornamental plant in the southern U.S. However, normally only a few cultivars, such as `Lady Finger' and `Orinoco', are grown in Georgia. Thirty-three primarily commercial cultivars of bananas were grown for two years near Savannah, Georgia to determine their suitability for ornamental and nursery production. Most plants were grown from tissue culture plugs. They where given rates of fertilization used for commercial banana fruit production. Most cultivars produced 10 to 14 leaves and grew to heights of 1.5 to 2.0 m. Some displayed desirable ornamental characteristics such as pink tinted pseudostems, colorful flowers, and large graceful leaves. Many of the cultivars flowered and began producing fruit in late summer, although only `1780', `Raja Puri' and `Sweetheart' produced palatable fruit before frost in November in some years. Cultivars were also rated for their ability to produce suckers that can be used for nursery production. In year two, `1780' and `Manzano' produced the largest number of high quality suckers for nursery production. For the planting as a whole, sales of suckers at a field day averaged $7/per plant in year 2 and $17/per plant in year 3.

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Jeffrey H. Gillman, Mark W. Rieger, Michael A. Dirr, and S. Kristine Braman

Two experiments were conducted to determine the effect of drought stress on the susceptibility of Buddleia davidii Franch. `Pink Delight' to the two-spotted spider mite (Tetranychus urticae Koch). In the first experiment, drought stress was imposed by withholding water until predawn xylem pressure potential fell below -1 MPa. Shoot growth was 75% less in drought-stressed than in nonstressed plants. Mite population densities were not affected, but noninfested leaf area was 14% higher, and degree of mite damage was lower, in nonstressed plants. Evidently, the greater amount of new growth in nonstressed plants leads to lower spider mite densities by diluting populations. In a second experiment, nonstressed B. davidii `Pink Delight' plants were watered every 1 to 2 days and drought-stressed plants were watered every 3 days. Spider mite populations were monitored by sampling newly expanded and mature foliage. Mite populations on mature foliage were not affected by stress, but stressed plants grew less and had larger spider mite populations on their newly expanded foliage than did nonstressed plants.

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Gerard Krewer, Esendugue Greg Fonsah, Mark Rieger, Richard Wallace, David Linvill, and Ben Mullinix

Bananas (Musa spp.) are a popular ornamental plant in the southern United States; however, only a few cultivars, such as Lady's Finger and Orinoco, are grown in Georgia. Thirty-three primarily commercial cultivars of bananas were grown for 2 years near Savannah, Georgia, to determine their suitability for ornamental and nursery production, and for 3 years for fruit observations. Most plants were grown from tissue culture plugs. They were given rates of fertilization used for commercial banana fruit production. Most cultivars produced 10 to 14 leaves and grew to heights of 1.5 to 2.0 m. Some displayed desirable ornamental characteristics such as pink-tinted pseudostems, colorful flowers, and large graceful leaves. Some of the most attractive tall-growing cultivars were Belle, Ice Cream, Kandarian, Manzano, Saba, and 1780. Some of the most attractive medium-height cultivars were Dwarf Namwah, Dwarf Orinoco, Goldfinger, Raja Puri, and Super Plantain. In the short category, the cultivars Dwarf Nino, Gran Nain, Kru, and Sum X Cross were among the most attractive ornamentals. Many of the cultivars flowered and began producing fruit in late summer, although only ‘Raja Puri’, ‘Sweetheart’, and ‘1780’ produced palatable fruit before frost in November. Cultivars were also rated for their ability to produce suckers that could be used for nursery production. In year 2, ‘Manzano’ and ‘1780’ produced more than six high-quality suckers for nursery propagation. Potential income for these cultivars was over $60 per plant. For the planting as a whole, sales of suckers at a field day averaged $7 per plant in year 2, and $17 per plant in year 3.

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Othmane Mandre, Mark Rieger, Stephen C. Myers, Ray Seversen, and Jean-Luc Regnard

Fruiting and nonfruiting `Washington' peach trees were grown in 2.4 (small) or 9-liter (large) containers to determine the influence of root confinement and fruiting on vegetative growth, fruit growth and quality, CO, assimilation (A), and carbohydrate content. Shoot length, fruit diameter, A, and leaf carbohydrates were measured weekly. Thirteen weeks after transplanting, trees were divided into roots, shoots, leaves, and fruit for dry weight measurement. The dry weight of all organs except fruit was reduced by root confinement, and only the weight of stems formed the previous season was not reduced by fruiting. Fruit dry weight was 30.0 g/tree for large- and small-container treatments, causing the yield efficiency (g fruit/g total dry wt) to be 50% higher for confined trees. Fruit red color, weight, and diameter were unaffected by root confinement, but higher flesh firmness and a more green ground color of the fruit surface from root-confined trees suggested that confinement delayed maturity. Vegetative growth was not reduced by lack of nonstructural carbohydrates in confined trees. A was reduced by root confinement on only the first of 11 measurement dates, whereas fruiting increased A on 5 of 8 measurement dates before fruit harvest. Fruit removal reduced A by 23% and 31% for nonconfined and confined trees, respectively, within 48 h of harvest. Leaf starch, sucrose, sorbitol, and total carbohydrate levels were negatively correlated with A when data were pooled, but inconsistent responses of A to carbohydrate content indicated that factors other than feedback inhibition were also responsible for the reduction in A on nonfruited trees. We hypothesized that a physiological signal originating in roots of confined trees reduced vegetativegrowth without reducing fruit growth.