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Festus K. Akinnifesi, Simon A. Mng'omba, Gudeta Sileshi, Thomson G. Chilanga, Jarret Mhango, Oluyede C. Ajayi, Sebastian Chakeredza, Betserai I. Nyoka, and France M.T. Gondwe

likely that rootstocks did not induce dwarfing effect on the grafted trees, but this might be clear at a later stage. Marcots possess lateral roots, but not the main root, and hence the roots might not have the same depth or spread as grafts or saplings

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I.L. Goldman

Few genes have been identified in red beet. A spontaneously occurring dwarf mutant was identified in the late 1970s and again in 1994 in several breeding populations. Mutant plants are characterized by extreme dwarfing of both root and shoot. Young leaves are narrow, thin and strap-like while older leaves are thicker and deeply veined. The shoot axis forms a compressed rosette. Neither the shoot axis nor the root axis of field-grown plants exceeds 3 cm in height. Genetic analysis of F2 and backcross populations revealed the dwarf phenotype is conditioned by a single recessive gene. Several experiments were conducted to determine if the dwarf phenotype was due to a lack of gibberellic acid (GA) production. Exogenous application of GA3 at concentrations ranging from 1 to 1000 ppm on dwarf plants a) following seeding and b) during reproductive growth revealed a linear increase in plant height. Control dwarf plants receiving a water-only treatment were 18% as tall as plants receiving regular application of 1000 ppm GA3. A wild-type phenotype during reproductive growth was recoverable following regular GA3 application.

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Mercy A. Olmstead, N. Suzanne Lang, Frank W. Ewers, and Shirley A. Owens

Xylem vessel anatomy was examined in tissues surrounding the graft union of sweet cherry (Prunus avium L.) scion (stem) and nondwarfing, semi-dwarfing, or dwarfing rootstock (root) combinations, to characterize potential changes in anatomical features during the initial stages of graft union formation. Vessel element length, frequency, diameter, lumen area (LAV), and mean vessel hydraulic diameter (VDh) were examined in `Rainier' (P. avium) scion wood grafted onto nondwarfing `Colt' (P. pseudocerasus L. × P. avium) or `F 12/1' (P. avium) rootstock and semi-dwarfing `Gisela 6' [`Gi 6' (P. cerasus L. × P. canescens L.)], or dwarfing `Gisela 5' [`Gi 5' (P. cerasus × P. canescens)] rootstock systems in: heterograft combinations (commercial scion-rootstock combinations); homografts (scion and rootstock are the same genetic material); and reciprocal heterografts (rootstock tissue grafted onto scion tissue). Vessel element length was not affected by rootstock, but vessel frequency and lumen area in graft union tissues were smaller in `Rainier'/`Gi 5' (dwarfing combination) than in `Rainier'/`Colt' (nondwarfing combination). The heterograft combination of `Rainier'/`Gi 5' had smaller scion LAV, lower VDh and narrower vessels than homograft or reciprocal heterograft combinations. As callus differentiated into vascular elements, xylem rays in `Rainer'/`Gi 5' tended to develop at an acute angle to the longitudinal axis of the tree and there was an increase in nonfunctional phloem in `Rainer'/`Gi 5' compared to `Rainer'/`F 12/1'. Collectively, the data provides further evidence that a combination of smaller and fewer vessels in the scion and graft union, as well as irregular vessel orientations in the vascular tissue within dwarfing combinations could contribute to hydraulic resistance in the graft union resulting in reduced scion growth (dwarfing).

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D.M. Glenn and R. Scorza

In reciprocal grafts of tall (`Elberta' and `Loring') and dwarf (`Empress' and `Juseito') peach (Prunus persica Batsch.) phenotypes, we measured dry-matter partitioning, resistance to root system water flow, and phytohormone content of xylem exudate. Scion characteristics determined the phenotype and growth characteristics of the tree irrespective of the rootstock. Tall phenotypes had higher dry weight and lower root resistance to water flow than dwarf phenotypes. Cytokinin-like activity and auxin levels in xylem sap were higher in dwarf than in tall phenotypes; whereas gibberellin-like activity was unaffected by either rootstock or scion. The scion of peach influenced phytohormone levels and resistance to water flow in the root system in addition to root and shoot growth.

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Mercy A. Olmstead, N. Suzanne Lang*, Frank W. Ewers, and Shirley A. Owens

Dwarfing rootstocks in sweet cherry (Prunus avium L.) have been planted worldwide. No single theory has emerged to answer why scion dwarfing occurs in fruit trees. This research examines the vascular pathway in a dwarfing cherry system to determine if physical limitations alter water transport as a possible dwarfing mechanism. Second-leaf `Lapins' trees grafted onto Gisela 5 (Gi5; dwarfing) and Colt (vigorous) rootstocks were field-grown in East Lansing, Mich. During maximum shoot elongation, trees were dug, placed into containers with safranin dye solution (0.1% w/v) for 6 hours and then removed for division (3-5 cm in length) based on location in scion, graft union, and rootstock tissue. Tissues were sectioned using a sliding microtome (120 μm) for examination with a laser confocal microscope (Zeiss LSM Pascal). Mean stem area and vessel diameter were measured; and mean hydraulic diameter was calculated for vessels in the area of dye translocation. Overall, Lapins/Gi5 stem area in the graft union was larger compared to Lapins/Colt; however dye translocation in Lapins/Gi5 was reduced compared to other tissues in the tree. Confocal microscopy indicated dye uptake through the grafted region was more uniformly distributed in Lapins/Colt than in Lapins/Gi5, with dye accumulation in areas of maximum translocation. Vessel diameter did not differ in these areas of translocation. However, in both combinations there was a reduction in mean hydraulic diameter of graft union sections, suggesting a reduction in vessel efficiency to translocate water in this region. Vascular system anomalies were more frequent in Lapins/Gi5, disrupting acropetal dye translocation. This suggests the greatest reduction in vascular transport is in Lapins/Gi5.

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A. Cedeño-Maldonado and E. Méndez

Control of tree size is an important consideration in developing commercial mango orchards. The use of dwarfing rootstock is one of the methods available for controlling tree size. The development of techniques for increasing the effect of dwarfing rootstock is the objective of the present research. Container size treatments consisted of planting seedlings of different rootstock in containers of 1 to 8 L in size. Pruning treatments consisted of removing the shoot, root or shoot-root (combined) tips of young seedlings previous to planting. Most significant changes were produced by using the smallest containers and combined pruning. Most significant fresh and dry weight reductions were obtained in the smallest containers. Combined pruning produced a significant increase in fresh weight-of both roots and shoots but no significant changes in dry weight. Plant height was not significantly affected but bark thickness was significantly reduced by decreasing container size. Eldon a monoembryonic variety was significantly bigger than polyembryonic varieties.

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A. Cedeño-Maldonado and E. Méndez

Control of tree size is an important consideration in developing commercial mango orchards. The use of dwarfing rootstock is one of the methods available for controlling tree size. The development of techniques for increasing the effect of dwarfing rootstock is the objective of the present research. Container size treatments consisted of planting seedlings of different rootstock in containers of 1 to 8 L in size. Pruning treatments consisted of removing the shoot, root or shoot-root (combined) tips of young seedlings previous to planting. Most significant changes were produced by using the smallest containers and combined pruning. Most significant fresh and dry weight reductions were obtained in the smallest containers. Combined pruning produced a significant increase in fresh weight-of both roots and shoots but no significant changes in dry weight. Plant height was not significantly affected but bark thickness was significantly reduced by decreasing container size. Eldon a monoembryonic variety was significantly bigger than polyembryonic varieties.

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I.L. Goldman and J.F. Watson II

A severe dwarf mutant affecting vegetative and reproductive growth arose spontaneously in our red beet (Beta vulgaris L. subsp. vulgaris) breeding nursery and was used in crosses with inbred lines to characterize its inheritance. Segregation data in backcross and F2 generations were collected. Chi-square goodness-of-fit tests did not deviate significantly from the expected ratios for a monogenic character for each genetic background-generation combination. We propose the symbol dw to describe the genetic control of this dwarf phenotype. Greenhouse experiments were conducted to determine whether the mutant was sensitive to exogenous application of gibberellic acid (GA). GA3 and GA4/7 in concentrations of 0 to 1000 ppm were applied to apical meristems during flower stem development in vernalized dwarf plants. Data on flower stem length and leaf length were collected over a 6-week period during reproductive growth. Recovery of wild-type flower stem length was obtained with application of both types of GA. A 30-fold increase in flower stem length over untreated plants was accomplished by GA application. Results of these phenocopy experiments suggest the mutant gene is involved in GA synthesis.

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Yun-Chan Huh, Seung-Ryong Cheong, and Jung-Myung Lee*

Common watermelons have an indeterminate growth habit with normal internode length, thus allowing the vine to grow indefinitely under the normal conditions. Watermelon breeders have identified four dwarf genes (dw-1, dw-1 s , dw-2, dw-3) and used these for developing dwarf watermelon cultivars. We discovered a naturally occurring new dwarf and seedless mutant (NDSM) from a landrace cv. Mudungsan that had been cultivated in the Mountain Mudung area nearby Gwangju City in Korea. The progenies of this mutant segregated in a ratio of 3 vine to 1 dwarf indicating a single recessive gene nature. Morphological characteristics of the NDSM were markedly different from those of the four known dwarf genotypes. NDSM plants grow shorter than 1 m in length with fan-shaped leaves and have fewer leaf lobes than normal plants, which could be clearly distinguished at 2 or 3 true leaf stage. Male and female flowers have just one petal and failed to open completely even at the anthesis. Even though there were some fertile pollen grains, the fruits of NDSM had no seed after fertilization. The F2 progenies, obtained from crossing `920533' (normal vine type) and NDSM, segregated in a ratio of 3 vine to 1 dwarf. All F1 plants from crosses between 2 dwarf types, `Sugar Bush' (dw-1dw-1) and `NH 9' (dw-2dw-2), and NDSM were normal, while F2 showed 9 vines, 3 dw-1 or dw-2 types, 3 NDSM types, and 1 double dwarf. The backcross generation segregated in a ratio of 1 vine to 1 dwarf. These results indicate that the genes for the NDSM and 2 dwarf types are non-allelic. We named this new dwarf genotype (NDSM) as dw-4 in addition to four dwarf genes previously identified.

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Qingrong Sun, Yinping Shi, Qiangsheng Wang, and Hongyan Sun

Laiyang dwarf cherry (Prunus pseudocerasus Lindl. Var. Laiyang dwary cherry) is an important dwarf type of Chinese cherry. It is both directly used for production and used as rootstock of sweet cherry (P. avium L.). Material used in this study was Laiyang dwarf cherry seed, which is open-pollinated. The fruit surface is sterilized with 70% ethanol on sterile working table, the seed is removed from fruit, and embryo is taken out of cracking seed shell. Explants include whole embryo, embryo without cotyledons, and cotyledons. Basal media are MS and 1/4MS major elements, sugar 30 g·L–1, agar 5.5 g·L–1, adjusted pH 5.8. Hormone used included 6-BA,IBA and 2,4-D for a total eight treatments. Culture temperature was 25° ±2°C, and photoperiod was 14 h. Results show 1) the plant regeneration rate from cotyledons is highest, second is from embryo without cotyledons, lowest is from whole embryo; 2) when BA is in the range of 0.7 to 5 ppm, plant regeneration is higher at a high concentration than that at low concentration; 3) at the same concentration, plant regeneration rate on the medium supplemented with 2,4-D is higher than that on the medium supplemented with IBA; 4) plant regeneration rate is higher on the medium containing cytokinin and auxin is higher than that on the medium only containing cytokinin; and 5) plant regeneration rate of continuous light culture cotyledons is higher than that of first dark culture 2 weeks cotyledons. Effects of light and dark/hormone and explant type on plant regeneration rate is discussed.