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Alyssa H. Cho, Carlene A. Chase, Danielle D. Treadwell, Rosalie L. Koenig, John Bradley Morris and Jose Pablo Morales-Payan

cultivars with growth habits that reduce light penetration to the soil surface and reduce weed seed germination. Branching in sunn hemp depends on the planting density and apical dominance of the plant. Planting density can influence the height and formation

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Rodomiro Ortiz and Dirk R. Vuylsteke

Apical dominance, i.e., the inhibition of lateral bud growth due to growth substances released by the terminal bud, has been considered as a limiting factor for the perennial productivity of plantains (Musa spp., AAB group). Segregation ratios in F1 and F2 plantain-banana hybrids suggest that inheritance of apical dominance is controlled by a major recessive gene, ad. The dominant Ad allele improved the suckering of plantain-banana hybrids, as measured by the height of the tallest sucker at flowering and harvest. At harvest, the ratoon crop of the diploid and tetraploid hybrids had completed 70% to 100% of its vegetative development, whereas the ratoon of the plantain parents, due to high apical dominance, was only at 50% of total pseudostem growth. Sucker growth rates are generally the result of gibberellic acid (GA3) levels, and it is suggested that the Ad gene regulates GA3 production. However, the Ad gene has incomplete penetrance, genetic specificity, and variable expressivity. Increased frequency of the Ad gene and a commensurate improvement in the suckering behavior of the diploid populations may be achieved by phenotypic recurrent selection.

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Senay Ozgen, James S. Busse and Jiwan P. Palta

; Sha et al., 1985 ). This condition is typified by browning and death of the shoot tip, loss of apical dominance, and axillary shoot development in an in vitro shoot culture. Transpiration is limited during in vitro culture by the high humidity that

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Bradley Dotson*, Camila Rey, Joonyup Kim and Sara Patterson

Cell separation regulates basic developmental processes such as abscission and dehiscence and is one of the horticultural traits first to be selected by mankind. Abscission is characterized by an active cell separation process where organs are detached from the main body of the plant through the dissolution of the middle lamella. Crops with early abscission can have significant reduction in yield. For example, canola, Brassica napus, loses 5% to 10% of crop due to early pod shatter. By screening T-DNA mutagenized populations of A. thaliana for delayed abscission, we have identified several genes that regulate cell separation, slm1-1 (slender lasting inflorescence and meristem) is one such genes. During our investigation of slm1-1 we have employed phenotypic, physiological, genetic, and molecular assays. Phenotypically, slm1-1 displays traits such as delayed abscission of floral organs, lack of anther pollen dehiscence (making slm1-1 functionally male sterile), delayed meristem arrest, and strong apical dominance. Phenotypic characterization includes scanning electron microscopy, bright field microscopy, and stereoscope microscopy. Physiological assays include reporter gene expression and break strength analyses. Genetically, slm1-1 is regulated by a single recessive gene. Molecular assays characterizing slm1-1 include TAIL-PCR, RT-PCR, and preliminary microarray of abscission zones. We have also begun to map based cloning of slm1-1. We believe that understanding genes that regulate cell separation in A. thaliana will contribute to crop improvement. Applications could include reducing loss during harvesting, regulation of pollination, changes in branching patterns, and longevity of flowering.

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Shiow Y. Wang, Miklos Faust and Michael J. Line

The effect of IAA on apical dominance in apple buds was examined in relation to changes in proton density (free water) and membrane lipid composition in lateral buds. Decapitation induced budbreak and enhanced lateral bud growth. IAA replaced apical control of lateral buds and maintained paradormancy. Maximal inhibition was obtained when IAA was applied immediately after the apical bud was removed; delaying application reduced the effect of IAA. An increase in proton density in lateral buds was observed 2 days after decapitation, whereas the change in membrane lipid composition occurred 4 days later. Removing the terminal bud increased membrane galacto- and phospholipids and the ratio of unsaturated to corresponding saturated fatty acids. Decapitation also decreased the ratio of free sterols to phospholipids in lateral buds. Applying thidiazuron to lateral buds of decapitated shoots enhanced these effects, whereas applying IAA to the terminal end of decapitated shoots inhibited the increase of proton density and prevented changes in membrane lipid composition in lateral buds. These results suggest that change in water movement alters membrane lipid composition and then induces lateral bud growth. IAA, presumably produced by the terminal bud, restricts the movement of water to lateral buds and inhibits their growth in apple.

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Shiow Y. Wang, Miklos Faust and Michael J. Line

The effect of Indole-3-acetic acid (IAA) on apical dominance in apple (Malus domestica Borkh.) buds was examined by studying changes In proton density (free water) and membrane lipid composition in lateral buds. Decapitation induced budbreak and enhanced lateral bud growth. IAA replaced apical control of lateral bud paradormancy. Maximal inhibition was obtained when IAA was applied immediately after the apical bud was removed. Delaying this application weakens the effect of IAA. An increase in proton density in lateral buds was observable 2 days after decapitation, whereas the change in membrane lipid composition occurred 4 days later. Decapitating the terminal bud induced an increase in membrane galacto- and phospholipids. and the ratio of unsaturated to corresponding saturated fatty acids. Decapitation also induced a decrease in the ratio of free sterols to phospholipids in lateral buds. Application of IAA to the terminal end of decapitated shoots inhibited the increase of proton density and prevented changes in the membrane lipid composition of lateral buds.

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James S. Busse, Senay Ozgen and Jiwan P. Palta

typified by the browning and death of the shoot tip, the loss of apical dominance, and axillary branch development in an in vitro shoot culture system ( McCown and Sellmer, 1987 ; Sha et al., 1985 ). Transpiration is limited during in vitro culture by high

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Dilma Daniela Silva, Richard C. Beeson Jr. and Michael E. Kane

Fisher’s least significant difference at P = 0.05. Although under influence of apical dominance, vegetative lateral buds of Rosa hybrida increased dry mass and developed new leaf primordia with age while still in the bud stage ( Marcelis-Van Acker

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Osamu Kawabata and Richard A. Criley

An aqueous solution of dikegulac-sodium at 0, 2000, 4000, 6000, or 8000 mg a.i./liter was sprayed on a mature Murraya paniculata hedge as the first leaves expanded on newly developing lateral shoots after trimming. The lateral shoots from each 0.09-m2 hedge surface elongated less and the coefficient of variation (cv) decreased as the growth regulator concentration increased. Application of dikegulac-sodium at 4000 mg a.i./liter to the most distal leaf on topped, single-leader seedlings inhibited the elongation of distal shoots while it enhanced proximal shoot growth. Dikegulac-sodium spray between 4000 and 6000 mg a.i./liter to the hedge decreased apical dominance among lateral shoots and enhanced uniform regrowth without causing visible damages. The cv reduction was attributed to the growth regulator-induced weakening of apical dominance. Chemical name used: sodium salt of 2,3:4,6-bis-O-(1-methylethylidene)-α-l-xylo-2-hexulofuranosonic acid (dikegulac-sodium).

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Amos Naor, Moshe Flaishman, Raphael Stern, Aharon Moshe and Amnon Erez

The relative contribution of various temperatures to dormancy completion of lateral vegetative apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] buds was studied quantitatively on whole container-grown trees. Trees were exposed continuously to 10 different temperatures and also to daily alternating temperatures in a 24-hour cycle. In addition, fully chilled vertically and horizontally positioned shoots were compared under forcing conditions. No budbreak occurred in shoots chilled above 12.5 °C. There was a steep increase in budbreak as the chilling temperature fell from 12.5 to 7.5 °C. There was little difference in the level of budbreak on shoots chilled between 7.5 and 0 °C. The relative contribution of temperature to chilling accumulation in apple found in our study differs from what has been proposed for stone fruit and for apple in previous studies, especially at temperatures <6 °C. The length of exposure to forcing conditions required to initiate budbreak diminished as the chilling temperature was reduced. No additional bud-break was apparent on shoots chilled longer than 2100 chilling hours. The chilling requirement found here for lateral vegetative buds is much higher than that needed for terminal vegetative and flower buds. Trees that were exposed to daily alternating temperatures had lower levels of budbreak when the high temperature in the diurnal cycle was greater than 14 °C. Practically no budbreak was apparent on trees that were exposed to diurnal cycles with a high temperature of 20 °C for 8 hours. Budbreak on horizontally positioned trees was more than twice that on the vertically positioned trees, emphasizing the magnitude of the apical dominance effect and its strong masking of the chilling effect on lateral buds in vertically grown apple trees. Based on the data collected here we propose a new response curve for vegetative budbreak in `Golden Delicious·apple, within a temperature range between 0 to 15 °C.