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Open access

Henry A. Robitaille

Abstract

Flowering in many plants can be geotropically influenced. Apple limbs are tied down as a commercial practice to increase flowering, and pineapple plants tipped on their sides flowered earlier than those grown upright (5). Flowering at the lower nodes of soybean plants was improved after weights were used to make stem tips grow downward (1). In apple (2) and pineapple (3) there is evidence that the geotropic stimulation may be mediated through ethylene. The effect of horizontal placement of plants on flowering of Phaseolus vulgaris was investigated, since ethephon treatments have increased flowering in that plant (4).

Open access

Henry A. Robitaille

Abstract

Increases in internal ethylene concentrations occurred after attached and detached apple (Malus domestica, Borkh. cv. Winesap) shoots were placed in a horizontal position. Ethylene was higher in the underside of the shoot, and decreased linearly in a basipetal direction along the axis. Bent and horizontally-oriented detached shoots did not increase in ethylene at 20°C, but increased to identical levels at 30°, suggesting that responses to the 2 types of stress were identical. Although gravitational stress alone increased ethylene levels, an experiment designed to separate bending stress from gravitational stress was inconclusive because rotation decreased shoot ethylene levels.

Open access

Edilson Paiva and Henry Robitaille

Abstract

The effectiveness of rest-breaking chemicals on detached shoots of apple (Malus domestica Borkh.) was dependent on the stage of bud rest at the time of chemical application. Thiourea (TU) and dinitro-ortho-cresol (DNOC) improved budbreak on September 9, the addition of gibberellic acid (GA3) to either chemical having no additional effect. On September 23 only DNOC was effective, and its effectiveness was increased with addition of GA3. DNOC + GA3 was the only effective treatment on both October 9 and October 25 at the start of the deep rest period. TU + GA3 improved budbreak on November 11, but no treatments were effective on December 4 and December 20. On January 14, as rest weakened, GA3 alone or in combination with DNOC effectively improved budbreak. By February 5, budbreak on control shoots was good and treatment effects were not significant.

Open access

Edilson Paiva and Henry A. Robitaille

Abstract

Buds of ‘Golden Delicious’ apple (Malus domestica Borkh.) were in deep rest between October 25 and December 20, 1976, although the uppermost bud on decapitated (apical and basal cuts) shoot segments developed at all sampling dates. Increases in rate of development of the uppermost bud, as well as in % break of lower buds after completion of deep rest, were both linear with time. Increases in % budbreak with time occurred in a basipetal direction, suggesting a gradient of increasingly deep rest from shoot apex to base. Development of the uppermost bud on decapitated shoots during deep rest was a result of the apical cut, and was not influenced by the original position of that bud on the intact shoot. The bud above the basal cut developed only on shoots with the terminal bud intact (basal cut only). Notching above a second bud on decapitated shoots did cause that bud to develop in addition to the uppermost bud. Results are discussed on the basis of a 2 stage rest model, rest gradient and apical dominance. There was no evidence for a role of ethylene in wounding or dinitro-orthocresol-stimulated budbreak. In addition, neither ethylene-releasing chemicals nor silver or cobalt had any influence on budbreak when applied to shoots at 2 dates during the rest period.

Open access

Henry Robitaille and R. F. Carlson

Abstract

Injections of 10 ppm of gibberellic acid (GA3) had increasing effects on terminal shoot growth of ‘Red Prince Delicious’ apple as rootstock vigor decreased. Injections of 100 ppm of abscisic acid (ABA) caused all trees to cease growth and to form terminal buds characteristic of summer dormancy. The most dwarfing trees responded more quickly to ABA than did the more vigorous trees.

Open access

Joyce G. Latimer and Henry A. Robitaille

Abstract

Many shoot sampling variables were found to influence bud rest of apple (Malus domestica Borkh.). Unlike terminal buds, uppermost lateral buds on decapitated stem pieces were stimulated by wounding and developed throughout the rest period under forcing conditions in a growth chamber and their rate of development was an accurate measure of depth of rest. Neither segment length nor location on the original intact shoot affected bud rest intensity, but single-node segments were avoided because they became desiccated. Rate of development of resting buds in darkness increased with increasing temperature between 20° and 30°C. Bud rest intensity decreased with decreasing shoot length. Shoot location in the tree crown (shaded interior vs. exterior) did not affect bud rest intensity, but there was a significant interaction between crop load and shoot height in the tree. Many interactions were significant between various shoot characteristics and bud rest intensity. Spurs were compared to shoots on the basis of the percent of a population showing terminal budbreak after 28 days of forcing. Terminal buds on spurs entered rest later and emerged earlier than did terminal buds on short, medium or long shoots.

Open access

Henry A. Robitaille and Robert F. Carlson

Abstract

Shoots on interstem-dwarfed apple (Malus domestica Borkh.) trees ceased growing earlier and showed a stronger tendency towards becoming summer dormant than did corresponding shoots on vigorous trees. Substances similar to gibberellic acid (GA3) and abscisic acid (ABA) were extracted from 1-year-old stems at 7 stages during the growing season using the centrifugation technique. ABA-like activity was higher in stems from dwarf than vigorous trees at full bloom, and remained higher until after the onset of summer dormancy. GA-like activity also was significantly higher in dwarf tree stems at the tight flower cluster stage. Injection experiments demonstrated that increasing ABA concn decreased terminal elongation in a linear manner. ABA-induced inhibition was reduced in direct proportion to the concn of GA3 in the injected solution.

Open access

Henry A. Robitaille and K. S. Yu

Abstract

Clones of peach (Prunus persica (L.) Batsch) were propagated from sprouted nodal cuttings made by undercutting sprouted buds 2 mm deep, from 5 mm above to 5 mm below each node when leaves were partially expanded. Rooting was greatest under mist when the basal hardwood tissue was dipped for 10 seconds in a solution of 100 to 500 ppm of indolebutyric acid (IBA) before insertion into a mixture of equal parts of sphagnum peat moss and perlite over underlying gravel. Rooting and transplanting success was equivalent to that for softwood cuttings without the problems of disease and late fall growth.

Open access

Henry A. Robitaille and R. F. Carlson

Abstract

Five apple species were grafted onto East Mailing VII, IX and 26 and Mailing Merton 104, 106 and 111 and 3 cherry species were grafted onto Mazzard F12/1 rootstock (a virus indexed clone) and grown in the field for one season. Determinations were made on graft success and plant response in the field. In October, representative graft unions of the 33 combinations were prepared, sectioned, and examined microscopically in an effort to find a rapid test for compatibility. The results of these microscopic examinations were correlated with the field results to see if one or more abnormalities could be found consistently in a significant number of grafts. Only a few grafts showed incompatibility characteristics which could be correlated to the field tests thus eliminating a quick microscope test. Anatomical abnormalities appeared to be the result of secondary influences suggesting that the biochemical factors probably control the compatibility or incompatibility of graft unions.

Open access

Henry A. Robitaille, Frank H. Emerson, and K. S. Yu

Abstract

Successful fruit thinning of apples (Malus domestica Borkh.) with CGA 15281, an ethylene-releasing compound, was demonstrated at 20 and 46 days past bloom. A rapid drop in fruit removal force was followed by fruit abscission on days 4 and 5. In contrast, ethephon often resulted in protracted abscission and complete fruit removal. Differential sensitivity to CGA 15281 existed among cultivars. Explants were less sensitive to ethylene than intact fruit, and could not be used for controlled experiments. No adverse effects of CGA 15281 were noted with the exception of some leaf drop in the interior of the tree.