Nigel C. Cook

Endogenous cytokinin distribution patterns at budburst in Granny Smith and Braeburn apple shoots in relation to bud growth

Abstract The possible relationship of branching habit to cytokinin content of apple shoots ( Malus × domestica Borkh.) was investigated. One-year old apple shoots are acrotonic (distal branching), more strongly so in Granny Smith than in Braeburn. In the first trial, long, 1-year old Granny Smith and Braeburn apple shoots were sprayed on 29 August 1995 to break rest with dinitro- o -cresol (DNOC) oil (5%). The cytokinin contents of the xylem sap, the combined bark and buds, and the wood were determined in distal and proximal shoot halves over the next 6 weeks. Budburst (terminal and lateral buds) was first visible (green tip) in both cultivars on 20 September 1995. A greater increase in cytokinin content of distal xylem sap, coupled with elevated cytokinin in the distal wood, reflect the overall acrotony of both cultivars. The strong acrotony of Granny Smith is reflected in the higher cytokinin concentration in distal portion 1 week before the proximal portion of the shoot. The differential distribution of cytokinin reflects the pattern of budburst and may be correlated with growth habit. In a subsequent trial, Granny Smith shoots chilled and forced in the absence of roots showed an increase in cytokinin content of the bark and buds, and the wood as growth resumed. This was roughly comparable in magnitude to the increase observed under field conditions. The cytokinin increase in rootless shoots and differential distribution of cytokinin prior to sprouting, support the hypothesis that shoot-derived, rather than root-derived, cytokinins act to trigger spring budburst.

Chilling period influences the progression of bud dormancy more than does chilling temperature in apple and pear shoots

Summary One year old, ca. 50.cm long shoots of ‘Doyenne du Comice’ pear (Pyrus communis L.) and ‘Granny Smith’ apple (Malus × domestica Borkh.) were selected randomly in autumn and winter 1999 from commercial orchards in either Elgin (348S, 320.m) or in Somerset West (34°S, 80.m), South Africa. Shoots were cold-stored at 1, 4, 7 or 10°C for periods of 0, 1, 2 or 3 months after a daily 12/12.h freezing temperature pre-treatment of 21/13°C (supposedly nonchilling temperatures) for periods of 0, 1, 2 or 3 weeks. In 2000, ‘Granny Smith’ apple and ‘Packham’s Triumph’ pear shoots were harvested in autumn from orchards in Elgin and cold-stored, without a freeze treatment, at 1, 4, 7, 10 or 13°C for periods of 0, 1, 2, 3 or 4 months. After the different treatments the shoots were forced to budburst at 25°C with continuous illumination. To determine the progression of bud dormancy, the rate of budburst, final percentage budburst, and the synchronization of budburst between shoots were used in 1999, but only the rate of budburst in 2000. In all the trials the storage period was the most important factor influencing the progression of dormancy. While in some cases the effects of both storage temperature and the freeze treatment were significant, the contribution to differences in the progression of dormancy was negligible. When our data were fitted to the chilling models currently used in South Africa the difference in temperatures between –1 to 13°C was over-emphasized relative to the period of exposure to these chilling temperatures.

Chilling response of 'Granny Smith' apple lateral buds inhibited by distal shoot tissues

Abstract Apple (Malus×domestica Borkh.) shoots were decapitated before or after chilling and then forced to budburst to determine the influence of distal inhibition (paradormancy) on the chilling response of lateral buds. Shoots were chilled and forced with the presence or absence of an inhibitory distal disbudded shoot piece. Endogenous cytokinins were determined from distal and proximal segments of shoots segmented before and after chilling and/or forcing. The isolation of lateral buds from distal inhibition by decapitation before chilling resulted in a dramatic increase in growth rate. This increase is greater than that observed in terminal buds during the normal development of acrotony on intact shoots. Distal shoot tissues appear to inhibit the chilling response of lateral buds. On intact shoots cytokinin increased more in the distal shoot tissues, but in decapitated shoots cytokinin increased more in the proximal shoot tissues. Increased cytokinin in the proximal stem segment following decapitation is possibly associated with an increased lateral bud growth rate.

Architectural analysis of 1-year-old apple seedlings according to main shoot growth and sylleptic branching characteristics

Abstract. Main shoot and sylleptic shoot growth characteristics were measured during and after the first year of growth of 255 Telamon x Braeburn apple seedlings. Although mean main shoot growth characteristics between branched and non-branched trees were significantly different, many non-branched trees expressed similar main shoot growth to branched trees. The variables describing length, number and position of the sylleptic shoots were used to classify branched trees into architecturally different groups. A continuum from trees with few and short shoots to trees with many long shoots is observed. The release of axillary buds from apical dominance is not under complete control by the apical meristem. Genetic seedling difference at the level of roots presumably plays an important role in sylleptic branching. Genetic variation in terms of number, position, and subsequent elongation of sylleptic shoots is clearly observed.

Effect of mineral oil and hydrogen' cyanamide concentration on dormancy breaking in 'Golden Delicious' apple trees

South African production areas receive insufficient winter chilling for apple production, necessitating the use of artificial means to break dormancy. Hydrogen cyanamide (HC) alone or in combination with mineral oil (oil) is used as a rest-breaking agent in many deciduous species. The effect of different concentrations of HC and oil on budburst, yield, fruit quality and vegetative growth of mature ‘Golden Delicious’ apple trees were evaluated; the objective was to determine the presence of interaction between the rest-breaking effect of HC and oil when combined at varied concentrations, and to determine appropriate concentrations of HC and oil, to enhance bud- burst, yield and fruit quality. Three trials were conducted in the Elgin valley (34 °S, 300 m) of the Western Cape, South Africa, in 1999 and 2000. The first trial evaluated four concentrations (0, 0.5, 1 and 2%) of Dormex® (hydrogen cyanamide 520 g-L−1) in combination with four concentrations of mineral oil (0, 1, 2, and 4%). The second trial used three concentrations (1, 2 and 4%) of Dormex® in combination with three concentrations of mineral oil (1, 2, 4%), plus an unsprayed control, and a treatment of 6% of DNOC Winter Oil®. The third trial included five treatments: 0.5% Dormex® + 3% oil, 1 % Dormex® + 4% oil, 6% DNOC Winter Oil®, 6% oil and a non-sprayed control. All of the treatments were applied at the first visible signs of budburst. No synergistic effect was observed between oil and HC. Mineral oil at 4% plus 1 to 2% Dormex® were sufficient to break dormancy. Dormex® at 4% (2.08% HC) reduced fruit set and yield.

Quantification of bearing habit on the basis of lateral bud growth of seven pear cultivars grown under conditions of inadequate winter chilling in South Africa

Abstract The bearing habit of seven pear cultivars was quantified according to the ontogenetic development from axillary buds, i.e. developmental changes in the terminal position of laterals on fruiting branches. Ten non-pruned branches of seven pear cultivars, i.e. Forelle (on Quince A and BP1 rootstocks), Abate Fetel, Flamingo, Packham’s Triumph, Golden Russet Bosc, Rosemarie and Beurre D’Anjou (all on BP1 rootstock) were observed. Description started with the development of the main fruiting branch, forming several leaves in the first year of growth (designated year Y), with meristems developing in the leaf axils. In the following season (year Y+1), these buds had five developmental alternatives: to remain as a latent bud (L), to develop as a vegetative bud (V), to become a flower bud without fruit (F), to become a flower bud setting a fruit (P), or to abort and leave a scar (S). Each year the development of these buds was observed and classified anew, giving rise to sequences describing up to 5 years of development. From 44% (‘Beurre D’Anjou’) to 79% (‘Flamingo’) of laterals remained in the growing phase (phase G comprising of V, F or P buds). This coincided with low proportions of buds remaining in the latent phase (L), with even fewer buds moving to the ending state (S; extinction mechanism). A relatively large proportion of growing laterals went from the growing phase back to the latent phase (up to 21% in the case of ‘Beurre D’Anjou’), probably due to inadequate winter chilling. The predominant bud type in the growing stage was vegetative (V). Although flowering was generally low, ‘Flamingo’ and ‘Abate Fetel’ became reproductive (F or P) in year Y+3. ‘Packham’s Triumph’ and ‘Rosemarie’ displayed F and P buds in year Y+1, corresponding to the ability of these cultivars to bear fruit on longer 1-year-old shoots. Although the occurrence of the extinction mechanism was low, the higher proportion of latent buds may aid in reducing the number of growing buds, thereby increasing the allocation of assimilates to fruiting structures. This, in combination with the tendency of ‘Packham’s Triumph’ and ‘Rosemarie’ to develop longer, more autonomous shoots, may explain their higher productivity and the occurrence of bourse-over-bourse bearing.

The effect of rootstock cultivar on the yield and fruit quality of 'Packham's Triumph', 'Doyenne du Comice', 'Forelle', 'Flamingo' and 'Rosemarie' pears.

Production data and questionnaire results concerning the long-term effects of various rootstocks on pear (Pyrus communis L.) yield indicated that BP1 and BP3 are currently the preferred rootstocks in the South African industry. Yield records of ‘Packhams Triumph’ pears indicated that BP3 and Old Home × Farmingdale 97′ (OH×F97) rootstocks were superior to other rootstocks. Yield of ‘Doyenne du Comice’ was highest on ‘Quince A’ (QA) and BP3. The yield and fruit quality of blushed pear cultivars, Rosemarie, Flamingo, and Forelle, were evaluated in replicated trials on different rootstocks in different areas, planted in 1996 and 1997. Parameters measured included trunk growth, yield, fruit quality, and fruit mineral content. ‘Rosemarie’ showed indications of incompatibility with QA and ‘Quince C51’ (QC51), but produced good initial yields on BP1 and QA with a ‘Beurre Hardy’ interstem. BP1 induced a slightly larger tree. ‘Flamingo’ on QA and QC51 produced the best yields. ‘Forelle’ on BP1, BP3, and QA produced similar yields up to the fourth leaf. Further long-term studies are needed.

Growth regulator manipulation of apple bud dormancy progressions under conditions of inadequate winter chilling

Elgin (34°S, 19°E; 305 m.a.s.l.), typical of South African apple growing regions, accumulates 745 Utah Chill Units (CU) p.a. The chilling requirement of ‘Golden Delicious’ apple (Malus xdomesf/ca Borkh.) is c.a. 1100 CU. Consequently, the chilling requirement is not satisfied and delayed foliation is common. The aim of this study was to use plant growth regulators (PGR’s) to manipulate the progression of dormancy in order to reduce the chilling requirement of ‘Golden Delicious’ shoots in mature commercial orchards. A trial was conducted in a commercial orchard in Elgin during the winters of 2004, 2006 and 2007. To advance the onset of dormancy, 250 mg −1 abscisic acid (ABA) was sprayed several times during April and May of 2004 and 2006. To induce a shallower state of dormancy, cytokinins were applied during April and May of 2006 and 2007. Benzyl adenine (BA) was applied at concentrations between 250 and 1000 mg −1 and forchorfenuron (CPPU) at 15 mg −1. Progression of dormancy was assessed by harvesting shoots every 2–3 weeks from initial spray date until commercial rest breaking agents were applied in the orchard. The time interval for 50% of the shoots to exhibit budburst under controlled conditions was used as a parameter for depth of dormancy. Although shoots were sprayed on c.a. the same calendar dates each year and before any significant CU had accumulated, the physiological state of the buds at application varied from shallow to deep dormancy depending on the season. Therefore calendar dates were not a good criteria for spray applications and CU accumulation was not a prerequisite for the onset of dormancy. PGR’s altered the dormancy progression of ‘Golden Delicious’ shoots. However, their efficacy was dependant on the dormancy status of the buds at the time of application. Furthermore, the effect was not sustainable. The trees appeared to “normalize” after a short period of time and consequently the PGRs had no effect on the dormancy release or budburst the following spring.

Effect of timing and concentration of rest breaking agents on budburst in 'Bing' sweet cherry under conditions of inadequate winter chilling in South Africa.

The effect of timing of various rest breaking agents (RBAs) on vegetative and floral budburst, and production efficiency was investigated on 4-year-old ‘Bing’ sweet cherry trees on ‘Gisela® 5’ rootstock. Two experiments were conducted, near Clarens (28°28’S; 28°19’E, 1860 m.a.s.l) and Reitz (28°0’S; 28°28’E; 1717 m.a.s.l) in the eastern Free State, during 2005 and 2006, respectively. The 2005 experiment evaluated five treatments; 1% and 2% Dormex® (49% hydrogen cyanamide, HCN); 1% Dormex® + 3% Budbreak® oil (869.3 g L−1 mineral oil); and 3% Lift® (thidiazuron and mineral oil) sprayed on whole trees on three dates preceding expected “green-tip”, and an unsprayed control. In 2006 four treatments were evaluated; 1% Dormex®; 1% Dormex® + 3% Bud-break® oil (869.3 g L−1 mineral oil); 3% Lift® applied on three dates, and an unsprayed control. Compared to the control, RBAs were most effective at improving vegetative budburst during both seasons while results of floral budburst and production efficiency varied between seasons indicating that time of RBA application should be based on chilling accumulation and bud development stage and not only calendar date. The time of application of RBAs had the most significant influence on budburst and production efficiency. No interaction was observed between time and treatment.

Genotypic response to chilling period of apple buds from two Western Cape localities

One-year-old, ca. 40 cm long shoots of various apple cultivars were selected from commercial orchards in both the Elgin [34°S, 305 m, ca. 750 Utah Chill Units (CU)] and Koue Bokkeveld (33°S, 945 m, ca. 1300 CU) regions of the Western Cape, South Africa. Shoots were forced at a constant 25°C with continuous illumination after chilling at 4°C. The effect of chilling period on the budburst of each cultivar in both regions was estimated in 1998 and 1999 by determining: 1) % budburst; 2) the % terminal budburst; and 3) the rate of budburst [1/(days to 25% budburst)]. It was found that these indices, in reaction to chilling, differed significantly between cultivars, and within cultivars between areas. The rate of budburst was the most consistent in describing the reaction of buds to chilling and may be more useful for grouping cultivars according to chilling response.