Search Results

You are looking at 1 - 10 of 13 items for

  • Author or Editor: Gerard Jacobs x
Clear All Modify Search
Free access

Daniel G. Malan and Gerard Jacobs

Incandescent light night break (NB) and day continuation (DC) prevented flower formation in Leucospermum R.Br. cv. Red Sunset. Natural short days (NSD) during winter were inductive for flowering of intact shoots until 28 Aug. (Southern Hemisphere), but only until 24 July for decapitated shoots. Vegetative axillary buds released from correlative inhibition by shoot decapitation were less responsive to inductive short days (SD) than distal axillary buds on intact shoots. At least 42 inductive SD cycles were required for normal flowering after cessation of shoot growth. The effective length of the NB depended on the length of the NSD of winter. A 2-hr NB prevented flowering in vegetative buds released from correlative inhibition by shoot decapitation on 3 Mar., but was inadequate for axillary buds on shoots decapitated on 1 May. When the NB was begun during winter and discontinued before natural day (ND) lengths became too long in spring, the flowering time was delayed.

Free access

Nigel C. Cook and Gerard Jacobs

In 1997, 1-year-old, unbranched, ≈1-m-long shoots of the apple (Malus ×domestica Borkh.) rootstock `Merton-Immune 793' (MI.793) were selected at random from two commercial stoolbeds in the Western Cape, South Africa, during the dormant period. One site has mild winters [307 Utah Chill Units (CU) in 1997, 34°S, 300 m] while the other is moderately cold (1497 CU in 1997, 33°S, 950 m). In 1998, `Granny Smith' shoots were collected from a mature orchard in another warm area (574 CU in 1998, 34°S, 116 m). Shoots were prepared and forced at 25 °C with continuous illumination. During dormancy the developmental rate was determined of the terminal bud, and of both distally and proximally situated lateral buds, with or without the inhibitory influence of a distal disbudded shoot piece (10 cm long). In the moderately cold area, the growth rate of the terminal bud increased shortly before spring budburst such that a weak acrotonic tendency was established. The shorter dormant period, as experienced with the mild winters common to the apple-growing regions of the Western Cape, impeded the full development of acrotony and subsequent apical control. With less chilling (mild areas) a basitonic tendency remained. Budburst was slower and more erratic, and inhibition by the distal shoot parts was accentuated. Delayed foliation may be due more to correlative inhibition than to endodormancy. When lateral buds are released from paradormancy they exhibit a growth potential similar to, or, with less chilling, even greater than, that of the terminal bud. This permits a greater expression of autonomy between shoots.

Free access

Karen I. Theron and Gerard Jacobs

Flowering-size Nerine bowdenii bulbs were sampled from a commercial planting at 2-week intervals from 13 Aug. 1991 to 14 June 1992. They were dissected, and the following variables were recorded: 1) number and dry weight of fully sheathing leaf bases or leaves of each growth unit, 2) length and dry weight of foliage leaves, 3) fresh weight of outermost inflorescence, and 4) dry weight of daughter bulbs. Bulb organs that served as sinks and sources changed as the bulb progressed in its growth and developmental cycle. Before the new foliage provided photosynthates, growth depended on reserves deposited and stored in leaf bases during the preceding seasons. Reserves were used for the development of new leaves (foliage and bases), roots, and daughter bulb enlargement. Once the foliage became the photosynthate source, reserves were stored in old and new leaf bases. The inflorescence became the major sink when elongation of the scape initiated. Thereafter, daughter bulbs became the dominant sinks, receiving photosynthates from the senescence foliage and the reserves stored in leaf bases. The decrease in dry weight of the leaf bases was prominent in bulbs that remained in situ.

Free access

Nigel C. Cook, Etienne Rabe and Gerard Jacobs

Syllepsis is the predominant mode of branching in young peach and nectarine trees [Prunus persica (L.) Batsch]. Cultivars differ considerably in expression of apical control of sylleptic shoots. This has practical implications regarding tree training. Four cultivars were selected for increasing apical control by the central shoot axis, viz., `Zaigina', `Mayglo', `Fiesta Red' (all nectarines), and `Oom Sarel' (clingstone peach), respectively. Young, actively growing shoots were harvested when ≈300 mm in length, at a time when development of sylleptic shoots (laterals) had begun. Length, crotch angle, and position (as distance from the apex) of the laterals were recorded. When length of the laterals was plotted against their position, two zones were observed. The gradient of length vs. position was shallower in the distal than in the proximal zone. Autonomy in lateral shoots can be described as their ability to grow independently of apical control by the apex of the branch. Autonomy of laterals near the branch apex increased with their length. In `Zaigina' this was established via a more distal start of the second zone, and in `Mayglo' via an increased gradient in the second zone. The early loss or maintenance of apical control regulates architecture in sylleptically branched peach and nectarine shoots. Crotch angle widening of laterals appears to be largely dependent on position, but in some cultivars, such as `Mayglo', other factors are also involved. The data provide evidence of correlative phenomena between actively growing shoots.

Free access

Evelyn Marais, Gerard Jacobs and Deirdre M. Holcroft

`Cripps' Pink' apples (Malus ×domestica Borkh.) subjected to 72 hours of postharvest irradiation developed a better red blush with high pressure sodium (HPS) (hue angle 56.5°) than with UV-B plus incandescent (UVB+I) lamps (hue angle 70.7°). Only HPS lamps were used in subsequent experiments. The increase in red color (hue angle decrease of 14.9°) in `Braeburn' apples held at -0.5 °C for 8 weeks prior to treatment was smaller than in fruit stored for 4 weeks (hue angle decrease of 23°). No increase in color or anthocyanin concentration was observed in `Forelle' pears (Pyrus communis L.) that were similarly treated. `Forelle' pears were harvested with or without attached stem and leaves to determine whether precursor availability restricted postharvest color development. Fruit were irradiated with HPS at 20/20 °C and 20/6 °C (day/night) for 168 hours. The absence of leaves hastened the decrease in hue angle, but this was due to yellowing and not to development of red blush. Since `Forelle' pears showed no response to light after harvest, two fully red cultivars, Bon Rouge and Red Anjou, were irradiated with HPS lamps for 72 hours. Hue angle was not affected by irradiation. Thus, anthocyanin synthesis was stimulated by postharvest irradiation with HPS lights in apples, but not in pears.

Free access

Iain A. Stephens, Celeste Meyer, Deirdre M. Holcroft and Gerard Jacobs

Glucose, fructose, sucrose, and starch concentrations were determined in leaves and inflorescences of protea cutflower cultivars soon after harvest and at the onset of leaf blackening while standing in water. At the onset of leaf blackening sugars and starch were lower in both inflorescences and leaves. Proportionately, sugars and starch decreased more in leaves than in inflorescences. Flower-bearing shoots of `Sylvia' were pulsed individually with 5% glucose solution until each shoot had taken up 10 mL solution. Water served for control treatment. Flowers were then stored for 21 days at 1 °C. After pulsing and after cold storage groups of flowering shoots were separated into inflorescence, leaf and stem components and glucose and starch content determined. Glucose content, determined upon completion of pulsing treatments, was significantly greater in all shoot components of shoots pulsed glucose compared with nonpulsed control shoots. Glucose content of leaves was significantly greater after storage for shoots pulsed than control shoots. Starch content of leaves determined upon completion of pulsing treatments was significantly greater in shoots pulsed with glucose than that of controls. There was a significant decrease in starch content for all tissue types during 21 days of storage. Pulsing flower stems of seven protea cultivars before 3 weeks cold storage significantly reduced the incidence of leaf blackening when assessed both on day 1, and again on day 7 after 3 weeks of cold storage. Supplementing holding solutions with 1% or 2% glucose reduced leaf blackening of proteas pulsed with glucose and cold stored for 3 weeks.

Free access

Caroline J. Poole, Audrey I. Gerber and Gerard Jacobs

Brunia albiflora (Pillans) is harvested commercially in South Africa as a cut flower for export to European markets. To compete with European cut flowers high quality and continuity of product during the marketing period are essential. Optimizing the cut-flower potential of B. albiflora requires an understanding of the flowering process and selection of clonal material. We present a series of scanning electron micrographs which show three-dimensional images of the developmental stages of the shoot apex during the transition from the vegetative to the reproductive state. In B. albiflora the inflorescence consists of more than 15 individual rotund inflorescences arising from lateral positions on the terminal portion of the shoot. Development of the apical meristem of axillary shoots was studied to determine the time and sequence of inflorescence initiation and development. These observations identified that flower initiation occurs in October, followed by flower development through summer, with anthesis being reached from February to March.

Free access

Audrey I. Gerber, Karen I. Theron and Gerard Jacobs

Inflorescence initiation in Protea cv. Lady Di (P. magnifica Link × P. compacta R. Br.) occurs predominantly on the spring growth flush when it is subtended by one or more previous growth flushes. Mature, over-wintering leaves are essential for induction of flowering in `Lady Di', and are also crucial to the early stages of inflorescence initiation and differentiation. Defoliation before elongation of the spring growth flush was complete prevented flowering, and shoots either remained vegetative or produced inflorescences that aborted. Levels of carbohydrates in the stem and leaves of overwintering shoots were low, and early growth and development of both the spring flush and inflorescence were, therefore, supported by current photosynthates from the mature leaves on the overwintering shoot. Likewise, reserve carbohydrates available in the flowering shoot were insufficient to account for the rapid increase in dry weight during the major portion of growth of the spring flush and inflorescence. This increase occurred after elongation of the spring flush was complete and was supported by current photosynthates from the leaves of the spring flush. Defoliation treatments that did not prevent inflorescence initiation had no effect on inflorescence development or on flowering time.

Free access

Audrey I. Gerber, Karen I. Theron and Gerard Jacobs

The date of pruning affected flowering time of Protea cv. Sylvia (P. eximia × P. susannae) by influencing the flush on which inflorescence initiation occurred, and the harvest could be manipulated to fall within the optimum marketing period for export to Europe. Flowers initiated on the spring flush reached anthesis in January-February, those on the first and second summer flushes in April-May and July-August, respectively, and those on the autumn flush in November-December. Thus, flowering shoots harvested within the optimum marketing period (September to February) initiated inflorescences on the autumn and spring flushes. Because shoots on the spring flush initiated inflorescences readily, many flowering shoots harvested in January and February (following initiation the previous spring) were short and therefore unmarketable. For commercial production, pruning in July is recommended to allow harvest in October-December of the following year. Since the vegetative and reproductive cycles necessary to produce inflorescences on long stems span more than a year, a biennial cropping system is recommended.

Free access

Paul J.R. Cronjé, Gerard Jacobs and Nigel C. Cook

Two-year-old apple branches, ≈50 cm long, were selected from a commercial `Royal Gala' orchard in the Ceres (Koue Bokkeveld) region of the Western Cape, South Africa [33 °S, 945 m, 1500 Utah model chilling units (CU)]. In 2000, the branches received either cold storage at 5 to 7 °C or natural chilling in the field. In 2001, the trial was repeated, but only with field chilling. The branches received five dormant pruning treatments: control (not pruned); pruning back to the fourth lateral shoot (heading) before or after chilling; and removal of the second and third lateral shoots (thinning) before or after chilling. After pruning and chilling, the branches were removed from the orchard or cold room every 2 weeks and forced in a growth chamber at 25 °C. The rate of budburst (1/days to budburst) was determined for the terminal buds of the lateral shoots. Lateral shoots on the 2-year-old branches were categorized according to position: the most distal extension shoot, and all other laterals grouped. Removing distal tissue by pruning (heading more than thinning) enhanced the effect of chilling on the terminal buds on the lateral shoots and promoted budburst. Pruning was more effective before than after chilling. Pruning enhanced the growth potential of the terminal buds on proximal shoots on 2-year-old branches.