Karen I. Theron

The effect of combined deficit irrigation and summer trunk girdling on the internal fruit quality of ‘Marisol’ Clementines

Abstract The influence of girdling, deficit irrigation, and a combination of girdling and deficit irrigation on fruit quality of ‘Marisol’ Clementines was investigated over two seasons, 1998 and 1999, at two different sites in South Africa. Girdling increased the total soluble solids (TSS) by 2–10%, however, the effect was not always significant. There were no trends observed for external fruit colour, fruit diameter, juice content (%), titratable acidity (TA) and TSS:TA ratios due to girdling. Deficit irrigation increased the TSS (by 2–17%) and the TA levels (by 9–13%) in both seasons and at both sites, increased the TSS:TA ratio at Citrusdal in 1998 (4–8%), and decreased the TSS:TA ratio (by 7%) in the second season (1999) at both sites due to the increased acidity levels (by 11–13%). Deficit irrigation had no major effects on external fruit colour and juice content in 1999, but decreased fruit diameter (by 10%) at Citrusdal. According to these results, deficit irrigation, summer trunk girdling, or a combination of both, can be used as cultural practices to improve internal fruit quality. The combination of girdling and deficit irrigation increased the TSS more than when the practices are executed on their own.

Evaluating the Effectiveness of Pre-Harvest Calcium Applications for Bitter Pit Control in ‘Golden Delicious’ Apples Under South African Conditions

ABSTRACT Pre-harvest foliar applications to increase fruit calcium (Ca) content and reduce bitter pit incidence, is a standard practice world wide. We re-evaluated the effectiveness of early season applications versus late applications of calcium nitrate (Ca(NO3)2) to reduce bitter pit in ‘Golden Delicious,’ under the present environmental conditions of the Western Cape, South Africa. Two periods of efficient uptake of external Ca were identified, viz., during cell division and the last few weeks before harvest. Foliar Ca applications during mid season (from 40 d after full bloom) were more effective in increasing fruit Ca content and reducing bitter pit incidence, than later applications. Thus, it is important to apply the bulk of the Ca(NO3)2 during the first few weeks after cell division to maximize the Ca content of ‘Golden Delicious’ apples.

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.

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.

Macro and micro element requirements of young and bearing apple trees under drip fertigation

‘Brookfield Gala’ apple trees on Merton 793 rootstock were planted on a loam sand soil at 2000 trees ha−1, trained to a central leader using the solaxe system and supplied with nutrients by drip fertigation. Trees were individually excavated at successive phenological stages in growth seasons two and three, and divided into structural components which were dried, weighed, and analysed to determine their macro and micro nutrient element concentrations. Losses in fruit, leaf drop and pruning were calculated, as were transfers to the permanent structure (fixation). To offset losses and fixation, second season trees needed to take up N, P, K, Ca, Mg and S at 34.5, 3.7, 23.8, 23.9, 5.8 and 1.8 g tree−1, respectively. Respective requirements for Mn, Fe, Cu, Zn, B and Mo were 118, 541, 18.1, 92.8, 79.4 and 2.1 mg tree−1. For third season trees producing 45.2 t ha−1, mineral requirements were within the ranges: N, 1.7–2.6; P, 0.3–0.4; K, 2.3–3.3; Ca, 0.5–1.9; Mg, 0.2–0.4; S, 0.2–0.2; Mn, 1.3–7.9; Fe, 28.7–32.6; Cu, 0.9–1.1; Zn, 3.0–5.5; B, 5.7–7.6 and Mo, 0.3–0.3 mg kg−1 yield.

Determining the probability of bitter pit in ‘Golden Delicious’ apples through the post-harvest mineral content of individual fruit

Summary Bitter pit fruit in commercial consignments of apples still poses an economic threat to exporters from South Africa. Mineral analysis of fruit has been used with variable success to predict bitter pit prior to harvest. The possibility of increasing the accuracy of existing predictive models by using analysis of individual fruit rather than pooled samples was investigated. By improving the normality of the distributions of the different minerals and decreasing the overlap between pitted and non-pitted fruit classes, we attempted to improve the reliability of predictions based on variable threshold values. Even though our model produced a correct classification of 85% for non-pitted fruit which can be useful, this was still below the required tolerance expected on the market which, at present, is less than 2% bitter pit in an overseas consignment. The classification for pitted fruit, 63%, was not satisfactory.

Differential dependence of apple (Malus domestica Borkh.) cultivars on the xanthophyll cycle for photoprotection

The dependence of fruit peel photosystems of ‘Granny Smith’, ‘Braeburn’, ‘Fuji’, ‘Golden Delicious’ and ‘Topred’ apple (Malus domestica Borkh.) peel on the xanthophyll cycle for photoprotection was studied under laboratory conditions. Mature fruit peel was either treated with 1 mm dithiothreitol (DTT) to inhibit the xanthophyll cycle or not treated. Fruit peel was subsequently exposed to photosynthetic active radiation stress of 1 500 µmol m−2 s−1 combined with heat stress of 45 °C for 3 h. Fruit peel photodamage was assessed by measuring the change in the maximum light use efficiency of photosystem II (Fv/Fm). The change in the concentration of the xanthophyll-cycle carotenoids zeaxanthin, antheraxanthin and violaxanthin plus lutein and β-carotene were analysed. The Fv/Fm of heat- and light-stressed DTT-treated (+DTT) ‘Granny Smith’ and ‘Braeburn’ peel had a low recovery after stress compared with the recovery Fv/Fm of similarly stressed peel not treated with DTT (−DTT). However, there was no difference in the recovery Fv/Fm between +DTT and −DTT ‘Fuji’, ‘Golden Delicious’ and ‘Topred’ peel. The photosystem of ‘Granny Smith’ and ‘Braeburn’ fruit therefore appear to have a higher dependency on the xanthophyll cycle for photoprotection than ‘Fuji’, ‘Golden Delicious’ and ‘Topred’ fruit.

The efficacy of 6-benzyladenine, gibberellins4+7 and prohexadione-calcium to increase fruit size in ‘Forelle’ and ‘Abate Fetel’ pear

Fruit size is an important marketing and quality parameter and has a significant effect on the economic value of fruit. The application of synthetic cytokinins are believed to enhance fruit size by stimulating and extending the cell division stage in fruit when applied at the correct time during fruit growth. In addition, a combination of prohexadione-calcium (P-Ca) with gibberellins4+7 (GA4+7) was used successfully on Japanese pear (Pyrus pyrifolia Nakai) and ‘Bing’ sweet cherry to improve fruit size. This combination of GA4+7 and P-Ca was evaluated and combined with 6-benzyladenine (6-BA) treatments on European pear (Pyrus communis L.) cultivars Forelle and Abate Fetel to determine whether fruit size can be improved under the Mediterranean-type growing conditions in South Africa. On both ‘Forelle’ and ‘Abate Fetel’ the combination of GA4+7 and P-Ca increased fruit size but the effect was more pronounced in ‘Abate Fetel’; further addition of 6-BA did not improve fruit size in these two cultivars over two consecutive seasons.

Rind texture and juice acid content of Citrus spp. as affected by foliar sprays of mono-potassium phosphate (MKP), urea ammonium phosphate (UAP) and mono-ammonium phosphate (MAP)

Rind coarseness of fruit of Citrus spp. is a general problem in the Citrusdal region of South Africa (Western Cape Province), often limiting the percentage of exportable fruit. Acid content of the juice of Citrus fruit is an important determinant of juice quality, and fruit with too high juice content are not preferred by consumers. The effects of foliar sprays of 1%, 3% and 5% mono-potassium phosphate (MKP), 2% urea ammonium phosphate (UAP) and 1% mono-ammonium phosphate (MAP) applied 4 or 6 weeks after full bloom (WAFB) on rind texture and juice acid content of selected Citrus spp. on rough lemon rootstock were evaluated from the 1998/99 to 2000/01 seasons. Foliar sprays of MKP and MAP reduced TA of ‘Thoro Temple’ tangor when applied 6 WAFB, and MKP, UAP and MAP improved rind texture of the tangors (‘Nouvelle’ and ‘Thoro Temple’) and oranges (‘Shamouti’ and ‘Valencia Late’) without affecting juice and SS contents.

Defoliation alters spring growth flush characteristics and inhibits flowering in Protea cv. Carnival

Abstract Inflorescence initiation in Protea cv. Carnival (P. compacta R. Br .×P. neriifolia R. Br.) starts at spring budbreak, and production of involucral bracts occurs concurrently with spring flush elongation. The presence of mature leaves on an over-wintering shoot is essential for inflorescence initiation on the spring growth flush of ‘Carnival’ indicating that conditions prevailing during winter, whether environmental or intra-plant factors, are conducive to flowering. Total defoliation applied 40 days before spring budbreak or earlier prevented flowering, and reduced the stem length and number of leaves on the spring growth flush. Later defoliation had a less marked effect on spring flush characteristics, and all shoots initiated flowers. Early defoliation prevented flowering either directly by a reduction in available carbohydrates or removal of photoperiod and temperature perceptive tissues, or indirectly by the resultant production of a weaker spring flush. Shoots are in the induced state and committed to flowering 6–7 weeks before inflorescence initiation begins. The induced state is retained for a period and then gradually lost.