Isa Bertling

Salicylic acid and methyl jasmonate improve chilling tolerance in cold-stored lemon fruit (Citrus limon).

Chilling injury (CI) is associated with the degradation of membrane integrity which can be aligned to phenolic oxidation activated by polyphenol oxidase (PPO) and peroxidase (POD), enzymes responsible for tissue browning. Phenylalanine ammonia-lyase (PAL) is a further enzyme prominent in the phenolic metabolism that is involved in acclimation against chilling stress. It was hypothesized that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by increasing the synthesis of total phenolics and PAL by activating the key enzyme regulating the shikimic acid pathway whilst inhibiting the activity of POD and PPO. Lemon fruit were treated with 10μM MJ, 2mM SA or 10μM MJ plus 2mM SA, waxed, stored at -0.5, 2 or 4.5°C for up to 28 days plus 7 days at 23°C. Membrane integrity was studied by investigating membrane permeability and the degree of membrane lipid peroxidation in lemon flavedo following cold storage. The 10μM MJ plus 2mM SA treatment was most effective in enhancing chilling tolerance of lemon fruit, significantly reducing chilling-induced membrane permeability and membrane lipid peroxidation of lemon flavedo tissue. This treatment also increased total phenolics and PAL activity in such tissue while inhibiting POD activity, the latter possibly contributing to the delay of CI manifestation. PPO activity was found to be a poor biochemical marker of CI. Treatment with 10μM MJ plus 2mM SA resulted in an alteration of the phenolic metabolism, enhancing chilling tolerance, possibly through increased production of total phenolics and the activation of PAL and inhibition of POD.

Anti-oxidant levels in various tissues during the maturation of ‘Hass’ avocado (Persea americana Mill.)

Summary Avocado is a fruit high in polyunsaturated fatty acids, with a high mineral content. However, no reports are available on how valuable avocado fruit could be as a source of anti-oxidants, depending upon their stage of maturity. Such a feature might become an important marketing tool in future. To evaluate the presence of anti-oxidant systems throughout avocado fruit development, various tissues were analysed for both their total and specific anti-oxidant compositions.Total anti-oxidant levels were found to be higher in the exocarp and in seed tissues than in the mesocarp, a possible reason for the susceptibility of avocado to post-harvest mesocarp disorders. While seed tissues contained predominantly ascorbic acid (AsA) and total phenolics (TP), the anti-oxidant composition of the mesocarp was characterised by the presence of the C7 sugar, D-mannoheptulose. Among the anti-oxidant enzymes assayed, peroxidase (POX) and catalase (CAT) were present in higher concentrations than superoxide dismutase (SOD) in mesocarp tissue. The low concentration of anti-oxidants in the mesocarp could be a major reason for the susceptibility of avocado fruit to physiological disorders.

Comparative performance of tomato cultivars cultivated in two hydroponic production systems

Cultivar selection for different hydroponic production systems is an important management decision, potentially impacting the tomato grower’s profitability. Knowledge on the performance of tomato cultivars, in specific hydroponic systems (open-bag and closed system) under South African conditions, is still very limited. The performance of four cultivars was evaluated in an open and a closed hydroponic (gravel-film technique) system. The commonly grown cultivars evaluated in each of the two hydroponic systems were ‘FA593’, ‘Malory’, ‘Miramar’ and ‘FiveOFive’. For each experiment a randomized complete block design was used with four replicates. Total, marketable and unmarketable yields, as well as internal fruit quality characteristics (total soluble solids (°Brix) and pH) were determined. Although no significant differences in total yield could be established - neither in the open nor in the closed hydroponic system - differences in marketable yield were observed. ‘Miramar’ and ‘Five-OFive’ produced the highest marketable yield in the closed system; the high unmarketable yield of ‘FA593’ and ‘Malory’ in the closed hydroponic system could be attributed to the high number of cracked fruit due to their inherent larger fruit size. There were no significant differences in °Brix between cultivars in the closed system. Cultivar ‘FiveOFive’, ‘FA593’ and ‘Miramar’ produced higher marketable yields than cultivar ‘Malory’ when grown in the open-bag system. ‘Malory’ and ‘FA593’ produced the highest number of fruit exhibiting fruit cracking in the open bag system. In the open system, only cultivar ‘Malory’ had a higher °Brix than ‘Miramar’ and ‘FiveOFive’. The most promising cultivars for local hydroponic tomato production, with regard to yield and quality, were identified as ‘Miramar’ and ‘FiveOFive’, with ‘FA593’ performing equally in the open system only. Further studies need to be undertaken on economical comparison of the two production systems.

Overhead irrigation increased winter chilling and floral bud production in Eucalyptus nitens

Eucalyptus nitens requires a sufficiently cold winter to produce flower buds. In areas in South Africa where E. nitens commercial plantations as well as breeding and production seed orchards are located, winter chilling is often insufficient for floral bud initiation. Hence, under such conditions, E. nitens floral bud and seed crops are poor and inconsistent. The local industry is almost entirely dependent on paclobutrazol (PBZ) applications for encouraging flowering in E. nitens seed orchards. Between 2008 and 2010, an experiment was conducted to investigate the potential of overhead irrigation (sprinkling) as a means of supplementing winter chilling to improve floral bud production in E. nitens. The treatments included three levels of sprinkling (nil, 10 weeks and 16 weeks duration), two levels of PBZ (nil, 0.025 g a.i. per mm basal stem circumference) and two grafted clones (prolific flowerer and shy-flowerer). Sprinkling reduced E. nitens daytime bud temperatures by as much as 16.2 °C on warm, dry winter days. In 2009 (cold winter) and 2010 (warm winter), sprinkling increased chilling accumulation by 44% and 72% (nil versus maximum sprinkling), respectively. In 2009, in the absence of PBZ, sprinkling resulted in a higher percentage of trees of either clone producing umbels (flower buds) compared with the control. In the warmer 2010 winter, sprinkling again increased flowering, with the number of flowering shoots and umbels per tree being significantly higher than the control at p < 0.05. In both 2009 and 2010, PBZ showed a strong additive effect to winter chilling on E. nitens floral bud production. The E. nitens clone × chilling × PBZ flowering interaction was complex and warrants more detailed investigation in future. Overhead sprinkling offers a practical method of supplementing winter chilling and improving floral bud production in high-chill-requiring temperate eucalypt species such as E. nitens.

Growth responses of tropical onion cultivars to photoperiod and temperature based on growing degree days

Onions (Allium cepa L.) are widely produced within the tropics, but little is known about the Eritrean cultivars Hagaz Red 1 and 2 (HR 1, and HR 2) and Red Creole (RC). Responses of the onion cvs. HR 1, HR 2 and RC to photoperiod and temperature on bulbing were compared. Plants were grown in growth rooms under combinations of photoperiod (11.5, 12, and 12.5 h) and day/night temperatures (25/12, 30/15 and 35/18°C). Growth responses were determined by growing degree day (GDD) base and the broken-stick regression model. There were relationships between bulb initiation and rate of leaf area growth under 12 and 12.5 h. Under the 12 h photoperiod, cultivars needed 343, 482, and 597 GDD before bulb initiation and 405, 432, and 431 GDD to increase rate of leaf area development at 25/12, 30/15, and 35/18°C, respectively. Under a 12.5 h photoperiod, cultivars needed 344, 423, and 432.2 GDD to initiate bulbing and 140, 411, and 579 GDD to increase leaf growth rates at 25/12, 30/15, and 35/18°C, respectively. Temperature induces variations in leaf number, plant height, leaf area, and affect bulbing and these responses may be used in the development of superior cultivars for tropical conditions.

Ripening physiology and quality of ‘Hass’ avocado (Persea americana Mill.) after cold storage at 1ºC

Summary The export of avocado (Persea americana Mill.) fruit into foreign markets has become a high priority for the South African industry due to increased competition in the European market. Some of these countries require phytosanitary treatment before access is granted. Cold disinfestation is the only suitable phytosanitary treatment available for avocado, but the fruit are generally considered to be sensitive to temperatures below approx. 5ºC. Phytosanitary disinfestation of South African avocados of acceptable fruit quality has been achieved by maintaining the pulp temperature below 2ºC for 28 d, but the effect on ripening physiology has not been determined. The effect of cold storage at 1ºC (air delivery temperature) on ‘Hass’ avocados with or without modified humidity packaging (MHP) for 28 d was examined. Compared to a conventional storage temperature of 5.5ºC for 28 d, fruit stored at 1ºC had significantly reduced rates of respiration and ethylene production, less softening, water loss, fresh weight (FW) loss, and mannoheptulose consumption measured immediately after cold storage. The use of MHP resulted in similar reductions at the conventional temperature and, when MHP was used in combination with 1ºC storage, these reductions were enhanced. Cold storage at either temperature resulted in increased cellulase and polygalacturonase activities during ripening when compared to non-stored fruit (kept at 21ºC until ripe), but no significant differences were recorded between the four cold storage treatments (i.e., 1ºC and 5.5ºC, with or without MHP) in the activities of cellulase, polygalacturonase, and pectin methylesterase during ripening. The number of days to ripen increased slightly after storage at 1ºC, but the percentage of marketable fruit was not significantly reduced. Significantly reduced FW and water losses from fruit stored under MHP resulted in significant reductions in the severity of skin spotting. The ripening physiology and quality of South African ‘Hass’ avocados were not negatively affected when fruit were stored at 1ºC for 28 d. When considered with previous results, this treatment appears to be suitable for future use.

Arbuscular mycorrhiza has limited effects on yield and quality of tomatoes grown under soilless cultivation

Abstract A study was conducted investigating the possible utilization of mycorrhiza to enhance yield and quality of tomatoes grown in a soilless culture system using sawdust or coir as growing media. The experiment was carried out in temperature-controlled and non-temperature-controlled (NTC) tunnels. Fertigation was applied at three levels (100%, 75%, and 50%) of the recommended nutrient concentration. Mycoroot™, containing four arbuscular mycorrhizal species (Glomus etunicatum, Paraglomus occultum, Glomus clarum, and Glomus mosseae), was applied at seeding, as well as transplanting. Growing tomatoes under reduced nutrient supply reduced the total soluble solids in the juice of the fruits, but improved total and marketable yield, as well as the number of marketable fruits. This effect was more substantial in the temperature-controlled tunnel than in the NTC tunnel. Fruit firmness and leaf chlorophyll concentrations were significantly higher in plants grown in the temperature-controlled tunnel. Growing tomatoes in sawdust improved the leaf Mn and Ca concentration over that of tomato plants grown in coir. Mycorrhiza colonization did not have a beneficial effect on tomato yield and quality. Further studies, including different media, nutrient composition, and concentration need to be carried out to investigate the possible effect of AMF failing to improve yield, despite AMF root colonization, and to reveal the cause of the poor beneficial effect of AMF on tomato plants grown under soilless culture.

Some initial changes in ‘Hass’ avocado (Persea Americana Mill.) physiology due to ethephon

Ethylene is used commercially to hasten ripening of avocados, but the presence of ethylene in the storage atmosphere can increase the risk of physiological disorders. To investigate the short term effect of ethylene on avocado physiology, fruit were dipped in a low concentration ethephon solution. The ethephon treatment significantly increased the ethylene production rate of avocados after 6 h, compared to untreated fruit. After 6 h, the respiration rate of treated fruit increased significantly during the investigation while that of the untreated fruit did not show a significant increase from the initial measurement. Lipid peroxidation increased after dipping, reaching a peak after 4 h, and then declined to a level similar to untreated fruit. Mannoheptulose declined significantly during the 6 h investigation in both the untreated and treated fruit. It is concluded that avocado fruit can undergo significant physiological changes, and quality loss, in a short amount of time, which is exacerbated by exposure to ethylene.

Yield and nutritional quality of different short-day onion cultivars as affected by nitrogen application

Onion is one of the most important vegetable crops in the world, cultivated and consumed for its flavour-enhancing and medicinal properties; hence, comprehensive understanding of crop responses to N application is of immense importance. To determine the effects of different nitrogen (N) levels on yield, quality and mineral content of onion, four short-day onion cultivars (‘Mata Hari’ [red onion], ‘Mikado’ [brown onion], ‘Cristalina’ [white onion] and ‘Star 5516’ [brown onion]) were subjected to six different N levels (0, 30, 60, 90, 120 and 180 kg ha−1). Application of 180 kg N ha−1 significantly increased total yield (46.51 t ha−1), whereas no N application resulted in poor yield (15.35 t ha−1). ‘Star 5516’ produced the highest marketable and total yield compared with the other cultivars, and ‘Mata Hari’ had the lowest marketable yield, possibly due to higher bolting percentage. An increase in N application resulted in an elevation of N, magnesium, manganese, zinc and sulphur concentrations in bulbs. Bulbs of the red onion cultivar had higher N, phosphorus, calcium, sodium, iron, zinc and sulphur levels than those of the other cultivars, with a significantly higher percentage total soluble solids and sulphur concentration than the white and brown onion cultivars. The results of this study show that increasing N fertilisation up to 180 kg N ha−1 increases the concentration of certain minerals in bulbs, as well as marketable yield, with ‘Star 5516’ yielding highest and ‘Mata Hari’ showing the best quality.

Monitoring eucalypt bud temperature using mobile temperature loggers

Winter chilling is a key environmental trigger of floral induction in temperate Eucalyptus. Over the past two decades, considerable site × eucalypt flowering interaction research has been undertaken over a range of high elevation (>1 100 m asl) forestry sites in the South African summer rainfall area. A practical method of accurately monitoring eucalypt bud temperature at remote sites in these areas was needed for this research. Utilisation of traditional methods of air and bud temperature measurement were not viable, due to significant risks of data and meteorological equipment loss posed by severe weather, vandalism and theft. Between 1996 and 2004, a robust structure for housing the miniature Hobo® temperature logger (hereafter termed ‘Hobo pole’) was designed and utilised for in-field monitoring of air temperature in the research trials. During 2009 and 2010, an experiment was conducted to investigate the relationship between E. nitens bud temperature (BudT), Hobo pole air temperature (HoboAT) and radiation screen air temperature (ScrnAT), and develop appropriate calibration models. Attempts to develop a single model for predicting hourly BudT from hourly HoboAT over the entire annual period yielded unsatisfactory results (maximum R 2 value 0.49). Separate winter and summer regression models were subsequently developed for predicting BudT from HoboAT, ScrnAT from HoboAT, and BudT from ScrnAT. In these regressions, R 2 values were generally slightly higher, and SE values lower, for mid-winter data than for mid-summer data. In mid-winter, BudT on HoboAT gave the highest R 2 value (0.99) and lowest SE value (0.49 °C) of all regressions. The Hobo logger/Hobo pole combination, together with developed regression models (presented in this paper), offers one practical, cost-effective solution for accurately monitoring eucalypt bud temperature at remote, high-elevation forestry sites in South Africa.