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The ripening of bananas is a process that needs to take place in accurately controlled conditions to ensure synchronization of all the metabolic processes that are involved with ripening so that the best quality product can be made available to the consumer. Optimal ripening requires the correct ambient room temperature and relative humidity, adequate ventilation and ethylene treatment. Banana ripening occurs at temperatures between 16º-18ºC, with 95% humidity and constant ventilation. Ripening is triggered when the bananas are exposed to ethylene for 24 hours. Fruit that ripens in ideal circumstances has a bright yellow colour, firm soft pulp and strong peel, are sweet tasting and have a pungent aroma. The conditions in the store and ripening rooms, especially temperature, are not always regulated to the optimum and subtropical fruits such as bananas are injured at temperatures below 12ºC. Phase transitions in the membranes, from a liquid-crystalline to a solid gel-like phase, are responsible for chilling injury. Chilling injury delays ripening, but does not noticeably influence the eating quality of the fruit. The bananas are still sweet with a firm soft pulp and strong peel. The aroma production is however reduced by the cold and undesirable colour changes occur. The chilling injured bananas are dull greyyellow in colour with brown or black spots on the peel. The peel sometimes becomes completely brown or black. The quality of the bananas is potentially influenced by this colour change since the consumer judges the fruit on visual appearance. Controlled atmosphere storage and modified atmosphere packaging reduce chilling injury and extends the life of the fruit due to delayed ripening. Chilling injured fruit consequently has less brown or black spots and sometimes even an improved yellow colour. A better quality product can be supplied to the consumer and the temperature control in the storeroom does not need to be 100% accurate. The advantages of these techniques are attributed to the modified atmospheres, namely reduced oxygen and increased carbon dioxide concentrations which influence respiration, ethylene synthesis and action. / Prof. C.S. Whitehead

Graduation date: 1972

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Initial activities were focused on isolation and characterization of fruit ripening-related genes from tomato. Screening of four tomato cDNA libraries at low stringency with 10 fruit development and ripening-related genes yielded ~3000 positives clones. Microarray expression analysis of half of these positives in mature green and breaker stage fruits resulted in eight ripening-induced genes. RNA gel-blot analysis and previously published data confirmed expression for seven of the eight. One novel gene, designated LeEREBP1, was chosen for further characterization. LeEREBP1 encodes an AP2/ERF-domain transcription factor and is ethylene inducible. The expression profiles of LeEREBP1 parallel previously characterized ripening-related genes from tomato. Transgenic plants with increased and decreased expression of LeEREBP1 were generated and are currently being characterized to define the function of LeEREBP1. A large public tomato EST dataset was mined to gain insight into the tomato transcriptome. By clustering genes according to the respective expression profiles of individual tissues, tissue and developmental expression patterns were generated and genes with similar functions grouped together. Tissues effectively clustered for relatedness according to their profiles confirming the integrity of the approach used to calculate gene expression. Statistical analysis of EST prevalence in fruit and pathogenesis-related libraries resulted in 333 genes being classified as fruit ripening-induced, 185 as fruit ripening-repressed, and 169 as pathogenesis-related. We performed a parallel analysis on public EST data for grape and compared the results for ripening-induced genes to tomato to identify similar and distinct ripening factors in addition to candidates for conserved regulators of fruit ripening. An online interactive database for tomato gene expression data - Tomato Expression Database (TED) was implemented. TED contains normalized expression data for approximately 12,000 ESTs over ten time points during fruit development. It also contains comprehensive annotation of each EST. Through TED, we provide multiple approaches to pursue analysis of specific genes of interest and/or access the larger microarray dataset to identify sets of genes that may behave in a pattern of interest. In addition, a set of useful data mining and data visualization tools were developed and are under continuing expansion.

Seasonal variations had a great influence on the ripening patterns of banana fruit. Normal ripening was enhanced or delayed by the effect of the maturing season on the fruit. Fruit maturing in summer (harvested in autumn) showed an increase in the rate of ripening, while fruit harvested in winter and spring showed a decrease in the rate of ripening. The banana fruit are sensitive to temperatures outside of their acclimated range. Due to ineffective storage and faulty handling procedures, cold and heat stress is a universal problem in bananas. Large amounts of fruit are lost due to heat and cold stress. Chilling injury occurs when bananas experience or are stored at temperatures below 13C, for a few hours to a few days. Temperatures slightly above 30C may cause extensive heat damage. The three main mechanisms suggested for the prevention of chilling and heat injury are the synthesis of small heat shock proteins, changes in the saturation of fatty acids of the membranes and suppression of the ethylene production and respiration rates. The prevention of cold and heat stress is investigated by monitoring the effect of short-term heat treatment and treatment with ethylene inhibitors on the ripening of the fruit. To determine changes in ripening the following eight ripening parameters are monitored: Yellowing, firmness of the pulp, sugar concentration, starch content, ethylene synthesis, respiration rate, rate of moisture loss and ion leakage. It was found that 1-methylcyclopropene (1-MCP) alleviates the symptoms of heat injury up to a point, but the symptoms of chilling injury were not affected. The effects of the three temperatures investigated: 35°C, 40°C and 45°C for different times. A short pre-heat treatment of 35˚C for 20 minutes proved successful to delay the ripening of the banana fruit, but did not protect the fruit against heat or chilling injury. The results of this study clearly indicate that both chilling and heat injury involve the action of ethylene and suggested that longer exposures to 1-MCP may be effective in preventing both types of injury. / Prof. C.S. Whitehead

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