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Institut für Kulturpflanzenwissenschaften

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    Premature fruit drop in mango (Mangifera indica L.) in Northern Vietnam
    (2011) Roemer, Malte G.; Wünsche, Jens Norbert
    Mango production in Northern Vietnam is mainly organized in farmer-owned, small-scale orchard operations. However, the production is limited due to excessive fruit drop presumably caused by unfavourable climatic conditions in combination with plant stresses during the fruit set period. There is a general belief that this phenomenon is caused by different combinations of stressing factors which may vary between regions and sites. In the mountainous area of Northern Vietnam, fruit drop of two main local cultivars ?Hôi? and ?Tròn? may be caused by environmental cues occurring particulary during fruit set. Environemntal stress factors may include excessive air temperature, low relative humidity, strong prevailing winds and little rainfall. These multiple stressors are likely associated with a time dependent change of the endogenous plant hormone auxin (indole-3-acetic acid, IAA) exported from fruit and within the pedicel. Field trials revealed, that fruit shedding could be reduced by irrigation and plant growth regulator (PGR) application throughout the 3-year experiment study; however, it remains unclear how climatic conditons might induce hormonal response and thus enhance fruit shedding at different stages of fruit development. The present research project consists of three studies. First it was to determined which single or multiple climatic cues trigger fruit shedding in mango. Second, it was attempted to alleviate fruit abscission by PGR spray application at post-bloom and early developmental stages of the fruit in comparison with regular irrigation scheduling to reduce extensive fruit abscission. Third, the morphological changes in the abscission zone of mango pedicel during fruit abscission were studied. The timely changes of plant tissue IAA concentration and its key role in the abscission process was also evaluated. The research work was conducted in a commercial orchard near the township of Yen Chau in 2007, 2008 and 2009. The experimental design consisted of 20 randomly selected 10-year-old mango trees of each of the cultivars ?Hôi? and ?Tròn?. Half of the trees were irrigated at 3-day-intervals by mircospinkler and the remaining trees served as non-irrigated controls. For the PGR applications, 3 trees of each cultivar in 2008, and 6 trees of each cultivar in 2009 were used. In both experiments, 10 randomly selected inflorescences per tree were labelled and counted twice per until end of the fruit drop period. Fruit tissue was collected on-site from irrigated and non-irrigated trees. The fruit export of the endogenous indole-3-acetic acid was analyzed by Radio-Immuno-Assay (RIA). Further, to clarify the morphological changes within the abscission zone (AZ) of mango pedicels, samples of irrigated and control trees were collected, fixed and embedded, using a modified dehydration and embedding technique by vacuum infiltration. The results of the first part of this study indicate that the onset of the hot, dry prevailing winds induced the fruit drop. Whether fruit drop was reduced by irrigation seems to depend on the level of soil water deficiency, hence the reduction of plant water potentials. The results of the second part of the study showed that PGR applications reduced excessive fruit drop. Although all chemical treatments indicated significant effects, a single spray application of N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU) and 1-naphtaleneacetic acid (NAA) effectively improved fruit retention. Furthermore, those spray applications led to a greater fruit set per inflorescennce than irrigation. However, IAA-export was not clearly affected by irrigation or PGR applications. The results of the third part of the study indicate positive effect of irrigation on fruit retention in both cultivars, which might be influenced by increased pedicel thickness thus increased carbon/nutrient availability to the fruit during critical environmental periods. Moreover, shortage of carboydrate supply to the fruit may be associated with a reduction of IAA-export out of the fruit and this in turn triggers the abscission process. In conclusion, this research proved that prevailing environmental conditons, particulary hot, dry winds, induce premature fruit drop in mango in Northern Vietnam. The identification of the physiological basis of premature fruit drop allowed the development of effective crop management strategies (e.g. PGR applications, irrigation) to overcome unfavourable environmental conditions and to reduce or even inhibit plant responses associated with premature fruit drop. The presented results suggest, that setting up an irrigation system increased fruit retention, which; however, is a great investment for farmers. It was shown that the ease of PGR application and the efficiency of the treatment is a promising alternative to irrigation in oder to prevent excessive fruit drop in mango. The development of effective, fruit drop reducing crop management strategies may also optimized crop loads and enhanced financial returns to growers.