Effects of Paclobutrazol and Uniconazole on the Growth and Development of Syzygium Campanulatum Korth

Syzygium campanulatum is a perennial indigenous species. It has been widely planted in urban areas as hedge plant, topiary and container planting. This ornamental species requires frequent trimming to maintain its shape due to its vigorous growth. However, the process of pruning or trimming is costly, time consuming and labour intensive. Such a practice may have to be scheduled as frequent as fortnightly to achieve the mentioned purpose, especially in the cases of maintaining the hedge plants and topiaries. Therefore, this study aims to discover an alternative approach in plant maintenance. The main objectives of the study are to determine the effects of plant growth retardants (PGRs) and the optimal dosage for controlling the growth of S. campanulatum for container purposes. This study also aims to determine the effects of PGRs on the appearance of this species after the treatment. The PGRs used were paclobutrazol and uniconazole. The first study screened paclobutrazol at ascending rates i.e. 0 gL-1, 1.25 gL-1, 2.50 gL-1 and 3.75 gL-1; and uniconazole at 0 mgL-1, 10 mgL-1, 20 mgL-1, and 30 mgL-1. The application of these PGRs inhibited the vegetative growth, reduced leaf area and increased the leaf area index significantly. Photosynthetic rate and transpiration rates of the plants treated with 3.75 gL-1 paclobutrazol were significantly reduced to 3.70 3molm-2s-1 and 1.43 mmolm-2s-1 respectively as compared to the control plants. On the other hand, uniconazole at 30 mgL-1 reduced transpiration rate and stomatal conductance to 1.26 3molm-2s-1 and 0.1 molm-2s-1 respectively as compared to the control plants. The most effective means for height suppression were application of paclobutrazol at 1.25 gL-1 and uniconazole at 10 mgL-1. The second study was carried out to determine the effects of these PGRs on plant tissue structure. The leaf and stem specimens of S. campanulatum were viewed under Scanning Electron Microscope, JSM 5610LV at an acceleration voltage of 15 kV. Cross sections of the treated leaf lamina of these PGRs showed that the palisade and spongy mesophyll cells were closely arranged due to the decreased leaf size. The palisade cell was found thicker following the treatment. As a result, the thickness of the treated leaf increased. Paclobutrazol at 3.75 gL-1 and uniconazole at 30 mgL-1 increased the palisade parenchyma by 35.17% and 37.56% respectively as compared to the control plants. Micrograph images of the stem cross section showed reduction in xylem thickness after the treatment with paclobutrazol or uniconazole. The xylem thickness were reduced by 48.56% and 39.38 after the application of paclobutrazol at 3.75 gL-1 and uniconazole at 30 mgL-1 respectively. This condition may affect water and nutrient uptake and ultimately slowed the growth of the treated plant. The third study was conducted to assess public preference on the appearance of the treated plants. A questionnaire form was designed and used in the survey. Results showed that 93.3% of the respondents agreed that the treated S. campanulatum plants had more attractive appearance and 96.7% of the respondents agreed that the aesthetic value of the treated plants increased. The results of these studies revealed that applications of paclobutrazol and uniconazole were very effective in controlling the growth of S. campanulatum in container, and hence extend the trimming cycle. This method can be a better alternative to overcome landscape maintenance problems such as shortage of funding, time and skilled gardeners.

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